Role of Abscisic Acid in Plant Stress

Abstract

The multifaceted role of Abscisic acid (ABA) as a phytohormone of great repute cannot be overstated. ABA right after its synthesis within plastids embark on a quest to find specific receptors. On binding these receptors a complex signaling cascade is triggered that ultimately modulates gene expression and other cellular processes, responsible for normal growth and development processes of plants. Under abiotic and biotic stresses ABA levels change tremendously, triggering a cascade of physiological responses that help the plant adapt to its environment. A deeper understanding of ABA’s mechanisms like understanding its metabolic pathways or its regulation at genetic and epigenetic levels hold the promise of enhancing crop productivity and resilience in the face of the daunting challenges posed by a changing climate. Use of gene editing techniques like CRISPER-Cas technology, regulating the ABA mediated stress responsive genes, using RNAi and modifying the intragenic and promoter regions of the genes involved in ABA biosynthesis are a few methods which can enhance the ABA production or ABA mediated response to tolerate the stress conditions. In essence, ABA is a paramount player in plant stress responses, and unlocking its mysteries holds the potential to revolutionize agriculture and safeguard food security.

1. Introduction

The continuously growing world population is expected to reach about nine billion by 2050. This situation may pose a significant challenge to meet the increasing food demands by our already struggling agriculture industry (Figure 1). Factors such as water scarcity, soil degradation, and a variety of biotic and abiotic stresses further worsen the situation, affecting agricultural productivity. These stresses have detrimental effects on plant growth, development, and reproductive processes, leading to substantial crop losses.

Figure 1.

Global map of physical and economic water scarcity areas. Image source: IWMI.

However, research on Abscisic acid (ABA) has emerged as a promising avenue to mitigate the detrimental effects of these stresses. ABA, a lipid hormone, find its role in regulating seed dormancy, leaf abscission, and the accumulation of nutrient reserves in seeds [1]. Studies have shown that ABA levels increase significantly during periods of water shortage, leading to stomatal closure and reduced water loss through transpiration [2]. This hormone helps plants to cope with drought, high salinity, oxidative stress, photo stress, low temperature, and other environmental stresses. Understanding various mechanisms and functions of ABA has the potential to enhance crop resilience and improve agricultural productivity in the face of abiotic stresses.

ABA is also known to interplay with other hormones, such as Gibberellins, Auxins, Cytokinins, Jasmonic acid, Salicylic acid etc. to influence plant growth and development. Jasmonic acid and Salicylic acid are the phytohormones which modulate the plant’s defense against biotic stresses like action of pathogens, insects, pests and herbivores. By understanding the role of ABA and its interactions with other hormones, researchers can develop novel approaches to boost crop resilience and alleviate the detrimental effects of abiotic and biotic stresses [3].

By investigating the role of ABA and elucidating its mechanisms of action, researchers are paving the way for innovative strategies to mitigate the harmful effects of abiotic stresses and improve crop productivity. Thus, the study of ABA and its outcome on plant responses to environmental stresses ensure for addressing the challenges faced by agriculture. In the present chapter, various strategies employed by the plant to cope up with these stress situations will be discussed. Further, strategies to improve stress tolerance in plants will also be considered.

2. Abscisic acid (ABA): a crucial phytohormone

2.1 Background of ABA

Abscisic acid (ABA) was first discovered in the 1960s by a team of researchers led by Frederick Addicott at the University of California, Davis. The team, while studying the physiological processes involved in fruit abscission, found a compound that triggered this process [4]. Initially referred to as “abscisin II” as it was the second compound to be identified in the abscission process, it was later renamed “abscisic acid” due to its acidic properties and involvement in various physiological processes beyond fruit abscission [5]. Since its discovery, ABA has been extensively studied for its role in plant growth and stress responses [2]. Soon after its discovery, it was revealed that ABA levels increase significantly when plants wilt and that ABA causes stomatal closure. These two findings highlighted the utility of ABA in environmental stress mediating responses plants. Later, it was revealed that ABA is also associated with accumulation of seed nutrient reserves and attainment of desiccation tolerance in seeds. Regardless of its name, ABA is not a major regulator of abscission, which is mainly controlled by ethylene [6].

Plant stress response is a highly complicated process involving numerous genes to act in sync and interact with one another during the process. Stress induced gene products regulate the biosynthesis of the well known plant growth regulators like ABA, salicylic acid, and ethylene, which can then start the second round of signaling. In this mechanism, small molecules like ABA are crucial [7]. During the past 40 years, the core components of ABA biosynthesis and signaling have been identified through molecular, genetic, biochemical, and pharmacological approaches.

2.2 ABA structure

ABA is a sesquiterpenoid compound with a complex chemical structure, containing a 15-carbon isoprenoid chain and a carboxyl group at one end, which modulates its acidic properties [4]. ABA holds several functional groups, including hydroxyl (-OH), ketone (=O), and carboxylic acid (-COOH) groups, which contributes to its biological activity (Figure 2).

Figure 2.

Structure of abscisic acid (a phytohormone). Adapted from Finkelstein [6].

ABA’s molecular structure has various important features that make its biological functions possible. The side chains with the two double bonds (Figure 2) and ABA’s stereocenter are two such chief properties. It is the stereocenter of the ABA which is responsible for its chirality and existence of ABA in two mirror image forms called as enantiomers. Due to differing spatial orientations, enantiomers are engaged in various biological interactions and functions. Exposure to ultraviolet (UV) light changes ABA’s conformation from active to inactive form. This can be due to due to photoisomerisation where rearrangement of atoms within molecule leads to different conformations. Exposure to UV radiations also lead to crosslinking of ABA with neighboring ABA molecule, reducing its conformational flexibility. However, prolonged UV exposure may lead to cleavage of chemical bonds within ABA molecules which may lead to its fragmentation or loss of Functional groups and loss of biological activity.

2.3 Biosynthetic pathway of ABA

ABA is synthesized mostly in plant chloroplast and plastid-containing cells [6]. However, it is also produced in animals including humans and fungi. It is a sesquiterpene (C15H24) manufactured from isopentenyl pyrophosphate (IPP) in the 2-C-methyl-D-erythritol-4-phosphate (MEP) pathway or Mevalonic acid pathway. IPP is firstly converted to Zeaxanthin through a serious of enzymatic reactions involving condensation of IPP with Dimethylallyl pyrophosphate (DMAPP) producing Geranylgeranyl pyrophospahte (GGPP). GGPP is further converted to Zeaxanthin through cyclization and hydroxylation reactions. Neoxanthin is created from IPP with intermediate products of zeaxanthin and violaxanthin via an intermediate (antheraxanthin) in plastids. All-trans-neoxanthin, all-trans-violaxanthin and 9-cisneoxanthin can proceed as precursors for xanthoxin in ABA synthesis. Subsequently, xanthoxin is oxidized to ABA aldehyde and then to ABA in the cytosol through various intermediate enzymatic reactions. The complex chemical structure of ABA and its involvement in stress responses have made it a subject of extensive research in the field of plant biology (Figure 3).

