The increase in de novo biosynthesis of ABA is due to the rise in abiotic stress which plays a role to inhibit its degradation and is thought to be stimulated by stress relief. Gene identified for ABA biosynthesis is ZEP and has been cloned and expressed in numerous plant species. This gene is found to be present in every plant part but is highly associated for basal expression in leaves (Xiong et al., 2002a). Moreover, level of ABA biosynthesis through ZEP gene is regulated not only in different plant portions and development phases but also in different plant species. The ZEP gene in Arabidopsis has a same basal transcript level in non-stressful conditions as in tobacco and tomato. These variations in the expression of ZEP genes are partly associated to basal transcript levels which also cover stress inducibility of genes as identified in different experiments. However, another ABA biosynthetic genes expression (NCED, AtAAO3, MCSU, and AtSDR1) are less debatable. The ABA biosynthesis is notably achieved after cleavage in the rate limiting step, and thus expression of NCED gene(s) has received a significant importance. NCED gene is found to be overexpressed in drought stress condition in maize (Zea mays; Tan et al., 1997), tomato (Lycopersicon esculentum; Burbidge et al., 1999), bean (Phaseolus vulgaris; Qin and Zeevaart, 1999), Arabidopsis (Iuchi et al., 2001), cowpea (Vigna unguiculata; Iuchi et al., 2000), and avocado (Persea americana; Chernys and Zeevaart, 2000). A remarkable rise in NCED transcript levels has been reported following 15–30 min of leaf extrication or induced dehydration (Xiong and Zhu, 2003), providing an evidence for the instant activation of NCED genes. Since ABA biosynthesis mechanism up-regulates drastically in response to stress, it can be deduced that protein levels of the related genes increases with the transcript levels, which were similarly noticed in NCED gene (Xiong and Zhu, 2003).

The end product ABA of the biosynthesis pathway negatively regulates the ABA accumulation via triggering its catabolic enzymes (Cutler and Krochko, 1999). The cytochrome P450 enzyme activity and ABA 8′- hydroxylase activity carries out the primary step of ABA degradation and was regulated by exogenous ABA accumulation. Since the product of NCED gene regulates the rate-limiting step in the ABA biosynthesis pathway, the information regarding the control of this gene product by ABA is very limited in terms of auto-regulation of ABA biosynthesis. In tomato plants and cowpea, this NCED gene was found to be not affected by exogenous ABA (Iuchi et al., 2000; Thompson et al., 2000). Hence, it can be concluded that ABA cannot stimulate its production but have the potential for its degradation.

The transcript measures 9-cis-epoxycarotenoid dioxygenase (NCED1) (an enzyme catalyzing the first step of ABA biosynthesis) and grows significantly in grape berries (Berli and Bottini, 2013). The resultant will be accumulation of ABA in different tissue pertaining to hormonal response ultimately causing a berry development and ripening (Wheeler et al., 2009).

However, Xiong and Zhu (2003) have shown that there is an up-regulation of ZEP, AAO3, and MCSU in Arabidopsis by ABA, even though being stimulated by stress. Exogenous ABA significantly regulates expression of such genes. Though, ABA biosynthesis production and degradation both are of greater significance in regulating ABA expression and adjusting plant stress responses and development strategies. As DRE- and ABRE-like cis elements are promoters of stress-inducible ABA genes (Xiong et al., 2001; Bray, 2002), consequently, these genes are in the same way controlled as DRE/CRT class of stress-responsive genes (Xiong et al., 2002b). Also, it has been shown that ABA induces second messengers activating defensive responses through the production of ROS (Sakamoto et al., 2008). Moreover, the expression of antioxidants enzyme gene and non-enzymatic defense systems genes are also activated by ABA signal induction mechanism (Jiang and Zhang, 2002).

With the help of ABA in stress response condition, screening can be done in vegetative tissues and would probably help to identify new loci which can be essential for regulating ABA metabolism (Zhang, 2014). ABA does specific types of acts which contain complicated regulatory mechanisms, production, degradation, signal perception, and transduction. Figuring out the crucial position of ABA in response to plant stress and their regulatory systems will help to formulate real-time techniques to procreate or genetically regulate crops with expanded tolerance to adverse environment stipulations.

Heavy metals are the foremost pollutants of environment, i.e., soil and water. Levels of some heavy metals like Cd, Cu, Pb, Hg, and Cr are high in agricultural and other natural areas which are because of human activities (Choudhary et al., 2010). Toxicity caused by heavy metals is the major cause of abiotic stress which leads humans, plants and animals to dangerous health effects (Singh et al., 2015, 2017; Tripathi et al., 2016a, 2017c). They have high reactivity and because of this they adversely influence energy synthesis, growth and senescence processes.

