Pisatin is the major phytoalexin made by the pea plant Pisum sativum. It was the first phytoalexin to be purified and chemically identified. The molecular formula is C17H14O6. The biosynthesis of pisatin begins with the amino acid L-phenylalanine. A deamination reaction then produces trans-cinnamate, which undergoes hydroxylation to form 4-coumarate. Acetyl-CoA is then added to form 4-coumaryl-CoA. Three malonyl-CoA moities are then added and cyclized to introduce a phenol ring. An isomerization reaction then occurs, followed by a hydroxylation and rearrangement of the phenol group to form 2,4',7-trihydroxyisoflavonone. This molecule can then follow one of two paths, both of which include the loss of water and a methylation to produce formononetin. This product then undergoes hydroxylation to form calycosin, followed by the formation of a dioxolane ring.
-R160.gif)