Constitutive and Induced Expression of Total Phenol and Phenol Oxidases in Wheat Genotypes Ranging in Resistance/Susceptibility to the Root-Lesion Nematode Pratylenchus thornei

Plant-derived phenolic compounds contribute to the defense against various pathogens, including root-lesion nematodes (Pratylenchus spp.). However, there are no reports on the role of phenolic compounds in wheat (Triticum aestivum) against Pratylenchus thornei. In this study, wheat genotypes ranging from resistant to very susceptible to P. thornei were used to investigate the level of total phenols and phenol oxidases, polyphenol oxidase (PPO), and peroxidase (POD) expressed in root tissues when grown in the presence and absence of P. thornei over time (2–8 weeks). Higher constitutive levels of total phenols were found in resistant synthetic hexaploid wheats CPI133872 (576 µg gallic acid equivalent (GAE)/g root) and CPI133859 (518 µg GAE/g root) at 8 weeks after sowing, compared with moderately resistant and susceptible genotypes (192 to 390 µg GAE/g root). The activity of PPO was induced in resistant (CPI133872) and moderately resistant (GS50a and its derivate QT8343) genotypes, becoming maximal at 4 weeks after P. thornei inoculation. The activity of POD was induced in CPI133872 at 6 weeks after P. thornei inoculation. Different genetic sources of resistance to P. thornei showed diverse defense mechanisms and differences in timing responses. The combined effects of total phenols and oxidative enzymes could be important for defense against P. thornei in some resistant wheat genotypes.