CYP90C1 and CYP90D1 are involved in different steps in the brassinosteroid biosynthesis pathway in Arabidopsis thaliana

Abstract

SummaryBrassinosteroids (BRs) are plant hormones that are essential for a wide range of developmental processes in plants. Many of the genes responsible for the early reactions in the biosynthesis of BRs have recently been identified. However, several genes for enzymes that catalyze late steps in the biosynthesis pathways of BRs remain to be identified, and only a few genes responsible for the reactions that produce bioactive BRs have been identified. We found that the ROTUNDIFOLIA3 (ROT3) gene, encoding the enzyme CYP90C1, which was specifically involved in the regulation of leaf length in Arabidopsis thaliana, was required for the late steps in the BR biosynthesis pathway. ROT3 appears to be required for the conversion of typhasterol to castasterone, an activation step in the BR pathway. We also analyzed the gene most closely related to ROT3, CYP90D1, and found that double mutants for ROT3 and CYP90D1 had a severe dwarf phenotype, whereas cyp90d1 single knockout mutants did not. BR profiling in these mutants revealed that CYP90D1 was also involved in BR biosynthesis pathways. ROT3 and CYP90D1 were expressed differentially in leaves of A. thaliana, and the mutants for these two genes differed in their defects in elongation of hypocotyls under light conditions. The expression of CYP90D1 was strongly induced in leaf petioles in the dark. The results of the present study provide evidence that the two cytochrome P450s, CYP90C1 and CYP90D1, play distinct roles in organ-specific environmental regulation of the biosynthesis of BRs.

Inhibition of Brassinosteroid Biosynthesis by Either a dwarf4 Mutation or a Brassinosteroid Biosynthesis Inhibitor Rescues Defects in Tropic Responses of Hypocotyls in the Arabidopsis Mutant nonphototropic hypocotyl 4

“…Interestingly, the former is involved in BR biosynthesis, catalyzing a reaction from typhasterol to castasterone, the last step in BR biosynthesis (Fujioka and Yokota, 2003;Kim et al, 2005).…”

CYP90A1/CPD, a Brassinosteroid Biosynthetic Cytochrome P450 of Arabidopsis, Catalyzes C-3 Oxidation

“…Additionally, most BR biosynthetic genes are up-regulated in BR-deficient mutants (16,17), thereby increasing the role of subsidiary reaction routes. Because of these intricacies, the use of indirect methods for the functional characterization of BR biosynthetic genes can be difficult and misleading, as has been the case with the cyp90c1 and cyp90d1 mutants (12,13).…”

Brassinosteroids, de-etiolation and the re-emerging art of plant hormone quantification

“…In the absence of direct evidence, expression levels of BR biosynthesis genes have often been used to support the widelyaccepted, but incorrect, assumption that BR levels are lower in light-grown than in dark-grown Arabidopsis seedlings. 5,6,27 The record has finally been corrected in our recent paper published in Planta, 1 which demonstrates, through direct measurement, that BR levels are in fact higher (not lower) in light-grown than in dark-grown Arabidopsis seedlings. This example demonstrates that even accurate gene expression data may be misleading if used as an indicator of plant hormone levels, and highlights the need for direct measurements.…”

“…Instead, the transcript (mRNA) levels of genes involved in hormone synthesis, catabolism and perception/signalling are often used to presuppose endogenous hormone levels, without confirmation by direct measurement (for example, see refs. [4][5][6][7][8][9].…”