Figure 3.

Abscisic acid (ABA) biosynthetic pathway.

Understanding the mechanisms underlying ABA synthesis and signaling pathways can have significant implications for improving crop yield and stress tolerance in plants. In the subsequent sections, we will focus on the role of ABA in plant growth, development and response to stress.

3. Role of ABA in plant growth and development

ABA plays a vital role in the general growth and development of plants by regulating various physiological processes. Here are some of the key functions of ABA in plant growth and development.

3.1 Seed dormancy

Seed dormancy is a crucial and essential process for plant survival and adaptation [8]. Seed dormancy can be considered as a stress tolerance response of the plant. It ensures that seeds remain dormant until favorable conditions for germination are present, increasing the likelihood of successful seedling establishment. This precise timing of germination is regulated by a complex network of genes and proteins. However, ABA is a key regulator to this process [9]. ABA accumulation in seeds promotes and maintains dormancy, while a decline in ABA levels triggers dormancy release and germination. Transcription factors, including Phytochrome Interacting Factors (PIFs), contribute to dormancy regulation. PIF1 interacts with ABA signaling components and controls the expression of genes involved in ABA biosynthesis and signaling. This interaction influences the timing of dormancy release and germination [8, 10].

The process of ABA induced seed dormancy involves the recognition of ABA by receptors called PYR/PYL/RCAR (Pyrabactin Resistance/PYR1-like/Regulatory Component of ABA Receptor) in the plasma membrane [2]. When ABA binds to these receptors, it triggers a series of signaling events. One of the key outcomes is the inhibition of protein phosphatases called PP2C (Protein Phosphatase 2C), which normally suppress the dormancy pathway [11, 12]. By inhibiting PP2C, ABA allows SnRK2 (Sucrose Non-Fermenting 1-Related Protein Kinase 2) to become active. These kinases phosphorylate ABF/AREB (ABA-Responsive Element Binding/ABA Response Element Binding) transcription factors, which then move to the nucleus and bind to specific DNA elements namely ABA-responsive elements (ABREs), activating or repressing genes related to seed dormancy and stress responses [13]. Apart from this receptor-mediated signaling, ABA also induces the production of LEA (Late Embryogenesis Abundant) proteins that help protect seeds from desiccation and other stresses during development and storage [14].

The coordination of various genes and proteins, such as ABA, ABI (Abscisic Acid Insensitive) proteins, DOG (Delay Of Germination) proteins, LEC (Leafy Cotyledons) genes, PIFs, and WRI1 (Wrinkled1), are crucial for seed dormancy regulation. This means that at higher concentrations of ABA, the activated ABF/ABRE gets associated with the above mentioned proteins and activates genes responsible for dormancy. However, on the onset of favorable conditions like light, moisture etc. ABA levels decline resulting a decrease in its inhibitory effects on germination. This allows ABF/AREB to become more active and able to attach strongly to the specific parts of genes involved in germination. The modified ABF/AREB moves into the nucleus and binds to specific regions in the genes called ABA-responsive elements (ABREs). In this situation, the modified ABF/AREB turns on genes that are needed for seedling growth, including genes that help break down stored nutrients, promote cell growth, and carry out other important processes for germination. This process can be aided by certain co-activators like CBP (CREB binding proteins); transcription factors NAC (NAM, ATAF1/2, CUC2) and chromatin remodeling factors like SWI/SNF (Switch/Sucrose Non-Fermentable). Understanding these mechanisms has significant implications for crop improvement and seed management strategies. By manipulating these genes and proteins, it is possible to develop crops with optimized germination characteristics [8, 9].

3.2 Seed germination

ABA helps to maintain seed dormancy by inhibiting the growth of the embryo until environmental conditions are favorable for germination. When conditions are suitable, ABA levels decrease, and the embryo begins to grow. ABA also has a part in changeable seed dormancy and seed germination with its interface with another plant hormone called gibberellins. Gibberellins are the hormones liable for signaling and stimulating remarkable increases in plant size, particularly in fruits and stems of plants. Both ABA and Gibberellins have reverse effects; plant hormones with opposite effects contribute to the balance within a plant obligatory for homeostasis. ABA also prevents loss of seed dormancy. Several ABA-mutant Arabidopsis thaliana plants have been identified and are available from the Nottingham Arabidopsis Stock Centre–deficient in ABA production and with altered sensitivity to its action. Plants that are hypersensitive or insensitive to ABA show phenotypes in seed dormancy, germination, stomatal regulation, and some mutants show stunted growth and brown/yellow leaves. These mutants reflect the importance of ABA in seed germination and early embryo development.

3.3 Fruit ripening

ABA plays quintessential role in regulating the ripening of fruit by promoting the breakdown of chlorophyll and the synthesis of pigments that give fruit its characteristic color. The changes in the levels of ABA concentration from very low in unripe fruits to tremendously high during fruit ripening depicts the ABA role in regulating fruit ripening.

In climacteric fruit such as apples, the level of ABA increases from maturation to harvest, while in non-climacteric sweet cherries, the level of ABA increases before maturation and thereafter decreases until harvest. Endogenous signals and environmental factors might affect ethylene biosynthesis primarily through ABA biosynthesis [15]. The differing ABA levels suggest that the role of ABA may vary between fruits. The application of ABA on fruit might be an effective tool for improving fruit quality and increasing health benefits. Fruit ripening being a complex process, sees dramatic changes in color, texture, flavor, and aroma of a fruit. There is much evidence that shows the role of ABA in fruit ripening and its involvement with fruit quality.

3.4 Root growth

Roots are crucial for the normal growth and development of plants. However, the architecture of the roots is significantly modified at the onset of environmental stresses. ABA controls root growth by preventing cell division and elongation at the root tip. This helps in regulating the root system’s depth and spread in response to environmental cues. Thus, it can be concluded that ABA influences root architecture by inhibiting root growth and promoting the formation of lateral roots, enabling plants to explore the soil for water and nutrients [16]. Depending on its dosage, ABA can either promote or hinder root growth. Even in well-watered environments, ABA can affect the root as well as shoot growth. ABA shows a biphasic effect on the growth and development of roots. Under favorable conditions, ABA is present in low concentrations, and it promotes root growth by enhancing cell division and elongation by acting in conjunction with auxin. However, under severe water stress conditions, the concentration of endogenous ABA rises and shows negative effects on the growth of roots. The modest biphasic effects of dry soil on root growth were enhanced by water deficit but hindered by severe water deficiency. Furthermore, Li et al. [17] showed that exogenous ABA had complex biphasic effects on root growth in well-watered settings also.

