African strains of Xanthomonas oryzae pv. oryzae (Xoo) contain less 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 Xoo strain 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 BAI3R vs. BAI3RΔtalC were performed to identify genes targeted by TalC. 120 up-regulated and 21 down-regulated genes were identified, among them Os11N3 which is a member of the MtN3/saliva family. Based on semi-quantitative RT-PCR and β-glucuronidase (GUS) reporter assays, we show that Os11N3 is directly up-regulated by TalC and identify a TalC DNA target box within the Os11N3 upstream sequence.
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