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Drought can be referred to as a meteorological period without significant rainfall and it is one of such major abiotic stresses that contributes to a huge reduction in crop yield throughout the world. Plant shows a broad range of physiological, morphological, and biochemical changes such as reduced photosynthetic accumulation, altered gene expression, etc. Under the drought stress which ultimately causes reduced growth as well as poor grain yield. Drought stressconditions trigger production of ROS, which disrupts the dynamic balance between ROS production and ROS scavenging systems and its accumulation depends on the intensity as well as duration of water stress, and it varies among species. A plant species that has a better inherited genetic response allowing it to rapidly deploy its antioxidant enzymatic and non-enzymatic defense system, can tolerate drought better than a plant species with a poor antioxidant defense system. Furthermore, enzyme and protein encoding drought specific genes have the ability to enhance drought tolerance. These two enzymatic and genetic engineering strategies are unique and vital tools, which can be used to help alleviate the world’s future problems related to energy, food, and environmental stresses, particularly drought. This chapter attempts to discuss developments in understanding effects of drought stress and underlying mechanisms in plants for its alleviation.
Bidhan Chandra Krishi Viswavidyalaya, Mohanpur, Nadia, West Bengal, India
Aditya Pratap Singh
Bidhan Chandra Krishi Viswavidyalaya, Mohanpur, Nadia, West Bengal, India
Saju Adhikary
Bidhan Chandra Krishi Viswavidyalaya, Mohanpur, Nadia, West Bengal, India
Subhasis Mondal
Bidhan Chandra Krishi Viswavidyalaya, Mohanpur, Nadia, West Bengal, India
Sujaya Dewanjee
Bidhan Chandra Krishi Viswavidyalaya, Mohanpur, Nadia, West Bengal, India
*Address all correspondence to: kousikatta1995@gmail.com
1. Introduction
Any inimical condition or substance that affects plant’s metabolism, growth and development is referred as stress. Basically, stress is an altered physiological condition caused by different living and non-living factors which disturb the equilibrium. Plants are frequently posed with a plethora of stress conditions such as drought, salinity, heat stress, low temperature, heavy metal toxicity, flooding and extremes of soil pH. Plants also face challenges from biotic factors like pathogens, insects etc. These types of abiotic and biotic factors limit plants growth and productivity. The non-living variable must impact the environment beyond its normal range of variation to unfavorably affect the population performance or individual physiology of the organism in a significant way.
Drought is a meteorological term and defined as a period without significant rainfall. Generally, drought stress occurs when the available soil-water becomes scanty and atmospheric conditions cause continuous loss of water by transpiration or evaporation. Water deficit is one of the major abiotic stresses, which adversely affects crop growth and yield. These changes are mainly associated with altered metabolic functions, one of those is either loss of or diminished synthesis of photosynthetic pigments, uptake and translocation of ion, carbohydrate biosynthesis, nutrient metabolism and synthesis of growth promoters. These changes in the metabolic functions and synthesis of photosynthetic pigments are closely related to biomass production in plant [1]. A common adverse effect of water stress on crop plants is the reduction in fresh and dry biomass [2]. Plant productivity under moisture stress is strongly associated with the processes of dry matter partitioning and temporal biomass distribution [3]. Previous study about different crop species faces huge yield reduction due to drought stress (Table 1). We have aimed to discuss the crops’ response and adaptive mechanisms to combat drought stress and also genetic interventions which may help developing cultivars suitable for water-scarce conditions.
During drought, Water scarcity occurs generally because of absence of water in the soil. But Physiological drought caused both lack of water in the soil, and also occurs when excess water is present in the soil. Thus, physiological drought is a situation where the plant cannot receive water [15, 16]. The responses of plants to water stress are diverse and may involve the contribution of various defense mechanisms or modification of physiology, morphology, anatomy, biochemistry, as well as short and long-term developmental and growth related adaptation processes [17].
Physiological reactions to moisture stress provides some escape mechanisms to the water stress comprise physiological and morphological adaptations [18]. Decreased leaf area (Figure 1), reduced stomatal number and conductance, enlargement of root system, increased leaf thickness, and leaf folding to lessen evapotranspiration are strictly associated with an adaptive response [17, 19, 20, 21]. Plant growth and productivity decreased under moisture stress, which are caused by alterations in plantwater relations, CO2 assimilation reduction, membrane damage of affected tissues, cellular oxidative stress, and inhibition of enzymes activity.
Figure 1.
Effects of different levels of drought stress on ricebean seedlings.
