RLB (Rice Lateral Branch) recruits PRC2-mediated H3K27 tri-methylation on OsCKX4 to regulate cytokinin homeostasis and lateral branching in rice (Oryza sativa L.)

Title : RLB (Rice Lateral Branch) recruits PRC2-mediated H3K27 tri-methylation on OsCKX4 to regulate cytokinin homeostasis and lateral branching in rice (Oryza sativa L.)

Abstract:

Lateral branches such as shoot and panicle are determining factors and target traits for rice (Oryza sativa L.) yield improvement. Cytokinin promotes rice lateral branching;, however, the mechanism underlying the fine-tuning of cytokinin homeostasis in rice branching remains largely unknown. Here, we report the map-based cloning of RICE LATERAL BRANCH (RLB) encoding a nuclear-localized, KNOX-type homeobox protein from a rice cytokinin-deficient mutant showing more tillers, sparser panicles, defected floret morphology as well as attenuated shoot regeneration from callus. RLB directly binds to the promoter and represses the transcription of OsCKX4, a cytokinin oxidase gene with high abundance in panicle branch meristem. OsCKX4 over-expression lines phenocopied rlb, which showed upregulated OsCKX4 levels. Meanwhile, RLB physically binds to Polycomb repressive complex 2 (PRC2) components OsEMF2b and co-localized with H3K27me3, a suppressing histone modification mediated by PRC2, in the OsCKX4 promoter. We proposed that RLB recruits PRC2 to the OsCKX4 promoter to epigenetically repress its transcription, which suppresses the catabolism of cytokinin, thereby promoting rice lateral branching. Moreover, antisense inhibition of OsCKX4 under the LOG promoter successfully increased panicle size and spikelet number per plant without affecting other major agronomic traits. This study provides insight into cytokinin homeostasis, lateral branching in plants, and also promising target genes for rice genetic improvement.

Audience Take Away Notes:

• RLB is a Homebox gene
• RLB can directly target osckx4 and inhibit its expression
• Osckx4 overexpression materials and RLB have similar phenotypes. Antisense inhibition of osckx4 can increase the number of secondary branches, and then increase the number of grains per spike
• RLB can interact with PRC2 member emf2b to introduce PRC2 into the osckx4 promoter, and inhibit its transcription, thereby inhibiting cytokinin metabolism

Biography:

Dr. Tong graduated from Zhejiang University with a bachelor's degree and studied plant pathology in ZheJiang University. She received her PhD degree in 2010. After that, she worked as a postdoctoral at in the Institute of Plant Physiology and Ecology, Chinese Academy of Sciences Shanghai Branch, and obtained the position of associate professor. Now she works at the China Rice Research Institute. She has published more than 30 research articles in SCI(E) journals.