Characterization of the Serralysin-Like Gene of 'Candidatus Liberibacter solanacearum' Associated with Potato Zebra Chip Disease

Differential expression of "Candidatus Liberibacter solanacearum" genes and prophage loci in different life stages of potato psyllid

Psyllid species, including the potato psyllid (PoP) Bactericera cockerelli (Sulc) (Triozidae) serve as host and vector of "Candidatus Liberibacter spp." ("Ca. Liberibacter"), which also infects diverse plant hosts, including citrus and tomato. Psyllid transmission of "Ca. Liberibacter" is circulative and propagative. The time of "Ca. Liberibacter" acquisition and therefore vector life stage most competent for bacterial transmission varies by pathosystems. Here, the potato psyllid-"Ca. Liberibacter solanacearum" (CLso) pathosystem was investigated to dissect CLso-prophage interactions in the tomato plant and PoP-psyllid host by real-time quantitative reverse transcriptase amplification of CLso genes/loci with predicted involvement in host infection and psyllid-CLso transmission. Genes/loci analyzed were associated with (1) CLso-adhesion, -invasion, -pathogenicity, and -motility, (2) prophage-adhesion and pathogenicity, and (3) CLso-lysogenic cycle. Relative gene expression was quantified by qRT-PCR amplification from total RNA isolated from CLso-infected 1st-2nd and 4th-5th nymphs and teneral adults and CLso-infected tomato plants in which CLso infection is thought to occur without SC1-SC2 replication. Gene/loci expression was host-dependent and varied with the psyllid developmental stage. Loci previously associated with repressor-anti-repressor regulation in the "Ca Liberibacter asiaticus"-prophage pathosystem, which maintains the lysogenic cycle in Asian citrus psyllid Diaphorina citri, were expressed in CLso-infected psyllids but not in CLso-infected tomato plants.

A computational and secretome analysis approach reveals exclusive and shared candidate type six secretion system substrates in Pectobacterium brasiliense 1692

The type 6 secretion system (T6SS) of Gram-negative bacteria (GNB) has implications for bacterial competition, virulence, and survival. For the broad host range pathogen, Pectobacterium brasiliense 1692, T6SS-mediated competition occurs in a tissue-specific manner. However, no other roles have been investigated. The aim of this study was to identify T6SS-associated proteins under virulence inducing conditions. We used Bastion tools to predict 1479 Pbr1692 secreted proteins. Sixteen percent of these overlap between type 1-4 secretion systems (T1SS-T4SS) and T6SS. Using label-free quantitative mass spectrometry of Pbr1692 T6SS active and T6SS inactive strains' secretomes cultured in minimal media supplemented with host extract, 49 T6SS-associated proteins with varied gene ontology predicted functions were identified. We report 19 and 30 T6SS primary substrates and differentially secreted proteins, respectively, in T6SS mutants versus wild type strains. Of the total 49 T6SS-associated proteins presented in this study, 25 were also predicted using the BastionX platform as T6SS exclusive and shared substrates with T1SS-T4SS. This work provides a list of Pbr1692 T6SS secreted effector candidates. These include a potential antibacterial toxin HNH endonuclease and several predicted virulence proteins, including plant cell wall degrading enzymes. A preliminary basis for potential crosstalk between GNB secretion systems is also highlighted.

A serralysin-like protein of Candidatus Liberibacter asiaticus modulates components of the bacterial extracellular matrix

Huanglongbing (HLB), the current major threat for Citrus species, is caused by intracellular alphaproteobacteria of the genus Candidatus Liberibacter (CaL), with CaL asiaticus (CLas) being the most prevalent species. This bacterium inhabits phloem cells and is transmitted by the psyllid Diaphorina citri. A gene encoding a putative serralysin-like metalloprotease (CLIBASIA_01345) was identified in the CLas genome. The expression levels of this gene were found to be higher in citrus leaves than in psyllids, suggesting a function for this protease in adaptation to the plant environment. Here, we study the putative role of CLas-serralysin (Las1345) as virulence factor. We first assayed whether Las1345 could be secreted by two different surrogate bacteria, Rhizobium leguminosarum bv. viciae A34 (A34) and Serratia marcescens. The protein was detected only in the cellular fraction of A34 and S. marcescens expressing Las1345, and increased protease activity of those bacteria by 2.55 and 4.25-fold, respectively. In contrast, Las1345 expressed in Nicotiana benthamiana leaves did not show protease activity nor alterations in the cell membrane, suggesting that Las1345 do not function as a protease in the plant cell. Las1345 expression negatively regulated cell motility, exopolysaccharide production, and biofilm formation in Xanthomonas campestris pv. campestris (Xcc). This bacterial phenotype was correlated with reduced growth and survival on leaf surfaces as well as reduced disease symptoms in N. benthamiana and Arabidopsis. These results support a model where Las1345 could modify extracellular components to adapt bacterial shape and appendages to the phloem environment, thus contributing to virulence.

