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Colonization of Rice Leaf Blades by an African Strain of Xanthomonas oryzae pv. oryzae Depends on a New TAL Effector That Induces the Rice Nodulin-3 Os11N3 Gene

¹UMR 5096 IRD-CNRS-U. Perpignan, Laboratoire Génome et Développement des Plantes, 911 Avenue Agropolis BP 64501, 34394 Montpellier Cedex 5, France; ²State Key Laboratory for Conservation and Utilization of Subtropical Agro-bioresources, Key Laboratory of Ministry of Education for Microbial and Plant Genetic Engineering, College of Life Science and Technology, Guangxi University, 100 Daxue Road, Nanning, Guangxi 530004, PR China; ³Department of Genetics, Martin-Luther-University Halle-Wittenberg, D-06120, Halle (Saale), Germany; ⁴Unité de Recherche en Génomique Végétale (URGV) UMR INRA 1165-CNRS 8114-UEVE 2, Rue Gaston Crémieux CP 5708 91057 Evry Cedex, France; ⁵UMR186 Résistance des Plantes aux Bio-agresseurs, Institut de Recherche pour le Développement, BP 64501, 34394 Montpellier Cedex 5, France

African strains of Xanthomonas oryzae pv. oryzae contain fewer TAL effectors than Asian strains, and their contribution to pathogenicity is unknown. Systematic mutagenesis of tal genes was used to decipher the contribution of each of the eight TAL effector paralogs to pathogenicity of African X. oryzae pv. oryzae BAI3. A strain mutated in talC was severely affected in the production of disease symptoms. Analysis of growth in planta upon leaf-clip inoculation showed that mutant bacteria multiplied only at the site of inoculation at the apex of the leaf, suggesting a requirement for talC during colonization of vascular tissues. Such tissue-specific effect of a tal mutant is a novel phenotype, which has not yet been characterized in other xanthomonads. Microarray experiments comparing the host response of rice leaves challenged with BAI3^R vs. BAI3^RΔtalC were performed to identify genes targeted by TalC. A total of 120 upregulated and 21 downregulated genes were identified, among them Os11N3, which is a member of the MtN3/saliva family. Based on semiquantitative reverse transcription-polymerase chain reaction and β-glucuronidase reporter assays, we show that Os11N3 is directly upregulated by TalC and identify a TalC DNA target box within the Os11N3 upstream sequence.