It is seemingly true that numerous physiological and developmental events are influenced by ABA. ABA notably increases freezing, chilling, drought, and salt tolerance in numerous plant species (Rikin and Richmond, 1976; Hsu and Kao, 2003). Heavy metals like Cd, Ni, Zn, and Al (Fediuc et al., 2005) have been revealed to raise ABA portions in plants (Table 1). One of the utmost toxic heavy metals is cadmium (Cd), a divalent heavy metal cation (Tripathi et al., 2012). Leakage of cadmium (Cd) in water, air and soil generally come as a discharge from mining, burning, industries and waste seepage, and by fertilization in fields with sewage sludge and phosphate. It is easily taken up by flora, which leads to lethal symptom such as reduced growth (Chen and Kao, 1995). It was also observed that cadmium damages photosynthesis of plants (Siedlecka and Baszynski, 1993), it lower chlorophyll level (Larsson et al., 1998), and inhibits the opening of stoma (Zhang, 2014). Fediuc et al. (2005) verified that roots have accumulated the Cd-induced ABA but it was not seen in shoots of Phragmites and Typha plants (Table 1).

Hsu and Kao (2003) reported the mechanism of cadmium resistance of rice crop from Taiwan. Their studies showed that at high temperature (30/35°C) in rice seedlings, ABA is involved for Cd resistance (Table 1). This result relies on observations that (a) in comparison of the level of endogenous ABA in Cd-sensitive cultivar (TN1) and Cd-tolerant cultivar (TNG67) it was found with increased level in latter one; (b) there is an increase in Cd tolerance of TN1 when exogenous dose of ABA was given; (c) whereas fluridone application reduced level of ABA, as well as cadmium tolerance level of TNG 67 seedlings; and (d) the fluridone effect on cadmium toxicity of TNG 67 seedlings was backed by the re-application of ABA. These results have shown the relationship that regulation of endogenous level of ABA biosynthesis can reduce Cd uptake in rice seedlings (Hsu and Kao, 2008). In a study by Wang et al. (2016), exogenous ABA was applied using two Solanum photeinocarpum ecotypes (mining and farmland) and found an increase in the Cd content in both ecotypes. Though, the association between Cd and ABA is dependent on plant species, application of ABA exogenously can produce results different from ABA production under Cd treatment (Table 1). Further, Pompeu et al. (2016) have shown that the mechanisms through which ABA is interacting with Cd involve evident histological and biochemical alterations. They have also indicated that the stress response of Cd is facilitated by ABA in tomato (Table 1). In the work of Fediuc et al. (2005), it’s proven that ABA mediated the Cd-precipitated stimulation of O-acetylserine (thiol) lyase (OASTL), the enzyme responsible for cysteine biosynthesis.

In a study carried out by Fusco et al. (2005), the expression of BjCdR39 (an aldehyde dehydrogenase) and BjCdR55 (RNA binding protein) was analyzed in ABA signaling. The expression of both BjCdR39 and BjCdR55, stimulated by cadmium in Brassica juncea, supported the involvement of ABA signal-transduction component in already present cross-talk between the cadmium-stimulated and water stress-stimulated signaling. The BjCdR51 and BjCdR49 denoting aquaporins PIP1 and PIP2 was found to be transcribed in B. juncea when exposed to Cd stress for a day along with expression of BjCdR55 and BjCdR39 (ABA- and drought-responsive gene). The above observation gave an indication that water stress is imposed by Cd and that Cd and ABA show synergistic relationship. BjCdR15 is a putative ortholog to Arabidopsis TGA3. TGA transcription factors belong to the group of bZIP transcription factors which are found in all eukaryotes. TGA factors bind specifically to TGACGTCA. BjCdR15 from B. juncea is up-regulated in plants treated for 6 h with cadmium (Fusco et al., 2005). When ABA treatment is given to the plant, both BjCdR15 and TGA3 responded to the treatment. However, more sensitivity was shown by TGA3 than BjCdR15 to ABA (Farinati et al., 2010).

