Arabidopsis BR Mutant Screening And Functional Analysis Of BSE1 Regulating Cell Elongation

Brassinosteroids (BRs), a group of important phytohormones, regulate multiple processes of Arabidopsis growth and development. In the past two decades, BR biosynthesis and signal transduction are hot topics of plant biology. In order to find new genes affecting BR biosynthesis, signal transduction and regulating plant growth and development together with BRs, we constructed a cDNA library using RNA isolated from wild-type Arabidopsis seedlings and overexpressed it into bril-5, a weak mutant of brassinosteroid receptor BRI1. We expected to find some genes to enhance or suppress the phenotypes of bril-5. A total of 11 mutants with changed phenotypes were isolated from about 40,000 transgenic lines. Gene cloning identified three of the eleven mutants with the same overexpressed gene which can obviously suppress the length of the petiole and the hypocotyls of bril-5, and the gene was then named as BSE1 (bril-5 suppressor with elongated cells 1). Microscopic observation results showed the elongation of the petiole and the hypocotyls dues to cell elongation but not cell division. A series of experiments proved that BSE1 regulating cell elongation is not directly through brassinosteoids, auxin or gibberellins mediated pathways, but by some downstream transcription factors controlling cell elongation. qRT-PCR results showed that the expression levels of PREs, transcription factors positively regulating cell elongation, and EXPANSINs, enzymes directly involved in cell expansion, are upregulated in mutants with BSE1 overexpression. Previous studies showed that BSE1 is a member of histone deacetylase complex that can suppress gene transcription. After transcription repressors binding to the promoter region of its target genes, the deacetylase complex may be recruited by these transcription factors, and then deacetylates histones near this region, thereby inhibits the expression of the target genes. We hypothesize that BSE1 may interact with transcription repressors that negatively regulate other transcription repressors of PREs, resulting in the elevated expression of PREs. More evidence will be needed in the future study to test this hypothesis. It is possible to find some new transcription regulators controlling cell elongation in Arabidopsis by revealing the molecular mechanisms of BSEl-regulated cell elongation.