Confocal microscopy reveals in planta dynamic interactions between pathogenic, avirulent and non‐pathogenic Pseudomonas syringae strains

Abstract

Recent advances in genomics and single-cell analysis have demonstrated the extraordinary complexity reached by microbial populations within their hosts. Communities range from complex multispecies groups to homogeneous populations differentiating into lineages through genetic or non-genetic mechanisms. Diversity within bacterial populations is recognized as a key driver of the evolution of animal pathogens. In plants, however, little is known about how interactions between different pathogenic and non-pathogenic variants within the host impact on defence responses, or how the presence within a mixture may affect the development or the fate of each variant. Using confocal fluorescence microscopy, we analysed the colonization of the plant apoplast by individual virulence variants of Pseudomonas syringae within mixed populations. We found that non-pathogenic variants can proliferate and even spread beyond the inoculated area to neighbouring tissues when in close proximity to pathogenic bacteria. The high bacterial concentrations reached at natural entry points promote such interactions during the infection process. We also found that a diversity of interactions take place at a cellular level between virulent and avirulent variants, ranging from dominant negative effects on proliferation of virulent bacteria to in trans suppression of defences triggered by avirulent bacteria. Our results illustrate the spatial dynamics and complexity of the interactions found within mixed infections, and their potential impact on pathogen evolution.

Inositol hexakisphosphate biosynthesis underpins PAMP‐triggered immunity to Pseudomonas syringae pv. tomato in Arabidopsis thaliana but is dispensable for establishment of systemic acquired resistance

“…Furthermore, the vascular and hydathode-specific expression of the IPS2 and IPS3 genes in arabidopsis (Donahue et al, 2010) strongly suggests a critical involvement of InsP 6 biosynthesis in specific cells and tissues that can act as barriers to pathogen ingress (hydathodes) or dissemination through the host (vasculature). Hydathodes are open pores on the leaf margin that provide an entry point for bacterial pathogens such as Pst into the apoplast (Rufián et al, 2018). Elevated levels of defence-related gene expression in cells surrounding hydathodes has been previously reported indicating a role of localized constitutive defence around these vulnerable openings (Macaulay et al, 2017;van den Burg et al, 2010).…”

Bacteria from four phylogroups of the Pseudomonas syringae complex can cause bacterial canker of apricot

“…This implies that large populations of bacteria can coexist in host plants, and that complex interactions between different strains can develop, as shown recently by Rufián et al . (). However, it is also possible that different mechanisms of host colonization and infection coexist in the P. syringae complex (Marcelletti & Scortichini, ); genetic traits giving the ability to cause apricot disease might have emerged independently in bacterial reservoirs.…”

The ecological genetics of Pseudomonas syringae from kiwifruit leaves

“…Studies exploring the dynamics of mixed infections have demonstrated inoculum density-dependent effects on colonization (Young, 1974;Macho et al, 2007). The ability of nonpathogenic P. syringae to colonize wider territories in the presence of a pathogenic strain was nicely demonstrated in a confocal microscopy study (Rufi an et al, 2017). It is possible that P. syringae G33C benefits from the virulence activity of Psa NZ54.…”

“…Studies of pathogen populations rarely take into consideration co-occurring commensal types and yet such types are likely to be important contributors to population structure and infection progress (Lindow and Brandl, 2003;Demba Diallo et al, 2012;Bartoli et al, 2015;Buonaurio et al, 2015;Rufi an et al, 2017). Here, with focus on P. syringae, we have combined traditional population genetic approaches with experiments designed to investigate interactions among members of an ecologically cohesive population.…”

“…in cankers, enhancing the severity of disease (Marchi et al, 2006;Moretti et al, 2011;Buonaurio et al, 2015). Synergistic interactions can also be exploitative: bacteria lacking virulence factors can reap benefits from co-existing pathogenic isolates (Young, 1974;Hirano et al, 1999;Macho et al, 2007;Rufi an et al, 2017).…”