Abiotic stresses, such as drought, salinity or heat, have become a major threat for crop plant survival and yield. Full understanding of the mechanisms ensuring plant adaptation to stress can assist in obtaining tolerant varieties. ABA, which is the major stress phytohormone, takes part in a plant’s adaptation to stress through regulation of physiological processes such as the biosynthesis of osmolytes and the detoxification of ROS (Reactive Oxygen Species); (reviewed by Roychoudhury et al., 2013; Mehrotra et al., 2014; Yoshida et al., 2014; Munemasa et al., 2015; Jones, 2016; Sah et al., 2016). Generally, ABA is considered to be a shoot and root growth inhibitor that acts to save water and energy under stress. However, in some cases ABA promotes root growth and enables the root system to access water in deeper soil layers (Sharp et al., 2004; Skirycz and Inzé, 2010; Zhao et al., 2014). ABA also prevents turgor loss under conditions of reduced water availability. One of the earliest protective processes that are observed in stressed plants is stomatal closure. The endogenous ABA level is precisely controlled by biosynthesis and the catabolism pathways. ABA biosynthesis is a multistage process that is dependent on many enzymes including NCED (CAROTENOID CLEAVAGE DIOXYGENASE). ABA catabolism requires ABA8′hydroxylase activity and results in the creation of phaseic acid (reviewed by Mehrotra et al., 2014; Sah et al., 2016). The perception of an ABA signal is mainly dependent on the core ABA signaling, which includes the action of PYR/PYL/RCAR (PYRABACTIN RESISTANCE PROTEINS/PYR-LIKE PROTEINS/REGULA TORY COMPONENTS OF ABA RECEPTOR) receptors, PPC (PHOSPHATASE 2C) phosphatases and SnRK2 (SNF1-RELA TED PROTEIN KINASE 2) kinases. Finally, AREB/ABF (ABS CISIC ACID RESPONSIVE ELEMENTS-BINDING FACTOR) transcription factors bind to ABRE (ABA RESPONSIVE ELEMENT) elements and regulate the expression of stress responsive genes (reviewed by Nakashima and Yamaguchi-Shinozaki, 2013; Yoshida et al., 2014; Daszkowska-Golec, 2016). ABA-dependent transcription factors are primarily members of large families such as bZIP (BASIC LEUCINE ZIPPER) (Banerjee and Roychoudhury, 2015), MYB (MYELOBLASTOSIS) (Golldack et al., 2011), WRKY (WRKY DNA-BINDING PROTEIN) (Chen L. et al., 2012) or AP2 (APETALA 2) (Golldack et al., 2011). ABI5 (ABA INSENSITIVE 5), which is a bZIP transcription factor, functions in the core ABA signaling.

The effect of mutation in ABI5 gene was firstly described by Finkelstein’s (1994) group. Arabidopsis insertional mutant abi5 was characterized as ABA insensitive in comparison to the wild type during seed germination. The mutation was mapped on 2 chromosome (Finkelstein, 1994). Transcriptomic analysis of abi5 mutant indicated the lower level of expression of stress responsive genes. It suggested the role of ABI5 in abiotic stress response (Finkelstein and Lynch, 2000). Further ABI5-related expression analysis confirmed its role in drought and salt adaption during seedling development (Finkelstein and Lynch, 2000; Lopez-Molina et al., 2001; Nakamura et al., 2001). The ABI5-regulated inhibition of seed germination and early seedling growth protects against plant development in adverse conditions. However, the ABI5 function is not only restricted to embryo tissues and its role was also described in the vegetative stage of development (Brocard et al., 2002; Kong et al., 2013). The target genes of ABI5 enable further adaptations to abiotic stresses.

In this review, we present the current understanding of the way ABI5 functions as (1) a regulator of abiotic stress responses and (2) an integrator of ABA crosstalk with other phytohormones. The results that are described primarily refer to research on Arabidopsis thaliana as the model plant.

The process of seed germination is a critical stage in the plant life cycle and therefore plants have evolved precise mechanisms for its regulation (Debeaujon et al., 2000; Manz et al., 2005; Fait et al., 2006; Finch-Savage and Leubner-Metzger, 2006; Nonogaki et al., 2010; Rajjou et al., 2012).

