Regulation of phenylalanine ammonia‐lyase activity and growth in lettuce by light and gibberellic acid

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Abstract
The effects of light and gibberellic acid (GA3) on growth and phenylalanine ammonia‐lyase (PAL) activity were studied in seedlings of lettuce (Lactuca sativa L.). Using an in vivo assay for PAL it was shown that wounding caused by excising hypocotyls results in an increase in PAL activity with time that can mask the effect of light on the activity of this enzyme. When hypocotyl sections were excised from light‐treated seedlings immediately prior to the in vivo assay of PAL, light was shown to cause a marked increase in PAL activity. Experiments with an inhibitor of PAL activity, α‐aminooxy‐β‐phenylpropionic acid (AOPP), confirmed that the volatile radioactive products measured in the in vivo assay resulted from the activity of PAL. Gibberellic acid suppresses the light‐induced increase in PAL activity and there is an inverse relationship between GA3‐induced growth and the activity of PAL. Over a wide range of GA3 concentrations, the activity of PAL is also inversely correlated with growth rate along the length of the hypocotyl section; the upper halves of sections elongate more rapidly and have lower levels of PAL than the lower halves. Despite the strong correlation between growth and PAL activity, experiments with AOPP and t‐cinnamic acid show that it is unlikely that elongation is regulated directly by products of PAL activity.