Influence of oxidative stress on deoxynivalenol biosynthesis-related genes in Fusarium graminearum in vitro and in planta

Accumulation of mycotoxin deoxynivalenol (DON) by Fusarium graminearum is significantly modulated by oxidative stress through the action of ROS molecules, such as hydrogen peroxide (H2O2). Several studies have investigated the influence of H2O2 in increasing DON production during liquid culture of F. graminearum. Although this phenomenon is well documented in in vitro experiments, evidence during infection of plant is lacking. In this thesis study, the responsiveness to H2O2-oxidative stress by Fusarium graminearum in terms of DON production was evaluated on two local field isolates and one strain taken from an institutional collection. For this evaluation, the expression level of two genes involved in DON production (tri5 and tri10) and of a fungal gene involved in H2O2 detoxification (FgCAT1) was analyzed during in vitro fungal growth, in presence or absence of H2O2, along with the quantification of DON. These genes resulted up-regulated for two of the three strains analyzed, instead oxidative stress had an opposite effect on one field isolate significantly reducing its DON production. Subsequently, to verify if an increased concentration of ROS in plant tissues could be sensed by the fungal pathogen affecting its mycotoxigenic activity, in planta experiments were conducted. For this purpose, wheat seedlings were treated with polyethylene glycol (PEG) to induce an osmotic stress condition and promote H2O2 production in tissues. An increase in H2O2 production was detected in plant tissues five days after PEG treatment. The treated plants were inoculated with F. graminearum and both plant oxidative stress-responsive genes and the expression of a key gene (tri5) for DON biosynthesis were analyzed. None of wheat genes analyzed showed a significant differential expression over time after PEG treatment, although for two wheat genes (FgCat1 and MnSOD) the effect of PEG treatment on their expression level seems to correlate positively with the increase in H2O2 level detected in tissues, and similarly was observed for the fungal tri5 gene involved in DON biosynthesis. In conclusion, it was found that the positive effect observed in vitro by H2O2 on DON production is not a trait shared by all strains of F. graminearum. Finally, further investigations are needed to confirm the close correlation between ROS concentration in plant tissues and mycotoxin DON produced during the infection.