Analysis of the mechanisms of action of Mycosphaerella fijiensis toxins during the development of Black Leaf Streak disease.

The goal of our programme is to understand the role of M. fijiensis toxins in Black Leaf Streak disease to facilitate early selection of banana genotypes resistant to the disease. Toxic effects induced by crude extracts, as well as by purified juglone released by the pathogen in culture filtrates, were analysed on two reference cultivars of banana. Injection of the pathogen metabolites into banana leaves showed a light-dependent toxicity revealed by the development of necrosis and a rapid decrease of the relative vitality index determined from chlorophyll fluorescence measurements. We demonstrated that juglone, a purified metabolite of M. fijiensis, has a direct inhibitory effect on the electron transfer properties of purified banana chloroplasts. Regardless of the bioassay performed, the susceptibility of the two reference cultivars to this metabolite was correlated with their respective sensitivity to the pathogen infection. On the other hand, some preliminary data showed that juglone initiates oxidation of ascorbic acid, the most abundant antioxidant system in plants. On this base, we hypothesized that oxidative damage could be involved in the action of M. fijiensis toxins.