Transcription factor HAT1 is phosphorylated by BIN2 kinase and mediates brassinosteroid repressed gene expression in Arabidopsis

Abstract

SUMMARYPlant 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 that modulate thousands of target genes. Arabodopsis thaliana homeodomain-leucine zipper protein 1 (HAT1), which encodes a homeodomain-leucine zipper (HD-Zip) class II transcription factor, 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 a reduced BR response that is stronger than the single mutants alone. 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 to activate BR-mediated growth. Expression levels of several BR-repressed genes are 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 a conserved homeodomain binding (HB) site and BR response element (BRRE) respectively, in the promoters of some BR-repressed genes. BES1 and HAT1 interact with each other and cooperate to inhibit BRrepressed gene expression. Furthermore, HAT1 can be phosphorylated and stabilized by GSK3 (GLYCOGEN SYNTHASE KINASE 3)-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.

Brassinosteroid Regulates Cell Elongation by Modulating Gibberellin Metabolism in Rice

“…Numerous BIN2/GSK2 substrates were identified in recent years, including a number of transcription factors (Vert et al, 2008;Ye et al, 2012;Zhang et al, 2012Zhang et al, , 2014aZhang et al, , 2014bCho et al, 2014), suggesting that the negative regulator BIN2/GSK2 acts as the central player in the BR response, similar to most other hormone-signaling pathways (Huq, 2006). Our analysis combined with previous reports implied that these transcription factors, (A) Physiological BR induces GA biosynthesis, inhibits GA inactivation, and stimulates self-biosynthesis to promote cell elongation.…”

Transcription factors involved in brassinosteroid repressed gene expression and their regulation by BIN2 kinase

“…HAT1 and HAT3 are positive regulators for BR-regulated growth as double mutants of hat1 hat3 have reduced BR response in hypocotyl elongation assays and overexpression of HAT1 leads to increased stem elongation at the early stage of vegetative growth. 30 We further found that HAT1 binds to a conserved DNA binding site (TAATAATTA), interacts with BES1 bound to BRRE site to cooperatively inhibit BR-repressed gene expression. The results suggest that HAT1 function as a co-repressor of BES1, providing a mechanism for BES1 to repress gene expression.…”

“…The results suggest that HAT1 function as a co-repressor of BES1, providing a mechanism for BES1 to repress gene expression. 30 Interestingly, HAT1 has more than 20 putative phosphorylation sites for BIN2 kinase and is phosphorylated by BIN2 kinase in vitro and likely in vivo as well. 30 Unlike BIN2 phosphorylation of BES1/ BZR1, BIN2 phosphorylation appears to stabilize HAT1 protein, 30 which is similar to BIN2 phosphorylation of MYBL2.…”

“…30 Interestingly, HAT1 has more than 20 putative phosphorylation sites for BIN2 kinase and is phosphorylated by BIN2 kinase in vitro and likely in vivo as well. 30 Unlike BIN2 phosphorylation of BES1/ BZR1, BIN2 phosphorylation appears to stabilize HAT1 protein, 30 which is similar to BIN2 phosphorylation of MYBL2. 22 What is the biological implication of BIN2 regulation of these transcriptional co-repressors?…”

A Recently Evolved Isoform of the Transcription Factor BES1 Promotes Brassinosteroid Signaling and Development in Arabidopsis thaliana

“…The unphosphorylated BES1 and BZR1 prefer to bind E-boxes (CANNTG) and/or BR response elements (CGTG T / C G) to regulate target gene expression (He et al, 2005;Yin et al, 2005). Furthermore, BES1 interacts with basic helix-loophelix (bHLH) transcription factors BIM1-3, histone demethylases ELF6 and REF6, histone methyltransferase SDG8, MYB family transcription factor AtMYB30, corepressors MYBL2, HAT1, and TPL-HDA19, and transcription elongation factor IWS1 to regulate gene expression (He et al, 2005;Yin et al, 2005;Yu et al, 2008;Li et al, 2009Li et al, , 2010Ye et al, 2012;Ryu et al, 2014;Zhang et al, 2014). Genome-wide studies have revealed that the BES1/BZR1 family regulates a transcription network involving thousands of genes (Sun et al, 2010;Yu et al, 2011).…”