Massetolide F Synthesized by <i>Pseudomonas extremorientalis</i> YL19 Inhibits the Wheat Pathogenic Fungus <i>Bipolaris sorokiniana</i>

Cyclic lipopeptides (CLPs), produced by diverse bacteria, including plant-associated <i>Pseudomonas</i> species, are bioactive metabolites with multiple functions that also benefit the host strain. Strain YL19, identified as <i>Pseudomonas extremorientalis</i>, showed strong antifungal activity against <i>Bipolaris sorokiniana</i>, the causal agent of wheat root rot. Its main secondary metabolite was purified through bioactivity-guided fractionation and identified as massetolide, marking the first report of its production by <i>P. extremorientalis</i>. In wheat pot experiments, the 100 mg massetolide F treatment reduced the disease index to 42.05, significantly lower than the diseased control. Massetolide F also displayed antitumor activity, with IC₅₀ values of 24 and 22 μg/mL against HeLa and HT1080 cells. Biosynthesis of massetolde F follows the classical Gac/Rsm pathway, but novel repressor of secondary metabolism (Rsm) binding sites were found in the 5'-UTR of <i>luxRU</i> and <i>luxRD</i> mRNA and LuxR regulators recognized unique promoter sequences unreported in other <i>Pseudomonas</i> that were identified, indicating a species-specific regulatory mechanism in YL19.