Changes in the root environment will consequently affect ABA-mediated responses on a local and systemic level [18]. Abiotic stress agents like drought, salt salt concentration and osmotic stressor lead to increased osmotic stress in roots. The fact that ABA increases water flow and ion flux in root tissues indicates that it modulates turgor by both boosting water influx into roots and decreasing transpiration [19].

4. Plant stress biology and role of ABA

Environmental changes are known to pose stress-like conditions for living organisms. Environmental stress can be classified as biotic or abiotic. The impact and intensity of the stress are two major criteria for living organisms to prompt the necessary response. Plants, being sessile, are readily prone to environmental stresses, which significantly affect plant growth and productivity in both natural environments and agricultural settings. Therefore, it is crucial to understand the different types of plant stresses and their underlying causes in order to develop effective strategies to mitigate their negative impacts.

4.1 Abiotic stress

Abiotic stress is the term used to describe the non-living components of environmental elements. These stressors can affect the plants negatively by impairing their growth, development and productivity. Plants must rapidly adapt to different abiotic stress conditions, such as drought, oxidative stress, salinity, cold and high temperature stress, as they are immobile. This sedentary life style makes them more vulnerable to effects of environmental stresses as compared to mobile organisms. To survive, plants have evolved various strategies to deal with these challenges [20]. As studied earlier ABA plays a significant role in facilitating plant responses to various abiotic stresses. By controlling physiological and molecular processes, ABA aids in plant adaptation and resilience in the face of challenging environmental conditions like drought, high temperatures, salinity, and extreme cold. Some major abiotic stresses where ABA has significant role are discussed in the subsections.

4.1.1 Drought stress

Drought stress is one of the most prevalent forms of plant stress, characterized by limited water availability. Drought can be defined as a situation when plant does not get sufficient water required for maximum growth and productivity [21, 22]. Water is an important component of plant’s transport system, as it helps in transfer of metabolites from one part of the plant to other. It also helps in maintaining the turgor pressure in the plant that allows the plant to grow in upright direction [23]. Plants use many strategies of drought avoidance, like possessing deep roots to increase the area of root zone effect [24], conservation of water by decreasing the total number of stomata, leaf widening, decreasing the leaf size, etc. (Figure 4). When drought stress is applied, the root system has been observed to adjust to the stress by aligning toward the wet patches in the pot [25]. Plants use many strategies to deal with unfavorable drought stress but a complete reprogramming of the transcriptional system is mainly required to show tolerance [26]. Since plants do not have a nervous system, therefore, hormones act as intraorganismal messengers, impacting the signal transduction, to make plants to deal with environmental changes. Decrease in water content can activate a set of physiological processes and bring about change in the localization of phytohormones, as adaptive response to osmotic stress [27].

Figure 4.

Plants strategies in response to stress.

Abscisic acid is a well characterized plant hormone, which plays a very important role during environmental catastrophes. Transpiration can be problematic for the plant during the drought stress. Stomatal closure and leaf wilting are the ways to diminish the effect of transpiration and save water. Abscisic acid plays a crucial role in stomatal closure and reduction in photosynthesis which in turn is one of the most important plants’ response against drought stress.

Abscisic acid accumulation has been a vital characteristic of drought tolerant plants as compared to drought sensitive plants [28]. Several studies have been conducted to check the endogenous level of ABA in stress affected plants. Exogenous spray of ABA was also found to enhance the drought tolerance in Bermuda grass [29].

4.1.1.1 ABA and stomatal closure

ABA helps to regulate the opening and closing of stomata, which are pores on the surface of leaves that allow for gas exchange. In response to drought or other environmental stresses, ABA triggers the closure of stomata, which helps to conserve water. This system regulates the absorption and release of certain gasses such as carbon dioxide, oxygen, and water vapor. Stomatal opening and closing occurs as a result of turgor pressure differences in the surrounding guard cells. ABA is formed in several plant parts, one of them being within the roots of plants. Roots produce ABA in response to identification of low or no moisture in the soil. ABA is then transferred from the roots to the leaves. ABA decreases the turgor of the guard cells causing them to close. This reduces plant transpiration, or the evaporation of water out of the stomata of leaves, thus prevents water loss and wilting in times of low moisture or drought. Furthermore, plants use stomata to absorb carbon dioxide out of the atmosphere which the plant needs to carry out photosynthesis. If the plant already has sufficient carbon dioxide, ABA signals the plant to close its stomata. The discovery and functional characterization of ABA-dependent candidate genes responding to drought was accelerated by recent developments in plant genomics. It was discovered that the MATE transporter gene AtDTX50 is implicated in ABA efflux and that mutants of dtx50 exhibit increased drought tolerance with decreased stomatal conductance in comparison to WT (wild type) plants [30].

4.1.1.2 Regulation of drought tolerance and ABA

Plants must consistently adjust ABA levels in response to shifting physiological and ecological situations. The synthesis of ABA is tightly regulated by various environmental factors, such as drought stress, which can induce the expression of genes involved in ABA biosynthesis [31]. The methods for raising ABA levels are still not fully understood. Both ABA-dependent and ABA-independent systems are involved in how plants respond to stress (Figure 5). To increase resistance to drought and salinity stress, a full understanding of how TF (transcription factors) pathways and ABA interact to generate stress responses is necessary. It is known that a number of transcription factors, including DREB2A/2B (Dehydration-Responsive Element Binding), AREB1, RD22BP1(Responsive to Dehydration 22 Binding Protein 1), MYC/MYB (Myelocytomatosis/ Myeloblastosis), and others, regulate the expression of the ABA-responsive genes by interacting with the corresponding cis-acting elements, DRE/CRT (Dehydration-Responsive Element/Cis-acting Regulatory Elements), ABRE (Abscisic Acid Responsive Element), and MYCRS/MYBRS (Myc-Responsive Cis-Element/Myb-Responsive Cis-Element), respectively. It’s important to comprehend these systems to improve crop plants’ ability to withstand stress.

Figure 5.

ABA dependent and ABA independent signaling in plants.

Abscisic acid responsive elements (ABREs) are short DNA sequences (consensus sequence ACGTG (G/T) (G/T) C) found in the promoter regions of many genes that are involved in ABA signaling pathways. ABREs are recognized by transcription factors, such as ABRE-binding factors (ABFs), which can bind to and activate gene expression. The role of ABREs in ABA signaling is to mediate the transcriptional response of genes to ABA. When a plant is exposed to stress conditions, such as drought or high salinity, the concentration of ABA in the plant increases. This increase in ABA concentration leads to the activation of ABFs, which in turn bind to ABREs and induce the expression of genes that are involved in stress responses, such as those that encode for osmoprotectants and stress-related enzymes.

