Of brassinosteroids

CLOUSE, S.D., FELDMANN, K.A., Molecular genetics of brassinosteroid action. In Brassinosteroids Steroidal Plant Hormones, (A. Sakurai, T. Yokota, and S.D. Clouse, eds,), Springer Verlag, Tokyo, 1999, pp 163-190. 

Be this as it may, the CH oxidation has become an attractive and promising preparative method in natural-product synthesis. For example, dioxiranes have been used to introduce selectively a hydroxy group at the C-25 position of steroids, a challenging task not readily achieved with other oxidants. A specific case is the TFD oxidation of brassinosteroid (equation 33) . 

The biosynthesis of brassinosteroids has been investigated by in vivo feeding of plant cell cultures, in addition to the analysis of native brassinosteroid-pattems in diverse plant species. The proposed pathway (Fig. (1)) leads from unpolar sterol precursors to the polyhydroxylated phytohormone brassinolide, that is known to exhibit the highest biological activity. 

Some biochemical functions defined by the Arabidopsis dwarf mutants were later confirmed by heterologous expression of genes and by in vivo conversion of postulated substrates [17-20]. As part of these physiological and biochemical studies, tomato cell suspension cultures have also been established to investigate intermediates and enzymes of brassinosteroid biosynthesis and metabolism [21-23]. Enzyme activities from partially purified protein extracts were first detected in this model system [24]. 

Most known brassinosteroid mutants are defective in genes that code for enzymes required for the biosynthesis of brassinosteroid precursors. This chapter will summarize the upstream part of brassinosteroid biosynthesis pathway. 

The Arabidopsis mutant dwf7/ste 1 is defective in C5-desaturation of episterol (Fig. (2)) [18], thus impaired in an enzyme function involved in a very early step of brassinosteroid precursor biosynthesis. The enzymatic block of dwf7/stel was determined by feeding experiments using l3C-labelled mevalonic acid and a subsequent analysis of endogenous sterol and brassinosteroid precursors. The mutant accumulates episterol with a simultaneous decrease of downstream intermediates (24-methylenecholesterol, campesterol, castasterone, brassinolide). 

This chapter deals with the conversion from cathasterone to brassinolide, the biologically most active brassinosteroid. Only two mutants have been found to be involved in the downstream subpathway. Nevertheless, enzyme activities from enriched protein fractions have first been detected in this part of brassinosteroid biosynthesis. 

The key step in the biosynthesis of brassinosteroids is the conversion of castasterone to brassinolide. This reaction is a lactonization of the steroidal B-ring or a Bayer-Villiger-oxidation. Tomato cell suspension cultures have been extensively studied in respect to the metabolism of 24-< / i-castasterone and 24-ep/-brassinolide [21, 22, 32, 33]. A microsomal fraction of tomato cell cultures, induced by 24-epi-castasterone, was able to convert this substrate into 24-epi-brassinolide (Fig. (11)). The specific enzyme activity was determined to be 230 fkat/mg protein with NADPH serving as the only accepted electrondonor [24],... 

The future will give us the chance to characterize more enzymes in the brassinosteroid pathway, leading not only to an improved understanding, but also to the possibility of modifying internal brassinosteroid levels, thus getting a new insight in the mode of action of brassinosteroids. 

Nakamura, A., Higuchi, K., Goda, H., Fujiwara, M. T., Sawa, S., Koshiba,T., Shimada, Y., and Yoshida, S. (2003). Brassinolide induces IAA5, IAA19, and DR5, a synthetic auxin response element in Arabidopsis, implying a cross talk point of brassinosteroid and auxin signalhng. Plant Physiol. 133,1843-1853. 

Takeuchi, Y., Omigawa, Y., Ogasawara, M., Yoneyama, K., Konnai, M., and Worsham, A. D. 1995. Effects of brassinosteroids on conditioning and germination of clover broomrape (Orobanche minor) seeds. Plant Growth Reg. 16, 153-160... 

Figure 31 Derivatives used for GC-MS analysis of brassinosteroids and their diagnostic ions for the side chain structures.

Figure 32 Structures of triazole inhibitors of brassinosteroid biosynthesis.

