Molecular, physiological, and biochemical properties of sclerotia metamorphosis in Rhizoctonia solani

Rhizoctonia solani Kuhn (Basidiomycota, Cantharellales) is the main causal agent of rice sheath blight (RSB), which causes serious yield losses worldwide. The lack of rice varieties with resistance against RSB, which has a high sclerotia (dense masses of hyphal cells that function as compact survival structures in the fungal life cycle, enabling the fungus to endure adverse conditions and serve as reservoirs for subsequent growth and reproduction) survival rate, and the wide host range of R. solani, create basic challenges in the control of RSB. Overwinter sclerotium is the primary source of infection during the tillering stage of rice growth. In R. solani, a loose type of sclerotia is present. The sclerotia are primarily formed of compact masses of monilioid cells, but they may also be composed of undifferentiated hyphae. Three stages of sclerotial metamorphosis process are based on phenotypic changes, including the mycelium stage, the initial sclerotia stage (formation of monilioid cells), and sclerotia maturation. The metamorphosis of sclerotia involves the energy metabolism pathways and signal transduction pathways in the cell. In addition, there is evidence that the expression of genes encoding cell cycle activities may be important for sclerotia formation. During sclerotia metamorphosis, R. solani significantly expresses genes that encode antioxidants and respond to stimuli. The oxidative bursts begin in the initial sclerotia metamorphosis stage; at this time, reactive oxygen species (ROS) are mostly produced at the hyphal branches. In this sense, two classes of proteins related to glycosyltransferases B and the RNA recognition motif superfamily play a critical role in the sclerotial metamorphosis process in R. solani as scavengers of free radicals. The analysis of metabolic differences during sclerotia metamorphosis indicates that the NO metabolism may play an important role in sclerotia metamorphosis. Moreover, an increase in glycerophosphoethanolamines (PE) and glycerophosphoserines (PS) levels may indicate an advanced state of differentiation in mature R. solani sclerotia. Understanding the mechanisms involved in the sclerotial metamorphosis of R. solani can introduce new strategies for the management of RSB. In this review, we discuss the putative signaling and regulation mechanisms (such as quorum sensing) involved in the metamorphosis of sclerotia.