Sclerotial development in Sclerotinia sclerotiorum: awakening molecular analysis of a "Dormant" structure

Sclerotia are hard, asexual, resting structures which can survive for years in soil. In Sclerotinia sclerotiorum, which provides a good model system for studying sclerotial development, sclerotial development has been traditionally divided to three macroscopically distinct stages (initiation, development and maturation). However, additional phases (which can be visualized microscopically) indicate a complex, multi-step, process is involved. Environmental changes, primary metabolism and secondary messengers have been well documented factors affecting sclerotial development, yet analysis of the molecular mechanisms involved in sclerotial development is in its infancy. Here, we review the current status of the known molecular components involved in sclerotial development, with an emphasis on phosphorylative regulation of sclerotial development in S. sclerotiorum. Components such as cAMP-dependent protein kinase, ERK-like mitogen-activated protein kinase and Ser/Thr phosphatases type 2A and 2B, shown to regulate other developmental processes in fungi, have recently been shown to also be involved in regulation of sclerotium development. Reversible protein phosphorylation, as well as additional regulatory mechanisms of gene expression such as DNA methylation and ribosome inactivation, most likely function in concert with secondary metabolites, reactive oxygen species, pH and light in order to regulate sclerotial development in different fungi. The diversity of sclerotium-producing fungi promises to yield exciting variations into the molecular mechanisms regulating this developmental process in different species.