DIFFERENT APPROACHES TO STUDY THE INTERACTION CITRUS/PHOMA TRACHEIPHILA REVEAL AN IMPORTANT ROLE PLAYED BY OXIDATIVE STRESS

Mal secco is a disease of Citrus caused by the mitosporic ascomycete Phoma tracheiphila, a pathogen that produces a phytotoxic complex known as “malseccin”. Based on a six peptides sequence identified by Fogliano et al. (1998) several couples of degenerated primers were designed for attempting the isolation of a unique gene sequence coding for malseccin, either from genomic DNA or cDNA. A number of fragments obtained by PCR amplification of both templates was cloned and the conceptual translation of the obtained sequences showed significant homologies (>50- 60%) with iron membrane transporter, NADPH-dependent reductase, and a monoammine oxidase. Considering these results it is impossible to assign a unique nucleotide sequence corresponding to malseccin. A MALDI-TOF/TOF analysis of extracellular proteins of P. tracheiphila was carried out to evaluate the possible presence of more than a single protein in the 60 kDa band found by Fogliano et al. (1998). It was found that a complex of proteins with different functions is present in the single 60 kDa band, suggesting that more than one toxic component is involved in the genesis of mal secco disease, even though also this analysis detected the presence of a monoammine oxidase in this mixture. Physiological experiments have disclosed the activity of different enzymes related to the oxidative burst in different filtrates of P. tracheiphila, indicating that, in the tolerant lemon cv Monachello this fungus undergoes oxidative stress that affects its ability to invade the host.