Genome mining and biosynthesis of a polyketide from a biofertilizer fungus that can facilitate reductive iron assimilation in plant

Significance Extensively used as a biofertilizer in agriculture, Trichoderma harzianum t-22 has been shown to promote plant fitness via secreting small molecules that have diverse functions. Tricholignan A, discovered via genome mining in this work, is a redox-active ortho -hydroquinone natural product that can facilitate reductive Fe assimilation in plant. The biosynthesis of the ortho -hydroquinone structure by a pair of polyketide synthases (PKSs) requires a critical Cα-methylation step that serves as the programming checkpoint. The responsible standalone methyltransferase requires chaperoning by a fused and inactive acyl carrier protein for activity. The use of an inactive carrier protein to regulate PKS function adds another layer of complexity to this already highly enigmatic family of enzymes.