Ascorbic acid might play a role in the sclerotial differentiation of Sclerotium rolfsii

Certain phytopathogenic fungi differentiate by forming sclerotia by an unclear biochemical mechanism. We have proposed that sclerotial differentiation might be regulated by fungal antioxidant defense. Part of this defense might be ascorbic acid, which in its reduced form is a well-known antioxidant. This natural antioxidant was studied in Sclerotium rolfsii in relation to oxidative-growth conditions, developmental stages and strain-differentiating ability. The transition of a sclerotial strain from the undifferentiated to the differentiated stage was accompanied by a sharp shift in the ratio of reduced/oxidized ascorbate toward the oxidized form. Ascorbate profiles and lipid peroxidation levels were different between the sclerotial strain grown under high- and low-oxidative stress conditions, as well as between a nonsclerotial S. rolfsii strain grown under high-oxidative stress conditions. In addition, the ratio of reduced/oxidized ascorbate in the nonsclerotial strain remained unchanged throughout growth. Lipid peroxidation under high-oxidative stress conditions in sclerotial S. rolfsii colonies one day before differentiation was 3.6-fold higher than in same-day colonies of this strain grown under low-oxidative stress conditions and 2.5-fold higher than in similar-day colonies of the nonsclerotial strain grown under high-oxidative stress conditions. Exogenous ascorbate caused a concentration-dependent reduction of lipid peroxidation and a proportional inhibition of the degree of sclerotial differentiation in the sclerotial strain grown under high-oxidative stress conditions by lowering its lipid peroxidation before differentiation to levels similar to the strain grown under low-oxidative stress conditions and to the nonsclerotial strain. Ascorbic acid might be produced by the sclerotial strain to reduce oxidative stress, although less efficiently than the nondifferenting strain. The data of this study support our theory that oxidative stress might be the triggering factor of sclerotial differentiation in phytopathogenic fungi.