Some like it hot: efficiency of the type III secretion system has multiple thermosensitive behaviors in the Pseudomonas syringae complex

The Pseudomonas syringae complex is an important group of ubiquitous bacteria containing plant pathogenic strains of which many cause damage and economic losses to a wide range of crops. Efforts to elucidate the determinants of host range have focused on the repertoires of effectors in the Type 3 Secretion System (T3SS). However, recently, we showed that the inability of a P. syringae pv. actinidiae strain to trigger an hypersensitive response (HR) in A. thaliana is due to an inefficient T3SS and not to the absence of a recognized effector. In this context, we compared the efficiency of the T3SS of several P. syringae strains belonging to different phylogroups. Through statistical analysis, we assessed the temporal dynamics of the induction of ion leakage, an HR indicator, in A. thaliana. We revealed that PtoDC3000 avrB and PmaM6 avrB consistently triggered a strong HR while other strains induced it at significantly different intensities depending on temperature. Among thermosensitive strains, both low and warm temperature-dependencies for T3SS efficiency were observed, irrespective of the in-vitro growth optimum of the strains. Surprisingly, differences were also observed among quasi-clonal strains. These results reveal a strong, strain-specific regulatory role of temperature in effector injection and reinforce the importance of environmental factors in the outcome of plant-bacteria interactions. Moreover, this work highlights the need to study bacterial virulence for a broader set of strains beyond model strains, such as PtoDC3000, that are not representative of the whole P. syringae complex.