Review on Role of Absciscic Acid for Adaptation and Performance of Crop Under Drought Condition

There are many biological regulators that enhance crop adaptation and performance under water deficit condition to realize sustainable crop production for food security and ample in put supply for Agro-industry. Hence, the current review conducted with the objectives to understand the effect of ABA for adaptation and performance of crops under drought stress. Absciscic acid (ABA) is one of Plant hormones that principally participate in crop response to drought stress. Its synthesis and catabolism for different function pass through many step in plant cell. After synthesis and catabolism ABA transported to different plant parts for regulatory purpose. ABA plays great role in plant adaptation and performance under drought starting from seedling emergency up to grain yielding by regulating plant cell, organ and organ system reaction to drought; this could be achieved through enhancing seed dormancy (inhibit seed germination during adverse condition) and reduction of water transpiration by promotion of stomata pore closure. Also via facilitating modification in root hydraulic conductivity, photosynthesis, biomass allocation between roots and shoots, plant water relations, osmolyte production, and synthesis of stress-responsive proteins and genes to confer stress tolerance ABA enhance crops performance under severe water deficit.


INTRODUCTION
Abiotic stresses such as drought is the major threats to agriculture production worldwide starting from vegetative growth up to giving yields (Bray et al., 2000). To provide tolerance against stresses, plants are equipped with several in built physiological and biochemical mechanisms occurring at cellular level. These are plant growth regulators (PGR) like auxins, gibberellins, cytokinins, ethylene and abscisic acid (ABA) (Upreti and Maryada, 2016).
The first target of manipulation of ABA signaling is dehydration tolerance. Stomata regulation and growth rate such as root elongation is also modified by ABA under dehydration (Spollen et al., 2000). In addition to survival under drought, water usage under well-watered conditions can be reduced by enhancement of ABA signaling (Kim and Van Iersel, 2011). Several studies show that over expression of ABA biosynthetic enzymes enhances drought tolerance (Iee et al., 2001). These may be mainly caused by lower transpiration rate. The yield of crop plants is also improved by the modification of ABA synthesis. A transgenic rice expressing ABA3/LOS5 is tolerant to drought, resulting in better yields under water deficit condition (Xiao et al., 2009).
Thus, mediation of the ABA pathway is a good scheme for the improvement of crops. ABA induces numerous responses in several aspects and the concentration of ABA is important under some conditions. For example, a higher concentration of ABA inhibits seedling growth, while a lower concentration (less than 1 μM) of ABA enhances it (Parcy et al., 1994). Since hypersensitivity to ABA confers not only tolerance to dehydration but also inhibition of seed germination and seedling growth; such expression only under stress may be useful.
ABA is important for seed dormancy. While easy and quick germination is good for cultivating, but unexpected germination, such as preharvest sprouting, causes substantial losses in seed yield and quality of cereal crops (Liu et al., 2013). Wheat mutants increasing ABA sensitivity show higher seed dormancy (Schramm et al., 2013). Temporally programmed manipulation or conditional induction of ABA sensitivity may provide controlled germination making significant profits enhancing crops performance under drought condition. Therefore, the objective of this review is to understand the effect of ABA for adaptation and performance of crops under drought stress.
proteins in seeds, promote seed desiccation tolerance and dormancy (Finkelstein et al., 2008) and inhibit seed germination. ABA is also involved in the control of lateral root formation and seedling growth (Xiong et al., 2006) as well as in the reduction of water transpiration through promotion of stomata pore closure (Kim et al., 2010). Moreover, ABA controls the expression of a large set of stress-responsive genes (Nemhauser et al., 2006).

