Sclerotinia sclerotiorum is a phytopathogenic ascomycete fungus with a host range of more than 400 plant species. S. sclerotiorum forms sclerotia, hard resting structures composed of interwoven hyphae with a melanin rind, which can stay dormant for years. Sclerotia formation is controlled by many environmental factors and internal signals, among them - Reactive Oxygen Species (ROS). ROS induces sclerotia formation, while antioxidants inhibit the process. Protein Tyrosin Phosphatases (PTPs) are susceptible to ROS, which can reversibly inhibit PTP activity. S. sclerotiorum ptp1 is highly expressed, specifically during sclerotia formation (30 fold when compared to hyphae). A ptp1 antisence mutant did not form sclerotia, and was not pathogenic. ptp1 expression increased significantly (more than 4 fold) as a result of sublethal 4mM H2O2 treatment. Growth of S. sclerotiorum in the presence of Pervanadate (1mM), a PTP inhibitor, was inhibited and accompanied by an increase in hyphal branching.
Differences in the hypersensitive response of different tomato varieties to S. sclerotiorum infection were associated with differences in the extent of the observed plant H2O2 burst. Though the expression of two fungal catalases was correlated with the levels of plant oxidative burst, no change in ptp1 transcript levels were observed. This indicates that the transcriptional response of ptp1 to ROS differs between the developmental and pathogenic processes. Taken together, ROS signaling/response may be differently modulated throughout the fungal life cycle.
