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PLANTS INTERACTING WITH OTHER ORGANISMS

NopL, an Effector Protein of Rhizobium sp. NGR234, Thwarts Activation of Plant Defense Reactions¹

Laboratoire de Biologie Moléculaire des Plantes Supérieures, Sciences III, Université de Genève, 1211 Genève 4, Switzerland (A.V.B., W.J.D., N.M.B., W.J.B., C.S.); Department of Microbiology, Centre of Legume Research, M409 Walters Life Science Building, University of Tennessee, Knoxville, TN 37996 (C.B.M., G.S.); and Federal Agronomy Research Station, Changins, 1260 Nyon, Switzerland (P.M.)

Bacterial effector proteins delivered into eukaryotic cells via bacterial type III secretion systems are important virulence factors in plant-pathogen interactions. Type III secretion systems have been found in Rhizobium species that form symbiotic, nitrogen-fixing associations with legumes. One such bacterium, Rhizobium sp. NGR234, secretes a number of type III effectors, including nodulation outer protein L (NopL, formerly y4xL). Here, we show that expression of nopL in tobacco (Nicotiana tabacum) prevents full induction of pathogenesis-related (PR) defense proteins. Transgenic tobacco plants that express nopL and were infected with potato virus Y (necrotic strain 605) exhibited only very low levels of chitinase (class I) and -1,3-glucanase (classes I and III) proteins. Northern-blot analysis indicated that expression of nopL in plant cells suppresses transcription of PR genes. Treatment with ethylene counteracted the effect of NopL on chitinase (class I). Transgenic Lotus japonicus plants that expressed nopL exhibited delayed development and low chitinase levels. In vitro experiments showed that NopL is a substrate for plant protein kinases. Together, these data suggest that NopL, when delivered into the plant cell, modulates the activity of signal transduction pathways that culminate in activation of PR proteins.

¹ This work was supported in part by the Swiss National Science Foundation (grant no. 3100–063893) and by the Université de Genève. Work in the Stacey laboratory was funded by the United States Department of Energy, Basic Energy Biosciences Program (grant no. DE–FG02–97ER20260).

² Present address: Centre National de la Recherche Scientifique, Institut des Sciences Végétales, Avenue de la Terrasse, F-91198 Gif sur Yvette, France.

³ Present address: Department of Microbiology and Immunology, School of Medicine, Universidad Central del Caribe, Call Box 60–327, Bayamon, Puerto Rico 00960–6032.

⁴ Present address: Department of Plant Microbiology and Pathology, 100 Waters Hall, University of Missouri, Columbia, MO 65211.

^* Corresponding author; e-mail william.broughton{at}bioveg.unige.ch; fax 41–22–3793009.

Received August 13, 2003; returned for revision September 15, 2003; accepted November 20, 2003.

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