Physiological Responses and Time-Course Transcriptomic Analysis of Morchella sextelata Strain Mel-21 During NaCl Exposure

Salt stress limits fungal growth and distribution, but the mechanisms underlying salt tolerance in Morchella remain unclear. Here, we combined physiological measurements, transcriptomic analysis, weighted gene co-expression network analysis (WGCNA), and heterologous expression assays to characterize salt-stress responses in Morchella sextelata. Among 24 preselected isolates representing four Morchella species, M. sextelata Mel-21 obtained the highest composite screening score under 1 mol·L−1 NaCl and was selected for detailed analysis. A concentration of 300 mmol·L−1 NaCl was subsequently selected as the representative stress condition. Salt stress increased electrical conductivity, osmotic pressure, Na+ accumulation, antioxidant enzyme activities, and proline content, while decreasing the K+/Na+ and Ca2+/Na+ ratios. Intracellular and extracellular polysaccharides showed distinct time-dependent accumulation patterns, and the corresponding crude extracts exhibited increased radical-scavenging activities under NaCl stress. Relative to the common 0 h baseline, RNA-seq identified 3654, 3843, 4289, 3566, and 2494 differentially expressed genes after 3, 6, 12, 24, and 48 h of NaCl exposure, respectively. These genes were enriched in pathways related to ribosomes, oxidative phosphorylation, protein processing in the endoplasmic reticulum, carbohydrate metabolism, and ABC transporters. Gene05935, encoding a predicted multi-pass transmembrane protein, showed consistently higher transcript abundance during NaCl exposure than at the common 0 h baseline. Heterologous expression of Gene05935 in Saccharomyces cerevisiae was associated with higher mean endpoint OD600 values under the tested high-salinity conditions, while the Arabidopsis thaliana assay provided preliminary qualitative observations of seedling appearance during NaCl exposure. Because time-matched untreated controls were not included, the transcriptomic differences cannot be attributed specifically to NaCl. Together, the physiological measurements and temporal expression profiles identify Gene05935 as a candidate for further functional characterization and may inform future cultivation-oriented evaluation of Morchella under saline conditions.