Discovery of the Hrp Type III Secretion System in Phytopathogenic Bacteria: How Investigation of Hypersensitive Cell Death in Plants Led to a Novel Protein Injector System and a World of Inter-Organismal Molecular Interactions Within Plant Cells.

In the early 1960s. Pseudomonas srringae and other host-specific phytopathogenic proteobacteria were discovered to elicit a rapid. resistanceassociated death when infiltrated at high inoculum levels into nonhost tobacco leaves. This hypersensitive reaction (or response: HR) was a useful indicator of basic pathogenic ability. Research over the next 20 years failed to identi fy an elicitor of the HR but revealed that its elicitation required contact between metabolically active bacterial and plant cells. Beginning in the early 1980%. molecular genetic tools were applied to the HR puzzle, revealing the presence in P syringae of clusters of h, ·p genes. so named because they are required for the HR and pathogenicity. and of aw· genes. so named because their presence confers HR-associated avirulence in resistant cultivars of a host plant species. A series of breakthroughs over the next two decades revealed that (i) hip gene clusters encode a type III secretion system (T3SS), which injects Avr (now "effector") proteins into plant cells, where their recognition triggers the HR: (ii) TJSSs, which are typically present iii pathogenicity islands acquired by horizontal gene transfers, are found in many bacterial pathogens of plants and animals and inject many effector proteins. which are collectively essential for pathogenicity; and (iii) a primary function of phytopathogen effectors is to subvert non-HR defenses resulting from recognition of conserved microbial features presented outside of plant cells. In the 2000s. Hrp system research shifted to extracellular components enabling effector delivery across plant cell walls and plasma tnembranes. regulation, and tools for studying effectors. [ABSTRACT FROM AUTHOR]