Soluble sinapoyl esters are converted to wall-bound esters in phenylalanine ammonia-lyase-inhibited radish seedlings

Abstract

Between the third and eighth day of radish seedling growth, wall-bound esters of (E)-sinapic and (E)-p-coumaric acids increase about fourfold, (E)-ferulic acid about sixfold, in the cotyledons. In the hypocotyl plus roots, wall-bound ferulic and p-coumaric acids increase twofold but wall-bound sinapic acid levels are unchanged. When grown with 100 μM 2-aminoindan-2-phosphonic acid (AIP), a competitive inhibitor of phenylalanine ammonia-lyase (PAL), cotyledons had levels of wall-bound sinapic acid similar to controls but there was no detectable wall-bound p-coumaric acid and only traces of wall-bound ferulic acid. As seedlings developed, their decrease in soluble sinapoyl esters greatly exceeded levels of wall bound sinapic acid accumulated. We interpret this as evidence that de novo hydroxycinnamic acid synthesis was blocked and preformed soluble sinapoyl esters stored in the cotyledons were mobilized and esterified into developing cell walls. Growth with 5 μM AIP inhibited wall-bound p-coumaric and ferulic acid accumulation more than 90% in the hypocotyl plus roots and slightly increased hypocotyl elongation, suggesting that wall-bound phenolics may normally play a role in restricting radish hypocotyl cell wall extensibility.