ABREs have been found to be involved in the regulation of many genes that are involved in stress responses in various crops, including rice, wheat, maize, and soybean. The discovery and characterization of ABREs have provided insight into the molecular mechanisms underlying ABA-mediated stress responses in plants. Studies in the promoter region of the Arabidopsis thaliana RD29A gene and rice OsRab16A gene contains contain one and three functional ABRE respectively, essential for the ABA-mediated induction of RD29A expression in response to dehydration stress and OsRab16A expression in response to salt stress [32, 33]. Similar studies in wheat have shown that the promoter region of the wheat TaERF1 gene contains multiple ABREs that are involved in the ABA-mediated induction of TaERF1 expression in response to drought stress [34].

Studies have further revealed that Farnesyl-transferase has some role in reducing the abscisic acid sensitivity of the plants; it shows negative correlation with the activity of abscisic acid. Therefore, it has been proposed that down regulation of farnesyl-transferase can help in increasing drought tolerance. In Arabidopsis, the down regulation of any subunit α- or β, of farnesyl-transferase, increases drought tolerance [35]. In canola, the down regulation of farnesyl transferase (α-subunit) by using drought responsive promoter of At HPR1 (Arabidopsis hydroxypyruvate reductase), driving an RNAi construct, resulted into enhanced closure of stomata thereby, minimizing the loss of water by transpiration and yield protection against drought in fields. The strategy can be used for enhancing drought tolerance in many crops [36].

4.1.2 Salinity stress

Salinity stress occurs when plants are exposed to high levels of salt in the soil or water, disrupting the osmotic balance within plant cells. This disrupts water uptake and causes ion toxicity, leading to reduced crop yield and limited cultivable lands [37]. According to a report by the Food and Agriculture Organization (FAO), approximately 20% of irrigated agricultural lands worldwide encounter salinity issues, leading to diminished crop productivity and economic setbacks (FAO, 2020) [38]. Salinity stress negatively impacts multiple facets of plant physiology, encompassing water balance, ion homeostasis, and gene expression [16]. To counteract the deleterious effects of salinity stress, plants have developed intricate mechanisms. Among these mechanisms, abscisic acid (ABA) emerges as a pivotal participant, acting as a phytohormone that regulates plant growth, development, and responses to stress. Salinity stress influences the biosynthesis of ABA, resulting in heightened ABA levels in various plant tissues, including roots and leaves [39].

Studies have shown that exogenous application of ABA can improve salt stress tolerance in various crops, such as wheat, maize, and rice. ABA application in Wheat plants exposed to salt stress, improved plant growth, reduced ion toxicity, and enhanced the activities of antioxidant enzymes, leading to reduced oxidative stress and improved salt stress tolerance [40]. Furthermore, genetic manipulation of ABA biosynthesis and signaling pathways has been used to enhance salt stress tolerance in crops. Over expression of genes involved in ABA biosynthesis, such as NCED (9-cis-epoxycarotenoid dioxygenase), and genes involved in ABA signaling, such as ABF (ABA-responsive element-binding factor), has been shown to enhance salt stress tolerance in Solanum lycopersicum L. [41].

Modulation of ABA levels and signaling pathways represents a promising approach for improving salt stress tolerance in crops. Elevated ABA levels serve as a signal for plants to initiate adaptive responses. ABA regulates stomatal closure, reducing water loss through transpiration, thereby aiding in water conservation and cellular hydration [4]. Moreover, it influences ion transport, facilitating the uptake of vital potassium (K+) ions while impeding the entry of toxic sodium (Na+) ions into plant cells, thus maintaining ion homeostasis [39].

At the molecular level, ABA acts as a transcriptional regulator, modulating the expression of stress-responsive genes. It activates genes encoding proteins such as late embryogenesis abundant (LEA) proteins and osmoprotectants, which play crucial roles in safeguarding plant cells against dehydration and preserving cellular integrity [16]. ABA regulates the expression of genes encoding antioxidant enzymes, counteracting the harmful effects of reactive oxygen species (ROS) generated during salinity stress [39]. ABA-mediated responses to salinity stress extend beyond cellular processes and encompass overall plant physiology and development. Additionally, ABA governs seed germination and dormancy, ensuring that germination occurs under favorable conditions while preventing premature seedling emergence in saline environments [39]. Thus it can be conclude ABA plays a central role in plants’ response to salinity stress by regulating stomatal closure, ion transport, gene expression, root architecture, and seed physiology. Understanding the mechanisms underlying ABA-mediated responses is vital for developing strategies to enhance salinity tolerance in crop plants and mitigate the adverse impacts of salinity stress on agricultural productivity.

4.1.3 Heavy metal stress

Some micro-nutrients essential for plant growth and development are heavy metals (HM), which are naturally occurring elements with large atomic weight and density. Excessive HM concentrations, however, adversely impact plant development and survival. Because of anthropogenic activities like industrialization, smelting, extensive mining, fertilizer application, polluted water irrigation, fossil fuel combustion, and vehicle emitted gasses and pollutants, the intensity of some heavy metals like Cd, Cu, Pb, Hg, and Cr is skyrocketing in agricultural and other natural areas [42]. Abiotic stress, mostly brought on by heavy metal toxicity, has harmful impacts on humans, plants, and animals’ health. Due to their high reactivity, they have a negative impact on the energy synthesis, growth and senescence processes. Elevated HM levels increase ROS generation, causing metabolic imbalance, disturbing ion homeostasis, disorganizing the antioxidant defense system, disrupting protein structure, pigment synthesis, enzyme activities and membrane integrity, increasing lipid peroxidation, and thus decreasing plant growth and productivity [43].

In essence, plants under HM stress evolve defense systems to mitigate its negative effects but not to prevent them. Yet, through boosting osmolyte accumulation and antioxidant machinery, exogenous ABA treatment is a strategy for improving plants’ resistance to HM toxicity. ABA regulates a number of physiological functions to assist plants in surviving the detrimental effects of HM stress [44]. ABA modifies the transfer of hazardous metals from roots to shoots. It inhibits the long-distance transmission of HM by closing stomata and decreasing transpiration rate [45]. Some plant species under cadmium (Cd) stress, including Oryza sativa L., Solanum tuberosum L., Brassica napus L., Triticum aestivum L., and others, activate genes involved in ABA production, increasing endogenous AB concentration [44, 45]. For instance, 10 and 20 mM foliar ABA spray increased growth, chlorophyll content, photosynthetic efficiency, catalase activity, and proline and protein levels in Lactuca sativa L. to reduce Cd inhibitory effects [46]. According to Deng et al. [47], treatment of 5 mM ABA reduced root damage caused by Cd by reducing cell death, hydrogen peroxide levels, and malondialdehyde concentrations. Moreover, through increasing chlorophyll content, osmolyte concentration, and antioxidant defense mechanisms, 10 mM foliar ABA action reduced Cd toxicity in Vigna radiata L. [48]. It is confirmed that ABA is essential for improving agricultural plant morphology, development, yield, and quality indices as well as reducing the risks associated with HM stress [42].