ABA biosynthesis in plant
With the exception of the conversion of xanthoxin to ABA in the cytoplasm, all the steps for the ABA synthesis occur in plastids (Seo and Koshiba, 2002) (Fig.1). The early C5 precursor of ABA, Isopentenyl pyrophosphate (IPP) is produced primarily in plastids via 1-deoxy-D-xylulose-5-phosphate (DXP) from pyruvate and glyceraldehydes-3-phosphate (Wasilewska et al., 2008). This leads to the sequential production of farnesyl pyrophosphate, geranylgeranyl pyrophosphate (GGPP), phytoene, carotene, lycopene and β-carotene. Β-carotene is converted to a xanthophyll, zeaxanthin, which is the first oxygenated carotenoid (Seo and Koshiba, 2002). Subsequent steps involve the synthesis of cis-isomers of violaxanthin and neoxanthin that are cleaved to form xanthoxin (the C15 precursor of ABA). This cleavage is catalyzed by the 9-cis-epoxycarotenoid dioxygenase (NCED) enzymes (Schwartz et al., 2003). Xanthoxin is presumed to migrate from the plastid to the cytosol (Nambara and Marion-Poll, 2005), where it is converted to ABA by three possible pathways: via abscisic aldehyde, xanthoxic acid or abscisic alcohol (Seo and Koshiba, 2002). ABA is derived from β-carotene (C40) through an oxidative cleavage reaction in plastids. The first step of ABA biosynthesis pathway is the conversion of zeaxanthin and antheraxanthin to all trans-violaxanthin, which will be catalyzed by zeaxanthin epoxidase (ZEP). Antheraxanthin is the intermediate product. All-transviolaxanthin is converted to 9-cis-violaxanthin or 9 -cis neoxanthin by the 9-cis-epoxy carotenoid dioxygenase (NCED), which yields a C15 intermediate product called xanthoxin. Xanthoxin is then converted into ABA by two enzymatic reactions. Finally, xanthoxin is converted to an ABA aldehyde by the enzyme, short-chain alcohol dehydrogenase/reductase (SDR), and then oxidation of the abscisic aldehyde to ABA is catalyzed by the abscisic aldehyde oxidase (AAO) (Finkelstein and Rock, 2002).

ABA catabolism for function
Cellular ABA levels can be lowered via two pathways: hydroxylation and conjugation (Nambara and Marion-Poll, 2005). ABA is hydroxylated via oxidation of the methyl groups of the ring structure at three positions, C-7′, C-8′, and C-9′, of which C-8′ is the primary site (Cutler and Krochko, 1999). The three forms of hydroxylated ABA exert significant biological activity (Zhou et al., 2004), but hydroxylation triggers further inactivation steps (Nambara and Marion-Poll, 2005). Cytochrome P450 monooxygenase (CYP707A) catalyzes the hydroxylation of ABA at the C-8′ to form unstable 8′-hydroxy ABA, which is subsequently converted to phaseic acid (PA) by spontaneous isomerization (Saito et al., 2004). PA is then converted to dihydrophaseic acid (DPA) by a soluble PA reductase (Gillard and Walton, 1976).
ABA conjugation apart from de novo biosynthesis, ABA conjugation/ deconjugation plays a critical role in the regulation of cellular ABA amounts under both normal and dehydration conditions (Xu et al., 2012). ABA and hydroxy ABA are conjugated with glucose for inactivation. ABA can be inactivated at the C-1 hydroxyl group by forming different conjugates. Of these conjugates, ABA glucosyl ester (ABA-GE) is the predominant form, which is produced by ABA glycosyl transferase (Cutler and Krochko, 1999). ABA-GE is stored in vacuoles and the apoplast (Dietz et al., 2000). The low membrane permeability makes ABA-GE suitable for long distance translocation and storage in vacuoles and the apoplastic space (Jiang and Hartung, 2008). Under dehydration conditions, ABA is released from the glucosyl ester form by β-glucosidases (Xu et al., 2012). The enzymatic activity of β-glucosidases to catalyze the hydrolysis of ABA-GE for releasing free ABA was first demonstrated in barley (Dietz et al., 2000).

ABA Transport for regulation purpose in plant cell
Stress-induced biosynthesis of ABA primarily occurs in vascular tissues but ABA exerts its responses in various cells, including distant guard cells (Kuromori et al., 2010). Thus, ABA responses require translocation from ABAproducing cells via intercellular transport to allow rapid distribution into neighboring tissues. Recently, cell-tocell ABA transport was shown to be mediated by two plasma membrane bound ATP-binding cassette (ABC) transporters (Kuromori et al., 2010) and a family of low-affinity nitrate transporters .Most ABC transporters are integral membrane proteins and act as ATP-driven transporters for a very wide range of substrates, including lipids, drugs, heavy metals, and auxin (Rea, 2007).

Role of Abscisic Acid in plants performance under drought
The ABA is an important chemical signal of plant responses to a range of abiotic stresses, including drought and salinity (Keskin et al., 2010). A dynamic balance between its biosynthesis and degradation, sensitized by developmental and environmental factors, determines the amount of available ABA (Cutler and Krochko, 1999). The functions of ABA in plants are multiple. High cellular ABA facilitate modifications in stomatal functioning, root hydraulic conductivity, photosynthesis, biomass allocation between roots and shoots, plant water relations, osmolyte production, and synthesis of stress-responsive proteins and genes to confer stress tolerance (Kim et al., 2010).