Plants can alter water relations to continue cellular mechanisms under drought stress conditions. Plants show osmotic adjustment by accumulating and integrating compatible solutes likely, proline, sugars and free amino acids [22]. Maintenance of turgor pressure as well as cell volume at low water potential is facilitated by osmotic adjustment and is vital for metabolic functions. Osmotic adjustment also plays role in recovery of metabolic activities post drought stress [23]. Previously, there are lot of studies investigated which showed the recovery of photosynthesis from moisture stress in various crop species and also recovered from drought stress in terms of oxidative stress, membrane stability index and antioxidative mechanisms [16, 24]. Osmolytes also have a significant role in drought stress recovery.
Drought stress at higher intensity decreases the activities of photosynthetic enzymes as well as leaf chlorophyll content which ultimately hampers the process of photosynthesis [20, 25]. Chlorophyll a/b proportion and synthesis of leaf chlorophyll altered during drought stress. A lower content of chlorophyll (Figure 2), inactivation of key proteins linked to the photosynthesis process, and alteration of thylakoid membranes happen as a result of drought stress. The decline in chlorophyll content is due to over production of O2− and H2O2 production, which ultimately results significant chlorophyll degradation and lipid peroxidation. During drought stress, in stomata and mesophyll cell the CO2 conductance declined as the decrease in the photosynthetic process. The decrease in photosynthetic activity also may be because of the reduction of stomatal movement [27]. Rubisco activity greatly affected by the loss of CO2 uptake, similarly it decreases the activity of sucrose phosphate synthase, nitrate reductase and RuBP production [20, 28]. Decline in photosynthetic activity, loss of photosystem II photochemical efficiency, reduction in chlorophyll content and alteration in stomatal movement results the reduction in plant productivity. As a consequence of reduction in photosynthetic activity in drought stress, it dismantles the production of carbohydrate in various way likely prevents the transport of sucrose into sink organs and reduces the level of sucrose in leaves, which in turn limits reproductive development. The free sugars and different metabolites thereof support plant growth under drought, and take up osmolytic role and compatible solutes to mitigate the drastic effect of the stress [29, 30].
Figure 2.
Visual effects of drought stress in rice. Source: [26].
The relative leaf water content (RLWC) is an estimate of leaf’s hydration status relative to its maximal water holding capacity at full turgid state. The relative leaf water content (RLWC) is one of the reliable parameters to know the water status in plants and it decreases gradually with increases in the severity of drought stress conditions. The decline of RLWC as a response to osmotic stress was earlier reported by several investigators under different stress conditions [31, 32, 33, 34]. The physiological traits considered for evaluating drought stress tolerance include root trait characteristics (root length, root density, root biomass, root length density, delayed canopy wilting (DCW) and leaf pubescence density (LPD) [35], delayed leaf senescence (DLS) [36], and recovery ability after wilting (RAW) [37]. Drought stress drastically affects seed germination and decreases the speed of germination (Figure 3). Apart from these, stomatal conductance, chlorophyll content and use of carbon isotope discrimination are also effective screening methods for drought stress tolerance and has been used for some food legumes.
Figure 3.
Ricebean response to varying levels of PEG as drought induction agent.
Plants have developed various adaptive mechanisms conferring tolerance to drought stress induced adversities through evolution [38]. Their survival strategies for drought stress can broadly be classified as escape, avoidance and tolerance. Hence, their drought stress response varies from molecular to plant level [39]. The mechanisms of plant escape, avoidance and tolerance (Figure 4) against drought stress are discussed as follows.
Figure 4.
Overall drought stress response in crop species.
3.1 Escape, avoidance and tolerance mechanisms
To escape the pernicious effects of drought stress on plant health and productivity, some plants utilize mechanisms involving shortening of the life cycle by rapid plant development, self-reproduction, and seasonal growth before the beginning of the drought season (Figure 4) [40]. Among all, early flowering is perhaps the best possible escape adaptive mechanism in plants [41]. However, this mechanism can connote a considerable reduction in the plant’s growing period compromising plant productivity in some cases [42].
