Synthetic Inhibitors of the Fungal Detoxifying Enzyme Brassinin Oxidase Based on the Phytoalexin Camalexin Scaffold

Abstract

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.

Substrate specificity and inhibition of brassinin hydrolases, detoxifying enzymes from the plant pathogens Leptosphaeria maculans and Alternaria brassicicola

“…BO was isolated from the wild‐type fungus and found to be competitively inhibited by the cruciferous phytoalexins cyclobrassinin and camalexin. Furthermore, recent results have shown that 5‐methoxycamalexin, a synthetic compound, was the most effective inhibitor of BO [29]. Recombinant SsBGT1 was isolated from Saccharomyces cerevisiae after the corresponding gene of S. sclerotiorum was cloned.…”

Plant Antimicrobial Agents and Their Effects on Plant and Human Pathogens

“…Other compounds probed to be better inhibitors of fungal growth, but they were also targets of degradation [27]. In a more recent work, new paldoxins were designed based on the camalexin scaffold because L. maculans is unable to metabolize it [92]. As a result, an even better inhibitor of brassinin oxidase was discovered but unfortunately, this compound also induced fungal pathways protecting the microorganism against oxidative stresses and brassinin toxicity.…”

Phytoalexins from Brassicaceae: News from the front

“…Thus, it is important to emphasize that, besides being a unique model extremophile for tolerance to abiotic stress (genome approximately twice that of A. thaliana, sequencing currently underway) (Amtmann et al, 2005;Amtmann, 2009), T. salsuginea presents a unique opportunity to discover new biosynthetic pathways of great significance to plant disease resistance and fitness. In addition, recourse to phytoalexin detoxification inhibitors, that is paldoxins, for treatment of specific fungal diseases needs further research (Pedras et al, 2009c). In particular, if inhibitors of fungal degradation of brassinin (1) can promote accumulation of plant self-defenses and control pathogens like L. maculans or A. brassicicola, new detoxification pathways of target phytoalexins need to be investigated.…”