Regulatory adaptation of an accessory gene controls fungal halotolerance and niche expansion.

Stress tolerance underpins ecological plasticity and niche expansion in fungi. Although Trichoderma is best known from sylvan, mycoparasitic, soil-, and plant-associated habitats, its occasional recovery from saline soils raises questions about the molecular basis of this adaptation. A coastal survey revealed limited but persistent Trichoderma diversity with frequent recovery of halotolerant Trichoderma asperelloides. Screening a salt-stressed T. asperelloides cDNA library in Saccharomyces cerevisiae identified 62 inserts, of which 12 were salt responsive in vivo. Among then, deletion of gld1, encoding an aldo-keto reductase, impaired halotolerance and glycerol accumulation. In a 27-species synthetic community, the Δgld1 mutant was competitively displaced by other species under salinity. Cross-species promoter replacement in a salinity-sensitive strain Trichoderma atroviride increased its halotolerance. A global tef1 haplotype network placed the coastal isolates within broader T. asperelloides diversity, consistent with recurrent expansion into saline/coastal soils. Together, these findings link accessory-gene regulation to niche expansion in fungi.