Constitutive Activation of Transcription Factor OsbZIP46 Improves Drought Tolerance in Rice

OsbZIP46 is one member of the third subfamily of bZIP transcription factors in rice (Oryza sativa). It has high sequence similarity to ABA-responsive element binding factor (ABF/AREB) transcription factors ABI5 and OsbZIP23, two transcriptional activators positively regulating stress tolerance in Arabidopsis (Arabidopsis thaliana) and rice, respectively. Expression of OsbZIP46 was strongly induced by drought, heat, hydrogen peroxide, and abscisic acid (ABA) treatment; however, it was not induced by salt and cold stresses. Overexpression of the native OsbZIP46 gene increased ABA sensitivity but had no positive effect on drought resistance. The activation domain of OsbZIP46 was defined by a series of deletions, and a region (domain D) was identified as having a negative effect on the activation. We produced a constitutive active form of OsbZIP46 (OsbZIP46CA1) with a deletion of domain D. Overexpression of OsbZIP46CA1 in rice significantly increased tolerance to drought and osmotic stresses. Gene chip analysis of the two overexpressors (native OsbZIP46 and the constitutive active form OsbZIP46CA1) revealed that a large number of stress-related genes, many of them predicted to be downstream genes of ABF/AREBs, were activated in the OsbZIP46CA1 overexpressor but not (even down-regulated) in the OsbZIP46 overexpressor. OsbZIP46 can interact with homologs of SnRK2 protein kinases that phosphorylate ABFs in Arabidopsis. These results suggest that OsbZIP46 is a positive regulator of ABA signaling and drought stress tolerance of rice depending on its activation. The stress-related genes activated by OsbZIP46CA1 are largely different from those activated by the other rice ABF/AREB homologs (such as OsbZIP23), further implying the value of OsbZIP46CA1 in genetic engineering of drought tolerance.