Designer phytoalexins: probing camalexin detoxification pathways in the phytopathogen Rhizoctonia solani

To probe the specificity of a camalexin detoxifying enzyme(s) produced by Rhizoctonia solani, the putative 5-camalexin hydroxylase (5-CAHY), the naturally occurring phytoalexin 1-methylcamalexin and designer phytoalexins in which the H-5 of camalexin was replaced with either a methyl group or a fluorine atom were synthesised. This investigation showed that biotransformation of 5-fluorocamalexin by R. solani was substantially slower than that of camalexin (12 days vs. six to eight hours), 5-methylcamalexin (5-6 days) or 1-methylcamalexin (5-6 days). Antifungal bioassays showed that 5-fluorocamalexin, 5-methylcamalexin and 1-methylcamalexin were more inhibitory to R. solani than camalexin, whereas their metabolic products displayed substantially lower inhibitory activity. It was concluded that detoxification via oxidation of the indole moiety of camalexins is predominant in the biotransformation of both camalexin and 5-methylcamalexin and likely catalysed by a specific 5-CAHY. By contrast, the pathways for detoxification of 1-methylcamalexin and 5-fluorocamalexin are likely catalysed by non-specific "house-keeping" enzymes. Most importantly, because 1- methylcamalexin showed stronger antifungal activity and was metabolised at substantially slower rate than camalexin this work suggested that, from a plant's perspective 1-methylcamalexin could be a more effective antifungal defence than camalexin.