Effects of Xanthomonas arboricola pv. juglandis on walnut fruit

Walnut bacterial blight (WBB), caused by Xanthomonas arboricola pv. juglandis, is one of the most damaging diseases in walnut production, capable of reducing yields by up to 50%. Despite its economic importance, tissue-specific defense mechanisms in walnut fruit remain poorly understood. In this study, we performed an integrative approach with metabolomic and biochemical analyses of walnut fruit responses to WBB, examining primary and secondary metabolites, antioxidant activities, and enzyme dynamics across affected and non-affected tissues of the husk, stalk, as well as kernel pellicle and flesh. Using LC-MS, GC-MS, and spectrophotometric assays, we identified distinct compartmentalized defense strategies. The husk showed strong phenolic-mediated responses, with naphthoquinone levels increasing by 125%. However, oxidative processes in necrotic areas may have reduced the stability of these compounds by 50%. In contrast, affected kernel tissues exhibited lower phenolic activity, suggesting that carotenoids and fatty acids could be involved as a protection mechanism, undergoing early oxidation to help preserve lipid integrity. Antioxidant enzymes were upregulated in husk and kernel pellicle, while volatile terpenes and sugars accumulated in infected tissues, indicating metabolic shifts toward localized defense. Overall, our findings reveal a tissue-specific, multi-layered defense network in walnut fruit that could constitute specific markers associated with disease progression.