Optimization of Protoplast Preparation and Establishment of PEG-Mediated Genetic Transformation Method in Cordyceps cicadae

<i>Cordyceps cicadae</i> (<i>C. cicadae</i>) is an important edible medicinal fungus; however, owing to its wild growth and lack of genome annotation, construction of a stable genetic transformation system in <i>C. cicadae</i> is greatly limited, impeding the extensive exploitation of <i>C. cicadae</i> in industry. Here, we successfully established an efficient plasmid transformation method within protoplasts of <i>C. cicadae</i> by PEG mediation using pCas9-EGFP as a marker plasmid. In order to overcome low transformation efficiency and acquire sufficient protoplasts for transformation, the influence of enzyme species, enzymatic hydrolysis time, enzymatic hydrolysis temperature, and fungal age on protoplast preparation were analyzed sequentially, and the optimal conditions for protoplast preparation were determined as follows: 2-day-old <i>C. cicadae</i> mycelia with 1.5% lywallzyme hydrolysis at 34 °C for 5 h. Our results indicate that no less than 5.1 × 10⁷ CFU/mL protoplasts could be acquired. Additionally, five osmotic pressure stabilizers including potassium chloride (KCl), sodium chloride (NaCl), glucose, mannitol, and sucrose were employed to enhance the regeneration of protoplasts, among which sucrose exhibited the highest regeneration rate of 10.43%. The transformation efficiency of plasmid was 37.3 CFU/µg DNA. On this basis, a genetic transformation method was successfully constructed, laying the foundation for further gene editing and metabolic engineering of <i>C. cicadae</i>.