Novel biosynthetic pathway of castasterone from cholesterol in tomato
- Authors
- Kim, Tae-Wuk; Chang, Soo Chul; Lee, June Seung; Takatsuto, Suguru; Yokota, Takao; Kim, Seong-Ki
- Issue Date
- Jul-2004
- Publisher
- AMER SOC PLANT BIOLOGISTS
- Citation
- PLANT PHYSIOLOGY, v.135, no.3, pp 1231 - 1242
- Pages
- 12
- Journal Title
- PLANT PHYSIOLOGY
- Volume
- 135
- Number
- 3
- Start Page
- 1231
- End Page
- 1242
- URI
- https://scholarworks.bwise.kr/cau/handle/2019.sw.cau/24816
- DOI
- 10.1104/pp.104.043588
- ISSN
- 0032-0889
1532-2548
- Abstract
- Endogenous brassinosteroids (BRs) in tomato (Lycopersicon esculentum) seedlings are known to be composed of C-27- and C-28-BRs. The biosynthetic pathways of C-27-BRs were examined using a cell-free enzyme solution prepared from tomato seedlings that yielded the biosynthetic sequences cholesterol --> cholestanol and 6-deoxo-28-norteasterone <----> 6-deoxo-28-nor-3-dehydroteasterone <----> 6-deoxo-28-nortyphasterol --> 6-deoxo-28-norcastasterone --> 28-norcastasterone (28-norCS). Arabidopsis CYP85A1 that was heterologously expressed in yeast mediated the conversion of 6-deoxo-28-norCS to 28-norCS. The same reaction was catalyzed by an enzyme solution from wild-type tomato but not by an extract derived from a tomato dwarf mutant with a defect in CYP85. Furthermore, exogenously applied 28-norCS restored the abnormal growth of the dwarf mutant. These findings indicate that the C-6 oxidation of 6-deoxo-28-norCS to 28-norCS in tomato seedlings is catalyzed by CYP85, just as in the conversion of 6-deoxoCS to CS. Additionally, the cell-free solution also catalyzed the C-24 methylation of 28-norCS to CS in the presence of NADPH and S-adenosylmethionine (SAM), a reaction that was clearly retarded in the absence of NADPH and SAM. Thus it seems that C-27-BRs, in addition to C-28-BRs, are important in the production of more active C-28-BRs and CS, where a SAM-dependent sterol methyltransferase appears to biosynthetically connect C27-BRs to C28-BRs. Moreover, the tomato cell-free solution converted CS to 26-norCS and [H-2(6)]CS to [H-2(3)]28-norCS, suggesting that C-28 demethylation is an artifact due to an isotope effect. Although previous feeding experiments employing [H-2(6)]CS suggested that 28-norCS was synthesized from CS in certain plant species, this is not supported in planta. Altogether, this study demonstrated for the first time, to our knowledge, that 28-norCS is not synthesized from CS but from cholesterol. In addition, CS and [H-2(6)]CS were not ;converted into BL and [H-2(6)]BL, respectively, confirming an earlier finding that the active BR in tomato seedlings is not BL but CS. In conclusion, the biosynthesis of 28-norBRs appears to play a physiologically important role in maintaining homeostatic levels of CS in tomato seedlings.
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