Mendoza-Herrera, Azucena

‘Candidatus Liberibacter solanacearum’ (Lso) is a phloem-limited bacterial plant pathogen infecting solanaceous plants in the Americas and New Zealand and is associated with diseases of apiaceous crops in Europe, Northern Africa, and the Middle East. This pathogen is also related to other Liberibacter species that infect other crops. In the USA, two haplotypes of Lso, LsoA and LsoB, are predominant and responsible for diseases in potato and tomato. Tobacco, Nicotiana tabacum, a model species to study plant defenses, is a host for Lso; therefore, the interaction between Lso and this host plant could be used to study Liberibacter−plant interactions. In this study, we characterized the infection associated with LsoA and LsoB in tobacco. Under laboratory conditions, LsoB caused more severe symptoms than LsoA, and LsoA and LsoB titers were dynamic during the 7 weeks of the experiment. We also measured SA and other metabolites, including oxylipins, at an early point of infection and found that SA was accumulated in plants infected with LsoB but not with LsoA; whereas ABA levels were reduced in LsoA- but not in LsoB-infected plants.

‘Candidatus Liberibacter solanacearum’ is a plant pathogen affecting the families Solanaceae and Apiaceae in different parts of the world. ‘Ca. L. solanacearum’ is a Gram-negative, fastidious α-proteobacterium that is vectored by different psyllid species. Plant-pathogenic bacteria are known for interfering with the host physiology or defense mechanisms, often by secreting bacterial effectors. Effector proteins are critical for virulence; therefore, the identification of effectors could help with disease management. In this study, we characterized the Sec-translocon-dependent ‘Ca. L. solanacearum’–hypothetical protein effector 1 (Lso-HPE1). We compared this protein sequence in the different ‘Ca. L. solanacearum’ haplotypes. We predicted the signal peptide and validated its function using Escherichia coli’s alkaline phosphatase fusion assay. Agrobacterium tumefaciens-mediated transient expression in Nicotiana benthamiana demonstrated that Lso-HPE1 from ‘Ca. L. solanacearum’ haplotypes A and B were able to inhibit the induction of cell death in plants. We also compared gene expression of the Lso-HPE1- transcripts in ‘Ca. L. solanacearum’ haplotypes A and B in tomato and in the vector Bactericera cockerelli. This work validates the identification of a Sec-translocon-dependent ‘Ca. L. solanacearum’ protein possibly involved in suppression of plant cell death.

‘Candidatus Liberibacter solanacearum’ is a plant pathogen associated with diseases affecting several crops of the Solanaceae and Apiaceae families. Two ‘Ca. L. solanacearum’ haplotypes (LsoA and LsoB) infect solanaceous crops in North America and are transmitted by the tomato psyllid Bactericera cockerelli. Although both ‘Ca. L. solanacearum’ haplotypes cause zebra chip in potato, the diseases associated with each haplotype in tomato (Solanum lycopersicum) have not been described. ‘Ca. L. solanacearum’-infected tomato plants exhibit symptoms resembling those of permanent yellowing disease (known in Mexico as “permanente del tomate”) and sometimes called psyllid yellows. In this study, the symptoms associated with each ‘Ca. L. solanacearum’ haplotype in tomato were compared, and the bacterial abundance in different nodes of the plants was measured by quantitative polymerase chain reaction. Surprisingly, both plant phenotype and bacterium distribution were different between LsoA- and LsoB-infected plants. Plants infected with LsoB died prematurely, whereas those infected with LsoA did not. Across the measured time points, LsoB abundance in infected plants was consistent with previous reports describing a sink to source gradient, while such gradient was only observed in LsoA-infected plants early after infection. This is the first report describing the differences in symptoms in tomato associated with two ‘Ca. L. solanacearum’ haplotypes, LsoA and LsoB.