Two splice forms of OsbZIP1, a homolog of AtHY5, function to regulate skoto- and photo-morphogenesis in rice

Plants possess well-developed light sensing mechanisms and signal transduction systems for regulating photomorphogenesis. ELONGATED HYOCOTYL 5 (HY5), a basic leucine zipper transcription factor, has been extensively characterized in dicot plants. In this study, we have shown that OsbZIP1 is a functional homolog of Arabidopsis HY5 (AtHY5) and is important for light-mediated regulation of seedling and mature plant development in rice. Ectopic expression of OsbZIP1 in rice reduces plant height and leaf length without affecting plant fertility, which is in contrast to OsbZIP48, another HY5 homolog we characterised earlier. OsbZIP1 is alternatively spliced and the isoform OsbZIP1.2 lacking COP1 binding domain regulates seedling development in dark; this is unique since AtHY5 lacking COP1 binding domain does not display such a phenotype. Rice seedlings overexpressing OsbZIP1 were found to be shorter than vector control under white and monochromatic light conditions whereas RNAi seedlings displayed completely opposite phenotype. While OsbZIP1.1 is light regulated, OsbZIP1.2 shows similar protein profile in both light and dark conditions. Due to its interaction with OsCOP1, OsbZIP1.1 undergoes degradation via 26S proteasome under dark conditions. Also, OsbZIP1.1 interacts with CASEIN KINASE 2 (OsCK2[a]3) and consequently undergoes phosphorylation. In comparison, OsbZIP1.2 did not show any interaction with COP1 and OsCK2[a]3. We propose that OsbZIP1.1 most likely works under low fluence of blue light (15 mol/m{superscript 2}/s) while OsbZIP1.2 becomes dominant as the fluence is increased to 30 mol/m{superscript 2}/s. Data presented in this study reveal that AtHY5 homologs in rice have undergone neofunctionalization and alternative splicing (AS) of OsbZIP1 has increased the repertoire of its functions. One sentence summaryAlternative spliced forms of OsbZIP1, an AtHY5 homolog in rice, regulate seedling development in response to light and dark