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

October 2013 , Volume 26 , Number  10
Pages  1,115 - 1,122

Min-Seon Choi,1 Wooki Kim,2 Chanhui Lee,3 and Chang-Sik Oh1

1Department of Horticultural Biotechnology, 2Department of Food Science and Biotechnology, and 3Department of Plant and Environmental New Resources, Kyung Hee University, Yongin, 446-701, Korea

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Accepted 28 May 2013.

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.

© 2013 The American Phytopathological Society