Three polyphenol oxidases from hybrid poplar are differentially expressed during development and after wounding and elicitor treatment

The expression of three polyphenol oxidase (PPO; EC 1.10.3.1) genes was investigated in hybrid poplar (Populus trichocarpa · P. deltoides). PtdPPO1 was previously isolated as a woundand herbivore-inducible PPO (Constabel et al. Plant Physiol 124: 285–295, 2000), whereas PtdPPO2 and PtdPPO3 are two novel hybrid poplar PPO genes isolated from stem and root tissue, respectively. Sequence analysis revealed that the three PPOs were 60–66% identical at the amino acid level. Using genespecific probes, the expression patterns of the three PPOs was investigated in various organs at different developmental stages. Under normal growing conditions, PtdPPO1 mRNA was absent from all organs tested, while PtdPPO2 was found to be expressed in mid-veins, petioles, stems and roots. PtdPPO3 was expressed only in roots. PtdPPO1 and PtdPPO2 were induced by mechanical wounding and methyl jasmonate, but in different tissues. Overall, the expression patterns suggest that the three PPO genes may have specialized physiological functions in hybrid poplar.