Brassinolides

No appreciable development of betacyanin pigment in the dark was observed in any of the weed seedlings during the course of the experiment. The development of betacyanin in the cotyledons of dark-grown A. retroflexus seedlings is reported to be induced by the plant growth regulator, kinetin (20), but not by brassinolide, a steroidal lactone (21). 

John Siddall was a prolific scientist who published a large number of articles and patents. He always maintained a complementary interest in insect physiology, as well as in chemical synthesis, particularly of chiral compounds. In fact, just before he entered the hospital, he and Steven Fung completed a total synthesis of the plant growth regulator Brassinolide, a complex steroid with many asymmetric centers. 

CS090 Abe, H., T. Morishita, M. Uchiyama, S. Takatsuto and N. Ikekawa. A new brassinolide-related steroid in the leaves of Thea sinensis. Agr Biol Chem... 

Sasa, S. Marumo and T. Kitsuwa. Microanalysis of brassinolide and its CS145 application to the identification of new brassinosteroids in plants. Proc 25th Symp on the Chem of Nat Prod Tokyo 1982 290-297. CS146... 

Baeyer-Villiger oxidation of the 5a-6-keto steroid 1 with trifluoroperacedc acid is 1000 times faster than oxidation with m-chloroperbenzoic acid and also is more regioselective. This oxidation was used in the last step in a synthesis of brassinolide (2), a natural steroid that promotes plant growth.2... 

FIGURE 12-28 Structural similarities between plant and animal signals. The plant signals jasmonate, indole-3-acetate, and brassinolide resemble the mammalian signals prostaglandin E1 serotonin, and estradiol. 

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. 

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],... 

Nevertheless, there are many questions still open because of problems to detect enzyme activities corresponding to each step of the pathway. The model of biosynthesis pathway was put together by studying the metabolism of exogenously applied intermediates in cell cultures of various origins and combining these results with data of native brassinosteroid patterns. It is more or less accepted that there are three pathways in parallel, the early and the late C6 oxidation pathway, as well as the 24/ -epimers follow ing the same route. Some observations in the analysis of native brassinosteroid patterns suggest a possible connection between the pathways. It was shown that seeds of Arabidopsis contain castasterone and 24-epi-brassinolide [34]. Also members of both 24-epimers, brassinolide and 24-epi-brassinolide were detected in tomato seeds [Winter, unpublished]. 

Morillon R, Catterou M, Sangwan RS, Sangwan BS, Lassalles J-P. 2001. Brassinolide may control aquaporin activities in Arabidopsis thalina. Planta 212 199-204. 

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