Pivotal roles of plant hormones in plant growth and development.
Climate change is an emerging issue for modern agriculture and has generated biotic and abiotic stresses for plants such as cold, high temperature, heat, drought, uneven rainfall, and UV radiations. In addition to these, serious stress factors are emerging related to water availability, nutrient cycling, salinity-sodicity, and pest attacks. In recent years, such phenomena have attracted the research community to avoid the fatal influence of climate change on crop production and obtain more food helping in fulfillment of increasing food demand of population surge. The anthropogenic activities in the agroecosystem are among the major causes for global warming and proportion in climate change. Therefore, it is assumed that identifying various plant hormones and their utilization to improve plant metabolic activities would help maintain plant growth, survival, and production under severe climate change circumstances. This chapter focuses on identifying the key aspects of plant hormones to retard the negative impacts of climate change and support sustainable agriculture.
- plant stress
- abiotic factors
- plant growth regulators
- agricultural production
Issues generated from changing climate are trending throughout the globe and have altered various Earth ecosystem processes . Climatic variability assumes about 60% of yield variability, and therefore, it is considered a critical factor to influence crop productivity and farmer’s income. In addition to crop yield and productivity, different natural resources such as land and water are also exploited and have a crucial impact on agricultural production . The primary reason for the substantial changes in climate in the last few decades is the excessive anthropogenic activities that transformed the composition of the global atmosphere. Since 1750, the greenhouses gases (GHGs) concentration increased, including methane (150%), carbon dioxide (40%), and nitrous oxide (20%), respectively .
The growing population has increased the demand for food that has resulted in intensive agricultural practices such as excessive fertilizer usage, manipulation of water resources, and livestock generation. Furthermore, such agricultural activities result in producing GHGs, thus polluting natural resources. Additionally, climate change adversely degrades the land resulting in increased desertification and having nutrient-deficient soils. The threat of growing land degradation day by day is another issue observed worldwide. Global Assessment of Land Degradation and Improvement (GLADA) reported that a quarter of the global land has not to be categorized as degraded. Climate change and anthropogenic activities are the key factors to deteriorate the 15 billion tons of fertile soil every year, and its 1.5 billion people are also affected .
The agriculture sector is also prone to climate change as it is sensitive to weather conditions and causes massive impacts on economics. The crop yield is also affected by changes in climatic events, including temperature and rainfall. Increase in temperature, changes in precipitation patterns, and CO2 fertilization vary due to the crop, location, and magnitude of change in the parameters (Figure 1). The increase in precipitation is likely to reduce the temperature, which ultimately reduces the crop yield. Moreover, humidity and wind speed also impact crop productivity and insect pests and diseases, which are more active in humid and warmer conditions . Climate change has also contributed to drought, increasing sea levels, intense storms, and floods, resulting in land degradation .
A collection of small molecules having a different structure that contributes to improving various plant growth and development processes and showing significant results against biotic and abiotic stresses are termed as plant hormones. Moreover, plant hormones regulate different mechanisms such as seed germination, cell differentiation, cell proliferation, senescence, stem elongation, organ formation and response to drought, pest attack, and wounding. Thus, plant growth and productivity, size, and architecture are controlled by plant hormones. In addition, it can be a possible solution to improve agricultural productivity and solve the worldwide food storage issues [5, 6].
Plant hormones can also be defined as the plant growth regulators either produced inside or by the plants, whereas plant growth regulators are synthetic materials that can alter plant biological processes and improve its growth [6, 7]. Major classical hormones included in the plant growth regulator category are abscisic acid, cytokinin, ethylene, auxin, jasmonate, gibberellin, salicylic acid, brassinosteroids—however, many more there to be discovered. For the last few years, the understanding of the plant hormones and their association with various plant processes, the mechanism behind them, and their signaling roles has been progressed .
In this chapter, we have aimed to understand plant hormones as a tool to achieve agricultural sustainability to tackle climate change. The chapter consists of eight sections, which describes the introduction (Section 1), interaction of plant hormones and plant physiology (Section 2), role of plant hormones in adaptation to salt stress (Section 3), participation of plant hormones in heat and cold stress tolerance (Section 4), the response of plant hormones to drought stress (Section 5), the involvement of phytohormones in resetting plant-pest interaction (Section 6), future prospects and challenges (Section 7), and the conclusion (Section 8), respectively.
