BarleyABI5(Abscisic Acid INSENSITIVE 5) Is Involved in Abscisic Acid-Dependent Drought Response

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 thatHvABI5is involved in ABA-dependent regulation of barley response to drought stress. We identified barley TILLING mutants carrying different alleles in theHvABI5gene and we studied in detail the physiological and molecular response to drought and ABA for one of them. Thehvabi5.dmutant, 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 ofhvabi5.dwas 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:HVA1andHVA22showed higher activity after drought, which may imply better adaptation ofhvabi5.dto stress. On the other hand, chlorophyll content inhvabi5.dwas lower than in WT, which was associated with decreased photosynthesis efficiency observed in the mutant after drought treatment. To verify thatHvABI5acts in the ABA-dependent manner we analyzed expression of selected genes related to ABA pathway inhvabi5.dand 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 ofHvNCED1, HvBG8, HvSnRK2.1, andHvPP2C4genes was 2-20 times higher inhvabi5.dcompared to "Sebastian". We also observed a faster stomatal closure inhvabi5.dand much higher induction ofHvNCED1andHvSnRK2.1genes after ABA treatment. Together, these findings demonstrate thatHvABI5plays 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 ofHvABI5action in regulating drought response and seed germination in barley.