The phytoalexins from cultivated and wild crucifers: Chemistry and biology

Abstract

Phytoalexins are antimicrobial secondary metabolites produced de novo by plants in response to stress, including microbial attack. In general, phytoalexins are important components of plant defenses against fungal and bacterial pathogens. The phytoalexins of crucifers are indole alkaloids derived from (S)-tryptophan, most of which contain a sulfur atom derived from cysteine. Beside their antimicrobial activity against different plant pathogenic species, cruciferous phytoalexins have shown anticarcinogenic effects on various human cell lines. This review focuses on the phytoalexins produced by cruciferous plants reported to date, with particular emphasis on their chemical synthesis, biosynthesis, metabolism by plant fungal pathogens and biological activities. A summary table containing all phytoalexins, their cultivated and wild cruciferous sources, their synthetic starting materials, biotransformation products and biological activities is provided.

Biosynthesis of cabbage phytoalexins from indole glucosinolate

“…The names of representative phytoalexin producers are given in italics. Solid arrows represent known enzymatic reactions, and dashed arrows signify proposed biosynthetic relationships, many supported by previous isotope feeding studies (9). *The feeding studies suggest that 1-methoxybrassenin B, sinalbin A, and erucalexin are derived from the 1-methoxy derivatives of brassinin and cyclobrassinin.…”

“…Over 30 known cruciferous phytoalexins are proposed to be derived from brassinin or brassinin analogs (9,32), and there is great interest in characterizing the biosynthetic pathway and biological function of each. To demonstrate the feasibility of using tobacco to test enzymes that modify brassinin en route to functionalized indole-sulfur phytoalexins, we added either CYP71CR1 or CYP71CR2 to the set of genes transiently expressed.…”

“…1) suggest that over 30 compounds arise from oxidative tailoring and rearrangement of this parent scaffold (9,12). Labeling studies (13,14) also indicate that brassinin itself is derived from indole glucosinolate via the activated product indole isothiocyanate, formed by the action of myrosinases (a class of thiospecific glucohydrolases) (Fig.…”

“…These compounds, called indole-sulfur phytoalexins, are a hallmark of the Brassicaceae with different subsets produced by different edible crucifers ( Fig. 1) (9). Like glucosinolates, indolesulfur phytoalexins have been linked to plant disease resistance and are thought to modulate human health: pure brassinin displays low-micromolar activities against plant pathogens (EC 50 = 10-140 μM for restriction of various Alternaria phytopathogen isolates) (10) and cancer cells (K i = 28 μM for anticancer target IDO) (11).…”

Spirocyclization reactions and antiproliferative activity of indole phytoalexins 1-methoxybrassinin and its 1-substituted derivatives

“…cabbage, turnip, Chinese cabbage, Japanese radish, wasabi, broccoli, rapeseed and arabidopsis). 4 The 44 cruciferous phytoalexins have been divided into six groups according to simple structural features. 4 A unique group of these natural products are spiroindoline structures containing a spirocyclic ring in the C-3 position [(S)-()-spirobrassinin [()-1], 5 …”