2. Interaction of plant hormones with plant physiology
Plant hormones are chemicals and should not be considered nutrients. These hormones are programmed to respond at a specific time or growth stage, or cycle. However, before or after that specific stage, the impact of hormones will diminish . Due to climate change, plants face various biotic and abiotic stresses, including salinity, cold and heat stress, drought, insects, and pathogens attack that adversely impact plant growth and yield. However, plant hormones in a small amount not only protect the plant from environmental stresses but also improve crop yield , as described in Table 1.
|Plant hormone||Identified primary function||References|
|Abscisic acid||Impacts various cellular processes (i.e.,) seed maturation and germination, stomatal movement, and leaf senescence.|||
|Cytokinin||Influence cell division in roots and shoots, delay tissues senescence.|||
|Ethylene||Promote, inhibit, or induce growth and development of leaves, flowers, and fruits.|||
|Auxins||It affects plant development as cell division, elongation, differentiation, flowering, and senescence.|||
|Jasmonates||It contributes to developing plant roots and reproductive systems, develops defense mechanisms, and triggers gene expression.|||
|Gibberellin||Facilitates plant growth, cell elongation, and development of flowers, fruits, and seeds.|||
|Develops resistance in plants and stimulates the production of antioxidants.|||
|Brassinosteroids||Regulate various physiological processes such as cell expansion and proliferation, vascular differentiation, timing senescence, and male fertility.|||
Plant hormones help to attain resistance in plants against induced stress (Figure 2). Abscisic acid is related to the plant stress responses such as cold, drought, salinity, and plant growth, including seed dormancy, embryo maturation, flower induction, and pest attack . Cytokinin is a hormone produced by plants as well as nearby insects and pathogens. However, it induces defense mechanisms in plants upon pathogen attack and impacts the plant physiological traits, including apical dominance, seed germination, leaf senescence, flower, and fruit development, whereas type of molecules of cytokinin hormones varies depending upon the plant growth stages and environmental conditions .
Ethylene has a significant role in leaf growth, development, senescence. However, the response depends on the concentration of hormones and the plant species. Therefore, the effect of ethylene on plant physiology might be an independent response or dependent on the interaction with other hormones. Moreover, ethylene has been shown to play an antagonistic role to auxins in the abscission of various organs. Furthermore, ethylene is vital for fruit ripening, plant aging, and protecting the seed until maturity . On the other hand, auxin is a growth hormone and involves in plant growth and development through cell elongation and expansion. It also regulates organ development, shoot, root branching, gravitropism, and phototropism .
Jasmonic acid is a plant hormone and is involved in plant defense mechanisms. Thus, the plant can perform effectively under stressful environmental conditions. Its interaction with ethylene improves root development and anthocyanin accumulation that could be related to its tolerance against abiotic stresses . Furthermore, it is involved in the inhibition of seed germination. The mechanism behind the role of Jasmonic acid varies depending upon the environmental constraints . Gibberellin is required by the plants at multiple stages and contributes to germination, the transition to flowering, and flower development, root elongation, and fruit development .
Salicylic acid is also a defense-boosting hormone as its level increases with pest attack . In addition, brassinosteroids regulate specific gene sets to help plants sustain during unfavorable environmental conditions . Moreover, it is also involved in various metabolic processes, including osmotic regulation, plant-water interaction, photosynthesis, nitrogen metabolism, and antioxidant metabolism .
3. Role of plant hormone in adaptation to salt stress
With the increasing population, it is challenging to fulfill the food requirements, as climate change has threatened the sustainability and productivity of the agriculture production system . Different types of abiotic stresses are present in nature, affecting the various processes in plants and indicating the unique and complex response against the stresses, depending on multiple factors such as degree of plasticity, including many morpho-physiological, cellular, and anatomical . According to an estimate, about 6% of the world’s total land area is affected by salinity. Among various biotic and abiotic stresses, salinity also plays a vital role in decreasing crop growth and productivity  that has been intensified due to poor irrigation practices, increasing population, and industrial pollution . Some other types of stress, including osmotic stress, ionic stress, and oxidative stress, occur in the plant because of salt stress. Consequently, to survive under such conditions, plants need to rely on such critical pathways, which help the plants reestablish the various processes, including ionic, osmotic, and reactive oxygen species . It is necessary for the plants that grow under salt stress conditions to re-adjust the different biochemical and physiological processes involved under salt stress, and plants have to adjust their physiological and biochemical processes, engaged in modifying not only the ionic but also the osmotic homeostasis .
Several studies have shown that every hormone present in the plant does not play only a biological role in the plant, but also plays a vital role in various important stages, such as tissue formation and other physiological processes [30, 31]. These phytohormones can act either near to or remove from the site of synthesis to regulate responses to environmental stimuli or genetically programmed developmental changes . So, hormones play a vital role in facilitating plant response to various abiotic stresses. The plant may try to spurt or survive under stress conditions and may also decrease plant growth. Thus, the plant focuses on its resources on withstanding the stress . Abiotic stresses may cause a different type of damage, often leading to adjustments in production, supply, and signal transductions of growth along with stress hormones that can promote definite protective mechanisms . Awareness of a stress signal generates the signal transduction pours in plants with hormones acting as the baseline transducers .
