A PHYSIOLOGICAL AND MOLECULAR APPROACH REVEALS A POSSIBLE MODE OF ACTION OF PHOMA TRACHEIPHILA IN ITS INTERACTION WITH LEMON

The mitosporic ascomycete Phoma tracheiphila causes “mal secco” disease of lemon (Citrus limon) and produces a 60 KDa toxic glycoprotein, denoted Pt60, responsible for most of the disease symptoms. These effects are enhanced when lemon leaves are illuminated, suggesting that the fungus may interfere with the photosynthetic process. Since ROS are hugely produced during photosynthesis, for disease induction P. tracheiphila has to cope with a highly oxidative environment. In vitro experiments were carried out inoculating P. tracheiphila in a culture medium supplemented with lyophilised lemon leaves (2% w/v). Antioxidative enzymatic activity [superoxide dismutase (SOD) and catalase (CAT)] was enhanced under these cultural conditions as compared with the control, suggesting that P. tracheiphila was under oxidative stress. To verify this hypothesis two lemon cultivars (Monachello and Femminello, kindly supplied by Dr. Germanà), with a different susceptibility to the pathogen, were tested. Enzymatic activities of P. tracheiphila [SOD, CAT, glutathione peroxidase (GPX), laccase, Mn peroxidase, and polygalacturonase)] and lemon [(lipoxygenase, polygalacturonase inhibiting protein (PGIP), SOD, CAT, GPX)] were analysed. A molecular approach was also followed to the aim of determining the complete sequence of the Pt60 gene, starting from six peptides previously identified (Fogliano et al., 1998). Three of the clones obtained showed some 50-60% similarity with proteins involved in redox reactions, i.e. a putative NADH-flavin reductase and a NADPHdependent reductase, supporting our hypothesis.