Synthetic inhibitors of the fungal detoxifying enzyme brassinin oxidase based on the phytoalexin camalexin scaffold.

Brassinin (1) is an essential phytoalexin produced in plants of the family Brassicaceae (common name crucifer) due to its role as a biosynthetic precursor of other phytoalexins and antimicrobial activity. The dithiocarbamate group of brassinin (1) is the toxophore responsible for its fairly broad antifungal activity. To the detriment of many agriculturally important crops, several pathogenic fungi of crucifers are able to overcome brassinin by detoxification. In this work, inhibitors of brassinin oxidase, a phytoalexin detoxifying enzyme produced by the plant pathogenic fungus Leptosphaeria maculans (asexual stage Phoma lingam ), were synthesized and evaluated. The camalexin scaffold was used for the design of brassinin oxidase inhibitors (i.e., paldoxins, phytoalexin detoxification inhibitors) because camalexin is a phytoalexin not produced by the Brassica species and L. maculans is unable to metabolize it. The inhibitory effect of camalexin and derivatives decreased as follows: 5-methoxycamalexin > 5-fluorocamalexin = 6-methoxycamalexin > camalexin > 6-fluorocamalexin; 5-methoxycamalexin was determined to be the best inhibitor of brassinin oxidase discovered to date. In addition, the results suggested that camalexin might induce fungal pathways protecting L. maculans against oxidative stress (induction of superoxide dismutase) as well as brassinin toxicity (induction of brassinin oxidase). Overall, these results revealed additional biological effects of camalexin and its natural derivatives and emphasized that different phytoalexins could have positive or negative impacts on plant resistance to different fungal pathogens.