4. Participation of plant hormones in heat and cold stress tolerance
Plants have established a different but extraordinary ability to acclimatize the harsh environment and flourish in habitats categorized by abiotic stresses, for example, temperature extremes . Both exogenous and endogenous environmental factors are involved in regulating plant growth and development . All the exogenous environmental factors (temperature, light, moisture, and atmospheric CO2) must be present at an optimal level to regulate the metabolic process. Temperature is one of the most important factors among all environmental factors because it plays a dynamic role in regulating the phenological development of a crop plant . Temperature beyond the “physiological optimum” that affects a plant’s optimal growth is usually known as “high temperature” for that particular plant. The participation of hormones in the plant response to heat stress can be examined in many different ways. Associate approaches such as short-term heat shock with enormously high-temperature and heat acclimation study reveal plants to minor heat stress previously imposing great heat stress and long-term high-temperature treatment .
Some important crops (annual) such as wheat, oats, barley, and pea show a substantial degree of intrinsic freezing tolerance, which can be further enhanced by using complex signaling events. On the other hand, some species such as maize, rice, and tomato, which belong to temperate or tropical zones, may face severe damage at a chilling temperature . Hormones are chemical messengers and low-molecular-weight complexes that transfer essential information from a production site to the place of action. Regulation of hormonal homeostasis is taken up by different processes such as biosynthesis, catabolism, and their transformation from one place to another. At the same time, the sensitivity is produced due to the presence and response of dedicated receptors determined by the receiving tissue, which pledge the signal transduction action to change the cellular process .
Various types and classes of hormones are available that are involved in the different and overlapping functions. Furthermore, hormones manifest different synergistic and antagonistic effects on the synthesis and signaling productions of the other hormones, generating a composite network of hormonal relations . The hormonal signaling system assimilates the peripheral information into endogenous development programs and initiates the stress-responsive pathway leading toward resistance. Hence, it may not be surprising that plants utilize phytohormones to signal cold stress. So, it may be accredited to the fact that hormonal behavior under cold stress conditions is inclined by cross-talk with signaling forces the consulting response to other environmental temptations including light  and is also obstructed by endogenous developing programs, especially those, which results in developmental phase transitions .
5. Response of plant hormones to drought stress
Although the green revolution has enhanced plant output, the ever-increasing global population and global warming (which is producing drought stress) are again putting a strain on our ability to feed the globe . As a result of catastrophic losses in agricultural output caused by drought stress during the previous few decades, there is a worldwide challenge to enhance the yield and plant drought resistance . Drought is a disaster for agriculture, humankind, and animals alike. Climate change is bringing us closer to a hotter and drier planet. Hence, we urgently need to produce drought-resistant, high-yielding crops . Drought is an important plant stressor that considerably influences plant growth and productivity, resulting in large agricultural production losses . Drought stress causes plants to undergo morphological, physiological, biochemical, and molecular changes .
Small compounds such as peptides or hormone are effective in agriculture for fine-tuning drought response pathways while maintaining production. Research initiatives that reveal the physiology of plant responses to drought in model systems and transfer these results to crops will result in novel water-saving measures . Even as the world’s population rises, finding solutions to alleviate agriculture’s “thirst” would lessen competition for freshwater supplies . Exogenous application of hormones improves the endogenous hormone contents that significantly help in improved photosynthetic fluorescence of leaves, plant enzymatic activities, regulates source-sink balance, yields maintenance, and enhances carbon metabolism under the drought stress environment .
To reduce the effect of drought stress on plants, these mechanisms may include the mitogen-activated protein kinases (MAPK) signaling system, calcium signaling route, transcription factor modulation, and higher levels of antioxidant enzymes and other chemicals . Exogenous application of chemicals (nitric oxide, 24-epibrassinolide, proline, glycine betaine), plant breeding, and transgenic approaches is under consideration by scientists to enhance these systems . Early-stage application helps to control drought stress, and spraying 6-BA has capability to regulate the content of endogenous hormones and improve photosynthetic characteristics of sweet potato, and thus effectively alleviates the loss of yield . Under drought condition, seed treatment of plant hormones, maize germination percentage, and seedling growth were enhanced significantly  amid to improved drought resistance.
6. Involvement of phytohormones in resetting plant-pest interaction
Many aspects of plant growth and responses to biotic and abiotic stressors are influenced by phytohormones . Jasmonates, ethylene, and salicylic acid are the three primary phytohormones that mediate defensive responses to pests and diseases. Other hormones’ involvement in defensive signaling has recently been discovered. Abscisic acid, a hormone generally linked with abiotic stress reactions, has been identified as a critical fine-tune regulator of defenses .