Seed Dormancy and Germination
Seed is a fundamental organ in higher plants and the transition to seed dormancy and germination signifies a key stage in the plant life cycle, which is an important factor in ecology and economical trait (Saroj, 2016). The ABA plays a central role in the induction and maintenance of seed dormancy. It also inhibits the transition from embryonic to germination growth which is very important during unfavorable condition to save the seedling. During the phase of desiccation tolerance, ABA metabolism must be regulated. For young and mature plant Sometimes, ABA translocates from the roots through xylem or phloem (Miransari and Smith, 2014) to the shoot system to exert its action there. 2.5.2. ABA induce gene expression ABA signaling leads to large changes in gene expression, which may involve changes in transcription, transcript processing, and stability (Cutler et al., 2010). In Arabidopsis, about 10% of all genes are regulated by ABA (Nemhauser et al., 2006). Expression of AREB1/ABF2, AREB2/ABF4, and ABF3/DPBD5 in Arabidopsis regulated by ABA in dehydration and high salinity stresses (Fujita et al., 2005). Over-expression of these factors in transgenic plants resulted in ABA hypersensitivity in germination of seed, growth, and also enhanced drought tolerance (Abdeen et al., 2010).
A number of genes that respond to stress at transcriptional level have been found to be induced by ABA (Delasny et al., 1994). However, not all the genes induced by stress are responsive to ABA. There are ABAdependent and ABA-independent signal transduction cascade between initial signal of stress and expression for specific gene. Gene expressed during stress helps in protecting cells from stress injury by producing proteins involved in signaling transduction mechanism (Shinozaki and Yamaguchi-Shinozaki, 1997). 2.5.3. ABA induces stomatal closure The increases in ABA enable plants to restrict their water loss through transpiration following closure of stomata and enhance plant water status following increase root hydraulic conductivity (Thompson et al., 2007). The sensitivity of stomata to ABA varies in plant species and cultivars and is dependent upon leaf age, climatic factors like temperature and relative humidity, plant nutritional status, ionic status of xylem sap, and leaf water status (Dodd et al., 1996). Such variations in ABA for stomata response are possibly the consequence of variations in the magnitude of ABA transportation to the active site at guard cell. Tardieu and Simonneau (1998) demonstrated that the xylem ABA concentration and stomata conductance are linearly inverse related and the slope of relationship varied diurnally. Exogenous application of ABA is effective to increase plant adaptive response to various stress conditions (Marcinska et al., 2013).
Light induces stomata ostiole opening, while ABA and elevated CO2 levels promote closure (Wasilewska et al., 2008) (Figure 2). This aperture regulation is under the control of guard cell turgor. Closing stimuli, including ABA, were shown to inhibit the uptake cellular machinery and trigger ion and water efflux. Elevated cytosolic Ca 2+ levels activate two types of anion channels that mediate anion release from guard cells; slow-acting sustained (S-type) or rapid transient (R-type) anion channels (Roelfsema et al., 2004). It was proposed that the S-type, and not the R-type, channel is responsible for ABA-mediated stomata closure (Joshi-Saha et al., 2011).
The membrane depolarization caused by anion efflux via ion channels activates outward-rectifying K + (K + out) channels and results in K + efflux from guard cells (Schroeder et al., 2001). The sustained efflux of both anions and K + from guard cells drives water efflux and contributes to the loss of guard cell turgor, leading to stomatal closure. ABA also inhibits ion uptake, which is required to initiate hyperpolarization of guard cell plasma membranes to induce stomatal opening. Under normal conditions, 2C-type protein phosphatase (PP2C) family members, which are negative regulators of ABA signaling, suppress open stomata 1 (OST1) kinase activity via physical interaction, leaving the S-type anion channel (SLAC1) with basal activity. Under conditions of stress, including drought, the ABA concentration in leaves increases rapidly (Hubbard et al., 2010). ABA perception occurs via regulatory component of ABA receptor (RCAR) family members functioning as ABA receptors in combination with PP2Cs (Cutler et al., 2010) ( Figure 2). The formation of the RCAR-PP2C complex breaks the PP2C linkage to OST1 complex, there by releasing active OST1 kinase from inhibition. In turn, OST1-mediated phosphorylation induces activation of the SLAC1 channel, then, releasing anions and depolarizing the membrane (Lee et al., 2009). This depolarization induces a further drop in turgor and closure of the stomata pores.

