Brassinosteroids function

S. Fujioka, Y. Aral et al. (2003). Brassinosteroid functions in a broad range of disease resistance in tobacco rice. Plant J. 33, 887-898. 

Isoprenoids that have hormonal and signaling functions form an important group. These include steroid hormones (1 = 6) and retinoate (the anion of retinoic acid 1 = 3) in vertebrates, and juvenile hormone (1 =3) in arthropods. Some plant hormones also belong to the isoprenoids—e.g., the cytokinins, absci-sic acid, and brassinosteroids. 

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]. 

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). 

Hong, Z., Ueguchi-Tanaka, M., Shimizu-Sato, S., Inukai, Y., Fujioka, S., Shimada, Y., Takatsuto, S., Agetsuma, M., Yoshida, S., Watanabe, Y., Uozu, S., Kitano, H., Ashikari, M. and Matsuoka, M. (2002) Loss-of-function of a rice brassinosteroid biosynthetic enzyme, C-6 oxidase, prevents the organized arrangement and polar elongation of cells in the leaves and stem. Plant., 32, 495-508. 

All the naturally-occurring brassinosteroids are known to be derivatives of 5a-cholestane. Diverse structural variations thus come from the kinds and orientation of functionalities on the skeleton. 

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). 

Of special interest for practical applications, as well as for future receptor studies, are the structure-activity relationships of the brassinosteroids. As postulated (6) the structural requirements for a high activity are the following (22R.23R)-vicinal diol moiety (24S)-methyl- or ethyl group 7-oxa-lactone or 6-oxo functionality 3a-hydroxy group, 2a,3a-vicinai diol or 3a,4a-vicinal diol and A/B-trans-fused ring junction. 

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. 

Microbial Transformation of Brassinosteroids. Until now, information concerning the microbial transformations of brassinosteroids has not been published. Such biotransformations could open novel pathways to additionally functionalized members for structure-activity investigations as well as provide information about possible metabolic processes of such compounds. Furthermore, the intermediate metabolites may have practical application. 

These results lead to the conclusion that for brassinosteroid activity the spirostan side chain moiety is able to substitute, at least partially, for the 22,23-vicinal diol function. 

Brassinosteroids - like other steroids - are lipophilic or amphiphilic some of them are mesogens. These properties cause them to accumulate in membranes or at membrane surfaces. Therefore, detailed insight into their role as plant growth substances might be obtained by studying their interference with membrane functions. Applied externally, the first membrane they meet is the plasmalemma. The functioning of the plasmalemma is crucial for transport in plants. Hereafter, some results on the plasmalemma are reported which reflect short-term effects of the substances applied. 

Brassinosteroids and ecdysteroids show similarities in their chemical structures (Figure 1). Molecules belonging to both groups are poly-hydroxylated steroids with carbonyl function at C-6. Presently, only C2g and C2Q brassinosteroids are found in plants, and thus differ from the animal ecdysteroids which contain mostly the 27 carbon skeleton of cholesterol Interestingly, C27 ecdysteroids are often the major ecdysteroids of plants, but overall there are also a number of C2g and C2g ecdysteroids in plants. Additional differences are ... 

Molecular modelling of ecdysteroids (7) and brassinosteroids is still in its infancy. A comparison of the three dimensional (3D-) structures of the two biologically most active representatives of brassinosteroids and ecdysteroids, i.e. of brassinolide and 20-hydroxyecdysone, suggests that functional groups relevant for bioactivity are found at similar positions in both molecules in spite of the structural differences mentioned above. More detailed comparative studies of the 3D-structures of ecdysteroids and brassinosteroids are eagerly awaited. 

Biochemical and Physiological Responses. Brassinosteroids have been reported to elicit several plant physiological functions, including the follow-ing 7) ... 

There is further evidence to support the claim that brassinosteroids are hormonal in their action (6). These are the effects of brassinolide on gravitropism (7), effects in conjunction with light quality (8), effects on photosynthate partitioning (9), probable effects on phytochrome (10), substitution for indole-3-acetic acid in soybean epicotyls (11), enhancement of xylem differentiation (11), stimulation of membrane permeability in cucumber hypocotyls (12), and stimulation of ATPase activity (12). Taken objectively, many of these specific physiological and biochemical functions which are attributed to brassinolide, and by inference to the brassinosteroids in general, have been attributed to the other plant hormones, especially indole-3-acetic acid and the gibberellins (6, 13). 

Abscisic acid (ABA) levels in rice plants, 308,31Or levels in squash hypocotyls, 315/.316 Active component of brassins identification, 9,lQf pilot plant extraction, 6,7/,8 solvent partition and column chromatography, 8 Adventitious root(s) development, 233,234r,235 formation, 247 Agriculture, application of 24-epibrassinolide, 280-290 22-Aldehydes, synthesis of brassinosteroids, 47-50f a hormone function, description for brassins, 4... 

The original structure of brassinosteroids and the requirement for reference compounds and sufficient amounts of brassinosteroids for biological studies has tremendously stimulated the synthesis of such phytohormones and their analogues up till now.2-4- 9 63 Much efforts have especially been focussed to developing convenient and effective methods for constructing the brassinosteroid side chain with (22R,23/ )-diol function, which is essential for a high bioactivity.63 Starting from suitable phytosterol precursors with a A22 double bond the alkyl... 

The structural determination of endogenous brassinosteroids, present only in minute amounts in plant material, requires the availability of corresponding reference standards. Thus, for the final identification of the new brassinosteroid secasterone (32) isolated from Secale cereale (see, Section 2), the four epimeric brassinosteroids with 2,3-epoxy function derived from castasterone (3) and 24-epicastasterone (13), respectively, were synthesized (Scheme 6).69... 

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