ROLE OF BRASSINOSTEROID CATABOLISM IN ARABIDOPSIS DEVELOPMENT
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BAS1 (phyB-4 ACTIVATION TAGGED SUPPRESSOR 1) and SOB7 (SUPPRESSOR OF phyB-4 7) 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 (bri1 enhanced 1-1) 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.