Effects of ABA on root growth
The ABA is also active in root-to-shoot communication in the plants suffered from stress. The ratio between the growth of root and shoot in a plant is sensitive to abiotic stresses, and there is coordination among them via long distance transport of substrates or signal (Munns and Crammer, 1996). Passioura and Stirzaker (1993) reported that the ABA acts as a feed-forward signal from the roots to the aerial plant parts under stress conditions. Jackson (1993) provided evidence for influence of the roots on shoot development via transport of hormones in the xylem. Further Saab et al. (1990) stated that the relationship between ABA and root growth is completely different from that in shoots, as higher ABA levels in roots promote root growth at low water potential (Watts et al., 1981). However, some investigations also reported inhibition in root expansion by exogenous applications of ABA (Cramer and Jones, 1996). ABA increase in roots tends to stimulate the water flow by increasing the root hydraulic conductivity and ion uptake, which causes increase in water potential gradient between soil and roots (Glinka and Reinhold, 1971). It also increases water absorbing area of roots and helps the plants transport more water and nutrient under stress situations.

Effects of ABA on shoot growth
ABA is generally regarded as an inhibitor of shoot growth (Davies, 1995). Initial studies of the effect of decreasing endogenous ABA levels in maize seedlings grown at low water potential (under conditions of near-zero transpiration) were consistent with this expectation (Saab et al., 1992). Opposite to this by studying on maize seedlings, Feng (1996) substantiated that the exogenous applications of ABA promotive effect on shoot growth at low water potential appeared to be specific to the water-stressed condition. Feng (1996) verified that a longer duration after transplanting revealed a greater length in the relationship of shoot growth under ABA at low water potential when compared with others like Fluridone (FLU, 10 µM) (Figure 3).

Effects of ABA on Photosynthesis Efficiency
Successive progress in drought stress of both wheat varieties resulted in a significant decrease in chlorophyll, chl a and b, and a significant increase in carotenoids (car) contents (Hala and Ghada, 2008) (Table1). Under severe water stress, ABA treatment resulted in a significant decrease in photosynthetic pigments contents of sakha 94 plants, whereas insakha 93 there was almost no change. Furthermore, the car/chl a+ b ratio were markedly increased in both wheat varieties with decreasing soil moisture, and spraying the plants with ABA resulted in a decrease in this ratio in sakha 93 and an increase in sakha 94 plants. Rehydration decreased the car/chl a+b ratio particularly in sakha 93. PSII of both wheat varieties decreased under drought stress compared to control well watered plants (Wise, 1995) (Table 1). Spraying the leaves with ABA resulted in a significant increase in PSII activity in severely stressed sakha 93 plants; the opposite trend was recorded for sakha 94 plants. Rewatering significantly increased the PSII activity only in sakha 93; the increase in sakha 94 was insignificant. Hence, ABA improves photosynthesis Efficience of Wheat under drought/water stress. Table 1. Changes in photosynthetic pigments (mg/g dry mass) and PSII activity (μmol reduced DCPIP/μg chl/mg FM) in the leaves of wheat seedlings (sakha 93 and 94) in response to drought, exogenous ABA and rewatering Major effect of drought in plant is reduction in photosynthesis, which is due to decrease in leaf expansion, impaired photosynthetic machinery, leaf senescence and finally reduction in assimilates production (Wahid and Rasul, 2005). Drought stress reduced the crop yield due to changes in photosynthetic pigments (Anjum et al., 2003) and diminished activities of Calvin cycle enzymes (Monakhova and Chernyadev, 2002  Drought stress caused significant reduction in yield components of cv. Punjab-96 but relatively the yield of cv. Chakwal-97 is affected less ( Table 2). The spike length, number of grains per spike and 100 grain weight were significantly decreased under drought stress in cv. Punjab-96. ABA priming completely ameliorated the drought induced inhibition in grain number and 100 grain weight. In cv. Chakwal-97, mild effect of drought stress was observed on the spike length and number of grains per spike. Generally; both varieties show good performance under ABA and ABA with drought when compared with under drought performance which implies that ABA enhancements of crop yield performance under drought stress or its ameliorating effects of drought stress on crop yield reduction.

CONCLUSION
Abiotic stresses such as drought become the major threat to agriculture production worldwide. To overcome such problem plants have several physiological and biochemical mechanisms occurring at cellular level. These are plant growth regulators like hormones. From these hormones Abscisic acid (ABA) is imperative in regulating root and shoots growth and other physiological process during crops subjected to water deficit. ABA follows process like biosynthesis, carboxylation and transportation for its availability and to carry out its function. ABA play great role in regulating plant physiological function to enhance crop adaptability and performance under adverse drought stress environments. It induces stomata closure under drought condition which is useful to inhibit water loss through evaporation. This also enhances plants water use efficiency under drought condition. Exogenous application of ABA also authenticated as it can increase root length and area coverage by root which helps plant to take up water from depth. ABA is important for inducing seed dormancy which inhibit unexpected seed sprout and seed emergence under unfavorable condition. This is crucial to avoid seed quality loss. Regulation of plant physiology and increase in water use efficiency by ABA enables crop to adapt and perform very well under water