Abstract
Summary Trans -cinnamic acid and other metabolites of the phenylpropanoid pathway, namely p -coumaric, caffeic, ferulic and sinapic acids, inhibited the increase in extractable phenylalanine ammonia-lyase (PAL) activity present in sunflower hypocotyls in response to light-sucrose and excision-light-sucrose treatments. Otherwise, in vitro inhibitors of PAL activity, such as D-phenylalanine, and 2-aminooxyacetic and phenylpropiolic acids, superinduced extractable PAL activity and prevented the enzyme activity decay typically observed in the light-sucrose induction system. In both inducing treatments, delayed transfer of hypocotyl tissue to t -cinnamic acid caused a rapid decline in PAL activity, while a superinduction effect was observed upon transfer of the tissue to phenylpropiolic acid. These results indicate that in sunflower hypocotyls cinnamic acid(s) could act as in vivo modulator(s) of PAL, affecting the mechanisms responsible for both the increase and decrease in enzyme activity.
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