Functional Analysis of Cryptochrome-Interacting Basic-helix-loop-helix1 (OsCIB1) in Controlling Leaf Angle and Grain Size in Rice (Oryza sativa)

Abstract

Cryptochrome-Interacting basic-helix-loop-helix (CIB), an critical transcription factor in plant, plays important roles associated several development processes, including hypocotyl elongation, flowering, and plastid development. It has been reported that a soybean CIB orthologue gene Cryptochrome-interacting bHLH1 (GmCIB1) promotes leaf senescence by activating transcription of senescence-associated genes such as WRKY DNA BINDING PROTEIN53b (WRKY53b). However, any functions of CIB1 have not been studied or reported yet in rice. In this study, we screened two T-DNA mutants to identify the function of OsCIB1. Especially, a rice gain of function mutant, oscib1-D, displayed wide leaf angles and slender grains, similar to plants with increased brassinosteroid (BR) levels or enhanced BR signaling. qRT-PCR analysis showed that gene in brassinosteroid signaling pathway are upregulated in oscib1-D, but there was no significant difference of the expression level of BR biosynthesis-related genes between WT and oscib1-D. In addition, osicb1-D showed more sensitive phenotype than WT to BR. Histological analysis revealed that increased cell length in adaxial surface of lamina joint is responsible for larger angles. Moreover, expression level of genes involved in cell elongation such as expansins and xyloglucan endotransglycosylase/hydrolase(XTH), two major cell wall-loosening enzymes, was significantly increased in oscib1-D. Thus, these results strongly suggest that OsCIB1 is involved in the BR signaling pathway and determines not only leaf inclination but also grain shape by regulating cell-elongation-related genes.