A conidial storage protein is an amino acid reserve for early-stage conidial germination

Dormant conidia are essential for fungal dispersal and survival under harsh environmental conditions. Upon encountering suitable conditions, they germinate to form intertwined mycelia, which then differentiate to generate new conidia. Protein synthesis is initiated rapidly upon conidial hydration, which precedes other metabolic activities and is a prerequisite for conidial germination. Here we report a small protein (SPIC) that serves as an amino acid source for this early protein synthesis in the endophytic insect-pathogenic fungus Metarhizium robertsii. SPIC is highly expressed during conidiation and accumulates abundantly in the cytoplasm of dormant conidia, constituting approximately 0.01% of conidial dry weight. Upon hydration, SPIC is degraded within one hour of germination. This degradation is accomplished by the subtilisin-like protease Pr1H. Pr1H is expressed as a precursor and becomes active via cleavage of its N- and C-terminal regions. The rapid liberation of amino acids from SPIC degradation by Pr1H timely supplies the early protein synthesis required for germination. SPIC degradation by Pr1H is autonomous and independent of external nutrient levels, thereby facilitating germination in the nutrient-poor environments where M. robertsii thrives, including insect cuticles and the plant rhizosphere. Our work identifies a storage protein and its degradation mechanism that supply amino acids for the critical early protein synthesis at the onset of conidial germination.