4.1.4 Temperature stress

Temperature stress encompasses both heat stress and cold stress. High temperatures can damage plant cells, impair photosynthesis, and accelerate water loss. Conversely, low temperatures can cause chilling injury, decrease metabolic activity, and affect nutrient uptake [49].

Cold stress presents a notable obstacle to the growth and development of plants, leading to decreased crop productivity and subsequent economic losses [50]. Low temperature causes cellular damage, disrupt vital physiological processes, and impair photosynthesis [51]. On the exposure to cold temperature, ABA synthesis and accumulation is triggered in plant tissues. ABA acts as a signaling molecule, initiating a cascade of physiological and molecular changes aimed at enhancing cold tolerance [52]. ABA regulates gene expression by activating specific stress-responsive genes involved in cold acclimation and the synthesis of protective proteins [53]. This activation helps plants adapt to the cold environment and mitigate cellular damage. One of the key roles of ABA in response to cold stress is the modulation of osmotic stress. ABA promotes the accumulation of cryoprotectants, such as soluble sugars and proteins, which safeguard cellular structures from freezing temperatures [54]. These cryoprotectants act as osmolytes, maintaining cellular hydration and preventing the formation of ice crystals that could damage cells.

Cold stress triggers ABA accumulation in guard cells, leading to stomatal closure and reducing water loss [55]. This process helps maintain cellular hydration, prevents excessive dehydration, and preserves cellular integrity. Furthermore, ABA interacts with other signaling molecules and hormones to coordinate the plant’s response to cold stress. These interactions contribute to the enhancement of cold tolerance in plants. Understanding the molecular mechanisms underlying ABA’s role in cold stress response is crucial for developing strategies to enhance plant tolerance and improve crop productivity. The identification of ABA signaling components and downstream target genes has provided valuable insights into the intricate regulatory networks involved in cold stress response [56].

High temperature is majorly associated with accelerated water loss and for such type of osmolytic stress, ABA accumulation has been observed as the key strategy of the plants to overcome its detrimental effects. Therefore, it can be concluded that temperature stresses namely heat stress and cold stress affects the osmotic balance of the plants in general. And, plants have evolved a common mechanism of ABA biosynthesis and signaling which leads as a plant’s response to the temperature stresses.

4.1.5 Oxidative stress

Oxidative stress occur when there is an imbalance between the production of Reactive oxygen species (ROS) and the antioxidant defense system in plants. ROS, including superoxide radicals, hydrogen peroxide, and hydroxyl radicals, are natural byproducts of various metabolic processes. Though ROS serve as signaling molecules in plant growth and development, their excess accumulation can lead to oxidative damage to cellular components [57]. The effects of oxidative stress on plants are diverse and can impact various physiological and biochemical processes. For instance, oxidative stress disrupts membrane integrity, resulting in lipid peroxidation and loss of cellular homeostasis [58]. It can also impair photosynthesis by damaging the photosynthetic machinery, including chlorophyll and photosystem proteins [59]. Moreover, oxidative stress can cause DNA damage and affect the stability and functionality of proteins [60].

To counteract the detrimental effects of oxidative stress, plants have evolved complex antioxidant defense systems. These systems include enzymatic antioxidants such as superoxide dismutase (SOD), catalase (CAT), and peroxidases, as well as non-enzymatic antioxidants like ascorbic acid (vitamin C), glutathione, and tocopherols (vitamin E). These antioxidants scavenge ROS and protect cellular components from oxidative damage [61]. In relation to oxidative stress, abscisic acid (ABA) has been found to play a crucial role in plant responses. ABA treatment has been shown to enhance antioxidant defense mechanisms and mitigate oxidative damage, thus improving plant tolerance to oxidative stress [62]. ABA regulates the expression of antioxidant genes and increases the activities of antioxidant enzymes leading to reduced ROS accumulation and lipid peroxidation [63]. Additionally, ABA promotes the accumulation of non-enzymatic antioxidants, such as ascorbate and glutathione, which contribute to enhanced ROS detoxification and protection against oxidative stress [64]. Understanding the relationship between ABA and oxidative stress is crucial for developing strategies to enhance plant stress tolerance and improve agricultural productivity. Continued research in this field will provide further insights into the molecular mechanisms underlying plant responses to oxidative stress and aid in the development of innovative approaches to mitigate its negative impacts.

4.1.6 Photo-stress

Photo-stress refers to the harmful effects caused by excessive light exposure in plants, leading to physiological and biochemical imbalances that can impact plant growth, development, and survival. Plants have developed adaptive mechanisms to cope with photo-stress, and one important factor in this response is abscisic acid (ABA). ABA acts as a signaling molecule and stress hormone, helping plants adapt to high light intensity and counteract its adverse effects. Abscisic acid deficient plants may arise due to mutation or epigenetic regulation at any level, in the abscisic acid pathway. In Arabidopsis, BCH1/BCH2 (Brassinosteroid insensitive 3) mutants showed a decrease in the production of zeaxanthin which encompasses a small proportion of β-carotene pool in the leaves. Zeaxanthin is a photo protective material and increases in quantity in response to sudden high light intensity via xanthophyll cycle. Zeaxanthin epoxidase (ZEP), a plastid imported protein located at ABA1 (AT5G67030) locus of Arabidopsis, shows spatial regulation in different species. Drought induced ZEP expression was observed in case of roots but absent in leaves of tomato [29, 30]. In high light conditions, ABA triggers the closure of stomata, reducing water loss through transpiration and preventing dehydration. This adaptive response allows plants to conserve water and maintain cellular integrity during periods of intense light exposure. Moreover, ABA also plays a role in regulating the antioxidant defense systems in plants. Excessive light exposure during photo-stress can generate reactive oxygen species (ROS) that cause oxidative damage to plant cells. ABA enhances the activity of antioxidant enzymes and promotes the synthesis of antioxidants, enabling the scavenging and neutralization of ROS. This protective mechanism helps shield plant cells from oxidative stress caused by excessive light. Extensive research has been conducted to understand the involvement of ABA in plant responses to photo-stress.

4.2 Biotic stress

Plants, with their sedentary lifestyle, face unique challenges when it comes to coping with biotic stress compared to animals. While animals can move to escape threats or actively defend themselves, plants being rooted in place, making them vulnerable to various pests and pathogens. Biotic stress in plants, which includes the attack of pests, pathogens, and herbivores, can have far-reaching consequences on plant growth, development, and overall fitness. Biotic stress can disrupt ecological balance by affecting plant populations and interactions with other organisms. The loss of a dominant plant species due to pest infestation can lead to changes in community structure and species composition, impacting ecosystem functions and services.

