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Membrane-Targeted HrpN_Ea Can Modulate Apple Defense Gene Expression

¹INRA, UMR1345 Institut de Recherche en Horticulture et Semences, F-49071 Beaucouzé, France; ²Université d'Angers, UMR1345 Institut de Recherche en Horticulture et Semences, SFR 4207 QUASAV, PRES L'UNAM, F-49045 Angers, France; ³AgroCampus-Ouest, UMR1345 Institut de Recherche en Horticulture et Semences, F-49045 Angers, France; ⁴INRA, Laboratoire des Interactions Plantes-Pathogènes, F-75231 Paris, France

Fire blight caused by Erwinia amylovora is the major bacterial disease of tribe Maleae, including apple. Among the proteins secreted by this bacterium, HrpN_Ea, also called harpin, is known to induce hypersensitive response in nonhost plants and to form amyloid oligomers leading to pore opening in the plasma membrane and alteration of membrane homeostasis. To better understand the physiological effects of HrpN_Ea in the host plant, we produced transgenic apple plants expressing HrpN_Ea with or without a secretion signal peptide (SP). HrpN_Ea expressed with a SP was found to be associated within the membrane fraction, in accordance with amyloidogenic properties and the presence of transmembrane domains revealed by in silico analysis. Expression analysis of 28 apple defense-related genes revealed gene modulations in the transgenic line expressing membrane-targeted HrpN_Ea. While apple transgenic trees displaying a high constitutive expression level of SP-HrpN_Ea showed a slight reduction of infection frequency after E. amylovora inoculation, there was no decrease in the disease severity. Thus HrpN_Ea seems to act as an elicitor of host defenses, when localized in the host membrane.