THE BIOCHEMICAL BASIS OF PATHOGENICITY AND HOST-SPECIFICITY OF PENICILLIUM DIGITATUM ON CITRUS

In this work we report that volatiles emitted from wounded citrus peel play a major role in host recognition by Penicillium digitatum. Volatiles of various citrus cultivars had a pronounced stimulatory effect on germination and germ tube elongation of green mold pathogen. When exposed to volatiles from grapefruit, the percentage of P. digitatum spores germinated on minimal media was 10-fold higher as compared to the control. In contrast, conidia germination and growth in non-host pathogens, Botrytis cinerea and Penicillium expansum, were either not affected or inhibited by the citrus peel volatiles. A GS-MS analysis of volatile compounds in the wound head space of various citrus fruit cultivars revealed that limonene is the major compound, suggesting it as a potential chemical regulator of germination in P. digitatum. After reaching the wound, to successfully colonize the entire fruit, P. digitatum needs to overcome defense mechanisms in the host tissue. Indeed, we found that P. digitatum actively suppresses a defense-related hydrogen peroxide (H₂O₂) burst in the citrus peel. In contrast, inoculation of citrus fruit with a non-pathogenic fungus, Penicillium expansum, triggers a massive production of H₂O₂ in host cells. Important, that initially (8 to 17h after inoculation) both fungi trigger an elevation in H₂O₂ levels in lemon peel disks. Later, approximately 25h after inoculation P. digitatum succeeds to significantly suppress H₂O₂ production by host cells, while in discs inoculated with P. expansum, the level of H₂O₂ was 2.5-fold above the control value at this time point. Suppression of H₂O₂ production in host tissue by exogenous citric acid significantly (P≤0.05) enhanced pathogenicity of P. digitatum and even allowed a non-pathogenic P. expansum to develop large lesions on lemons, oranges and grapefruits. These results, together with recent reports suggesting the potential involvement of citric acid in green mold pathogenesis, indicate that the ability to suppress hydrogen peroxide production in host tissue plays an important role in pathogenicity of P. digitatum on citrus fruit.