HrpN of Erwinia amylovora functions in the translocation of DspA/E into plant cells

Abstract

The type III secretion system (T3SS) is required by plant pathogenic bacteria for the translocation of certain bacterial proteins to the cytoplasm of plant cells or secretion of some proteins to the apoplast. The T3SS of Erwinia amylovora, which causes fire blight of pear, apple and other rosaceous plants, secretes DspA/E, which is an indispensable pathogenicity factor. Several other proteins, including HrpN, a critical virulence factor, are also secreted by the T3SS. Using a CyaA reporter system, we demonstrated that DspA/E is translocated into the cells of Nicotiana tabacum'Xanthi'. To determine if other T3-secreted proteins are needed for translocation of DspA/E, we examined its translocation in several mutants of E. amylovora strain Ea321. DspA/E was translocated by both hrpW and hrpK mutants, although with some delay, indicating that these two proteins are dispensable in the translocation of DspA/E. Remarkably, translocation of DspA/E was essentially abolished in both hrpN and hrpJ mutants; however, secretion of DspA/E into medium was not affected in any of the mentioned mutants. In contrast to the more virulent strain Ea273, secretion of HrpN was abolished in a hrpJ mutant of strain Ea321. In addition, HrpN was weakly translocated into plant cytoplasm. These results suggest that HrpN plays a significant role in the translocation of DspA/E, and HrpJ affects the translocation of DspA/E by affecting secretion or stability of HrpN. Taken together, these results explain the critical importance of HrpN and HrpJ to the development of fire blight.

Pseudomonas syringae Effector Avirulence Protein E Localizes to the Host Plasma Membrane and Down-Regulates the Expression of the NONRACE-SPECIFIC DISEASE RESISTANCE1/HARPIN-INDUCED1-LIKE13 Gene Required for Antibacterial Immunity in Arabidopsis

“…One of the founding members of this gene family, NDR1, is a PM protein required for immunity as well as abiotic stress responses (Knepper et al, 2011). Another member of the NHL gene family, HIN1, was shown to be induced by harpins (Gopalan et al, 1996), which are the extracellular components of the T3SS (Kvitko et al, 2007;Bocsanczy et al, 2008;Boureau et al, 2011). In our study, we not only found that NHL13 is downregulated by AvrE and HopM1 but also found that the nhl13 mutants are compromised in disease resistance, especially to the avrE hopM1 mutant (Fig.…”

Protein Export According to Schedule: Architecture, Assembly, and Regulation of Type III Secretion Systems from Plant- and Animal-Pathogenic Bacteria

“…In contrast to Xanthomonas campestris pv. vesicatoria, several predicted translocon proteins from other plant-pathogenic bacteria are not essential for pathogenicity, suggesting that additional proteins such as harpins are involved in effector protein translocation (47,300,364,438). Harpins are small, heat-stable T3S substrates from plant-pathogenic bacteria that are rich in glycine and can elicit plant defense responses when infiltrating the plant apoplast at high concentrations.…”

Translocation of NopP by Sinorhizobium fredii USDA257 into Vigna unguiculata Root Nodules

“…T3SS-dependent translocation of effectors from phytopathogens into leaf mesophyll cells has been confirmed using the calmodulin-dependent adenylate cyclase toxin from Bordetella pertussis as a reporter (5,6,21,24,28,30). When Cya is fused to the C-terminal end of an effector, the resulting hybrid protein is delivered by the T3SS into host cells, where it binds to the eukaryotic protein calmodulin and produces cyclic AMP (cAMP) from ATP (33).…”