Figure 33 Biosynthetic pathways of brassinosteroids. The larger the arrows, the more principal the routes. Arabidopsis enzymes involved in the biosynthetic reactions are indicated in bold letters.

Figure 34 Structures of brassinosteroid receptors and current model for brassinosteroid signaling.

Takao Yokota is Professor of the Department of Biosciences, Teikyo University. He received his Ph.D. degree from the University of Tokyo in 1971. He became an Assistant Professor in the University of Tokyo in 1970, then a Lecturer in 1988. In 1990, he was assigned to the present position. He has been involved in brassinosteroid research for nearly 30 years. His major research interests concern the biosynthesis and physiological roles of brassinosteroids. 

Bajguz, A. and Tretyn, A. (2003) The chemical characteristic and distribution of brassinosteroids in plants. Phytochemistry, 62,1027-46. 

Fujioka, S. and Yokota, T. (2003) Biosynthesis and metabolism of brassinosteroids. Annu. Rev. Plant Biol, 54,137-64. 

Suzuki, H., Inoue, T., Fujika, S., Saito, T., Takatsuko, S., Yokota, T., Murofushi, N., Yanagisawa, T. and Sakurai, A. (1995) Conversion of 24-methylcholesterol to 6-oxo-24-methylcholestanol, a putative intermediate of the biosynthesis of brassinosteroids, in cultured cells of Catharanthus roseus. Phytochemistry, 40,1391-7. 

Wang, Z., Nakano, T., Gendron, J., He, J., Chen, M., Vafeados, D., Yang, Y., Fu-jioka, S., Yoshida, S., Asami, T. and Chory, J. (2002) Nuclear-localized BZRl mediates brassinosteroid-induced growth and feedback suppression of brassinosteroid biosynthesis. Dev. Cell, 2, 505-13. 

Park, S.C. Kim, T.W. Kim, S.K. Identification of brassinosteroids with 24-R-methyl in immature seeds of Phaseolus vulgaris. Bull. Korean Chem. Soc. 2000, 21, 1274-1276. 

Chemical synthesis of brassinosteroids, phytohormones with a seven-membered O-heterocycle in some of them 02KPS99. 

The beginning of brassinosteroid research in Japan was initiated with Distylium factor research. Distylium factors were discovered in Distylium racemosum leaves, when bioassayed, using... 

Brassinosteroids are steroidal plant hormones which promote plant growth. To date the presence of over sixty kinds of brassinosteroids has been verified from various plant sources and thirty one of them fully characterized. In this chapter, the distribution and structural characteristics of the naturally-occurring brassinosteroids are discussed. 

Microanalytical techniques, consisting of methaneboronation of the vicinal hydroxyls ( bismethaneboronate or methaneboronate-TMS-ether ) have been developed by Takatsutoet al. for the GC/MS or GC/SIM of brassinosteroids (16). Consequently brassinone (15), 24-ethylbrassinone (12) and 28-norbrassinolide (14) were detected by GC/MS without isolation from Chinese cabbage ( Brassica campestris ), green tea ( Thea sinensis ) and chestnut insect galls ( Distilium racemosum ) (17-19). The presence of brassinolide and castasterone in the tissues of these plants has been also detected. 

The presence of brassinosteroids in monocots has also been demonstrated. Typhasterol ( 2-deoxycastasterone ) from cattail ( Typha latifolia ) pollen (35), castasterone and dolichosterone from rice shoots ( Oryza sativa ) (36) and castasterone, typhasterol and teasterone from com (Zea mays ) pollen (37) were identified. 

The occurrence of brassinosteroids in gymnosperms has been reported from conifers. Yokota ef al. isolated typhasterol ( 2-deoxycastasterone ) from pollen of Japanese black pine ( Pinus thunbergii ) (38) and identified castasterone and typhasterol from shoots of sitka spruce (Picea sitchensis ) (39). Kim ef al. also identified castasterone and brassinolide from cambial scrapings of Scots pine (Pinus silverstris ), using a modified dwarf rice lamina inclination assay which showed a synergistic response of brassinosteroids with indole-3-acetic acid (40). 

---