Brassinosteroids biological activity

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. 

T. Yokota K. Mori, Molecular Structure and Biological Activity of Brassinolide and Related Brassinosteroids. In Molecular Structure and Biological Activity of Steroids M. Bohl, W. L. Duax, Eds. CRC Press Boca Raton, FL, 1992 pp 317-340. 

Among all naturally-occurring brassinosteroids, brassinolide and castasterone are considered to be the most important brassinosteroids because of their wide distribution as well as their potent biological activity. 

Brassinosteroids are grouped into C, Cg, and steroids like the typical phytosterols. These classifications result basically from different alkyl substitutions in the side chain, that is no substituent in steroids (14, 15 ), a methyl (1, 3,5, 7-9, 16-22,29) or an exomethylene ( 2,4, 6) at C24 in steroids, and an ethyl at C24 (12 )or an ethylidene at C24 ( 10,11, 13) or an exomethylene at C24 with a methyl at C25 ( 23-28 ) in steroids. The structure-activity relationship reveals that alkylation in the side chain is necessary for biological activity. More especially, the presence of a saturated alkyl ( a methyl or an ethyl ) at C24 and a methyl at C25 makes brassinosteroids biologically more active. Thus, 25-methylbrassinolide bearing both 24-methyl and 25-methyl functions has been chemically synthesized and it is more potent than brassinolide which, until now, had been considered to be the most potent brassinosteroid (44). 

This brief review covers the period of the last ten years in the development of brassinosteroid chemistry in our laboratory. Our interest has been focused on the synthesis of natural brassinosteroids (brassinolide, homobrassinoli-de, epibrassinolide, norbrassinolide, etc.), their analogues and intermediates starting from stigmasterol, ergosterol, or pregnenolone. Some aspects of biological activity are discussed. 

For an interaction to occur with a putative brassinosteroid receptor, both stereospeciflcally arranged vicinal diol functions in the ring A, as well as in the side chain, can be assumed as important. In our synthetic program for modified brassinosteroids, we were especially interested in new types of analogs with the ring A- and B-seco structure to study the influence of the geometry of this molecular adaptation on biological activity. 

Table L Biological Activity of Selected Brassinosteroid Analogs in the Rice Lamina Inclination Test...

Following the discovery of brassin in 1970 (1 ), the isolation and chemical characterization of the active ingredient brassinolide (BR) in 1979 (2) and the subsequent identification of a number of other naturally-occurring brassinosteroids (BS) (see 2) considerable research effort has been directed towards determining the taxonomic and morphological distribution and biological activity of these compounds in plants. 

Our synthetic work on brassinosteroids showed that the stereochemistry in the steroid molecule is very important to retain the biological activity. We found that, of the 13 asymmetric centers in the molecule, only 3 asymmetric centers (at C-22, C-23, and C-24) can be altered. On the basis of this information, we prepared brassinolide and all the analogs (23) by using the general synthetic scheme (Figure 3). Additionally, we prepared compounds having different substituents (H, CH3, and C2H5) at C-24 (2, 7). 

Some specificity was demonstrated in that 22S,23S-homobrassinolide induced the delay, whereas 22S,23S-homocastasterone (Figure 4) had no effect. Once again, this pioneer work leads to several possibilities including the potential biological activity of brassinosteroid derivatives, and it is certain that as a result of this research other laboratories will be evaluating this class of compounds in entomological systems. 

Since the discovery of brassinolide in rape pollen, 11 additional biologically active isomers have been isolated from higher plants. With the exception of pollen tissues, all of the brassinosteroids that were isolated were found only in young, immature, actively growing and differentiating plant tissues, including immature seeds and shoots of a variety of plants. 

It is suggested that the biological activity of some of these brassinosteroids might be due to their conversion to brassinolide by the plant tissue during the bioassay (15). This will require further investigation. 

Biologically active brassinosteroids by themselves cause little curvature however, if a brassinolide treatment is co-applied with auxin the ensuing curvature is greater than the sum total of lAA and brassinolide applied separately (Figure 3). 

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