Disruption of photosynthetic machinery leads to reduced photosynthetic efficiency which is one of the most common consequences of Biotic stress. Pests and pathogens may damage chloroplasts, interfere with photosynthetic pigments, or induce the production of reactive oxygen species (ROS), thereby impairing the plant’s ability to convert sunlight into chemical energy. Insects and pathogens on the other hand can feed on plant tissues, leading to tissue loss, reduced fruit set, or even complete crop failure. The economic impact of yield losses due to biotic stress is substantial, affecting farmers’ income and food security at both local and global scales. Certain pests like nematodes can cause root galling, affecting nutrient uptake, while fungal pathogens can colonize the root system and disrupt nutrient transport pathways. Nutrient deficiencies can further weaken the plants, making them more susceptible to additional stressors.

4.2.1 Biotic stress and phytohormones

In response to these stressors, plants have evolved refined defense mechanisms and signaling pathways to mitigate the damage caused by biotic stress. These mechanisms involve both physical barriers and intricate signaling networks. Interestingly, while biotic stress can have negative consequences, it can also trigger Induced Systemic Resistance (ISR) in plants. ISR is a defense mechanism where localized stress responses in one part of the plant activate systemic defense responses throughout the entire plant. This response helps plants to ward off subsequent attacks by pests or pathogens. Abscisic acid (ABA) is a key player in regulating plant responses to both biotic and abiotic stresses. ABA interacts with other signaling pathways, forming a complex network that fine-tunes plant responses to biotic stress. Abscisic acid (ABA), jasmonic acid (JA), and salicylic acid (SA) are three key phytohormones that play crucial roles in fine-tuning plant defense responses to biotic stressors. While ABA primarily regulates plant responses to abiotic stresses, it also interacts with JA and SA pathways to modulate defense against biotic stress agents.

This crosstalk is often fine tuned as per stress condition. Under certain severe abiotic stress conditions, increased ABA levels can suppress JA signaling and defense responses associated with herbivore attacks. ABA promotes the accumulation of protein phosphatases, which dephosphorylate and inactivate key components of the JA signaling pathway, such as MYC2 transcription factors. This antagonistic interaction helps plants conserve resources by down-regulating defense mechanisms against herbivores when facing severe abiotic stress. However, while concurrently dealing with abiotic and pathogen attack sometime ABA can positively regulate Jasmonic acid responsive genes for action against necrotrophs [65].

Similarly, in case of salicylic acid there is a complex cross talk with ABA. ABA is generally known to suppress the activities of SA. Higher levels of ABA suppresses the expression of pathogenesis related (PR) genes which are in turn induced by SA. However it can act synergistic or independently to SA for making response against biotic stress more refined. ABA has been shown to negatively regulate SA-mediated defense responses against pathogens. Increased ABA levels can suppress the expression of pathogenesis-related (PR) genes that are typically induced by SA. This antagonistic interaction allows plants to allocate resources away from SA-mediated defenses and prioritize stress tolerance under severe abiotic stress conditions. However, under moderate stress conditions, ABA and SA pathways may act synergistically to enhance plant defense against pathogens. This synergy involves the coordination of various defense mechanisms, including the activation of PR genes, production of antimicrobial compounds, and reinforcement of physical barriers. ABA, JA, and SA signaling pathways converge on common transcription factors and regulatory elements, enabling coordinated gene expression. Transcription factors, such as MYC2, NPR1 (Non expressor of Pathogensis-Related gene), and WRKYs, act as hubs for integrating signals from ABA, JA, and SA pathways, regulating the expression of downstream defense-related genes. These transcription factors facilitate the synergistic or antagonistic interactions between the hormone pathways, determining the outcome of plant defense responses.

4.2.2 ABA and plant microbiome

The plant microbiome, composed of various microorganisms such as bacteria, fungi, and viruses, plays a critical role in plant growth, health, and adaptation to different environmental conditions [66]. Understanding the mechanisms that regulate the plant microbiome is essential for enhancing crop productivity and sustainability. One significant mechanism involved in modulating the plant microbiome is the influence of abscisic acid (ABA) [67]. ABA affects the plant microbiome through its influence on root exudation. It can modify the composition of root exudates, which are organic compounds released by plant roots into the soil. These exudates serve as an energy source for microorganisms and can attract or repel specific microbial taxa [68]. By altering the composition of root exudates, ABA indirectly affects the abundance and diversity of microbial communities in the rhizosphere. Furthermore, ABA can directly affect microbial growth and activity. Some studies have demonstrated that ABA can promote the growth of beneficial microbes, such as plant growth-promoting rhizobacteria (PGPR), while inhibiting the growth of pathogenic microorganisms [69]. This direct effect of ABA on microbial populations further contributes to the modulation of the plant microbiome.

Understanding the role of ABA in modulating the plant microbiome has significant implications for agriculture and plant health. Manipulating ABA levels in plants could potentially be used as a strategy to enhance beneficial microbial colonization, improve nutrient uptake, and confer resistance against pathogens. However, further research is needed to unravel the complex interactions between ABA, plants, and the microbiome, and to develop practical applications for agricultural systems.

5. ABA and crosstalk with other phytohormones

Phytohormones are chief controllers of plant growth, development, and stress response. ABA plays a role in the modulation of other phytohormones, such as auxin, cytokinins, and gibberellins. The interplay between these phytohormones is critical for maintaining proper balance and coordination of plant growth and development.

ABA and auxin have antagonistic effects on root growth, where ABA inhibits root growth, while auxin promotes it. ABA also inhibits the biosynthesis and transport of auxin, thereby reducing its concentration and activity in the plant [70]. In contrast, ABA and cytokinins have synergistic effects on stress responses, particularly in enhancing the antioxidant defense system in plants. Studies have shown that the application of ABA and cytokinins together can increase the activity of antioxidant enzymes and reduce oxidative damage under stress conditions [62]. Moreover, ABA and gibberellins have been found to interact in the regulation of seed germination and dormancy. ABA promotes seed dormancy, while gibberellins promote germination. The balance between ABA and gibberellins is critical for proper seed development and germination.

As discussed earlier, the ABA can act synergistic, antagonistic or independent to Salicylic acid and Jasmonic acid regulating their effects in biotic stress responses. It is this crosstalk or interplay between these phytohormones that makes plant to recognize and differentiate the microorganisms as symbiotic or pathogenic partners. Overall, the modulation of other phytohormones by ABA is a complex and dynamic process that contributes to the regulation of plant growth, development, and stress responses.

6. Strategies to develop stress tolerant varieties

6.1 ABA, crop breeding and genetic engineering

Crop breeding has been a traditional approach to develop stress-tolerant varieties of crops. Recent advances in molecular biology and genetic engineering have allowed for more targeted and efficient approaches to crop breeding, including the use of ABA signaling pathways. A recent study published in the Journal Plant Physiology in 2018 reported the successful development of a stress-tolerant tomato (Solanum lycopersicum) variety using a gene-editing approach to enhance ABA signaling. The resulting plants showed improved drought and salt tolerance, as well as increased yield under stress conditions [71].

