Exudates secreted: Penicillium expansum's means of conquering the external environment

Abstract

Penicillium expansum, the causal agent of apple blue rot, produces patulin, a well-known mycotoxin regulated in many countries. When grown on various synthetic media, P. expansum secretes exudates. The release of the latter has been previously described in fungi and contribute to their virulence by secreting toxic specialized metabolites (SMs) into the external environment. Exudates are no longer produced in a mutant deleted for PatL, a transcription factor essential for patulin biosynthesis, suggesting a link between patulin and exudate production. In this context, the content of exudates produced by P. expansum was characterized by metabolomics and proteomics. These analyses revealed 20 SMs such as patulin and citrinin, and 194 proteins in the secretome, mainly involved in carbohydrate metabolism to degrade the plant wall, and to build or modify the fungal wall. Four of the secreted proteins (PatE, PatB, PatO, PatF) are involved in patulin biosynthesis, especially PatE, the last enzyme in the patulin biosynthetic pathway. The fact that its substrate, ascladiol, was also present suggests that the final step of patulin secretion may occur outside the fungus. In parallel, a comparative transcriptomic study of wild-type and null mutant ΔpatL strains unveiled that PatL negatively or positively regulated the expression of 294 genes. Among them, 21 genes encode proteins present in exudates, i.e. 11 % of the exudates, including four proteins involved in patulin biosynthesis, but also enzymes involved in starch degradation. Our results suggest a broader role for PatL than the sole regulation of patulin biosynthesis.