Inhibition of mRNA levels and activities by trans-cinnamic acid in elicitor-induced bean cells

Addition of trans -cinnamic acid to bean cell suspension cultures after their treatment with fungal elicitor resulted in the loss of induced phenylalanine ammonia-lyase (PAL) enzyme activity. In contrast elicitor-induced chalcone synthase (CHS) activity was arrested but did not decline, whereas chalcone isomerase (CHI) activity was relatively unaffected. However, translational activities of extracted polysomal mRNAs encoding these three enzymes were depressed by cinnamate treatments as were levels of PAL and CHS polysomal mRNAs and rates of transcription of these genes measured in isolated nuclei. Treatment of elicitor-induced cultures with l -α-aminooxy-phenylpropionic acid (AOPP), a potent inhibitor of PAL activity (and therefore cinnamate production) in vivo , resulted in increased production of PAL and CHS mRNAs. Addition of cinnamate to elicitor-treated cultures inhibited the appearance of a number of polypeptides translated in vitro from polysomal mRNA, although at least nine polypeptides were specifically induced by cinnamate treatment. We conclude that cinnamic acid potentially could act as an in vivo modulator of the synthesis of phenylpropanoid pathway enzymes although it is not yet fully possible to rule out less specific inhibitory effects.