Brassinosteroid precursor biosynthesis

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). 

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 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. 

Another dwarf mutant of Arabidopsis, sax], defines a step upstream of DWF1 in the brassinosteroid biosynthesis pathway [27]. Rescue experiments with intermediates showed that saxl is involved in the oxidation and isomerization of 3P-hydroxyl,A5 6 precursors to 3-oxo-A4 5 steroids (Fig. (4)). 

Fig. 2. Biosynthesis of typical plant sterols. Boxed sterols are probable precursors of various brassinosteroids. Among these sterols, sitosterol, stigmasterol, campesterol, 24-epicampesterol and cholesterol are the most common end-of-pathway sterols in plants. Italic letters refer to the lesions in the biosynthesis mutants of Arabidopsis (dim) and pea (Ikb).

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