An early C-22 oxidation branch in the brassinosteroid biosynthetic pathway

Area of Science:

• Plant biochemistry and molecular biology
• Steroid hormone biosynthesis
• Brassinosteroid metabolism

Background:

• 22-hydroxylated steroids are critical plant hormones involved in growth and development.
• Understanding their biosynthesis pathways is essential for plant science.
• The det2 mutant in Arabidopsis thaliana is known to have defects in brassinosteroid biosynthesis.

Purpose of the Study:

• To investigate the natural occurrence of 22-hydroxylated steroids in Catharanthus roseus cells and Arabidopsis seedlings.
• To elucidate the metabolic fate of 22-hydroxycampesterol (22-OHCR) in plants.
• To identify the specific defect in brassinosteroid biosynthesis in the det2 mutant.

Main Methods:

• Gas chromatography-mass spectrometry (GC-MS) for identification of steroids.
• Metabolic experiments using deuterium-labeled 22-OHCR.
• Analysis of endogenous brassinosteroid levels in wild-type and det2 Arabidopsis seedlings.
• Phenotypic rescue experiments with exogenous application of steroids.

Main Results:

• Several 22-hydroxylated steroids, including 22-hydroxycampesterol (22-OHCR), were identified in C. roseus and Arabidopsis.
• Metabolic studies revealed a subpathway: 22-OHCR → 22-hydroxyergost-4-en-3-one (22-OH-4-en-3-one) → 22-hydroxy-5alpha-ergostan-3-one (22-OH-3-one) → 6-deoxocathasterone (6-deoxoCT).
• The det2 mutant accumulates 22-OH-4-en-3-one and shows reduced levels of downstream compounds, indicating a block in the conversion to 22-OH-3-one.
• Exogenous application of 22-OH-3-one and 6-deoxoCT rescued det2 mutant phenotypes.

Conclusions:

• A novel brassinosteroid biosynthesis subpathway involving 22-hydroxylated intermediates has been substantiated.
• The det2 mutant is defective in the conversion of 22-OH-4-en-3-one to 22-OH-3-one, a key step towards brassinolide.
• This study provides critical insights into brassinosteroid metabolism and its regulation in plants.