Gibberellic acid (GA) and ABA are the main phytohormones that participate in the regulation of the seed germination process (reviewed by Jacobsen et al., 2002; Daszkowska-Golec, 2011). GA is well known as a positive regulator of seed germination. An environment that is favorable for seed germination leads to the activation of the GA biosynthesis genes – GA3OX1 (GIBBERELLIN 3-OXIDASE 1) and GA3OX2, which results in a higher content of the active GA pool (Ogawa et al., 2003; Mitchum et al., 2006). Next, GA responsive genes encoding key regulators and enzymes for germination are activated. The increase of GA during seed germination is associated with a decrease of ABA. The low ABA level is caused by the activity of CYP707A2 (CYTOCHROME P450) encoding ABA8′-hydroxylase and is responsible for ABA catabolism in seeds (Kushiro et al., 2004; Miransari and Smith, 2014). On the contrary, seed dormancy, which prevents germination under unfavorable conditions, is connected with a high ABA and low GA content (Okamoto et al., 2006; Liu et al., 2014). To summarize, the complex ABA and GA interplay enables seed germination at the appropriate moment.

Sometimes, seeds start to germinate prior to the occurrence of abiotic stress. The inhibition of germination and prevention of germinating embryo from dryness is conducted through the ABA-dependent pathway described above. However, plants develop a special subtype of ABA action, which requires the action of seed specific, ABA-dependent transcription factors such as ABI3, ABI4, and ABI5. Mutations in these ABI loci lead to an ABA, salt and osmotic stress insensitivity during seed germination (Finkelstein, 1994; Carles et al., 2002; Nambara et al., 2002). ABI3 has a B3 domain and activates genes via an Ry/Sph, seed-specific enhancer element that is present in the promoter sequences. Its function is mainly attributed to the maintenance of embryogenesis and seed dormancy. The loss of this function results in damaged seed development (Finkelstein and Somerville, 1990). ABI3 induces the expression of MIR159 encoding the negative regulator of MYB33 and MYB101. Both of these act as positive components of ABA signaling. This indicates that ABI3 is responsible for the negative feedback regulation of the ABA cascade in germinating seeds that are under abiotic stress (Reyes and Chua, 2007). ABI4, a transcription factor with an AP2 domain, also functions during seed germination under abiotic stress. The target genes of ABI4 have CE1 (COUPLING ELEMENT 1) in their promoter regions. ABI4 was shown to play a significant role in the ABA-dependent inhibition of lateral root development (De Smet et al., 2003; Shkolnik-Inbar and Bar-Zvi, 2010). Additionally, it has a negative influence on the expression of photosynthesis-related genes and mediates the plant response to glucose (Acevedo-Hernández et al., 2005; Bossi et al., 2009). ABI4 is also assumed to participate in the regulation of ABA signaling through the activation of the MIR159b expression and it also targets mature miR159 – MYB33 and MYB101 (Daszkowska-Golec et al., 2013). ABI3 and ABI4 were observed to act together with ABI5 in the regulation of abiotic stress responses (Nakamura et al., 2001; Reeves et al., 2011).

In germinating seeds and young seedlings, ABA signal perception is also governed by the ABI5 bZIP transcription factor. ABI5 shows a high homology to AREBs and also functions in a similar manner (Finkelstein and Lynch, 2000; Kim, 2006; Yoshida et al., 2010). Its expression is activated by drought and salt stress during seed germination within a short developmental window, which occurs between 48 and 60 h after stratification. Therefore, ABI5 activity causes the inhibition of seed germination or early seedling growth. A correlation was shown between ABA sensitivity, the expression of ABI5 and the re-establishment of desiccation tolerance (Maia et al., 2014). Growth inhibition of the embryo and root at a later stage of development ensures more water retention and thus drought tolerance (Lopez-Molina et al., 2001; Brocard et al., 2002; Maia et al., 2014). Under stress, SnRK2.2, SnRK2.3 and SnRK2.6 phosphorylate the ABI5 trans-activation domain in vegetative tissues. The phosphorylation of ABI5 changes its conformation and enables its further interactions with other proteins (Nakamura et al., 2001). The first targets of ABI5 that were identified were the genes encoding LEA proteins, EM1 (EARLY METHIONINE-LABELED 1), EM6 and LEAD34 (Finkelstein and Lynch, 2000). Additionally, in a Yeast One-Hybrid assay, ABI5 was shown to bind directly with the ABRE sequence in the EM6 promoter (Nakamura et al., 2001; Carles et al., 2002).

