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The increase in extractable phenylalanine ammonia-lyase (PAL;EC 4.3.1.5.) activity induced in French bean cell suspension cultures in response to treatment with autoclaved ribonuclease A was inhibited by addition of the phenylpropanoid pathway intermediates cinnamic acid, 4-coumaric acid or ferulic acid. The effectiveness of inhibition was in the order cinnamic acid&gt;4-coumaric acid&gt;ferulic acid. Cinnamic acid also inhibited the PAL activity increase induced by dilution of the suspensions into an excess of fresh culture medium. Addition of low concentrations (&lt;10(-5)M) of the pathway intermediates to cultures at the time of application of ribonuclease gave variable responses ranging from inhibition to 30-40% stimulation of the PAL activity measured at 8 h. Following addition of pathway intermediates to cultures 4-5 h after ribonuclease treatment, rapid increases followed by equally rapid declines in PAL activity were observed. The cinnamic acid-stimulated increase in enzyme activity was unaffected by treatment with cycloheximide at a concentration which gave complete inhibition of the ribonuclease-induced response. However, cycloheximide completely abolished the subsequent decline in enzyme activity. Treatment of induced cultures with α-aminooxy-β-phenylpropionic acid (AOPPA) resulted in increased but delayed rates of enzyme appearance when compared to controls not treated with the phenylalanine analogue. The results are discussed in relation to current views on the regulation of enzyme levels in higher plants.

Modulation of L-phenylalanine ammonia-lyase by pathway intermediates in cell suspension cultures of dwarf French bean (Phaseolus vulgaris L.)