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Insect-vectored plant bacterial pathogens are gaining attention in recent years due to crop threatening outbreaks around the world. Candidatus Liberibacter spp. are infecting crops of different botanical families: Solanaceae, Rutaceae, and Apiaceae and are vectored by psyllids. Five genetic haplotypes (A-E) have been described thus far for the species Ca. Liberibacter solanacearum (Lso). Haplotypes A and B infecting solanaceous plants, haplotypes C-E infecting Apiaceae crops. To better understand the genetic basis that governs host specificity of Lso haplotypes, we sequenced the genome of haplotype D (LsoD). The LsoD genome size is 1.23 Mbp, with a GC content of 34.8% and 1167 predicted genes. Enzyme Commission (EC) numbers were assigned using the JGI software tool and 358 ECs were identified. ECs were mapped to metabolic pathways and compared with other sequenced Liberibacters. Phylogenetic analysis based on ECs and assigned metabolic pathways shows that LsoD groups together with Lso haplotypes (A and B) and is clearly different than Liberibacter species infecting citrus. Differences between LsoD and LsoA/B haplotypes were also found, hinting on host specific enzymes. The LsoD genome was also scanned to identify putatively secreted proteins using the SignalP tool. Thirty-one putative genes were identified, most of them with unknown function. While some genes have homologous in other Lso haplotypes, some were unique to LsoD. By quantitative-PCR we examined the expression of the putatively secreted proteins in the different hosts; the psyllid vector Bactericera trigonica, and carrot. Several genes with significantly higher expression levels in carrot compared with psyllid and vice versa were identified. These genes may have host specific functions. Overall, our analyses reveal genetic and metabolic elements differentiating the carrot-infecting Lso from Lso haplotypes infecting potato/tomato. Research is underway to identify the function of these elements.

In this review, we summarise the current knowledge on three pathogens that exhibit distinct tissue specificity and modes of pathogenesis in citrus plants. Xanthomonas axonopodis pv. citri causes canker disease and invades the host leaf mesophyll tissue through natural openings and can also survive as an epiphyte. Xylella fastidiosa and Candidatus Liberibacter are vectored by insects and proliferate in the vascular system of the host, either in the phloem (Candidatus Liberibacter) or xylem (X. fastidiosa) causing variegated chlorosis and huan-glongbing diseases, respectively. Candidatus Liberibacter can be found within host cells and is thus unique as an intracellular phytopathogenic bacterium. Genome sequence comparisons have identified groups of species-specific genes that may be associated with the particular lifestyle, mode of transmission or symptoms produced by each phytopathogen. In addition, components that are conserved amongst bacteria may have diverse regulatory actions underpinning the different bacterial lifestyles; one example is the divergent role of the Rpf/DSF cell-cell signalling system in X. citri and X. fastidiosa. Biofilm plays a key role in epiphytic fitness and canker development in X. citri and in the symptoms produced by X. fastidiosa. Bacterial aggregation may be associated with vascular occlusion of the xylem vessels and symptomatology of variegated chlorosis. Fil: Vojnov, Adrián Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Ciencia y Tecnología "Dr. César Milstein". Fundación Pablo Cassará. Instituto de Ciencia y Tecnología ; Argentina Fil: Do Amaral, Alexandre Morais. Ministerio da Agricultura Pecuaria e Abastecimento de Brasil. Empresa Brasileira de Pesquisa Agropecuaria; Brasil Fil: Dow, John Maxwell. National University of Ireland; Irlanda Fil: Castagnaro, Atilio Pedro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto Superior de Investigaciones Biológicas. Universidad Nacional de Tucumán. Instituto Superior de Investigaciones Biológicas; Argentina Fil: Marano, María Rosa. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Biología Molecular y Celular de Rosario. Universidad Nacional de Rosario. Facultad de Ciencias Bioquímicas y Farmacéuticas. Instituto de Biología Molecular y Celular de Rosario; Argentina

Insect-vectored plant bacterial pathogens are gaining attention in recent years due to crop threatening outbreaks around the world. Candidatus Liberibacter spp. are infecting crops of different botanical families: Solanaceae, Rutaceae, and Apiaceae and are vectored by psyllids. Five genetic haplotypes (A-E) have been described thus far for the species Ca. Liberibacter solanacearum (Lso). Haplotypes A and B infecting solanaceous plants, haplotypes C-E infecting Apiaceae crops. To better understand the genetic basis that governs host specificity of Lso haplotypes, we sequenced the genome of haplotype D (LsoD). The LsoD genome size is 1.23 Mbp, with a GC content of 34.8% and 1167 predicted genes. Enzyme Commission (EC) numbers were assigned using the JGI software tool and 358 ECs were identified. ECs were mapped to metabolic pathways and compared with other sequenced Liberibacters. Phylogenetic analysis based on ECs and assigned metabolic pathways shows that LsoD groups together with Lso haplotypes (A and B) and is clearly different than Liberibacter species infecting citrus. Differences between LsoD and LsoA/B haplotypes were also found, hinting on host specific enzymes. The LsoD genome was also scanned to identify putatively secreted proteins using the SignalP tool. Thirty-one putative genes were identified, most of them with unknown function. While some genes have homologous in other Lso haplotypes, some were unique to LsoD. By quantitative-PCR we examined the expression of the putatively secreted proteins in the different hosts; the psyllid vector Bactericera trigonica, and carrot. Several genes with significantly higher expression levels in carrot compared with psyllid and vice versa were identified. These genes may have host specific functions. Overall, our analyses reveal genetic and metabolic elements differentiating the carrot-infecting Lso from Lso haplotypes infecting potato/tomato. Research is underway to identify the function of these elements.