Sclerotial metamorphosis in filamentous fungi is induced by oxidative stress

Sclerotium-forming filamentous fungi are of great agriculturaland biological interest because they can be viewed as modelsof simple metamorphosis. They differentiate by asexually producingsclerotia but the processes involved in sclerotial metamorphosiswere poorly understood. In 1997, it was shown for the firsttime that the sclerotial differentiation state in Sclerotiumrolfsii concurred with increasing levels of lipid peroxides.This finding prompted the development of a theory supportingthat sclerotial metamorphosis is induced by oxidative stress.Growth factors that reduce or increase oxidative stress areexpected to inhibit or promote sclerotium metamorphosis, respectively.This theory has been verified by a series of published dataon the effect of certain hydroxyl radical scavengers on sclerotialmetamorphosis, on the identification and quantification of certainendogenous antioxidants (such as ascorbic acid, ß-carotene)in relation to the fungal undifferentiated and differentiatedstates, and on their inhibiting effect on sclerotial metamorphosisas growth nutrients. In 2004–2005, we developed assaysfor 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 toinitiate studies on the exact role of glutathione, hydrogenperoxide, and superoxide radical on sclerotial metamorphosis.The emerging data, combined with similar data from other better-studiedfungi, allowed us to make some preliminary postulations on theROS-dependent biochemical signal transduction pathways in sclerotiogenicfilamentous fungi.