Mycotoxins as host tissue colonization factors

The biological role of mycotoxins remains unclear. It is believed that producing fungi might be better protected against organisms sharing the same trophic niche. However, some mycotoxins seemed to enhance fungal aggressiveness during host exploitation. For example, Alternaria alternata produces many toxic secondary metabolites, of which the most relevant is alternariol (AOH) and its derivative monomethyl ether (AME). Recently, the central role of the polyketide synthase gene pksI for the biosynthesis of AOH and AME has been established (Wenderoth et al., 2019, Mol. Microbiol. 112, 131-146). Moreover, the pksI-deleted Alternaria strain displayed reduced virulence on tomato, citrus and apple, suggesting AOH as virulence and colonization factor. A polyketide synthase is also the first step of the biosynthesis of the mycotoxin patulin, mainly associated to Penicillium expansum. The disruption of the 6-methyl-salicylic acid synthase allowed to obtain mutants that produced less patulin than their wild-type (WT) strain and showed a significantly reduced virulence on apples (Sanzani et al., 2012, Int. J. Food Microbiol. 153, 323-331). Moreover, when patulin was exogenously restored, mutants recovered their virulence as compared to that of the WT. Finally, mutants were susceptible to the antioxidant quercetin at 1/100 of the concentration needed for the WT. Finally, the quinone menadione was used as stressing agent for uncovering the molecular determinants driving Aspergillus flavus in challenging oxidative stress conditions by the host (Zaccaria et al., 2015, Toxins 7, 4315- 4329). Metabolic and transcriptional analyses were conducted. Under oxidative stress conditions, A. flavus proved to activate several metabolic processes for limiting the ROS associated detrimental effects, as well as for triggering adaptive and escape strategies, including aflatoxin B1 production. The results reported herein encourage investigation of mycotoxins from a plant pathologist perspective