Brassinosteroids, microtubules and cell elongation in Arabidopsis thaliana. I. Molecular, cellular and physiological characterization of the Arabidopsis bul1 mutant, defective in the Delta(7)-sterol-C5-desaturation step leading to brassinosteroid biosynthesis

Although cell elongation is a basic function of plant morphogenesis, many o f the molecular events involved in this process are still unknown. Tn this work an extremely dwarf mutant, originally named bul, was used to study one of the main processes of plant development, cell elongation. Genetic analy ses revealed that the BUL locus was linked to the nga 172 marker on chromos ome 3. Recently, after mapping the new dwf7 mutation of Arabidopsis, which is allelic to ste1, it was reported that dwf7 is also linked to the same ma rker. Sterol analyses of the bull-1 mutant indicated that bull-1 is defecti ve in the Delta (7)-sterol-C5-desaturation step leading to brassinosteroid biosynthesis. Considering these findings, we designated our bul mutant as b ul1-1/dwf7-3/ste1-4. The bul1z-1 mutant was characterized by a very dwarf p henotype, with delayed development and reduced fertility. The mutant leaves had a dark-green colour, which was probably due to continuous stomatal clo sure. The bull-1 mutant showed a partially de-etiolated phenotype in the da rk. Cellular characterization and rescue experiments with brassinosteroids demonstrated the involvement of the BUL1-1 protein in brassinosteroid-depen dent plant growth processes.