Type III secretion systems (T3SS) in phytopathogenic bacteria were first described in the 80s. However, whereas numerous characterization studies have revealed the basic processes of assembly, structure and function in animal/human pathogenic systems, our knowledge about these processes in plant type III biosystems is considerably small in comparison. Nonetheless, in recent years we have witnessed important breakthroughs in our understanding on how phytopathogens employ, build and regulate their T3SS: new master transcriptional regulators have been discovered, the activity of already described regulators of the system have been thoroughly investigated, quorum sensing regulators and population dynamics have been found to determine the fine activation of the system, new plant-derived signals have been found to upregulate the phytopathogenic T3SS, and more. Moreover, a considerable weaponry of effectors targeting and tuning the plant responses have been identified and protein components of the core machinery have been structurally described. This Research Topic aims to highlight the increasing efforts put into determining how these complex machineries work to deliver their effectors to manipulate plant responses, and hopes to help in integrating current and new findings into a better understanding of these systems. Manuscripts that deal with the structure of the apparatus, the secretion and translocation processes, or their regulation at any level are highly welcome.

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