Polyphenol oxidase from hybrid poplar. Cloning and expression in response to wounding and herbivory

The inducible expression of polyphenol oxidase (PPO), a presumed antiherbiv ore enzyme, was examined in hybrid poplar (Populus trichocarpa x Populus de ltoides). Following mechanical wounding simulating insect damage, PPO activ ity increased dramatically in wounded and unwounded leaves on wounded plant s beginning at 24 and 48 h, respectively. A hybrid poplar PPO cDNA was isol ated and its nucleotide sequence determined. On northern blots, PPO transcr ipts were detected within 8 h of wounding, and reached peak levels at 16 an d 24 h in wounded and unwounded leaves, respectively. Methyl jasmonate spra y and feeding by forest tent caterpillar also induced PPO expression. The i nduction of PPO was strongest in the youngest four leaves, which were gener ally avoided by caterpillars in free feeding experiments. This wound- and h erbivore-induced expression of PPO in hybrid poplar supports the defensive role of this protein against insect pests.