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OsbZIP48, a HY5 transcription factor ortholog, exerts pleiotropic effects in light-regulated development.
Plants have evolved an intricate network of sensory photoreceptors and signalling components to regulate their development. Among the light signalling components identified till date, HY5, a bZIP transcription factor, has been investigated extensively. However, most of the work on HY5 has been carried out in Arabidopsis, a dicot. In this study, based on homology search and phylogenetic analysis, we could identify three homologs of AtHY5 in monocots; however, AtHYH (HY5 homolog) homologs are absent in monocots analysed. Out of the three homologs identified in rice, we have functionally characterized OsbZIP48. OsbZIP48 was able to complement Athy5 mutant. OsbZIP48 protein levels are developmentally regulated in rice. Moreover, OsbZIP48 protein does not degrade in dark grown rice and Athy5 seedlings complemented with OsbZIP48, which is in striking contrast to AtHY5. In comparison to AtHY5, which does not cause any change in hypocotyl length when over-expressed in Arabidopsis, the over-expression of full-length OsbZIP48 in rice transgenics reduced the plant height considerably. Microarray analysis revealed that OsKO2, which encodes ent-kaurene oxidase 2 of GA biosynthesis pathway, is down-regulated in OsbZIP48OE and up-regulated in OsbZIP48KD transgenics as compared to the wild type. Electrophoretic mobility shift assay showed that OsbZIP48 directly binds to the OsKO2 promoter. The RNAi lines and T-DNA insertional mutant of OsbZIP48 showed seedling lethal phenotype despite the fact that roots were more proliferative during early stages of development. These data provide credible evidences that OsbZIP48 performs more diverse functions in a monocot system like rice in comparison to its Arabidopsis ortholog, HY5.
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