Evaluation of the role of autophagy in fungal development and pathogenesis

Abstract

Autophagy is a conserved bulk degradation process in eukaryotic cells. It serves as a major survival function during starvation stress and is important for proper growth and differentiation. The targets for autophagic degradation can be non-selective or highly selective, depending on the specific biological circumstance and/or the specific inducer involved. Autophagy was first identified and characterized in yeast and in animal cells. In recent years, autophagy has been studied in filamentous fungi and evidence shows that it plays an important role at multiple stages of fungal development. Autophagy induction in differentiated structures (including fused filaments, aerial hyphae, germ tubes, appressorium) in fungi has been observed, but limited information is available on the mechanism and functional role of autophagy in such processes.

In this study, an essential role for autophagy-assisted glycogen breakdown (glycogen autophagy) is described in the rice-blast fungus Magnaporthe oryzae. Glycogen is an important carbon store in the cell. During differentiation, increased demand for energy and/or cellular material may trigger large-scale glycogen breakdown. The conidiation defect in autophagy-deficient mutant, atg8-delta (ATG8 short for AuTophagy related Gene 8), could be significantly restored by exogenous addition of glucose or glucose phosphate-6 (G6P), indicating a role for carbon source utilization for autophagy in Magnaporthe. Characterization of a deletion mutant of GPH1 (Glycogen PHosphorylase 1), encoding glycogen phosphorylase, further suggests that vacuolar, but not cytosolic, degradation of glycogen is important for Magnaporthe conidiation. A vacuolar glucoamylase, Sga1 (Sporulation-specific GlucoAmylase 1), was identified as the enzyme carrying out vacuolar hydrolysis of glycogen and dependent on autophagy for getting access to the substrate. A cytosolic variant of Sga1 could bypass the requirement of autophagy for glycogen breakdown and thus restored conidiation in the atg8? mutant. Besides being important for conidiation, autophagy was also found to be essential for Magnaporthe pathogenesis.

To uncouple the various functions of macroautophagy (such as pexophagy, cytoplasm to vacuole transport etc), the Atg20 (Autophagy related gene 20) protein was characterized and found to be essential for Magnaporthe conidiation and pathogenesis. Although mediated by Atg20, pexophagy was not important for Magnaporthe development. It was unclear whether the Cvt pathway exists in Magnaporthe. In this study, the specific target(s) for Atg20 mediated transport were uncovered and Atg20-dependent endosomal sorting and retrieval trafficking were key to conidiation and/or pathogenesis in Magnaporthe.