Harpins are glycine-rich and heat-stable proteins that are secreted through type III secretion system in gram-negative plant-pathogenic bacteria. Many studies show that these proteins are mostly targeted to the extracellular space of plant tissues, unlike bacterial effector proteins that act inside the plant cells. Over the two decades since the first harpin of pathogen origin, HrpN of Erwinia amylovora, was reported in 1992 as a cell-free elicitor of hypersensitive response (HR), diverse functional aspects of harpins have been determined. Some harpins were shown to have virulence activity, probably because of their involvement in the translocation of effector proteins into plant cytoplasm. Based on this function, harpins are now considered to be translocators. Their abilities of pore formation in the artificial membrane, binding to lipid components, and oligomerization are consistent with this idea. When harpins are applied to plants directly or expressed in plant cells, these proteins trigger diverse beneficial responses such as induction of defense responses against diverse pathogens and insects and enhancement of plant growth. Therefore, in this review, we will summarize the functions of harpins as virulence factors (or translocators) of bacterial pathogens, elicitors of HR and immune responses, and plant growth enhancers.

Programmed cell death (PCD) is triggered when Pto, a Ser-Thr protein kinase, recognizes either the AvrPto or AvrPtoB effector from Pseudomonas syringae pv tomato. This PCD requires mitogen-activated protein kinase kinase kinase (MAPKKKa) as a positive regulator in tomato (Solanum lycopersicum) and Nicotiana benthamiana. To examine how PCDeliciting activity of the tomato MAPKKKa protein is regulated, we screened for MAPKKKa-interacting proteins in tomato and identified a 14-3-3 protein, TFT7. Virus-induced gene silencing using the TFT7 gene in N. benthamiana compromised both Pto-and MAPKKKa-mediated PCD, and coexpression of TFT7 with tomato MAPKKKa enhanced MAPKKKa-mediated PCD. TFT7 was also required for PCD associated with several other disease resistance proteins and contributed to resistance against P. syringae pv tomato. Coexpression of TFT7 with MAPKKKa in vivo caused increased accumulation of the kinase and enhanced phosphorylation of two MAP kinases. TFT7 protein contains a phosphopeptide binding motif that is present in human 14-3-3«, and substitutions in this motif abolished interaction with MAPKKKa in vivo and also the PCDenhancing activity of TFT7. A 14-3-3 binding motif, including a putative phosphorylated Ser-535, is present in the C-terminal region of MAPKKKa. An S535A substitution in MAPKKKa reduced interaction with TFT7 and both PCD-eliciting ability and stability of MAPKKKa. Our results provide new insights into a role for 14-3-3 proteins in regulating immunity-associated PCD pathways in plants.

The bacterial plant pathogen, Erwinia amylovora, causes the devastating disease known as fire blight in some Rosaceous plants like apple, pear, quince, raspberry and several ornamentals. Knowledge of the factors affecting the development of fire blight has mushroomed in the last quarter century. On the molecular level, genes encoding a Hrp type III secretion system, genes encoding enzymes involved in synthesis of extracellular polysaccharides and genes facilitating the growth of E. amylovora in its host plants have been characterized. The Hrp pathogenicity island, delimited by genes suggesting horizontal gene transfer, is composed of four distinct regions, the hrp/hrc region, the HEE (Hrp effectors and elicitors) region, the HAE (Hrp-associated enzymes) region, and the IT (Island transfer) region. The Hrp pathogenicity island encodes a Hrp type III secretion system (TTSS), which delivers several proteins from bacteria to plant apoplasts or cytoplasm. E. amylovora produces two exopolysaccharides, amylovoran and levan, which cause the characteristic fire blight wilting symptom in host plants. In addition, other genes, and their encoded proteins, have been characterized as virulence factors of E. amylovora that encode enzymes facilitating sorbitol metabolism, proteolytic activity and iron harvesting. This review summarizes our understanding of the genes and gene products of E. amylovora that are involved in the development of the fire blight disease.