Plants have their defense systems to combat pests’ invasions. It has limited pre-existing defense systems, and the majority of the defense response is triggered only after an insect or disease has infected it. Pests’ attacks are repelled both directly and indirectly by plants. Many of these defenses are controlled by signaling pathways in which phytohormones play a significant role. Insects evolve methods to overcome the plant obstacles simultaneously, resulting in an intriguing co-evolution of plant-pest interactions. Biological immune systems require the ability to sense and respond, but to what degree can plants recognize and respond to pests specifically is integral? The jasmonate route has emerged as a critical signaling mechanism for integrating data from the plant-pest contact into broad-spectrum defensive responses . A proper defense reaction to a biotic danger requires early detection. Pathogens are identified when pattern recognition receptors (PRRs) on the surface of the host plant cell detects conserved patterns of microbial molecules termed microbe- or pathogen-associated molecular patterns (MAMPs or PAMPs), resulting in PAMP-triggered immunity (PTI). PRRs also identify damage-associated molecular patterns (DAMPs) that are endogenous chemicals generated by the plant after infection and induce defensive responses . Pathogens can avoid this innate immune response by using effector proteins that decrease PTI when delivered into the host cell. Disease resistance proteins identified in some plant genotypes precisely detect pathogen effectors, resulting in effector-triggered immunity (ETI) . However, both PTI and ETI models are considered as a generalization . Involved receptors and ligands in molecular identification have empowered conclusions about the specificity of recognition in plant-pathogen interactions. Usually, ETI is activated by highly pathogen-specific chemicals, whereas PTI is based on the non-specific detection of common microbial molecules .
In contrast to diseases, insects are vastly complex multicellular animals with a wide range of lives and cognitive behaviors. The plant may use cues from these patterns to identify the threat of herbivory and establish appropriate defense responses . When plant tissue integrity is disrupted during insect feeding, jasmonoyl-L-isoleucine (JA-Ile) is produced, and a well-defined signal transduction chain is activated, leading to the transcriptional activation of defensive responses . Beyond jasmonates, how can plants fine-tune their defensive mechanism to mount herbivore-specific responses? There are two possible responses to this topic. The first is that plants may employ jasmonates-independent, parallel pathways to generate unique response patterns. Second, specificity may be mediated by the activation of jasmonates-response spatiotemporal modulators. The first premise is supported by research on the tomato’s recognition and response mechanism to the potato aphid
The capacity of one participant to notice and respond to cues provided by the other is a recurring topic in all realms of plant-pest biology. This information flow serves as a great focal point for revealing fundamental chemical and molecular principles of plant-pest interactions. Nonetheless, the research supports several broad conclusions concerning plant-pest interactions’ distinctiveness. To begin, plants recognize distinct arthropods by combining various environmental signals, ranging from mechanical stimulation by insects moving on plant surfaces to contact with glandular components during feeding. Second, herbivore sensing activates regulatory responses involving several phytohormones, with the Jasmonate pathway playing a pivotal role in host resistance. Third, despite the significant conservation of Jasmonate signaling, it is becoming increasingly evident that several hormone response pathways interact to transform initial sensory events into suitable responses that promote plant fitness in the face of aggressive aggressors. Anyhow, understanding the impacts of phytohormones at the whole plant level is undoubtedly important, and the future study presents an intriguing challenge.
7. Future prospects and challenges
Plant hormones have proven themselves effective against biotic and abiotic stresses that emerged due to climate change. However, it is a challenge for researchers and industrialists to prepare cost-effective products that can be used on a large scale. Moreover, salicylic acid is a defensive hormone, but its biosynthesis pathway for salicylic acid is still incomplete; thus, this section requires attention. Furthermore, various parts of the globe face more biotic/abiotic stress, but we still lack understanding of the response of jasmonates against multiple stress conditions. In addition, optimizing the endogenous levels of plant hormones to improve the stress-responsive crosstalk mechanisms between multiple hormones is still a research gap.
The increasing population requires the scientific community to suggest effective technologies for food security. Besides this, climate change is a global problem that adversely impacts the agriculture sector and creates hurdles for agriculture sustainability. Plant hormones within the plants or prepared by the plants have a promising role in tackling these challenges. It enables the plants to tolerate the biotic and abiotic stresses as well as improve agriculture productivity. Therefore, understanding the mechanism and endogenous application of plant hormones can be an effective tool against climate change and its driven problems.
Conflict of interest
The authors have no conflict of interests to declare.
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