Gram-negative phytopathogenic bacteria, all hemibiotrophs after all?

International audienceIrrespective of the bacteria considered, after an initial biotrophic phase, in which the bacteria avoid the elicitation of or suppress plant defences, a switch to necrotrophic stage, in which the bacteria actively kill plant cells, is observed. Both phases are required to induce disease although the length of time of each phase will vary between bacteria. The length of each phase will also vary for the same bacteria between experimental conditions, as the inoculum concentration, age of the plant and the inoculation procedure will influence the outcome of the interaction. Another difficulty arises because the switch between the initial biotrophic phase and the later necrotrophic phase cannot be visualised through microscopic examination, as it has been performed with fungi. This makes the necrotroph/biotroph classification highly hazardous for phytopathogenic bacteria. Furthermore, all phytopathogenic bacteria possess weapons to supress or avoid premature elicitation of plants defences, indicating they initially develop on living hosts, and weapons involved in cell death induction, probably resulting in nutrient acquisition. Therefore, phytopathogenic bacteria should always be considered as hemibiotrophs. Although the biotrophic phase has been thoroughly described for type III-dependent phytobacteria, it has been overlooked for type II-dependent soft rot pectinolytic bacteria, probably because the asymptomatic biotrophic phase is highly dependent on environmental conditions and is difficult to reproduce in laboratory conditions. Moreover, because the zig-zag model has been so powerful in understanding the biotrophic phase of type III-dependent phytobacteria, their necrotrophic phase is often underestimated. To achieve a better comprehension of plant-bacterial interactions, the hemibiotrophic nature of these interactions should be recognized