Some Like it Hot: Efficiency of the Type III Secretion System has Multiple Thermosensitive Behaviours in the Pseudomonas syringae Complex.

The Pseudomonas syringae species complex is an important group of ubiquitous bacteria containing plant-pathogenic strains of which many strains cause damage and economic losses to a wide range of crops. Efforts to elucidate host range determinants have focused on the effector repertoires in the type 3 secretion system (T3SS). However, recently, we showed that the inability of a P. syringae pv. actinidiae strain to trigger effector-triggered immunity (ETI) in Arabidopsis thaliana is due to an inefficient T3SS and not to the absence of a recognised effector. We thus compared the T3SS efficiency of several P. syringae strains belonging to different phylogroups. We assessed the temporal dynamics of their ability to induce ion leakage, an indicator of the hypersensitive response (HR), in A. thaliana Col-0, as a proxy for T3SS function. Though not a direct measurement of T3SS efficiency, the use of a robust statistical model allowed us to reveal that P. syringae strains DC3000 avrB and M6 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 their in vitro growth optimum, even among quasiclonal 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 beyond model strains such as DC3000 and B728a that are not representative of the diversity of the P. syringae species complex.