Discovery and Biosynthesis of Antifungal Alteramide C from the Biocontrol Agent <i>Lysobacter</i> sp. SR01

Soybean anthracnose, caused by the fungus <i>Colletotrichum truncatum</i>, poses a serious threat to global soybean production. In this study, we isolated and characterized <i>Lysobacter</i> sp. strain SR01, a novel bacterium that exhibits strong antagonistic activity against <i>C. truncatum</i>. 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 <i>atc</i> gene cluster, which notably lacks the cyclase genes <i>OX1</i> and <i>OX4</i> required for the canonical heat-stable antifungal factor pathway. The heterologous expression of <i>OX4</i> 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.