In another work, researchers reported the successful development of stress-tolerant rice (Oryza sativa) using a combination of conventional breeding and molecular breeding approaches. The resulting rice varieties showed improved tolerance to drought, salinity, and submergence. The molecular breeding approach involved the identification of genes involved in ABA signaling and the development of molecular markers for these genes to enable more efficient selection of stress-tolerant plants during breeding [72]. Generally, the use of ABA signaling pathways in crop breeding is a promising approach to developing stress-tolerant varieties of crops, and ongoing research in this area is likely to lead to further advances in the coming years. Overall, the multifaceted roles of ABA in plant growth, development, and stress responses make it a crucial component of plant physiology and a key target for research aimed at improving crop productivity and stress tolerance.

6.2 Epigenetic changes and crop improvement

Term Epigenetics was coined by Waddington [73] referring to inherited changes, meiotic or mitotic [74] in terms of chromatin structure, DNA cytosine methylation and histone modifications. All these modifications create various global and locus specific epialleles [75]. These changes can be passed on from generation to generation and can be influenced by environmental factors, including stress. Better understanding of epigenetic mechanisms and modifying the epigenetic framework of the plant could help us in developing better varieties. Drought stress or ABA treatment is known to transform the pattern of histone epigenetic marks which directly influence the genes action [76]. H3K14 acetylation has been observed in Arabidopsis and tobacco cultured cells when exposed to salinity stress or ABA treatment [77].

According to recent research, plants’ responses to abiotic stressors such drought, salinity, heat, and cold are significantly influenced by epigenetic alteration leading to changes in plant’s epigenome [78]. In this chapter, we will discuss the epigenetic control of stress signaling networks associated with ABA. One approach for developing stress-tolerant crops using CRISPR-Cas9 is epigenome editing. This technology allows for precise modifications to be made to the epigenetic marks on specific genes to enhance stress tolerance.

A recent study has compiled all the aspects of CRISPR-Cas9 mediated epigenetic regulations in various plants [79]. Another approach is through the use of stress-induced epigenetic changes. For example, a study published in the Journal Plant Cell in 2019 explained that exposing maize (Zea mays) plants to mild drought stress during seed development resulted in changes to DNA methylation patterns that enhanced drought tolerance in the resulting plants [80]. Mostly, the use of epigenetic changes to develop stress-tolerant crops is still a comparatively new field of research, and further studies are needed to fully understand the potential of this approach. However, early studies have shown promising results and suggest that epigenetic editing could be a valuable tool in developing crops that are better able to cope with abiotic stresses.

7. ABA quantification techniques

The amount of ABA concentration present in different plant tissues can be quantified using a wide variety of techniques. Researchers can understand the dynamics of ABA levels and how they affect plant growth and response by quantifying ABA.

Different types of methods executed for ABA measurement are:

• Enzyme-linked immunosorbent assay (ELISA): This technique makes use of specific antibodies that bind to ABA. An indirect measurement of the concentration of ABA is provided by the binding, which is measured using colorimetric or fluorescent signals. Dubas et al. [81] measured ABA in microspores of Brassica napus using indirect ELISA. The MAC 252 was used as antibody (Babraham Technix, Cambridge, UK). Absorbance was calculated by microplate reader Model 680 (Bio-Rad Laboratories, Inc.) at the wavelength of 405 nm.

• Gas chromatography-mass spectrometry (GC-MS): GC-MS uses gas chromatography to isolate ABA from other components in the sample and mass spectrometry to identify it. With its exceptional sensitivity and specificity, this approach produces accurate measurements.

• High-performance liquid chromatography (HPLC): HPLC uses liquid chromatography to extract ABA from other chemicals. With the help of methods like UV absorption or fluorescence, the separated ABA is detected, enabling accurate measurement.

• Immunoassays: Immunoassay techniques rely on the particular binding of antibodies to ABA, as in RIA (Radioimmunoassay) and FIA (Fluorescence Immunoassay). To characterize the binding and calculate the concentration of ABA, radioactive or fluorescent markers are used. RIA has an effective range of 0.165–13.2 ng ABA and has been used to find out ABA levels in extracts of leaves of tomato, Lycopersicon esculentum Mill. cv. Ailsa Craig (wild type) and three wilty mutants, notabilis, flacca and sitiens [82].

• Biosensors: To transform the binding of ABA into quantifiable signals, biosensors combine biological components with transducers. It is possible to quantify the concentration of ABA by detecting the interaction between it and the biological component. Typically biosensors are used for detecting analytes in low concentrations. There are two types of biosensors that have been developed for finding ABA viz., Localized surface plasmon resonance (LSPR) and Förster resonance energy transfer (FRET) sensors. LSPR sensors develop the interaction of light with metal nanoparticles to identify substances, whereas FRET sensors depend on energy transfer between a donor and acceptor molecule. LSPR sensors possess a remarkable sensitivity that enables the detection of substances without the need for labels, and they also have the capability to simultaneously detect multiple targets. On the other hand, FRET sensors exhibit exceptional specificity and can be customized for various applications. In the field of biosensors, aptamers have emerged as a cost-effective and sensitive substitute for antibodies. Recently, scientists have developed genetically encoded FRET sensors for the purpose of detecting ABA concentration and uptake in plants. These sensors demonstrate a high signal-to-noise ratio and enable precise measurements of ABA levels in living tissue [83].

• Fluorescence resonance energy transfer (FRET): Many of the sensor variants did not respond to the addition of ABA; however, FRET pair variants with improved dimerization fluorescent protein versions consistently responded to the addition of ABA. Understanding the pertinent metabolic fluxes can be gained from the kinetics of FRET sensor responses. ABI1aid and PYL1 domain interaction results in a “closed” state after ABA binding, which brings the fluorophores closer together and boosts FRET effectiveness. When the sensor closes, fluorescent proteins with improved dimerization propensities will improve the sensor’s closed state through dimerization. Additionally, a high FRET efficiency is anticipated as a result of the heterodimer state’s parallel orientation [83].

Among these methods, the choice depends on factors such as sensitivity, specificity, ease of use, and available resources. GC-MS is often considered the gold standard method due to its high sensitivity and specificity, allowing for accurate quantification. However, ELISA and HPLC are also widely used due to their simplicity and availability of commercial kits. Immunoassays offer high sensitivity and have been extensively used in research. Biosensors, while still developing, hold promise for rapid and on-site analysis.

