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Published on: March 24, 2012
The tomato DWARF enzyme catalyses C-6 oxidation in brassinosteroid biosynthesis
G J Bishop1, T Nomura, T Yokota
1Frontier Research Program, Institute of Physical and Chemical Research, RIKEN, Wako-shi, Saitama 351-0198, Japan.
Summary
The tomato Dwarf gene (D) encodes a cytochrome P450 enzyme crucial for brassinosteroid (BR) biosynthesis. This enzyme catalyzes the C-6 oxidation of 6-deoxocastasterone to castasterone, a key step in plant hormone production.
Area of Science:
- Plant Biology
- Molecular Genetics
- Biochemistry
Background:
- Brassinosteroids (BRs) are essential steroidal plant hormones regulating growth and development.
- Mutations in BR biosynthesis or perception pathways typically result in dwarf phenotypes.
- The tomato Dwarf (D) gene, encoding a cytochrome P450, was previously identified and linked to BR biosynthesis.
Purpose of the Study:
- To elucidate the specific enzymatic function of the tomato DWARF (D) protein in brassinosteroid biosynthesis.
- To identify the substrate and reaction catalyzed by the DWARF enzyme.
- To confirm the role of DWARF in the C-6 oxidation step of BR production.
Main Methods:
- Complementation of the tomato extreme dwarf (dx) mutant phenotype using D cDNA.
- Exogenous application of brassinosteroid intermediates to dx plants to assess hypocotyl elongation.
- Quantitative analysis of endogenous brassinosteroid levels in wild-type (D) and mutant (dx) tomato plants.
- Functional expression of the DWARF gene in yeast to confirm enzymatic activity.
Main Results:
- The D cDNA successfully complemented the dwarf phenotype of dx tomato mutants.
- C-6 oxidized BR intermediates, but not 6-deoxocastasterone (6-deoxoCS), rescued hypocotyl elongation in dx plants.
- dx mutants accumulated high levels of 6-deoxoCS and lacked castasterone (CS), while D plants showed the opposite pattern.
- Yeast expression confirmed that DWARF catalyzes the C-6 oxidation of 6-deoxoCS to CS and identified 6alpha-hydroxycastasterone as an intermediate, indicating a dual catalytic role.
Conclusions:
- The tomato DWARF gene encodes a cytochrome P450 enzyme that catalyzes the C-6 oxidation of 6-deoxocastasterone to castasterone.
- This enzymatic activity represents a critical step in the brassinosteroid biosynthesis pathway.
- The DWARF enzyme appears to catalyze two sequential oxidation steps in BR production.
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