Monitoring Indole Alkaloid Production by Penicillium digitatum During Infection Process in Citrus by Mass Spectrometry Imaging

Penicillium digitatum, responsible for the green mold disease, is the most severe postharvest pathogen of citrus fruits. Several phytopathogenic fungi produce secondary metabolites that have been associated to the pathogenicity. For P. digitatum, indole alkaloids, such as tryptoquialanines A and B, were reported as secondary metabolites, however, the tryptoquialanines are not required for P. digitatum pathogenicity and their exact biological role is still unknown.
Mass Spectrometry Imaging (MSI), a powerful technique that allows correlating compounds to their spatial distribution, was applied here to monitor the secondary metabolites produced by P. digitatum on the orange surface during infection process. Through MSI it was possible to report, for the first time, the high production of tryptoquialanines A and B and the intermediates of the tryptoquialanines biosynthetic pathway described for the first time in P. digitatum. It was also possible to observe the accumulation of the indole alkaloids on the fruit surface from 4 to 7 days post inoculation.
The biological role of tryptoquialanines was investigated, since they are intensely produced on citrus surface and are not essential for infection. Based on structure similarity, tryptoquialanine A can be considered a tremorgenic mycotoxin, acting on central nervous system of vertebrate animals. Thus, tryptoquialanine A was submitted to insecticidal bioassays. In 24 hours after the exposure to tryptoquialanine A, the mortality rate of Aedes aegypti larvae was 37 % and increased during the days, reaching 81 % on the fourth day. This result gives the first insights about the tryptoquialanines biological role, indicating a very pronounced insecticidal activity and suggesting that indole alkaloids may be important as a biocontrol against insects during orange decay and therefore are intensely produced on fruits surface.