In avoidance strategy, high plant water potential is maintained through transpiration loss reduction and the increased water uptake from well-established root systems [43]. Xeromorphic features such as the presence of hairy structure on leaves and cuticles in some cases do help to maintain high water potentials in plant tissues [44]. It is notable that overdevelopment of these structures may lead to reduced productivity and reduced decreased size of vegetative and reproductive parts [45]. On the contrary, an adaptive tolerance mechanism at the photosynthetic level involves reductions in the plant’s total leaf area and limited expansion of new leaves. Likewise, trichrome production on leaves is an attribute that enables the plant to tolerate water deficits in dry environments [46]. There is an increase in rate of light reflection in the leaf reducing the leaf temperature as well as trichomes provide additional layer of resistance to the water loss thereby reducing the rate of water loss through transpiration [47]. Changes in root system-size, density, length, proliferation, expansion and growth rate, constitute the preliminary strategy for drought-tolerant plants to cope against drought [48]. Osmotic adjustment, antioxidant defense mechanism, metabolic and biochemical dynamics of stomatal closure, solute accumulation and increment in root shoot ratio are other common strategies that aid to drought stress resilience [49].
Drought stress triggers an array of biochemical mechanisms including fluidity of the plasma membranes, osmolytes production, lipid peroxidation, reactive oxygen species (ROS) generation, rigidity of the cellular membranes and activation of different enzymes which are involved in oxidative defense system [50, 51]. Previously, in various crop species ROS generation instigates significant damage to cellular components and also causing damages to lipid peroxidation, proteins [52]. The drought stress induced ROS generation had calamitous effects on lipid membrane and protein. Among all the ROS superoxide radical (O2•−), hydrogen peroxide (H2O2), singlet oxygen (1O2) and hydroxyl radical (OH•) are mainly produced by enzymatic or non-enzymatic processes during photosynthesis (Figure 5). Their production occurs also in components of electron transport system in the mitochondria by partial reduction or oxidation of atmospheric oxygen [53]. In some current studies, it has been shown that ROS have dual role in plant biology; involvement in vital signaling processes and as toxic by-products of aerobic metabolism [53].
Figure 5.
Production of various ROS by energy transfer (or) sequential univalent reduction of ground state triplet oxygen.
4.2 Enzymatic and non-enzymatic antioxidants
There are several componentsutilized by plantsto cope up with oxidative stress, which are involved in ROS homeostasis modulation [54]. Plants produces various reactive oxygen species (ROS) continuously as bi-products of various metabolic pathways in different cellular compartments like chloroplast, mitochondria, and peroxisome. ROS have partially reduced forms of atmospheric oxygen and under normal conditions, their production in plant cells is balanced by their effective scavenging through enzymatic and non-enzymatic cascade (Figure 6). ROS can cause damage to different biomolecules namely DNA, proteins and lipids, and therefore by creating oxidative injury; it leads to a reduction in plant growth and development [56]. The equilibrium between the production and the scavenging of ROS may be perturbed by various stress factors. Thus, the disturbances of cellular homeostasis resulted in a sudden rise in intracellular levels of ROS leading to oxidative stress which in turn can cause substantial damage to cell structure and membrane integrity. To mask themselves from these toxic oxygen intermediates, plant cells contain both enzymatic and non-enzymatic components. Among them enzymatic antioxidantsare superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR), monodehydroascorbate reductase (MDHAR) and ascorbate (AsA), glutathione (GSH), carotenoids, glycine betaine, proline, α-tocopherol and flavonoids are the non-enzymatic antioxidants [51, 57]. Hence, stress induced oxidative damage of ROSs can only be counteracted by increased level of enzymatic and nonenzymatic antioxidants [54].
Figure 6.
ROS scavenging mechanism by antioxidant defense system in different stress conditions. Source: [55].
5. Molecular and genomic prospects for improvement of drought tolerance
Traditionally, there have been several efforts to develop drought-tolerant crop genotypes through usual breeding methods [58, 59]. In this method, two groups of plants with desirable traits are selected and crossed to obtain offsprings having new genetic arrangements [60]. Drought resistance is directly or indirectly incorporated in the crop species via genetic variability of traits and thus selection in breeding is ought to be useful. Important traits to target in plant breeding might include water-extraction efficiency, water-use efficiency, conductance of water, osmo-elastic adjustments and leaf area modulation [15]. Genetic data improves the efficiency of the breeding method. Polymorphisms based on molecular markers that occur naturally in the DNA like restriction fragment length polymorphisms (RFLPs), sequence characteristic amplified regions (SCARs), random amplified polymorphic DNA (RAPDs), simple sequence repeats (SSRs), amplified fragment length polymorphism (AFLPs), and others have been effectively utilized. The use of plant breeding methods has an enormous potential to accelerate drought-tolerant plant production and help drought management assist these plants [15].
