Variations in type III effector repertoires, pathological phenotypes and host range of X anthomonas citri pv. citri pathotypes

Summary The mechanisms determining the host range of X anthomonas are still undeciphered, despite much interest in their potential roles in the evolution and emergence of plant pathogenic bacteria. X anthomonas citri pv. citri ( X ci ) is an interesting model of host specialization because of its pathogenic variants: pathotype A strains infect a wide range of R utaceous species, whereas pathotype A */ A W strains have a host range restricted to M exican lime ( C itrus aurantifolia ) and alemow ( C itrus macrophylla ). Based on a collection of 55 strains representative of X ci worldwide diversity assessed by amplified fragment length polymorphism ( AFLP ), we investigated the distribution of type III effectors ( T3E s) in relation to host range. We examined the presence of 66 T3E s from xanthomonads in X ci and identified a repertoire of 28 effectors, 26 of which were shared by all X ci strains, whereas two ( xopAG and xopC1 ) were present only in some A */ A W strains. We found that xopAG (= avrGf1 ) was present in all A W strains, but also in three A * strains genetically distant from A W , and that all xopAG ‐containing strains induced the hypersensitive response ( HR ) on grapefruit and sweet orange. The analysis of xopAD and xopAG suggested horizontal transfer between X . citri pv. bilvae , another citrus pathogen , and some X ci strains. A strains were genetically less diverse, induced identical phenotypic responses and possessed indistinguishable T3E repertoires. Conversely, A */ A W strains exhibited a wider genetic diversity in which clades correlated with geographical origin and T3E repertoire, but not with pathogenicity, according to T3E deletion experiments. Our data outline the importance of taking into account the heterogeneity of Xci   A */ A W strains when analysing the mechanisms of host specialization.