b HLH 122 is important for drought and osmotic stress resistance in A rabidopsis and in the repression of ABA catabolism

Summary Although proteins in the basic helix‐loop‐helix (b HLH ) family are universal transcription factors in eukaryotes, the biological roles of most b HLH family members are not well understood in plants. The A rabidopsis thaliana b HLH 122 transcripts were strongly induced by drought, N a C l and osmotic stresses, but not by ABA treatment. Promoter:: GUS analysis showed that b HLH 122 was highly expressed in vascular tissues and guard cells. Compared with wild‐type ( WT ) plants, transgenic plants overexpressing b HLH 122 displayed greater resistance to drought, N a C l and osmotic stresses. In contrast, the bhlh122 loss‐of‐function mutant was more sensitive to N a C l and osmotic stresses than were WT plants. Microarray analysis indicated that b HLH 122 was important for the expression of a number of abiotic stress‐responsive genes. In electrophoretic mobility shift assay and chromatin immunoprecipitation assays, b HLH 122 could bind directly to the G ‐box/ E ‐box cis ‐elements in the CYP 707 A 3 promoter, and repress its expression. Further, up‐regulation of b HLH 122 substantially increased cellular ABA levels. These results suggest that b HLH 122 functions as a positive regulator of drought, NaCl and osmotic signaling.