Functional characterization of OsbZIP 48 in rice

44 Plants have evolved an intricate network of sensory photoreceptors and signalling components 45 to regulate their development. Among the light signalling components identified till date, HY5, a 46 bZIP transcription factor, has been investigated extensively. However, most of the work on HY5 47 www.plantphysiol.org on August 25, 2017 Published by Downloaded from Copyright © 2017 American Society of Plant Biologists. All rights reserved. 3 has been carried out in Arabidopsis, a dicot. In this study, based on homology search and 48 phylogenetic analysis, we could identify three homologs of AtHY5 in monocots; however, AtHYH 49 (HY5 homolog) homologs are absent in monocots analysed. Out of the three homologs 50 identified in rice, we have functionally characterized OsbZIP48. OsbZIP48 was able to 51 complement Athy5 mutant. OsbZIP48 protein levels are developmentally regulated in rice. 52 Moreover, OsbZIP48 protein does not degrade in dark grown rice and Athy5 seedlings 53 complemented with OsbZIP48, which is in striking contrast to AtHY5. In comparison to AtHY5, 54 which does not cause any change in hypocotyl length when over-expressed in Arabidopsis, the 55 over-expression of full-length OsbZIP48 in rice transgenics reduced the plant height 56 considerably. Microarray analysis revealed that OsKO2, which encodes ent-kaurene oxidase 2 57 of GA biosynthesis pathway, is down-regulated in OsbZIP48 and up-regulated in OsbZIP48 58 transgenics as compared to the wild type. Electrophoretic mobility shift assay showed that 59 OsbZIP48 directly binds to the OsKO2 promoter. The RNAi lines and T-DNA insertional mutant 60 of OsbZIP48 showed seedling lethal phenotype despite the fact that roots were more 61 proliferative during early stages of development. These data provide credible evidences that 62 OsbZIP48 performs more diverse functions in a monocot system like rice in comparison to its 63 Arabidopsis ortholog, HY5. 64