Protocatechuic Acid, Ferulic Acid and Relevant Defense Enzymes Correlate Closely with Walnut Resistance to Xanthomonas arboricola pv. Juglandis

Abstract

Background: Juglans regia L. is an important nut tree that has a wide range of distribution in temperate regions of the world. In some walnut orchards, walnut blight can become a problematic disease that affects the growth of walnut trees. To explore the correlation between biochemical response and walnuts resistance, we inoculated four walnut cultivars with Xanthomonas arboricola pv. Juglandis (abv. Xaj). The walnut cultivars were, namely, ‘Xiangling’, ‘Xiluo 2’, ‘Yuanfeng’ and ‘Xifu 2’. Total phenol content (TPC) and total flavonoid content (TFC) were measured, whereby nine major phenolic compounds and several relevant enzymes were identified. Results: The results showed that the most resistant and susceptible walnut varieties were ‘Xiluo 2’ and ‘Xifu 2’ respectively. The reaction of walnut to Xanthomonas arboricola pv. Juglandis was characterized by the early accumulation of phenolic compounds in the infected site. The accumulation of phenolic compounds after infection was accompanied by an increase in reactive oxygen species (ROS). This was reflected in the catalytic effect of peroxidase (POD) on ferulic acid dimerization which contributed to walnut resistance against disease. This also meant a series of defense-related functions with a key role of polyphenol oxidase (PPO). The results revealed that the competitiveness of flavonoids were weakened in biosynthesis, thereby strengthening the synthesis of chemicals catalyzed by phenylalanine ammonia lyase (PAL) against walnut blight. The resistant variety ‘Xiluo 2’ had a significantly higher protocatechuic acid content, compared to other varieties, when measured on the 4th day after inoculation (dpi). Conclusion: Our findings confirmed that walnut blight resistance is a collective function of interactions between the accumulation of phenolic compounds in the phenylpropane metabolic pathway and the action of relevant defense enzymes.