Altered lignin composition in phenylalanine ammonia-lyase-inhibited radish seedlings: implications for seed-derived sinapoyl esters as lignin precursors.

We earlier reported that when phenylalanine ammonia-lyase (PAL) activity in radish seedlings was inhibited by the competitive inhibitor 2-aminoindan-2-phosphonic acid (AIP), soluble sinapoyl esters carried over from the seed were converted to wall-bound esters in young cotyledons. We now report that these soluble sinapoyl esters may also be converted into lignin in the cotyledons. When radish seedlings were grown in the presence of 100 microM AIP, lignin formation (determined as lignothioglycolic acid) was inhibited ca. 74% in the cotyledons and ca. 80% in hypocotyls plus roots. The syringyl to guaiacyl (S/G) ratio in the lignin of AIP-grown plants, as determined by alkaline cupric oxidation and from Fourier-transform infrared (FT-IR) spectra, was higher in cotyledons, but lower in hypocotyls plus roots, as compared to plants grown on distilled water. These results support the view that soluble sinapoyl esters preformed in seeds may contribute to the syringyl moiety of lignin in cotyledons during early seedling development and that there is no appreciable transport of soluble sinapoyl esters from cotyledons to the hypocotyls and roots.