Citrinin as an accessory establishment factor of P. expansum for the colonization of apples

Penicillium expansum is the causal agent of blue mold decay of apples. This fungal species can produce the two important mycotoxins patulin and citrinin. It was previously shown that patulin represents a colonization factor for the infection of apples. No definitive information about the importance of citrinin for the colonization of apples is currently available. The pksCT gene of the citrinin cluster codes for the citrinin polyketide synthase. Mutants of P. expansum in which the pksCT was inactivated showed a drastic decrease in the citrinin production. In addition, the pksCT mutants were also reduced in the ability to colonize apples. Externally added citrinin restored the capacity of the mutants to colonize apples roughly to that of the wild type. A kinetic analysis of the expression of the two respective pks genes of patulin (patK) and citrinin (pksCT) revealed that both genes are highly expressed in the first phase during the colonization process. The production of patulin in the apple matrix coincides with the expression of the patK gene. Almost no citrinin could be identified analytically during the first phase but only at a later stage of the colonization. It could be demonstrated that citrinin is degraded in apples and can tightly be bound to pectin. Overall the results suggest that citrinin may have an accessory function for the establishment of the colonization guided by other factors.