Plant and animal-pathogenic bacteria utilize phylogenetically distinct type III secretion systems (T3SS) that produce needle-like injectisomes or pili for the delivery of effector proteins into host cells. Pantoea stewartii subsp. stewartii (Pnss), the causative agent of Stewart's bacterial wilt and leaf blight of maize, carries phylogenetically distinct T3SSs. In addition to an Hrc-Hrp T3SS, known to be essential for maize pathogenesis, Pnss has a second T3SS (PSI-2) that is required for persistence in its flea beetle vector, Chaetocnema pulicaria (Melsh). PSI-2 belongs to the Inv-Mxi-Spa T3SS family typically found in animal pathogens. Mutagenesis of the PSI-2 psaN gene, which encodes an ATPase essential for secretion of T3SS effectors by the injectisome, greatly reduces both the persistence of Pnss in flea beetle guts and the beetle's ability to transmit Pnss to maize. Ectopic expression of the psaN gene complements these phenotypes. In addition, the PSI-2 psaN gene is not required for Pnss pathogenesis of maize and is transcriptionally up-regulated in insects compared to maize tissues. Thus, the Hrp and PSI-2 T3SSs play different roles in the life cycle of Pnss as it alternates between its insect vector and plant host.
Valdir R. Correa, Doris R. Majerczak, El-Desouky Ammar, Massimo Merighi, Richard C. Pratt, Saskia A. Hogenhout, David L. Coplin and Margaret G. Redinbaugh