Sclerotial metamorphosis in filamentous fungi is induced by oxidative stress

Abstract

Sclerotium-forming filamentous fungi are of great agricultural and biological interest because they can be viewed as models of simple metamorphosis. They differentiate by asexually producing sclerotia but the processes involved in sclerotial metamorphosis were poorly understood. In 1997, it was shown for the first time that the sclerotial differentiation state in Sclerotium rolfsii concurred with increasing levels of lipid peroxides. This finding prompted the development of a theory supporting that sclerotial metamorphosis is induced by oxidative stress. Growth factors that reduce or increase oxidative stress are expected to inhibit or promote sclerotium metamorphosis, respectively. This theory has been verified by a series of published data on the effect of certain hydroxyl radical scavengers on sclerotial metamorphosis, on the identification and quantification of certain endogenous antioxidants (such as ascorbic acid, β-carotene) in relation to the fungal undifferentiated and differentiated states, and on their inhibiting effect on sclerotial metamorphosis as growth nutrients. In 2004-2005, we developed assays for the measurement of certain redox markers of oxidative stress, such as the thiol redox state, the small-sized fragmented DNA, and the superoxide radical. These new advances allowed us to initiate studies on the exact role of glutathione, hydrogen peroxide, and superoxide radical on sclerotial metamorphosis. The emerging data, combined with similar data from other better-studied fungi, allowed us to make some preliminary postulations on the ROS-dependent biochemical signal transduction pathways in sclerotiogenic filamentous fungi.