Ralstonia solanacearum is a devastating, soil-borne plant pathogen with a global distribution and an unusually wide host range. It is a model system for the dissection of molecular determinants governing pathogenicity. We present here the complete genome sequence and its analysis of strain GMI1000. The 5.8-megabase (Mb) genome is organized into two replicons: a 3.7-Mb chromosome and a 2.1-Mb megaplasmid. Both replicons have a mosaic structure providing evidence for the acquisition of genes through horizontal gene transfer. Regions containing genetically mobile elements associated with the percentage of G+C bias may have an important function in genome evolution. The genome encodes many proteins potentially associated with a role in pathogenicity. In particular, many putative attachment factors were identified. The complete repertoire of type III secreted effector proteins can be studied. Over 40 candidates were identified. Comparison with other genomes suggests that bacterial plant pathogens and animal pathogens harbour distinct arrays of specialized type III-dependent effectors.

anthomonas oryzae pv. oryzae (Xoo) is the etiological agent of bacterial blight disease in rice. The disease is most severe in southeast Asia but is increasingly damaging in west African countries, and results in substantial yield loss 1. TALes from Xoo are injected by a type III secretion system into plant cells and recognize effector-binding elements (EBEs) in cognate SWEET host gene promoters, which results in induction of SWEET genes and production of sugars that enable disease susceptibility in rice 2,3. An array of central repeats, which are 34-35 amino acids long, are present in each TALe and interact with EBEs via two repeat variable di-residues (RVDs) at the 12th and 13th position of each repeat 4,5. Aberrant repeats, longer than 35 amino acids, are hypothesized to allow looping out of the repeat to accommodate alternate sequence binding for a particular TALe 6. Bacterial blight depends on TALe-mediated induction of at least one member of a family of sugar-transporter genes. Although rice has more than 20 SWEET genes, only those of clade III are reported to be induced by Xoo 7-10. Although all five of the known clade III SWEET genes in rice can function as susceptibility genes for bacterial blight, only three are known to be targeted in nature 10. More specifically, SWEET11 expression is induced by strains encoding the TALe PthXo1, SWEET13 by PthXo2 and SWEET14 by any one of several TALes, namely AvrXa7, PthXo3, TalC and TalF (originally Tal5) 7,9-15 (Table 1). Effectors of Xoo that target clade III SWEET genes are referred to as major TALes owing to their strong virulence effect. Naturally occurring resistance has arisen as the result of nucleotide polymorphisms in EBEs of SWEET promoters. EBE alleles of SWEET11 that are not recognized by PthXo1 are collectively referred to as the recessive resistance gene xa13. Rice varieties containing xa13 are resistant to strains that solely depend on PthXo1 for SWEET induction. Most indica rice varieties carry a SWEET13 allele that contains four adenines in the EBE for PthXo2, and rice lines carrying this allele are susceptible to PthXo2-dependent strains 12. A rare exception is the recessive resistance allele xa25, which contains three adenines in the EBE for SWEET13 in the indica cultivar Minghui 63, conferring resistance to strains that depend solely on PthXo2 16. A similar recessive resistance allele in japonica rice varieties is equally effective against strains relying on PthXo2 (ref. 12). Additional naturally occurring recessive EBE polymorphisms that confer resistance to strains carrying PthXo2, and the overlapping EBEs for PthXo3, TalF and AvrXa7 have subsequently been identified in the promoters of SWEET13 and SWEET14, respectively, in germplasm collections 17,18. Rice susceptibility genes are good targets for genome editing for disease resistance. TALe-mediated susceptibility is particularly modifiable. For instance, transcription-activator-like effector nuclease (TALEN)-directed mutations in SWEET14 created lines resistant to strains carrying PthXo3/Avr...