Beyond the Halo: Molecular Mechanisms and Ecological Aspects of the Infection Strategy of Pseudomonas savastanoi pv. phaseolicola

Pseudomonas savastanoi pv. phaseolicola, the causal agent of halo blight of bean, is a useful model for studying bacterial adaptation to plant infection. Although its toxins, effectors, population dynamics, and epidemiology have been investigated for decades, these aspects have often been treated separately. This narrative synthesis reinterprets the available literature within a stage-based infection framework encompassing host-surface colonization, establishment of a compatible apoplastic habitat, evasion of host immune responses, multiplication, and spread to another compatible host. Particular attention was given to studies published after the last major review on this pathovar, whereas older studies were retained when they represented primary reports, classical experimental evidence, or information that has not been superseded. The review shows that successful infection is best understood as involving opportunistic entry, rapid transition from an epiphytic to a pathogenic lifestyle, effector-dependent immune suppression, phaseolotoxin-mediated manipulation of host primary metabolism, the exploitation or establishment of hydrated apoplastic conditions, and environmentally driven dissemination. It also distinguishes experimentally supported mechanisms from unresolved or hypothesis-generating areas, including stomatal reopening, micronutrient acquisition, population-level division of labor, alternative hosts, and symptomless reservoirs. The major synthesis emerging from this review is that the infection biology of P. savastanoi pv. phaseolicola cannot be reduced to individual virulence factors because disease development depends on how these factors are coordinated with host-derived resources, immune constraints, and environmental opportunities.