Genome-wide study of KNOX regulatory network reveals brassinosteroid catabolic genes important for shoot meristem function in rice.

Katsutoshi Tsuda,Hajime Ohyanagi,Sarah Hake,Nori Kurata

doi:10.1105/tpc.114.129122

Abstract

In flowering plants, knotted1-like homeobox (KNOX) transcription factors play crucial roles in establishment and maintenance of the shoot apical meristem (SAM), from which aerial organs such as leaves, stems, and flowers initiate. We report that a rice (Oryza sativa) KNOX gene Oryza sativa homeobox1 (OSH1) represses the brassinosteroid (BR) phytohormone pathway through activation of BR catabolism genes. Inducible overexpression of OSH1 caused BR insensitivity, whereas loss of function showed a BR-overproduction phenotype. Genome-wide identification of loci bound and regulated by OSH1 revealed hormonal and transcriptional regulation as the major function of OSH1. Among these targets, BR catabolism genes CYP734A2, CYP734A4, and CYP734A6 were rapidly upregulated by OSH1 induction. Furthermore, RNA interference knockdown plants of CYP734A genes arrested growth of the SAM and mimicked some osh1 phenotypes. Thus, we suggest that local control of BR levels by KNOX genes is a key regulatory step in SAM function.

Highlights

In contrast to animals that complete organ formation during embryogenesis, plants continue to produce organs throughout their life cycle
At the T2 generation, we selected lines homozygous for the transgene and examined their phenotypes with and without DEX treatment. 35S:Oryza sativa homeobox1 (OSH1)-glucocorticoid receptor (GR) plants grown on DEX-containing media for 3 weeks showed DEX-dose-dependent phenotypes
At 0.1 mM DEX, the short sheath phenotype was still observed and leaf blades were erect (Figures 1C and 1D). These transgenic plants showed no abnormality without DEX (Figure 1E), and control nontransgenic plants grown with DEX were normal, confirming these leaf phenotypes were caused by induced overexpression of OSH1

Summary

Introduction

In contrast to animals that complete organ formation during embryogenesis, plants continue to produce organs throughout their life cycle. Formation of aboveground organs relies on the shoot apical meristem (SAM). The SAM is an indeterminate structure comprised of self-renewing stem cells in its center and daughter cells at its periphery. Lateral organs such as leaves and flowers initiate from the flank of the SAM at the expense of stem cells. Cells in the SAM are nucleocytoplasmic, small, and divide slowly, whereas those within lateral organs are vacuolated, elongate, and have increased rates of cell division (Steeves and Sussex, 1989). The transition from the indeterminate SAM to determinate lateral organs is likely to include various cytological and physiological changes

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