A BRASSINOSTEROID-INSENSITIVE MUTANT IN ARABIDOPSIS-THALIANA EXHIBITSMULTIPLE DEFECTS IN GROWTH AND DEVELOPMENT

Brassinosteroids are widely distributed plant compounds that modulate cell elongation and division, but little is known about the mechanism of action of these plant growth regulators. To investigate brassinoste roids as signals influencing plant growth and development, we identifi ed a brassinosteroid-insensitive mutant in Arabidopsis thaliana (L.) H enyh. ecotype Columbia. The mutant, termed bri1, did not respond to br assinosteroids in hypocotyl elongation and primary root inhibition ass ays, but it did retain sensitivity to auxins, cytokinins, ethylene, ab scisic acid, and gibberellins. The bri1 mutant showed multiple deficie ncies in developmental pathways that could not be rescued by brassinos teroid treatment, including a severely dwarfed stature; dark green, th ickened leaves; male sterility; reduced apical dominance; and de-etiol ation of dark-grown seedlings. Genetic analysis suggests that the Bri1 phenotype is caused by a recessive mutation in a single gene with ple iotropic effects that maps 1.6 centimorgans from the cleaved, amplifie d, polymorphic sequence marker DHS1 on the bottom of chromosome IV. Th e multiple and dramatic effects of mutation of the BRI1 locus on devel opment suggests that the BRI1 gene may play a critical role in brassin osteroid perception or signal transduction.