8. Conclusion

Plants are absolute necessity for life on earth serving as cornerstone for survival and sustenance of mankind on this planet. Cultivated plants also called as crops not only serve as source of food and nutrition but also make the foundation of a big agricultural industry. However, plants face numerous pressures on a constant basis including biotic and abiotic elements, which offer serious obstacle to their growth, development and productivity. Plants have developed a variety of defense mechanisms that cooperate to lessen the detrimental effects of these stimuli. ABA has been found as a fundamental regulator of plant stress tolerance. ABA supports the Plant’s ability to adapt to the challenging environmental cues and preserve cellular homeostasis by activating a specific signaling pathways.

Improved crop resilience and productivity under numerous stresses have been made possible by better understanding the role of ABA in stress tolerance. The introduction or modification of genes involved in ABA production, signaling, and subsequent stress response is possible due to genetic engineering techniques. Scientists are working to create crops that are more resistant to stress by increasing the synthesis of ABA or changing the expression of ABA-responsive genes. For instance, it has been demonstrated that many crops may tolerate stress better when genes associated with ABA biosynthesis or transcription factors involved in ABA signaling are over-expressed. Promoter region of various genes involved in ABA biosynthesis and stress responses are modified to regulate the expression of these specific genes. Site specific mutagensis or gene editing using CRISPER-Cas9 technology has been serving as key members in the race for crop improvement in terms of stress avoidance, resilience or tolerance. Though stress is a complex process but its response is even more complex where many pathways crosstalk with one another. Still ABA acts as Hub molecule modulating other stress responsive molecules as spokes making it overall a hub and spoke model for stress tolerance.

Another tactic is cross-breeding or hybridization, where hybrid plants with improved stress tolerance are selected utilizing the inherent genetic diversity found in crop species. Breeders can add advantageous features associated to ABA-mediated stress responses by mating several kinds or species. Using this method, crops with enhanced resistance to environmental stresses have been successfully developed.

The plant’s ability to cope with stress is influenced by both genetic and epigenetic alterations. Plants’ gene expression and stress responses can be affected by epigenetic alterations such DNA methylation and histone modifications. Discovering these epigenetic mechanisms and how they interact with ABA signaling pathways is opening up new possibilities for creating crops that can withstand stress. Editing the epigenome of a plant using CRISPER-Cas technology with guide RNA targeted to a specific gene or epigenetic modification is nowadays a commonly used technique to enhance crop stress resistance.

Researchers from Seoul National University in South Korea found that ABA receptors in Arabidopsis thaliana which were modified via agrochemical engineering were more responsive to ABA, increasing the plants’ tolerance to salt and drought [84, 85]. The agrochemical engineering strategy, according to the researchers, might be used to increase other crops’ tolerance to salt and drought. Since salt stress and drought are becoming more prevalent due to climate change, this could aid in addressing the growing issue of food poverty.

Plant stress biology is a wide and complicated field with numerous posed threats and unlimited opportunities. Future developments in cross-breeding methods, epigenetic research, agrochemical and genetic engineering hold considerable promise for creating crops with increased resistance to Environmental stresses. The potential ecological and ethical ramifications of these strategies must be carefully considered, though. To ensure the safe and responsible use of genetically modified crops, thorough risk analyzes and regulatory frameworks are required. Furthermore, the scientific community is not the only one who must address the impacts of Environmental stresses on crop production. The demand for sustainable agriculture practices and policies that put climate resilience and food security first is growing as the world’s population expands. To assist farmers in implementing climate-smart agricultural methods and stress-reduction techniques, investments in R&D, infrastructure, and education are crucial. In order to secure food security in the face of shifting climatic conditions, it is essential to strike a balance between technological breakthroughs and ethical issues.

9. Future perspectives

Various points can be kept in mind for ABA future research:

• Future study on biotic and abiotic plant stress will deepen our grasp of the complex mechanisms underpinning plant stress tolerance. The hormone ABA, which is crucial in modulating stress reactions, is one compound whose production is of special interest. Despite extensive research has been carried out on various aspects of ABA, insights into how plants experience and react to stress will be gained from more research into the biosynthetic pathways of ABA and the control of its synthesis.

• The manipulation of genes involved in ABA production, signaling, and subsequent stress responses will be the main focus of research. Researchers can improve the expression of ABA-responsive genes or increase ABA production by precisely altering these genes, which will increase the ability of crops to withstand stress. The creation of stress-tolerant cultivars will also be accelerated by improvements in gene-editing technologies like CRISPR-Cas9. These technologies allow for more precise and focused alterations.

• Crop breeding presently is and will be a key factor in improving stress tolerance, especially through cross-breeding and hybridization. Breeders can selectively integrate ABA-mediated stress response features into commercial varieties by taking use of the inherent genetic diversity seen among crop species. Advanced phenotyping tools and genomic selection strategies shall be used in conjunction with this strategy to help breeders more effectively identify and select plants with desired stress tolerance features.

• Stress tolerance studies are increasingly focussing on epigenetic changes, such as variations in DNA methylation and histone acetylation. The epigenetic changes connected to ABA signaling networks and stress responses will open up new possibilities for enhancing crop resilience. By examining how alterations in the epigenome affect gene expression and stress-related metabolic pathways, researchers will pave the road for novel methods to improve stress tolerance through epigenetic adjustments.

• Modifications of promoter regions of ABA biosynthesis genes and ABA-responsive components will be another topic of future study. Researchers can fine-tune the ABA signaling system and improve stress tolerance by finding and modifying particular promoter sequences and cis-regulatory regions that regulate the production of stress-responsive genes. This strategy will make it possible to precisely control how genes are expressed in response to stressful situations, enabling crops to employ more potent stress mitigation techniques.

• Additionally, a thorough understanding of plant stress reactions will be possible through clarifying the interactions between ABA signaling and other hormone pathways. The complex relationships between ABA and other hormones, including salicylic acid, jasmonate, and ethylene, can be uncovered by researchers in order to pinpoint critical regulatory sites and create plans for improving crop stress responses. The ability of plants to tolerate stress more generally could be increased by manipulating the interactions between hormone pathways.

• Future developments in omics technologies, like as genomes, transcriptomics, proteomics, and metabolomics, will considerably increase our understanding of the mechanisms underlying stress tolerance. With the use of these technologies, scientists will be able to thoroughly examine the molecular alterations brought on by stress on plants. Combining data from several omics levels will enable the identification of critical targets for enhancing stress tolerance and offer a comprehensive perspective of stress responses.

• Overall, a combination of genetic engineering, crop breeding, epigenetic changes, and a better comprehension of ABA signaling networks and their interactions will enable the development of crops that can withstand stress. Using this potential and turning it into workable solutions for sustainable agriculture would require continued study and cooperation among scientists, breeders, and policymakers. We can secure food security and agricultural sustainability in the face of a changing climate and an expanding global population by tackling the problems caused by biotic and abiotic pressures.