Some like it hot: efficiency of the Type 3 Secretion System and activation of plant defenses have multiple thermosensitive behaviors in thePseudomonas syringaecomplex

The Pseudomonas syringae species complex is a very important plant pathogenic bacterium, causing damage and economic losses on many crops. Research efforts for the understanding of the determinants of the host-range of a given strain are considerable. Recently, we showed that the inability of a P. syringae pv. actinidiae strain to trigger ETI in A. thaliana is due to an inefficient T3SS and not to the absence of a recognized effector. In this context, we compared several P. syringae strains belonging to different phylogroups and carrying the same plasmid-born avirulence gene for their ability to induce an HR in A. thaliana Col-0, as a marker of T3SS efficiency. Pto DC3000 AvrB and Pma M6 AvrB consistently triggered a strong HR while other strains induced it at different intensities significantly depending on temperature. Both behaviors of low and warm temperature-dependency for T3SS efficiency were observed among thermosensitive strains, irrespective of their in-vitro growth optimum. Surprisingly, differences were also observed among quasi-clonal strains. These results reveal a strongly strain-specific regulatory role of temperature in effector injection and reinforce the notion that the presence/absence of effectors is not sufficient to predict the outcome of plant-bacteria interactions. Moreover, this work highlights the necessity to study bacterial virulence in a broader set of strains insofar as Pto DC3000 is a reliable model strain but not representative of the P. syringae complex. Author summary Pseudomonas syringae is a bacterium ubiquitous in the environment and responsible for important crop losses worldwide. Moreover, P. syringae serves as a model organism for studying plant-pathogen interactions. Bacteria rely on various virulence mechanisms to successfully infect their host, one of the most important of which is the Type III Secretion System (T3SS). Although it is acknowledged that environmental conditions play a significant role in the plant-pathogen interactions, our understanding of their influence on both partners is still limited. Regarding temperature, contrasting findings have been reported about its influence on T3SS efficiency. While most efforts have focused on a few strains of P. syringae , the phylogenetic diversity of this bacterium is enormous. Consequently, we decided to characterize the role of temperature on T3SS efficiency in P. syringae in a broader way. Our results showed that bacterial behavior in response to temperature is highly variable among strains, even among very closely related ones. Our study provides new insights into the role of temperature in regulating bacterial virulence and further evidence of the need to investigate the diversity of pathogenic organisms beyond model strains.