Another major heavy metal is copper. Food chain is continuously infiltrated by Cu²⁺ contamination through soil which is a major threat to human health and has become an important concern for environment sustainability (Chary et al., 2008). It is essential in low quantity for usual plant growth and development but it lead to phytotoxicity at high concentrations (Berenguer et al., 2008). Against various biotic and abiotic stresses, plants develop a strong protection mechanism which consists the action of antioxidative enzymes by phytohormones such as auxins (Park et al., 2007), cytokinins (Zhang and Ervin, 2008), ABA (Staneloni et al., 2008), ethylene, salicylates, jasmonic acid, brassinosteriods, and polyamines (Choudhary et al., 2010).

Oxidative stress in plant’s metabolic reactions can be caused by metal induced effects of both Cr⁶⁺and Cu²⁺ which ultimately results in discharge of oxidants and free radicals. It is also shown that whenever there is Cr⁶⁺and Cu²⁺ stress there is increase in the synthesis of exogenous or endogenous ABA level. It further shows that the participation of ABA (Table 1) in heavy metal stress tolerance (Choudhary et al., 2010). The application of silicon (Si) has been known to increase the plant’s tolerance capacity against abiotic stresses. In a study, Kim et al. (2014) showed that Si significantly improved the growth and biomass of rice (Oryza sativa) plants and reduced the toxic effects of Cd/Cu after different stress periods. It was found that reduction in uptake of metals led to modulation of the ABA phytohormone involved in response to stress.

Srivastava et al. (2015) have mentioned the three essential ABA-related genes which includes majorly ABA-induced PP2C1 (HAI1), ABA insensitive 1 (ABI1), and ABA interactive protein 2 (AIP2) in B. juncea. GPX is a redox-related gene that is up-regulated under arsenic and copper stress in response to ABA. Under copper stress, GPX6 form (glutathione peroxidase gene encoding isoforms in cytosol and mitochondria) was observed to be intensely up-regulated in Arabidopsis (Milla et al., 2003). In another study, GPX3 had double roles in plant, i.e., homeostasis of hydrogen peroxide and signal relaying in guard cells. In turn, this signal is responsible for regulating stomata according to ABA (Miao et al., 2006). Therefore under arsenic stress, over-expression of GPX6 and GPX3 at separate time durations controlled the level of ROS and opening of stomata.

Drought is one of the principal abiotic stresses that negatively influence the growth of plant and yield (Tripathi et al., 2016b). Greater than half of the terrestrial region, consisting of the widespread portion of arable land, is susceptible to drought (Kogan, 1997). ABA, a phytohormone involved in the regulation of abiotic stress pathways in plants, is responsible for the plant response toward stress situations and additionally involved in other developmental process for example seed dormancy (Cutler et al., 2010; Fujita et al., 2011, 2013, 2014). Drought conditions create osmotic stress in organisms, which eventually cause desiccation and resistance to water uptake in plants. During osmotic stress conditions, ABA accrues (Table 1) and acts as a controller in stress response and tolerance of plants (Yamaguchi-Shinozaki and Shinozaki, 2006; Nakashima and Yamaguchi-Shinozaki, 2013). ABA is known to positively affect stress tolerance following exogenous application or through overexpressing genes for its increased endogenous content in plants. In a recent study, it was seen that when ABA, γ-aminobutyric acid (GABA) and salicylic acid (SA) were applied exogenously, it effectively improved the drought-induced damages in creeping bentgrass (Agrostis stolonifera) by maintaining membrane stability and leaf water status (Li et al., 2016). After analyzing its metabolic profile, it was found that ABA, GABA, and SA have influenced the common metabolic pathways and also caused differential changes in metabolite accumulation under drought stress (Li et al., 2016).

Some of the work has provided an evidence that water scarcity can have an effect on the expression of core ABA signaling constituents, equivalent to ABA, PYR/PYL/RCAR ABA receptors, protein phosphatases 2C (PP2Cs), and subclass III SnRK2 protein kinases (Weiner et al., 2010). Osmotic stresses, including drought, cold, and high salinity can cause cellular dehydration at the time of seed perfection in vegetative development (Fujita et al., 2011), and hence results in hyper-activation of plant ABA metabolism (Nambara and Marion-Poll, 2005) and transport (Kuromori et al., 2010). It was thought that drought induced-stress consequently enforce the high ABA level in Arabidopsis leaves and safeguard the plant against disease symptoms associated with an avirulent strain of Pseudomonas syringae pv. tomato (Mohr and Cahill, 2003; Tossi et al., 2009). In proliferating tissues, amount of ABA is 40-fold higher when going through drought and salt stress (Zeevaart and Creelman, 1988). Mutants having non-functional ABA bio-synthesis are extra vulnerable to environment deviations when compared to transgenic plants which can induce much hormonal response and forecast higher tolerance against abiotic stress than the wild type (Iuchi et al., 2001; Qin and Zeevaart, 2002). Huang et al. (2016) studied that ramie BnbZIP3 gene was upregulated in the presence of drought, high saline and ABA in ramie. BnbZIP3 gene belongs to the group of bZIP transcription factors. This might be due to the reason that the promoter of BnbZIP3 may contain number of cis-acting elements that are involved in ABA signaling and different stress responses.