Many ABI5 target genes are associated with the germination process, e.g., PGIP1 (POLYGALACTURONASE INHIBITING PROTEIN 1) and PGIP2 genes (Kanai et al., 2010) (Figure 1). The induction of their ABI5-mediated expression leads to the inhibition of the activity of polygalacturanases, which results in the retardation of seed coat rupture and thus germination. In these processes, ABI5 is under the negative regulation of PED3 (PEROXISOME DEFECTIVE 3), which encodes the ATP-binding cassette transporter that is associated with fatty acid β oxidation. Thus, ABI5 acts as a crucial regulator of germination through its involvement in physiological and the biochemical aspects of this process (Kanai et al., 2010) (Figure 1).

Some reports indicate that the ABI5 function is not only restricted to germination and post-germinative growth, but also includes the vegetative tissues. ABI5 expression has been observed in root tips, nodes and leaf veins at the seedling stage, while in older plants, its activity has been shown at the edges of leaves and in flowers. Additionally, many target genes are regulated by ABI5 at the later stages of plant development. ABA-induced expression of EM1 was decreased in the vegetative tissues of an abi5 mutant and ABI5 overexpression in 2-week-old plants was observed to have some positive impact on the expression of the ABA-responsive genes Cor6.6, Cor15a and Rab18 (Finkelstein and Lynch, 2000; Brocard et al., 2002). In 7-day-old seedlings, ABI5 regulates the expression of DGAT1 (DIACYLGLYCEROL ACYLTRANSFERASE 1) in the presence of salt and osmotic stress in an ABA-dependent way. DGAT1 encodes a key enzyme in TAG (TRIACYLGLYCEROL) biosynthesis, which is accumulated in stressed plants (Kong et al., 2013). The involvement of ABI5 in the regulation photosynthesis is another example of its function in vegetative tissues. The genes that are responsible for chlorophyll catabolism, SGR1 (STAYGREEN1) and NYC1 (NON-YELLOW COLORING 1), contain an ABRE element in their promoters and are positively regulated by ABI5 together with another transcription factor, EEL (ENHANCED EM LEVEL) (Sakuraba et al., 2014) (Figure 1). These results indicate that ABI5 acts as a negative regulator of photosynthesis through the activation of chlorophyll degradation.

Lateral roots development is also regulated by ABI5. Similar to ABI4, ABI5 has an influence on the lateral root length. An abi5 mutant showed a weakened response to ABA- and nitrate-mediated lateral root growth inhibition (Signora et al., 2001). Additionally, ABA induces ABI5 expression in the lateral root tips. These results indicate that ABI5 acts as a negative regulator of lateral root development in the presence of stress (Brocard et al., 2002; De Smet et al., 2003; Shkolnik-Inbar and Bar-Zvi, 2010).

ABI5 and ABI4 not only act together during lateral root formation. The ABI5-regulated gene, DGAT1 encoding the TAG (TRIACYLGLYCEROL) biosynthesis enzyme is also activated by ABI4 (Kong et al., 2013). Furthermore, many genes encoding LEA, dehydrins and oleosins were shown to be regulated by both ABI4 and ABI5. Additionally, ABA pathway-related transcription factors and regulators were identified in the group of genes/proteins that are commonly regulated by ABI4 and ABI5. Among them, negative ABA regulators such as AHG1 (ABA-HYPERSENSITIVE GERMINATION 1) were also detected. Thus, both ABI transcription factors are simultaneously a part of a positive and negative feedback loop in the ABA-signaling. Furthermore, ABI5-specific targets seem to demand other co-regulators compared to ABI4-regulated genes (Reeves et al., 2011).

Recently, ABI5 was shown to take a part in a dark-induced leaf senescence process. It negatively and directly regulates ABR (ABA RESPONSIVE PROTEIN) expression in the darkness. ABR, a gene encoding a LEA protein, is a stress responsive gene that is associated with leaf rescue from the senescence process. The probable mechanism was shown to be related to the photosynthesis proteins. Thus, ABI5 plays a positive role in leaf senescence through its negative impact on the photosynthesis process (Su et al., 2016) (Figure 1).

Regulation of the ABI5 expression is complex and is mediated through many regulators. ABI5 expression is under the control of multiple transcription factors and proteins that belong to other functional groups (Figure 2).