Marker assisted selection (MAS) and genomic selection (GS) are the two well versed approaches of genomic assisted breeding. For the first approach, foremost step is to identify the molecular markers linked to the trait of interest so that selection can be performed in breeding programs. However, GS depends on progress of selection models based on genetic markers present on the whole genome and selection of genome estimated breeding values (GEBVs) in breeding populations through phenotyping of “training population”.
MAS utilizes molecular markers in identification of quantitative trait loci (QTL) or specific genes that are linked with the target trait and are used to identify the individual with desirable alleles (Figure 7) [61]. Through these methods, QTLs for the traits linked with drought resistance are identified in various crops i.e., rice, wheat, maize, sorghum, pearl millet, soybean and many other crops [62, 63, 64, 65, 66, 67].
Figure 7.
Model for the role of signaling factors in stomatal closure and retrograde signaling during water stress. Source: [16].
Genomic selection utilizes all the markers available for a population of GEBVs and GS models are used for selection of elite lines without phenotyping [61]. Contrary to MAS, the information about QTLs is not the prerequisite for GS [68]. However, GS requires denser marker data than MAS. GS is being applied for breeding in maize tolerant to drought by the international maize and wheat improvement center (CIMMYT) [69]. Research efforts through this approach are progressing in other crops i.e., sugarcane, legumes and wheat [70, 71, 72].
Many studies have elucidated molecular responses in plants related to drought-induced transcription signaling pathways. In recent times, various stress-responsive genes and transcription factors having potential to mitigate drought-induced oxidative stress have been identified [73]. The TFs operate specific interaction with the cis-elements present in the genes’ promoter region and, stimulate the expression of stress-inducible genes of various signaling pathways upon binding [74, 75]. These TFs are categorized into different families based on their conserved motifs that code their DNA binding domain (DBD), viz., APETALA 2 (AP2)/ethylene-responsive element binding factor (ERF); dehydration-responsive element binding protein (DREB); no apical meristem/Arabidopsis transcription activation factor, cup-shaped cotyledon (NAC); related to abscisic acid insensitive (ABI3)/VIVIPAROUS 1 (VP1) (RAV); WRKY; auxin response factor (ARF); and SQUAMOSA-promoter binding protein (SBP). The DBDs of the AP2/ERF, DREB, NAC, SBP, and WRKY are named as per the names of their respective TFs family, whereas DBDs of ABI3/VP1 and ARF family of TFs are named as B3 family [76].
Biochemical and molecular factors involved in the induction of processes to alleviate the detrimental impacts of water stress include transcription, stress responsive genes like TaNAC69 (wheat), AP37 & OSNAC10 (rice), NF-YB2 (maize) and abscisic acid [16]. Transgenic expression of different stress responsive genes has been also utilized to confer increased tolerance to draught defecits. [77, 78]. The increased expression of these genes is frequently associated with a decreased plant growth rate and this could narrow down its practical use (Table 2) [79]. In this sense, genomic and related molecular tools could accentuate the genes that mitigate the stress effect so that efforts may help maintaining those genes in breeding programs [104]. Marker assisted breeding combined with traditional breeding as an integrated approach is the best approach for the improvement of the drought stress tolerance in plants. [105, 106].
Sustainable crop production to feed exponentially growing population is the major challenge to the scientific communities in the current global climate change scenario. Out of many productivity-limiting factors, drought stress is one of the most critical factor and of prime importance in the context of decreasing water availability for crop production. Water deficit leads to cellular damage and triggers an array of signaling pathways which in turn activate synthesis of gene transcripts associated with protective functions. In general, wilting occurs owing to physiological responses such as reduced turgor pressure, gaseous exchange, mineral assimilation and overall growth. The prominent result of these is reduced photosynthetic rate Many plant species are inherently equipped with drought tolerance mechanisms such as reduction in leaf area and canopy resistance. Both these mechanisms induce tolerance by cutting off excessive absorption of indecent light as a result of reduced surface area exposed to the incident radiations. In order to select for a tolerant genotype and/or traits conferring tolerance, robust phenotyping is a must. Marker assisted breeding to incorporate drought tolerance conferring quantitative trait loci (QTL) has proven to be effective and efficient. In addition, the knowledge generated by “OMICS” techniques (genomics, proteomics, transcriptomics, epigenomics and metabolomics) and transgenomics are potent and significant tools that would enable a researcher to develop an effective strategy for crop improvement programs in a less time-consuming cost-effective manner. So, an integrated approach will provide better understanding of mechanisms underlying drought stress and plants’ response to it, and help in developing genotypes for dry environments in order to reduce the threat to global food security.
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