Effects of Salinity on Bacterial Spot Disease, Physiology, Growth, Fruit Quality, and Transcriptomic Responses in Tomato Plants

Soil salinity and bacterial spot of tomato (BST), caused by Xanthomonas perforans, are major abiotic and biotic stresses limiting tomato production, particularly in Florida. While their individual effects are well documented, the impact of soil salinity on BST has not yet been investigated. This greenhouse study evaluated how increasing irrigation water salinity (electrical conductivity [EC] = 0.5, 3, 5, or 7 dS m−1) affected tomato growth, physiology, BST severity, fruit quality, and transcriptomic responses. Salinity reduced plant growth and BST severity but did not directly affect X. perforans populations. Results indicated that reduced plant physiological activity (net CO2 assimilation [A], transpiration [E], and stomatal conductance [gs]) contributes to lower disease levels. Increased salinity led to more solute concentrations, altered sugar metabolism, and improved perceived taste, as supported by taste panel, osmolality, and transcriptomic analyses. They also showed that transcriptional responses to salinity (EC = 7 dS m−1) and X. perforans infection were strongly time-dependent. Salt-treated plants exhibited fewer differentially expressed genes following inoculation, whereas comparisons between EC 7-treated and control plants revealed extensive salinity-induced reprogramming. KEGG analysis indicated enrichment of photosynthesis, carbon metabolism, amino acid biosynthesis, and ribosome pathways, while defense-related pathways, including MAPK signaling and plant–pathogen interaction, were downregulated, suggesting that tomato prioritized adaptation to salinity over pathogen defense.