ABI5 also undergoes epigenetic regulation. Together with ABI3, ABI5 is under the control of PKL (PICKLE), a SWI/SNF class chromatin-remodeling factor. PKL negatively regulates the expression of ABI5 and ABI3 because of the increase in the histone methylation (H3K9 and H3K27) of their promoters and chromatin repression in an ABA-dependent manner. PKL releases the germinating embryos from this inhibition and enables seedling growth under unfavorable conditions (Perruc et al., 2007). Another example of ABI5 epigenetic regulation is the ABA-activated action of HLS1 (HOOKLESS 1), a putative histone acetyltransferase. HLS1 directly interacts with the ABI5 sequence, mediates H3 acetylation and positively regulates ABI5 expression. Furthermore, HLS1 acts cooperatively with MED18 (MEDIATOR 18), a subunit of MEDIATOR complex, to increase ABI5 expression. HLS1 and MED18 interact physically and probably are the part of the same complex. Using transgenic Arabidopsis plants it was proved that HLS1 and MED18 are engaged in ABA signaling and are tightly related to ABI5 action (Liao et al., 2016).

An increased activity of TE (Transposable Elements) was observed under unfavorable environmental conditions (McCue et al., 2012; Le et al., 2014; Makarevitch et al., 2015). Recently, the mechanism of the modulation of ABI5 expression via TE has been described. Heat stress-activated retrotransposon, ONESEN, was integrated into ABI5 and disrupted its expression, possibly through the incorrect progress of the transcription (Ito et al., 2016). Moreover, the ABA insensitivity, which was mediated through the insertion of ONESEN in the ABI5 gene, was heritable. On the other hand, an ABA insensitive phenotype can be recovered by the IBM2 (INCREASE IN BONSAI METHYLATION 2) mediated epigenetic regulation of ONESEN. It is possible that ONESEN has an influence on ABA-related genes thus ensuring not only a short-term response to abiotic stress, but also an evolutionary plant adaptation to adverse environmental conditions (Ito et al., 2016).

ABI5 activity is regulated at the protein level via protein interaction and posttranslational modification (reviewed in Daszkowska-Golec, 2016; Table 1).

Although ABA is considered to be the main stress plant hormone, other phytohormones such as auxin, cytokinins (CKs), gibberellic acid (GA), brassinosteroids (BRs) and jasmonic acid (JA) also regulate plant adaptation to an adverse environment. Many of the components that are responsible for the biosynthesis and signaling of auxin, CKs, GA, BRs, and JA were identified as taking part in the plant response to drought and salt stress. The precise action of the regulators that belong to different phytohormone pathways ensures a balanced reaction of a plant to abiotic stress. The auxin, CKs, GA, BRs and JA regulation of drought and salt stress includes crosstalk with ABA. Many ABA-related genes are under the control of other phytohormones. This usually requires an interaction with the ABA signaling components. Among the many ABA-related regulators, ABI5, which integrates various phytohormone pathways and thus enables appropriate plant stress response, appears to be one of the most important (Table 2).

Arabidopsis ABI5 orthologs have been identified in other dicot plants and their sequences are available in bioinformatic databases. However, their precise function under abiotic stresses was described only for a few species. In Brassica oleracea, BolABI5 was found as a close homolog of AtABI5. BolABI5 is expressed mainly in flowers. BolABI5 overexpression in abi5 rescued ABA insensitive phenotype during seed germination. Additionally, application of ABA, drought, salt and osmotic stress activated BolABI5 expression and indicated its role in abiotic stress adaptation. Similar to AtABI5, BolABI5 activates expression of target genes through binding to ABRE elements present in their promoters (Zhou et al., 2013). BolABI5 possesses the ability to interact with BolOST1, a homolog of SnRK2.6/OST1. Probably, BolOST1 is responsible for its phosphorylation and activation as a transcription factor (Wang M. et al., 2013). Two AtABI5 orthologs, BrABI5a and BrABI5b, were identified in Brassica rapa. Both are able to reverse abi5 phenotype in the presence of ABA during seed germination. BrABI5a and BrABI5b show a high similarity to ABI5 including conservation of phosphorylation sites. Expression of BrABI5 genes is activated by ABA treatment; however they act differently in the presence of abiotic stresses. Only BrABI5b is induced by drought and salt. Probably they play a different role in the adaption to unfavorable conditions (Bai et al., 2016).

