Role of brassinosteroid catabolism in Arabidopsis development

BAS1 (phyB&barbelow;-4 A&barbelow;CTIVATION TAGGED S&barbelow;UPPRESSOR 1&barbelow; ) and SOB7 (S&barbelow;UPPRESSOR O&barbelow;F phyB&barbelow;-4 7&barbelow; ) are brassinosteroid-catabolizing P450s in Arabidopsis thaliana that synergistically/redundantly modulate photomorphogenic traits such as flowering time. This study investigates the role of BAS1 and SOB7 in photomorphogenesis by studying null-mutant genetic interactions with the photoreceptors phyA, phyB and cry1 with regard to seed-germination and flowering time. The removal of BAS1 and/or SOB7 rescued the low germination rate of the phyA-211 phyB-9 double-null mutant. With regard to floral induction, bas1-2 and sob7-1 showed a complex set of genetic interactions with photoreceptor-null mutants. Histochemical analysis of transgenic plants harboring BAS1:BAS1-GUS and SOB7:SOB7-GUS translational fusions revealed overlapping and distinct expression patterns. BAS1's expression in the shoot apex increases during the phase transition from short-to-long-day growth conditions and requires phyB in red light.;Application of this kind of genetic analysis approach to study other BR catabolic genes requires availability of the loss-of-function mutants. Our analysis shows that the ben1-1 (b&barbelow;ri1 e&barbelow;n&barbelow;hanced 1&barbelow;-1&barbelow; ) mutant (T-DNA intronic insertion mutant of BEN1) is not suitable for multiple mutant analyses which involve combining two or more T-DNA insertion mutations. We generated a genetic triple-mutant from a cross between the bas1-2 sob7-1 double-null (T-DNA exonic insertion mutants) and ben1-1. The single ben1-1 line behaves as a transcript null. However, in the triple-mutant background ben1-1 was reverted to a partial loss-of-function allele. The enhanced expression of BEN1 remained stable when the ben1-1 single-mutant was re-isolated from a cross with the wild-type. In addition, the two genetically identical pre-triple and post-triple ben1-1 mutants also differed phenotypically. Restriction endonuclease analysis with methylation sensitive enzymes demonstrates that the recovered ben1-1 mutant is epigenetically different from the original ben1-1 allele.