Discovery and Biosynthesis of Antifungal Alteramide C from the Biocontrol Agent Lysobacter sp. SR01

Soybean anthracnose, caused by the fungus Colletotrichum truncatum, poses a serious threat to global soybean production. In this study, we isolated and characterized Lysobacter sp. strain SR01, a novel bacterium that exhibits strong antagonistic activity against C. truncatum. Combined chemical and genomic analyses identified alteramide C and its precursor, 3-deOH alteramide C, as the key antifungal metabolites. These compounds are biosynthesized by a PoTeM-type atc gene cluster, which notably lacks the cyclase genes OX1 and OX4 required for the canonical heat-stable antifungal factor pathway. The heterologous expression of OX4 in SR01 redirected metabolite production toward HSAF-type scaffolds, demonstrating the genetic plasticity of the cluster. Purified alteramide C and 3-deOH alteramide C strongly inhibited mycelial growth and conidial germination in vitro and significantly reduced lesion development on soybean pods under both protective and curative conditions. Our findings reveal a lineage-specific metabolic shortcut that generates structurally simplified antifungal compounds, reflecting an evolutionary adaptation for efficient biosynthesis through a truncated pathway, and highlight the potential of these simplified molecules as sustainable candidates for the control of soybean anthracnose.