A Brassinosteroid - lnsensitive Mutant in Arabidopsis thaliana Exhibits Multiple Defects in Growth and Development ' Steven

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 brassinosteroids as signals influencing plant growth and development, we identified a brassinosteroid-insensitive mutant in Arabidopsis thaliana (L.) Henyh. ecotype Columbia. The mutant, termed bril , did not respond to brassinosteroids in hypocotyl elongation and primary root inhibition assays, but it did retain sensitivity to auxins, cytokinins, ethylene, abscisic acid, and gibberellins. The bril mutant showed multiple deficiencies in developmental pathways that could not be rescued by brassinosteroid treatment, including a severely dwarfed stature; dark green, thickened leaves; male sterility; reduced apical dominance; and de-etiolation of darkgrown seedlings. Cenetic analysis suggests that the Bril phenotype is caused by a recessive mutation in a single gene with pleiotropic effects that maps 1.6 centimorgans from the cleaved, amplified, polymorphic sequence marker DHSl on the bottom of chromosome IV. The multiple and dramatic effects of mutation of the BRIl locus on development suggests that the BRIl gene may play a critical role in brassinosteroid perception or signal transduction.