Lso-HPE1, an Effector of 'Candidatus Liberibacter solanacearum', Can Repress Plant Immune Response

'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.

The First Purification of Functional Proteins from the Unculturable, Genome-Reduced, Bottlenecked α-Proteobacterium 'Candidatus Liberibacter solanacearum'

'Candidatus Liberibacter solanacearum' is an unculturable α-proteobacterium that is the causal agent of zebra chip disease of potato-a major problem in potato-growing areas, because it affects growth and yield. Developing effective treatments for 'Ca. L. solanacearum' has been hampered by the difficulty in functionally characterizing the proteins of this organism, largely because they are not easily expressed and purified in standard expression systems. 'Ca. L. solanacearum' has a reduced genome and its proteins are predicted to be prone to instability and aggregation. Among intracellular-dwelling bacteria, chaperone proteins are conserved and overexpressed to buffer against problems in protein folding. We mimicked this approach for expressing and purifying 'Ca. L. solanacearum' proteins in Escherichia coli by coexpressing them with chaperones. Neither of the representative 'Ca. L. solanacearum' enzymes, dihydrodipicolinate synthase (key in lysine biosynthesis) and pyruvate kinase (involved in glycolysis), were overexpressed in standard E. coli expression plasmids or strains. However, soluble dihydrodipicolinate synthase was successfully coexpressed with GroEL/GroES, while soluble pyruvate kinase was successfully coexpressed with either GroEL/GroES, dnaK/dnaJ/grpE, or a trigger factor. Both enzymes, believed to be key proteins for the organism, were purified by a combination of affinity chromatography and size-exclusion chromatography. Additionally, both 'Ca. L. solanacearum' enzymes are active and have the canonical tetrameric oligomeric structure in solution, consistent with other bacterial orthologs. This is the first study to successfully isolate and functionally characterize proteins from 'Ca. L. solanacearum'. Thus, we provide a general strategy for characterizing its proteins, enabling new research and drug discovery programs to study and manage the pathogenicity of the organism.

Candidatus Liberibacter solanacearum patógeno vascular de solanáceas: Diagnóstico y control

Candidatus Liberibacter solanacearum (CLso) es una bacteria fitopatógena Gram-negativa, limitada al floema en solanáceas y no cultivable in vitro. Es transmitida de manera vertical y horizontal por el psílido Bactericera cockerelli. En México se asocia como responsable de la enfermedad "permanente del tomate", "punta morada de la papa" (Zebra chip) y "variegado del chile". Los síntomas causados por la bacteria varían según el cultivar y la etapa de crecimiento del hospedante pero consisten principalmente en amarillamientos y deformación de la lámina foliar, debido a la alimentación del vector y la colonización del patógeno. Las infecciones ocasionadas por CLso reducen la calidad del producto y el valor comercial en el mercado. La presencia de esta bacteria ha sido detectada en los estados de Coahuila, Sinaloa y Guanajuato, México a través de técnicas moleculares; mientras que el control de la enfermedad se encuentra enfocado en el vector, mediante prácticas culturales y la aplicación de agentes químicos y biológicos. Por lo anterior el objetivo del trabajo es puntualizar la situación actual de la distribución de CLso en México, los métodos de diagnóstico y las estrategias para el manejo integrado de la enfermedad y el vector.

Infection by Candidatus Liberibacter solanacearum' haplotypes A and B in Solanum lycopersicum 'Moneymaker'

'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.