Role of the exopolysaccharide levan in the life cycle of Pseudomonas syringae pv. actinidiae biovar 3, a phytopathogenic member of the bacterial community of Pseudomonas sp. associated with kiwifruit

The spread of plant pathogens represents a growing threat to global agriculture, particularly in the context of changing environmental conditions. Among these, Pseudomonas syringae pv. actinidiae biovar 3 (Psa3), the causal agent of bacterial canker of kiwifruit, represents one of the most significant threats to kiwifruit production worldwide. The success of this pathogen is probably closely linked to its ability to adapt to fluctuating environmental conditions and to persist on plant surfaces, where complex interactions with abiotic factors and microbial communities occur. In this context, exopolysaccharides (EPSs) play a key role in bacterial ecology, contributing to surface colonization, protection against environmental stresses, and biofilm formation. Among EPSs, levan, a fructan polymer synthesized by levansucrase enzymes, has been suggested to be involved in the epiphytic and pathogenic lifestyle of several P. syringae strains. However, the occurrence of non-levan-producing strains within the P. syringae species complex indicates that levan is not universally required for pathogenicity but rather contributes to specific aspects of bacterial fitness. Moreover, its specific role and the mechanisms of its production and regulation remain only partially understood and are still poorly supported by experimental evidence. This study investigated the genetic and functional basis of levan metabolism in Psa3. After assessing the distribution and conservation of lsc genes within Psa3, the contribution of levan to biofilm formation, the ability to utilize levans as carbon sources, the effects of chemical and physical stressors on bacterial survival and the involvement of γ-levan and β-levan in leaf colonization were investigated. Overall, this work provides new insights into the role of levan in Psa3, improving our understanding of the mechanisms underlying bacterial survival and persistence in the kiwifruit environment and offering a basis for future studies aimed to a better understanding of pathogen ecology and to the improvement of disease management practices.