Genome-Wide Study ofKNOXRegulatory Network Reveals Brassinosteroid Catabolic Genes Important for Shoot Meristem Function in Rice  

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.

Ideal crop plant architecture is mediated by tassels replace upper ears1, a BTB/POZ ankyrin repeat gene directly targeted by TEOSINTE BRANCHED1

“…Plant tissue including 3-wk-old tiller buds as well as young ear primordia smaller than 5 mm were dissected from field grown B73 or tb1-r. About 1 g of tissue per biological replicate was then fixed in 1% formaldehyde. Nuclei extraction and chromatin immunoprecipitation using α-TB1 antibody were performed as described previously (56). Normal goat anti-Guinea pig IgG (sc-2711; Santa Cruz Biotechnology) was used as a negative control.…”

Maize YABBY Genes drooping leaf1 and drooping leaf2 Regulate Plant Architecture

“…Our findings indicate that the drl genes regulate a suite of traits in the leaf and stem similar to those affected by BR; however, a connection between these two important developmental regulators has not been established. drl1 mutant alleles together with drl2 natural variant and mutant alleles condition a phenotype that strongly resembles aspects of rice mutants that have elevated BR levels or enhanced BR signaling (Zhang et al, 2009;Tong et al, 2012;Tsuda et al, 2014;Sun et al, 2015): drl mutants have narrow leaves, increased leaf angle, expanded auricles, reduced sclerenchyma, and elongated internodes (Figures 1 to 3; Supplemental Figure 3). One peculiar phenotype observed in drl1-R; drl2-M plants was complete fusion along the sheath margins at some adult nodes Supplemental Figure 3I), a phenotype related to BR hypersensitivity in Arabidopsis (Bell et al, 2012;Gendron et al, 2012).…”

“…BRs influence leaf bending and internode elongation in maize (Hartwig et al, 2011;Makarevitch et al, 2012;Kir et al, 2015) and rice (Wada et al, 1981;Cao and Chen, 1993;Yamamuro et al, 2000;Hong et al, 2002;Zhang et al, 2009;Tong et al, 2012;Tsuda et al, 2014;Sun et al, 2015), regulate adaxial cell elongation in auricles (Wada et al, 1981;Cao and Chen, 1993), reduce abaxial sclerenchyma proliferation (Sun et al, 2015) in rice, and cause organ fusion in Arabidopsis (Bell et al, 2012;Gendron et al, 2012). Our findings indicate that the drl genes regulate a suite of traits in the leaf and stem similar to those affected by BR; however, a connection between these two important developmental regulators has not been established.…”

A Rice KNOX Transcription Factor Represses Brassinosteroid Production in the Shoot Apical Meristem

“…Early observations that the rice KNOX gene OSH1 increases cytokinin and decreases gibberellin levels in transgenic tobacco plants prompted Kusaba et al (1998) to hypothesize that OSH1 affects plant hormone metabolism. Subsequent studies showed that KNOX genes activate cytokinin biosynthesis (Yanai et al, 2005) and repress gibberellin production (Bolduc and Hake, 2009).In an effort to decipher KNOX regulatory networks in rice, Tsuda et al (2014) examined the effect of altering the expression of OSH1 using an inducible OSH1 overexpressor and a loss-of-function osh1 mutant. Whereas overexpression of OSH1 caused brassinosteroid (BR) insensitivity, resulting in plants with twisted, erect leaf blades and severely shortened leaf sheaths, OSH1 loss-of-function mutants resembled BR overproducers, exhibiting increased coleoptile growth, defects in boundary formation between the SAM and first leaf primordium, and an increased lamina joint angle of the second leaf.…”

“…In an effort to decipher KNOX regulatory networks in rice, Tsuda et al (2014) examined the effect of altering the expression of OSH1 using an inducible OSH1 overexpressor and a loss-of-function osh1 mutant. Whereas overexpression of OSH1 caused brassinosteroid (BR) insensitivity, resulting in plants with twisted, erect leaf blades and severely shortened leaf sheaths, OSH1 loss-of-function mutants resembled BR overproducers, exhibiting increased coleoptile growth, defects in boundary formation between the SAM and first leaf primordium, and an increased lamina joint angle of the second leaf.…”