The Arabidopsis dwf7/ste1 mutant is defective in the Delta(7) sterol C-5 desaturation step leading to brassinosteroid biosynthesis

Lesions in brassinosteroid (BR) biosynthetic genes result in characteristic dwarf phenotypes in plants. Understanding the regulation of BR biosynthesi s demands continued isolation and characterization of mutants corresponding to the genes involved in BR biosynthesis. Here, we present analysis of a n ovel BR biosynthetic locus, dwarf7 (dwf7). Feeding studies with BR biosynth etic intermediates and analysis of endogenous levels of BR and sterol biosy nthetic intermediates indicate that the defective step in dwf7-1 resides be fore the production of 24-methylenecholesterol in the sterol biosynthetic p athway. Furthermore, results from feeding studies with C-13-labeled mevalon ic acid and compactin show that the defective step is specifically the Delt a(7) sterol C-5 desaturation, suggesting that dwf7 is an allele of the prev iously cloned STEROL1 (STE1) gene. Sequencing of the STE1 locus in two dwf7 mutants revealed premature stop codons in the first (dwf7-2) and the third (dwf7-1) exons. Thus, the reduction of BRs in dwn is due to a shortage of substrate sterols and is the direct cause of the dwarf phenotype in dwf7.