Effects of walnut bacterial blight on walnut fruit : insights into the tissue-specific metabolic response

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, knowledge of tissue-specific defense mechanisms in walnut fruit remains limited. In this study, we performed an integrative metabolomic and biochemical analysis of walnut fruit responses to WBB, focusing on primary and secondary metabolites, antioxidant activities, and enzyme dynamics across affected and non-affected tissues of the husk, stalk, as well as kernel skin and flesh. Using LC-MS, GC-MS, and spectrophotometric assays, we identified distinct compartmentalized defense strategies. The husk displayed strong phenolic-mediated responses, including naphthoquinone accumulation by 125%, though oxidative stress in necrotic areas reduced their stability and content by 50%. In contrast, kernel tissues showed reduced phenolic activity but relied on carotenoids and fatty acids, with carotenoids oxidizing early to preserve lipid integrity. Antioxidant enzymes were upregulated in husk and kernel skin, while volatile terpenes and sugars accumulated in infected tissues, indicating metabolic shifts toward localized defense. Overall, our findings reveal a tissue-specific and multi-layered defense network in walnut fruit, providing insights for breeding programs and future WBB management strategies.