Studies of wound-induced phenylpropanoid metabolism in fresh-cut Romaine lettuce and its modification by heat shock

Wounding induces significant changes in the phenylpropanoid metabolism and therefore the quality of fresh-cut lettuce. A putative wound signal is though to be produced by wounding that stimulates phenylpropanoid metabolism; the first enzyme of which is phenylalanine ammonia-lyase (PAL). Exogenous applications of some currently known stress signaling molecules (i.e., jasmonic acid (JA), methyl jasmonate (MeJA), abscisic and salicylic acid) to fully mature leaves of Romaine lettuce did not induce changes in PAL activity similar to that induced by wounding. However, application of JA, McJA, or SA to the less mature, central leaves increased PAL activity; although the increase was far below that induced by wounding. The wound signal(s) produced in fresh-cut lettuce that induces PAL activity remains unknown.; A wound-inducible PAL gene (LsPAL1) was cloned and characterized. The protein sequence predicted for LSPAL1 is very similar to those predicted for other putative PAL genes of other species. Escherichia coli was transformed with an LsPAL1-maltose binding protein (MBP) construct. The fusion protein PAL-MBP had significant PAL activity. Polyclonal antibodies were raised against PAL-MBP and the antiserum was used in subsequent analyzes. Northern analyses demonstrated that after wounding PAL mRNA accumulates before the peak in induced PAL activity by ca. 12 h at 10°C. Tissue printing analysis showed that the wound-induced accumulation of lettuce PAL transcript(s) was associated with tissue close to the epidermis and vascular bundles in lettuce mid rib tissue.; The effect of heat-shock (HS) treatments was studied as an approach to lessen the impact of wounding on fresh-cut Romaine lettuce. A 2 min HS at 45°C caused a delay in the increase in PAL activity in wounded tissue. Experiments with anti-PAL polyclonal antibodies suggested that HS reduces the increase in wound-induced PAL protein in parallel with its effect on enzyme activity. However, northern blot analysis showed that HS did not reduce the accumulation of PAL mRNA following wounding. Therefore, the mechanism by which HS modifies the increase in PAL activity following wounding of lettuce remains unresolved.