The prime ABA-facilitated signaling pathway which incorporates PYR/PYL/RCARs, PP2Cs, SnRK2s, and bZIP transcription factors are fortunately classified under in vitro condition (Fujita et al., 2014). In short, major ABA signaling components actively regulates both fast and slow ABA communication pathway to tackle dehydration. Previous work reported the relation between osmotic stress by high salt or drought and two cellular pathways; one is ABA-dependent and another one ABA-independent. Cold stress response persuade through altered gene expression via an ABA-independent pathway. ABA-dependent pathway depends upon the availability of cis-acting element called ABRE element (ABA-responsive element) (Tuteja, 2007). Gene related studies indicate a co-relation between ABA-dependent and ABA-independent pathways and a cross talk or association of involved molecules in the signaling pathway. Calcium acts as a secondary messenger in response to stress and a potential candidate to assist cross communication. An ample amount of research work has proved that ABA, drought, cold and high salt induces a sudden rise in calcium levels in plant cells (Mahajan and Tuteja, 2005; Tuteja, 2007).

The solar electromagnetic spectrum having wavelength between 200 and 400 nm belongs to the category of Ultraviolet (UV) radiation. The UV radiation is characterized as having a shorter wavelength in comparison with the photosynthetically active radiation (PAR) which has a wavelength between the range of 400–700 nm which in some extent over imposes with that people understand as violet. According to international standardization, the ultra violet rays encompasses three different types of radiation viz., UV-C, UV-B, and UV-A. The UV-C rays have wavelength between 200 and 280 nm a portion of shorter wavelength and consequently emit high energy photons which was absorbed completely by the ozone layer and not able to reach the surface of Earth.

Plants are categorized as sessile organisms which are attached to one place and required sunlight for their growth and therefore, they are certainly exposed to UV rays which contain approximately 7% of the electromagnetic radiation emanated from the sun (Tossi et al., 2009). Most of the UV-B radiations are captivated in the ozone layer in stratospheric layer and the remaining radiations are transmitted to the Earth’s surface (Tossi et al., 2009). A high percentage of UV-B rays stimulate the reactive oxygen species (ROS) which instigates damage to biomolecules and destroys membrane integrity, cell morphology as well as plant physiology and consequently affects plant growth and development which may be visible in several plant species (Frohnmeyer and Staiger, 2003; Tripathi et al., 2017a,b,c).

Abscisic acid functions as the main stimulating means of stomata closure, but in addition it imparts an important role in plants for their adaptations to drought conditions and to UV-B radiations (Sangtarash et al., 2009). The reaction mechanism of ABA involved the inhibition of ethylene synthesis in plant and enhances plant growth (Zhang, 2014). Ethylene production in plants increases by ultraviolet radiations (UV-B) as well as in water deficient conditions (Table 1). The plant species that experiences the exposure of UV-B radiations were found to be more tolerant to drought conditions and therefore ABA works in many ways toward plant’s reaction to drought conditions (Li et al., 2014).

It was studied previously that exposure to UV-B radiations in Arabidopsis cause resistance to pathogen named Hyaloperonospora parasitica (Kunz et al., 2008). It was also reported that plant exposed to UV-B radiations when treated with ABA restrict the expression of plant defensin 1.2, a gene involved in defense mechanism and get up-regulated by UV-B (Mackerness et al., 2001). A novel cis-regulatory element, i.e., UVBoxANAC13 was found to be regulated by UV-B radiation but suppressed during other environmental stress situations (Safrany et al., 2008). A study of Tossi et al. (2009) in maize plant shows that their leaves cause an increase in the production of ABA when exposed to UV-B radiation, and the concentration of H₂O₂ and NO was also enhanced in this particular condition. In addition ABA is needed for nitric oxide involved diminution of the deleterious effects generated by UV-B (Table 1). This proves that the upsurge in amount of ABA is generally the quickest response involved in the signaling pathways which is affected by UV-B radiations in maize leaves (Table 1).

