Brassinolide activity

Receptor requirements for brassinolide activity include a hydrophilic site for the 2a 3a vicinal diol, a site which accepts the 6-0x0 group [70], and a side chain domain which needs further specification. Because of its relative non-specificity, the side-chain moiety may be a suitable target for the design of receptor probes. 

Brassinolide was tested on 17 bioassays for growth substance. The results led to claims that brassinolide possesses a broad spectrum of biological activity, including gibberellin-, auxin- and cytokinin-like activity (32,34). These claims must be treated with some caution however, since the claimed "specificity of some of the bioassays selected is questionable. At present three bioassay techniques (35,36,37) are used routinely for the detection of brassinolide activity. All three assays are sensitive to auxin, which is a prerequisite for the detection of brassinolide-like compounds. This is not to say that brassinolide has auxin-like activity, but rather there seems to be an interaction of cooperative action between auxin and BR. 

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. 

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. 

Among the compounds that have been isolated from the brassins were a new group of glucosyl esters of fatty acids, the structures of which were established by spectral methods (254, 255) as well as by chemical and biochemical synthesis (256, 257, 258). Although these esters were much less active than brassinolide in the bean second internode bioassay, their presence may be essential for seed germination since the enzymatic synthesis of these esters was correlated to germination (257, 258). 

Structure-activity relationship Biosynthesis and Metabolism Biosynthesis of castasterone Biosynthesis of brassinolide from castasterone Regulation of biosynthesis Metabolism and its regulation Physiology and Signal Transduction Physiology Signal transduction Jasmonates and Oxylipins Introduction Chemistry Chemical structure Chemical stability... 

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. 

Brassinolide (BL) has a seven-membered lactone ring that is formed by a Baeyer-Villiger oxidation of its immediate precursor CS. Tomato CYP85A3 catalysed the Baeyer-Villiger oxidation to produce BL from CS in yeast, in addition to the conversion of 6-deoxocastasterone to CS (Nomura et al, 2005). Arabidopsis CYP85A2, which was initially characterized as CS synthase (see above), also has BL s)mthase activity. A microsomal enzyme preparation from cultured cells oiPhaseolus vulgaris catalysed a conversion from CS to BL. This enzyme preparation also catalysed the conversions of 6-deoxocastasterone and typhasterol to CS. 

The structure of the unique biologically active plant growth promoter, brassinolide, from pollen of the rape plant (Brassica napus L.) was published in 1979 (1). That... 

The isolation and structural determination of brassinolide [BR] was reported in Nature by Dr. N. B. Mandava and co-workers at the USDA in 1979(5), and was subsequently announced in Chem, Eng, News 6). The steroidal skeleton of the chemical structure of BR prompted us to examine the effect on the LJT. (225,235)-homobrassinolide was synthesized by Mori in 1980( 7) at our request just after the structural determination of BR. BR itself was also produced by Ikekawa and his co-workers the same year(3). Wada, a co-worker in our group, tested the activity of BR and homobrassinolide on rice lamina inclination and found that both compounds dramatically induced the inclination of laminae (see Table I) as expected(5). These findings strongly suggested... 

From 40 kg of rape pollen, 4 mg of an active compound was eventually isolated and shown to be the novel plant growth substance brassinolide (1) (3). The structure, as determined by X-ray crystallography, was [ (2a, 3a, 22R, 23R)-tetrahydroxy- 24 a-methyl-B-homo-7-oxa-5a-cholestane-6-one]. This structure was unique in possessing a 24a-methyl, a 7-oxalactonic B ring, and vicinal hydroxyls on the A ring (C2a and C3a) plus a side chain (C22R and C23R ). 

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