Harpins, Multifunctional Proteins Secreted by Gram-Negative Plant-Pathogenic Bacteria

Abstract

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.

Comparative Secretome Analysis of Ralstonia solanacearum Type 3 Secretion-Associated Mutants Reveals a Fine Control of Effector Delivery, Essential for Bacterial Pathogenicity

“…All together, these data led us to hypothesize that RipX, RipW, RipAB, and RipAM could be involved, in an Hpa protein-independent manner, in the first stages of Rip translocation process. This hypothesis is in accordance with a polymutant approach in Pst DC3000 which highlighted a consortium of redundant translocators required to promote Pst T3E injection into plant cells (61,62). RipF1, RipW, RipX, RipAB, and RipAM all belong to the R. solanacearum core effectome (11).…”

Plant Aquaporin AtPIP1;4 Links Apoplastic H2O2 Induction to Disease Immunity Pathways

“…In plants, the rapid production of ROS, especially H 2 O 2 , represents the successful recognition of pathogen infection and pathogen-associated molecular patterns (PAMPs; Torres, 2010). Well-known examples of PAMPs are invariant microbial epitopes like fungal chitin (Kaku et al, 2006) and bacterial flagellin (Zipfel et al, 2004) and harpin (Sang et al, 2012;Choi et al, 2013) proteins. These PAMPs can be recognized by plasma membrane (PM) integral pattern receptors to induce immune responses (Ausubel, 2005), including H 2 O 2 production in plants (Felix et al, 1992;Levine et al, 1994Newman et al, 2013Galletti et al, 2011).…”

Grapevine Downy Mildew <b><i>Plasmopara viticola</i></b> Infection Elicits the Expression of Allergenic Pathogenesis-Related Proteins

“…While β-1,3-glucanases have been extensively described as allergens, no record describing the potential allergenicity of HBP1s has been found. The targets of HBP1 are harpins [11], protein elicitors produced by a number of phytopathogenic bacteria [11,12], that are considered virulence factors and can induce a strong hypersensitive reaction in plant tissues. HBP1s are expressed upon bacterial infection, but, in general, their upregulation can be also be a consequence of different stress conditions or treatments independent of the presence of harpin [13,14].…”