Heat shock treatments delay the increase in wound-induced phenylalanine ammonia-lyase activity by altering its expression, not its induction in Romaine lettuce (Lactuca sativa) tissue

Wounding lettuce (Lactuca sativa L., var. Longifolia) leaves induced an eight-fold increase in the activity of phenylalanine ammonia-lyase (PAL; EC 4.3.1.5), and the subsequent accumulation of phenolic compounds and tissue browning at 10 C. PAL is a key enzyme in the synthesis of phenolic compounds. A PAL cDNA was previously isolated by reverse-transcription PCR using total RNA from wounded lettuce leaves. RNA gel blots showed that maximum accumulation of both PAL mRNA and PAL enzyme activity occurred 24 h after wounding. A 2-min heat shock at 45 C administered within 5min of wounding delayed the woundinduced increase in PAL activity, but did not delay the increase in wound-induced PAL mRNA. Changes in the content of PAL protein were also followed by immunoblot using anti-PAL antibody raised against the bacterially expressed protein from the cDNA. Immunoblots showed that the level of PAL protein in wounded lettuce tissue was significantly reduced by the heat shock treatment. These data suggest that heat shock reduces the rise in woundinduced PAL enzyme activity by reducing the translation of wound-induced PAL mRNA, or by increasing the turnover of the induced PAL protein.