The role of ABI5 homologs in other dicots was also reported. The expression of SlABI5 in tomato (Solanum lycopersicum) increased in seeds in reponse to ABA (Sun et al., 2015). The function of Medicaco truncatula AtABI5 homolog, MtABI5, was described in more detail, using an insertional mtabi5 mutant. Similarly to atabi5, mtabi5 was ABA-insensitive during seed germination. Furthermore, the level of seed dormancy was decreased in mtabi5. Expression analysis showed reduced expression of LEAs, and SIP1 (SEED IMBIBITION1) encoding raffinose synthase in mtabi5 seeds, while photosynthesis-related genes, e.g., LHCA1 (PHOTOSYSTEM I LIGHT HARVESTING COMPLEX GENE 1) and PsaD-2 (PHOTOSYSTEM I SUBUNIT D-2) were up-regulated. MtABI5 acts as the seed development regulator through the control of RFO (RAFFINOSE FAMILY OLIGOSACCHARYDES) and LEA synthesis. Additionally, it serves as a repressor of photosynthesis and accumulation of chlorophyll and carotenoids (Zinsmeister et al., 2016). Recently, action of FaABI5 in strawberry (Fragaria x ananassa Duchesne) has been also described. It regulates fruit ripening and responses to such as salinity and UV-B radiation (Li et al., 2016). The above results confirm the conserved role of ABI5 orthologs in dicot species. However, further studies are needed in order to reveal their accurate function.

Nowadays, the abiotic stress tolerance of monocot plants is a very important issue. An accurate understanding of the stress-related mechanisms in monocots can help to improve cereal growth and yield in adverse environments. One of the areas of interest is the ABA-dependent regulation of the stress response in cereals, which includes the action of ABI5.

Although AtABI5 orthologs have been identified in monocot plants, their precise function still remains elusive. HvABI5 is a barley bZIP transcription factor that has high similarity to AtAREB2 and AtABI5 (Casaretto and Ho, 2003). The potential phosphorylation sites of HvABI5 are strongly conserved. The role of barley HvABI5 in abiotic stress responses is very poorly understood. It is already known that HvABI5 directly, in ABA-dependent way, activates the HVA1 and HVA22 expression through binding to the ABRC (ABA RESPONSE PROMOTER COMPLEX) elements within their promoters. However, HvVP1 (VIVIPAROUS1), a homolog of AtABI3, is also required for the activation of HVA1 and HVA22 expression. HVA1 and HVA22 encode, respectively, a group 3 LEA protein and a protein that is involved in vesicular trafficking. Their activity was shown to ensure tolerance to low water availability during seed germination (Casaretto and Ho, 2003). Induction of HvABI5 expression has also been detected in leaves after drought treatment in different barley varieties (de Mezer et al., 2014). In rice, OsABI5 shows a high homology to ABI5 and HvABI5 and had the ability to bind the G-box element (Zou et al., 2008). Like ABI5, OsABI5 interacts with the AtABI3 ortholog OsVP1. Overexpression of OsABI5 in an abi5 Arabidopsis mutant rescued ABA-insensitive phenotype. This could be proof that AtABI5 and OsABI5 share similar functions (Zou et al., 2007). OsABI5 expression is induced by ABA and salt, but drought and cold stress represses its activity. Forty-five-day-old plants of OsABI5-overexpression lines showed a faster turgor loss, chlorosis and growth inhibition in the presence of salt. Conversely, OsABI5-antisense plants were described as being salt tolerant with a changed expression of SalT, a salt-responsive gene and SKC1, which is a QTL (Quantitative Trait Locus) encoding a sodium transporter. Possibly, OsABI5 acts as an ABA-dependent negative regulator of stress tolerance in rice. Furthermore, the fertility of OsABI5-antisense lines was lower than in the wild type due to unsettled pollen formation. It is possible that OsABI5 can also regulate pollen maturation (Zou et al., 2008).

In wheat, TaABI5 is closely related to HvABI5. Its expression is induced by ABA, drought and low temperature. Two-week-old seedlings of TaABI5-overexpression tobacco (Nicotiana tabacum) lines showed a better survival rate in freezing temperature (Kobayashi et al., 2008). Additionally, 7-day-old seedlings of transgenic lines were more tolerant to salt and osmotic stress and had a higher percentage of green cotyledons compared to the wild type. The opposite effect of the enhanced expression of TaABI5 and OsABI5 on abiotic stress tolerance may be the result of the different developmental stages of the analyzed plants. Furthermore, their function in ABA-activated signaling cannot be equivalent. The root growth of TaABI5-overexpresssion seedlings was ABA-hypersensitive (Kobayashi et al., 2008). Additionally, TaABI5-regulated stress-responsive genes, TaDHN13, TaRAB18 and TaRAB19, were identified (Kobayashi et al., 2008).

