Deciphering Enzymatic and Phenolic Defence Signatures in Beans in Response to Halo Blight and Common Bacterial Blight Pathogens

Pseudomonas savastanoi pv. phaseolicola (Psp) and Xanthomonas axonopodis pv. phaseoli (Xap) are among the most economically significant bacterial pathogens of common bean (Phaseolus vulgaris L.), responsible for severe yield reductions worldwide. In this study, we investigated the biochemical and physiological basis of differential defence responses in the susceptible cultivar Aras 98 and the resistant genotype 36K following infection with Psp and Xap. Four-week-old plants of the resistant genotype 36K and susceptible cv. Aras 98 were separately inoculated with Psp (510-p and 522-p) or Xap (120-x and 145-x) at 108 CFU mL−1, with mock-inoculated plants serving as controls. Three independent biological replicates were used at each sampling time point, and leaf tissues were collected at 0, 12, 24, 36, and 72 h post-inoculation. The activities of Superoxide Dismutase (SOD), Polyphenol Oxidase (PPO) and Phenylalanine Ammonia-Lyase (PAL), as well as proline and total phenolic compounds, were subsequently evaluated. In addition, leaf surfaces collected 24 h after inoculation were examined using SEM to visualize bacterial colonization and biofilm formation. Results revealed stronger and time-dependent defence responses in 36K, where PPO reached 617.00 ± 16.62 Abs min−1 g−1 at 24 h after Xap (120-x). SOD, PAL, proline, and total phenolic responses also showed genotype- and time-dependent variation, with 36K generally exhibiting earlier or more pronounced defence-related responses than cv. Aras 98. The findings highlight that enhanced activation of biochemical defence mechanisms is closely associated with resistance to bacterial pathogens in common bean. These results further emphasize that the deployment of genetically resistant cultivars represents an effective, sustainable strategy for the management of bacterial diseases in bean production systems.