Several studies attempt to relate the interaction incorporating UV-B and ABA, but some of it concludes that the presence of ABA enhance the tolerance of grapevines to UV-B (Berli et al., 2010, 2011). The same effect was observed by Rakitin et al. (2008) in leaves of Arabidopsis creating a positive effect of UV-B on ABA synthesis in tissues during high exposure to UV-B. Whereas additional studies reported that the interaction between water stress conditions to UV-B radiation observes a less sensitivity to UV-B in various plant species during drought conditions (Berli and Bottini, 2013)

It was observed in previous studies that ABA defends maize leaves during exposure of UV-B irradiation. The outcomes were observed by utilizing the vp14 maize mutant which is not active for ABA synthesis. The VP14 gene transcribes a 9-cis-epoxycarotenoid dioxygenase (NECD) enzyme (Tan et al., 1997). NECD cleaves epoxycarotenoids which converts into xanthoxin, which was further modified by a short-chain dehydrogenase/reductase (SDR1) to form abscisic aldehyde which is then converted into ABA by enzyme aldehyde oxidase (AO). The relation of ABA to nitric oxide regulation is still an issue of discussion. It was previously reported that either ABA- or UV-B induced NO is largely formed by NOS-like activity (i.e., ABA or UV-B might show NOS like activity to synthesize NO) (Qu et al., 2006). Whereas, several other pharmacological data suggest that nitrate reductase (NR) is the chief source of nitric oxide in guard cells in reaction to ABA-mediated H₂O₂ synthesis (Bright et al., 2006).

In terms of plant productivity, it is surely considered that water scarcity is a chief restraining factor for plant growth under field conditions since plants are exposed to varying levels of water stress daily. Water shortage destroys many plant capabilities similar to photosynthesis, transpiration, stomatal conductance, and metabolite accumulation (Sangtarash et al., 2009), and therefore results in extensive decrease in plant growth and productivity (Reddy et al., 2004). ABA, a growth hormone in plant serves, many physiological processes in plants. Plant responses to drought incorporate alterations in morphology and biochemistry leading to acclimation in non-severe circumstances, and cause harm to plant and plant ingredients, in extreme cases (Li et al., 2014). Since water-stressed vegetation have more concentration of ABA than the good-watered crops, Sangtarash et al. (2009) speculated that the consequences of ABA will be highly enhanced for the sufficiently watered plants than for the water-stressed plants (Table 1).

It was assumed for the past 25 years that an increasing concentration of ABA in drought resistant plants also restricts growth of the plant; predominantly inhibit shoot growth (Trewavas and Jones, 1991). Several research mentioning the relationship between the ABA concentration of plant tissue or xylem sap and growth inhibition recommended that the enhanced concentration of endogenous ABA in drought resistant plant was enough to support a portion of plant growth, not all the growth inhibition resulted from water stress (Sangtarash et al., 2009).

Numerous studies have shown that due to the ABA content the specific mRNA and proteins accumulated late during seed embryogenesis in different plant species (Mundy and Chua, 1988). Physiological studies have proved that the escalation in endogenous ABA content in plant tissue resulted either from high osmoticum, NaCl, or drying conditions in water stressed plants (Li et al., 2014). Due to this, proteins and nucleic acid gets accumulate and cause intra-cellular osmolarity or have various defensive functions.

It has also been reported that ABA potentially inhibits shoot growth in appropriately watered plants (Table 1). Nevertheless, solely a sequence of reports was found previously that shows ABA deficiency in plant under drought conditions causes enhancement of shoot growth which is also in consistent with the expectation that endogenous ABA accumulation is responsible for plant growth inhibition. It was reported that the rate of shoot elongation was high in ABA deficient maize seedlings (fluridone-treated or vp5 mutant) in comparison with the control (Sharp et al., 1994). A study by Bray (2002) illustrated that an excessive awareness of ABA is required to hinder an overload ethylene creation from tissues below water stress stipulations. As a result, ABA accumulation in the course of drought could result in retaining shoot progress as well as root development, instead of avoiding growth which is more likely believed (Table 1).

Abscisic acid is known to regulate the balance between intrinsic growth and environmental responses. AtABCG25 acts as cell-membrane ABA transporter exporting ABA from cytoplasm to outside of the cells. The plants with over-expressed AtABCG25 shows a reduced transpiration phenotype without any growth retardation. In a work by Kuromori et al. (2016), it was observed that AtABCG25 over-expression stimulated a local ABA response in guard cells. Furthermore, it was seen that AtABCG25 overexpression increased the drought tolerance, probably resulting from maintenance of water contents over the common threshold for survival after drought stress treatment (Kuromori et al., 2016).