ZmABI5 in maize shows a homology to AtABI5 and HvABI5. The expression of ZmABI5 is activated by ABA, SA (salicylic acid) salt, cold and heat stress. On the other hand, ZmABI5 was down-regulated by drought and wounding in the leaves. However, drought and wounding stress activated ZmABI5 in roots, while ABA and salt treatments repressed its activity. Twenty-one-day-old seedlings of tobacco ZmABI5-transgenic lines showed a lower tolerance to drought, salt, heat and cold compared to the wild type. Chlorophyll content, proline accumulation and the activity of antioxidant enzymes, POD (PEROXIDASE) and SOD (SUPEROXIDE DISMUTASE) were decreased in transgenic lines under drought, salt, cold and heat stress (Yan et al., 2012). After stress application, ZmABI5-overexpressed plants also had a higher amount of MDA (MALONDIALDEHYDE), which is an indicator of oxidative damage. Furthermore, salt and heat caused a lower induction of stress-related genes such as CAT1, APX, ERD10A-D (EARLY RESPONSE TO DEHYDRATION) and PR5 (PATHOGENESIS-RELATED 5) in plants with an overexpression of ZmABI5 compared to the wild type, while under osmotic and cold stress, the expression of these genes was up- or down-regulated. The changed expression of stress-response genes in transgenic plants indicates the complex role of ZmABI5 in responses to adverse conditions depending on the type of the stress that is applied. It can be assumed that ZmABI5, like OsABI5, acts as a negative regulator of the abiotic stress response (Yan et al., 2012). The results presented above indicate that monocot AtABI5 orthologs may participate in abiotic stress responses in different ways. Possibly, their action is related to the stage of plant development. Additionally, the mechanism of action of particular orthologs may not be the same in different monocot species. Furthermore, AtABI5 orthologs can also function in different ways depending on the type of stress.

The multiple sequence alignment presents the level of similarity between the described dicot and monocot orthologs (Figure 3). Across the species, the most conserved domain in AtABI5 is a bZIP domain. Conversely, C1 conserved region seems to be weakly preserved, especially in monocots. In other species, AtABI5 phosphorylation sites are mainly maintained in conserved domains. Interestingly, the site for the ubiquitination seems to be characteristic only for the dicots. Probably, another amino acid position is ubiquitinated in monocot plants. The sumoylation site is conserved across AtABI5 homologs. The analysis of ABI5 sequences in different species shows the divergence between dicot and monocot ABI5 forms what can result in its different action in grasses. The relationship between AtABI5 dicot and monocot orthologs is also included on the phylogenetic tree (Figure 4). AtABI5 seems not to show higher similarity to any from described dicot orthologs. The further distance between TaABI5 and OsABI5 may reflect their contrast function under abiotic stress.

Plant abiotic stress responses require a complex and accurate regulation. This is achieved through the operation of many regulators that have a modulated activity depending on the water status in the environment. ABI5 seems to be an ABA signaling regulator that integrates many different signals and influences the expression of stress-responsive genes. Many processes such as seed germination, seedling growth, photosynthesis and lateral root development were shown to be regulated by ABI5. The mode of the ABI5 gene and ABI5 protein regulation is complex and requires many transcription factors, enzymes and protein regulators. ABI5 expression is the outcome of the action of many transcription factors such as WRKY and MYB as well as epigenetic events. Additionally, multiple protein interactions and posttranslational modifications such as phosphorylation, ubiquitination, sumoylation and S-nitrosylation modulate ABI5 activity at the protein level. The action of ABI5 is also related to components of auxin, CK, GA, BR and JA signaling and metabolism pathways. The complicated network of ABI5 connections often results in a positive or negative ABA signaling feedback loop. Finally, the elaborate ABI5 functions ensure a balanced and adequate, respective to the intensity of stress response to adverse conditions. Further studies that are performed using monocot ABI5 homologs and investigations of their precise actions in the presence of abiotic stress can be very helpful in obtaining cultivars that have a better stress tolerance.

AS wrote the manuscript and prepared figures. AD-G and IS contributed to the writing of manuscript and revised it critically for important intellectual content.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.