First published online September 6, 2002; 10.1104/pp.008722
Plant Physiol, October 2002, Vol. 130, pp. 930-939
An Early C-22 Oxidation Branch in the Brassinosteroid
Biosynthetic Pathway
Shozo
Fujioka,*
Suguru
Takatsuto, and
Shigeo
Yoshida
RIKEN (The Institute of Physical and Chemical Research), Wako-shi,
Saitama 351-0198, Japan (S.F., S.Y.); and Department of Chemistry,
Joetsu University of Education, Joetsu-shi, Niigata 943-8512, Japan
(S.T.)
The natural occurrence of 22-hydroxylated steroids in
cultured Catharanthus roseus cells and in Arabidopsis
seedlings was investigated. Using full-scan gas chromatography-mass
spectrometry analysis, (22S)-22-hydroxycampesterol
(22-OHCR),
(22S,24R)-22-hydroxyergost-4-en-3-one (22-OH-4-en-3-one),
(22S,24R)-22-hydroxy-5 -ergostan-3-one
(22-OH-3-one), 6-deoxocathasterone (6-deoxoCT),
3-epi-6-deoxoCT, 28-nor-22-OHCR, 28-nor-22-OH-4-en-3-one,
28-nor-22-OH-3-one, 28-nor-6-deoxoCT, and
3-epi-28-nor-6-deoxoCT were identified.
Metabolic experiments with deuterium-labeled 22-OHCR were performed in
cultured C. roseus cells and Arabidopsis seedlings (wild
type and det2), and the metabolites were analyzed by gas
chromatography-mass spectrometry. In both C. roseus
cells and wild-type Arabidopsis seedlings,
[2H6]22-OH-4-en-3-one,
[2H6]22-OH-3-one,
[2H6]6-deoxoCT, and
[2H6]3-epi-6-deoxoCT were
identified as metabolites of [2H6]22-OHCR,
whereas the major metabolite in det2 seedlings was [2H6]22-OH-4-en-3-one. Analysis of endogenous
levels of these brassinosteroids revealed that
det2 accumulates 22-OH-4-en-3-one. The levels of downstream compounds were remarkably reduced compared with the wild
type. Exogenously applied 22-OH-3-one and 6-deoxoCT were found to
rescue det2 mutant phenotypes, whereas 22-OHCR and
22-OH-4-en-3-one did not. These results substantiate the existence of a
new subpathway (22-OHCR  22-OH-4-en-3-one 22-OH-3-one 6-deoxoCT) and reveal that the det2 mutant is defective
in the conversion of 22-OH-4-en-3-one to 22-OH-3-one, which leads to
brassinolide biosynthesis.
*
Corresponding author; e-mail sfujioka{at}postman.riken.go.jp;
fax 81-48-462-4959.
© 2002 American Society of Plant Physiologists
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