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 ofwhich many strains cause damage and economic losses to a wide range of crops. Efforts to elucidate host range determinantshave focused on the effector repertoires in the type 3 secretion system (T3SS). However, recently, we showed that the inability ofa P. syringae pv. actinidiae strain to trigger effector-triggered immunity (ETI) in Arabidopsis thaliana is due to an inefficient T3SSand not to the absence of a recognised effector. We thus compared the T3SS efficiency of several P. syringae strains belonging todifferent phylogroups. We assessed the temporal dynamics of their ability to induce ion leakage, an indicator of the hypersensi-tive response (HR), in A. thaliana Col- 0, as a proxy for T3SS function. Though not a direct measurement of T3SS efficiency, theuse of a robust statistical model allowed us to reveal that P. syringae strains DC3000 avrB and M6 avrB consistently triggered astrong HR while other strains induced it at significantly different intensities depending on temperature. Among thermosensitivestrains, both low and warm temperature dependencies for T3SS efficiency were observed, irrespective of their in vitro growthoptimum, even among quasiclonal strains. These results reveal a strong, strain-specific regulatory role of temperature in effectorinjection and reinforce the importance of environmental factors in the outcome of plant–bacteria interactions. Moreover, thiswork highlights the need to study bacterial virulence beyond model strains such as DC3000 and B728a that are not representativeof the diversity of the P. syringae species complex.