Abstract
ABA INSENSITIVE 5 (ABI5) is a basic leucine zipper (bZIP) transcription factor which acts in the abscisic acid (ABA) network and is activated in response to abiotic stresses. However, the precise role of barley (Hordeum vulgare) ABI5 in ABA signaling and its function under stress remains elusive. Here, we show that HvABI5 is involved in ABA-dependent regulation of barley response to drought stress. We identified barley TILLING mutants carrying different alleles in the HvABI5 gene and we studied in detail the physiological and molecular response to drought and ABA for one of them. The hvabi5.d mutant, carrying G1751A transition, was insensitive to ABA during seed germination, yet it showed the ability to store more water than its parent cv. “Sebastian” (WT) in response to drought stress. The drought-tolerant phenotype of hvabi5.d was associated with better membrane protection, higher flavonoid content, and faster stomatal closure in the mutant under stress compared to the WT. The microarray transcriptome analysis revealed up-regulation of genes associated with cell protection mechanisms in the mutant. Furthermore, HvABI5 target genes: HVA1 and HVA22 showed higher activity after drought, which may imply better adaptation of hvabi5.d to stress. On the other hand, chlorophyll content in hvabi5.d was lower than in WT, which was associated with decreased photosynthesis efficiency observed in the mutant after drought treatment. To verify that HvABI5 acts in the ABA-dependent manner we analyzed expression of selected genes related to ABA pathway in hvabi5.d and its WT parent after drought and ABA treatments. The expression of key genes involved in ABA metabolism and signaling differed in the mutant and the WT under stress. Drought-induced increase of expression of HvNCED1, HvBG8, HvSnRK2.1, and HvPP2C4 genes was 2–20 times higher in hvabi5.d compared to “Sebastian”. We also observed a faster stomatal closure in hvabi5.d and much higher induction of HvNCED1 and HvSnRK2.1 genes after ABA treatment. Together, these findings demonstrate that HvABI5 plays a role in regulation of drought response in barley and suggest that HvABI5 might be engaged in the fine tuning of ABA signaling by a feedback regulation between biosynthetic and signaling events. In addition, they point to different mechanisms of HvABI5 action in regulating drought response and seed germination in barley.
Highlights
Abscisic acid (ABA) is the crucial regulator of plant responses to abiotic stresses
This presumption is supported by the fact that HvABI5 shows the highest level of similarity to monocot BASIC LEUCINE ZIPPER (bZIP) such as wheat wABI5, rice TRANSCRIPTION FACTOR RESPONSIBLE FOR ABA REGULATION1 (TRAB1), OsABF4 and OsABF2, and maize ZmABI5, which act as ABA-dependent transcription factors. wABI5, TRAB1, and OsABF2 were described as positive regulators of response to abiotic stresses, including drought in vegetative tissues of these species (Kagaya et al, 2002; Kobayashi et al, 2008; Hossain et al, 2010a)
We found that HvABI5 takes part in regulation of processes associated with drought stress tolerance, such as membrane protection, flavonoid accumulation, and stomatal closure
Summary
Abscisic acid (ABA) is the crucial regulator of plant responses to abiotic stresses. In the presence of unfavorable conditions, the precise regulation and function of ABA-dependent signaling components ensure the appropriate activity of stress-responsive genes (reviewed by Yoshida et al, 2019), and the regulation of physiological processes, such as photosynthesis, stomatal closure (Song et al, 2016; Cai et al, 2017; Saito and Uozumi, 2019), and osmoprotectant biosynthesis (Jones, 2016; Sah et al, 2016; Martignago et al, 2020).In Arabidopsis thaliana, ABA INSENSITIVE 5 (ABI5) encodes the ABA-dependent, BASIC LEUCINE ZIPPER (bZIP) transcription factor, composed of C1, C2, C3, and bZIP conserved domains (Lopez-Molina et al, 2001; Nakamura et al, 2001). The formation of ABAPYRABACTIN RESISTANCE1/PYR LIKE/REGULATORY COMPONENT OF ABA RECEPTOR-PHOSPHATASE 2C complex (ABA-PYR1/PYL/RCAR-PP2C) promotes ABI5 phosphorylation mediated by the SNF1-RELATED PROTEIN KINASE2s (SnRK2s) and its activation. AtABI5 binds ABA RESPONSIVE ELEMENTs (ABRE cis-elements) present in the promoters of regulated genes and activates or represses their transcription, often in the interaction with other regulatory proteins (reviewed by Daszkowska-Golec, 2016; Sah et al, 2016; Dejonghe et al, 2018; Yoshida et al, 2019)
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