Zhang D, Ye H, Guo H, Johnson A, Zhang M, Lin H, Yin Y

Plant Steroid hormones, Brassinosteroids (BRs), play essential roles in modulating cell elongation, vascular differentiation, senescence and stress responses. BRs signal through plasma membrane-localized receptor and other components to modulate the BES1/BZR1 (BRI1-EMS SUPPRESSOR 1/BRASSINAZOLE RESISTANT 1) family of transcription factors, which modulate thousands of target genes. HAT1, encoding a homeodomain-leucine ...

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zipper (HD-Zip) class-II transcription factor from Arabidopsis thaliana, was identified through chromatin immunoprecipitation (ChIP) experiments as a direct target gene of BES1. Loss-of-function and gain-of-function mutants of HAT1 display altered BR responses. HAT1 and its close homolog HAT3 act redundantly as the double mutant hat1 hat3 displayed reduced BR response, which is stronger than the single mutants. Moreover, hat1 hat3 enhanced the phenotype of a weak allele of the BR receptor mutant bri1 and suppressed the phenotype of constitutive BR response mutant bes1-D. These results suggest that HAT1 and HAT3 function activating BR-mediated growth. Expression levels of several BR-repressed genes increased in hat1 hat3 and reduced in HAT1OX, suggesting that HAT1 functions to repress the expression of a subset of BR target genes. HAT1 and BES1 bind to conserved HB-binding site and BRRE, respectively, in the promoters of some BR-repressed genes. BES1 and HAT1 interact with each other and cooperate to inhibit BR-repressed gene expression. Furthermore, HAT1 can be phosphorylated and stabilized by GSK3-like kinase BIN2 (BRASSINOSTEROID INSENSITIVE 2), a well-established negative regulator of the BR pathway. Our results thus revealed a previously unknown mechanism by which BR signaling modulates BR-repressed gene expression and coordinates plant growth. This article is protected by copyright. All rights reserved.

Date: Oct. 27, 2013

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