During surveys carried out in 2014 and 2015 in Algerian potato fields, severe bacterial tuber soft rot was observed. Twenty-one Gram-negative pectinolytic bacterial isolates were obtained from diseased potato tubers. These induced a hypersensitive reaction and soft rot symptoms when inoculated in tobacco leaves and potato tubers, respectively. PCR amplification using the Y1/Y2 primers demonstrated that all of the bacterial isolates belonged to the genus Pectobacterium. Phylogenetic analysis using partial malate dehydrogenase (mdh) gene sequences revealed that six and 15 isolates grouped with Pectobacterium carotovorum subsp. brasiliense and Pectobacterium carotovorum subsp. carotovorum reference strains, respectively. Multilocus sequence analysis of concatenated partial sequences of the acnA, atpD, gyrB and infB genes carried out on selected bacterial isolates confirmed the results obtained with the mdh gene sequences. PCR amplification with the BR1f/L1r primers that are specific for P. carotovorum subsp. brasiliense confirmed that six of the Algerian isolates belonged to this taxon.

This is an open access article under the terms of the Creat ive Commo ns Attri butio n-NonCo mmerc ial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

The main measure worldwide adopted to manage plant bacterial diseases is based on the application of copper compounds, which are often partially efficacious for the frequent appearance of copper-resistant bacterial strains and have raised concerns for their toxicity to the environment and humans. Therefore, there is an increasing need to develop new environmentally friendly, efficient, and reliable strategies for controlling plant bacterial diseases, and among them, the use of nanoparticles seems promising. The present study aimed to evaluate the feasibility of protecting plants against attacks of gram-negative and gram-positive phytopathogenic bacteria by using electrochemically synthesized silver ultra nanoclusters (ARGIRIUM‑SUNCs®) with an average size of 1.79 nm and characterized by rare oxidative states (Ag2+/3+). ARGIRIUM‑SUNCs strongly inhibited the in vitro growth (effective concentration, EC50, less than 1 ppm) and biofilm formation of Pseudomonas syringae pv. tomato and of quarantine bacteria Xanthomonas vesicatoria, Xylella fastidiosa subsp. pauca, and Clavibacter michiganensis subsp. michiganensis. In addition, treatments with ARGIRIUM‑SUNCs also provoked the eradication of biofilm for P. syringae pv. tomato, X. vesicatoria, and C. michiganensis subsp. michiganensis. Treatment of tomato plants via root absorption with ARGIRIUM‑SUNCs (10 ppm) is not phytotoxic and protected (80%) the plants against P. syringae pv. tomato attacks. ARGIRIUM‑SUNCs at low doses induced hormetic effects on P. syringae pv. tomato, X. vesicatoria, and C. michiganensis subsp. michiganensis as well as on tomato root growth. The use of ARGIRIUM‑SUNCs in protecting plants against phytopathogenic bacteria is a possible alternative control measure. Key points • ARGIRIUM‑SUNC has strong antimicrobial activities against phytopathogenic bacteria; • ARGIRIUM‑SUNC inhibits biofilm formation at low doses; • ARGIRIUM‑SUNC protects tomato plants against bacterial speck disease.

The purpose of this study was to determine whether zinc phosphate treatments of tomato plants (Solanum lycopersicum L.) can attenuate bacterial speck disease severity through reduction of Pseudomonas syringae pv. tomato (Pst) growth in planta and induce morphological and biochemical plant defence responses. Tomato plants were treated with 10 ppm (25.90 µM) zinc phosphate and then spray inoculated with strain DAPP-PG 215, race 0 of Pst. Disease symptoms were recorded as chlorosis and/or necrosis per leaf (%) and as numbers of necrotic spots. Soil treatments with zinc phosphate protected susceptible tomato plants against Pst, with reductions in both disease severity and pathogen growth in planta. The reduction of Pst growth in planta combined with significantly higher zinc levels in zinc-phosphate-treated plants indicated direct antimicrobial toxicity of this microelement, as also confirmed by in vitro assays. Morphological (i.e. callose apposition) and biochemical (i.e., expression of salicylic-acid-dependent pathogenesis-related protein PR1b1 gene) defence responses were induced by the zinc phosphate treatment, as demonstrated by histochemical and qPCR analyses, respectively. In conclusion, soil treatments with zinc phosphate can protect tomato plants against Pst attacks through direct antimicrobial activity and induction of morphological and biochemical plant defence responses.

No abstract

Selenium (Se) is a micronutrient which plays a beneficial role in plants. Its adsorption at low doses can stimulate plant growth and enhance the plant’s capacity to respond to abiotic stresses, such as salinity, drought, cold and hot temperature, probably due to its antioxidant properties. Here, the effect of selenium supplied in soil-drench treatments as sodium selenate (Na2SeO4) at the dose of 4 mg L−1 (21.17 µM) per plant was studied on tomato (Solanum lycopersicum L.; cv. Rio Grande) against Pseudomonas syringae pv. tomato (Pst), the causal agent of tomato bacterial speck. Sodium selenate treated tomato plants challenged with Pst showed a reduction in disease severity expressed as percentage of diseased area and number of lesions per leaf. Furthermore, Pst bacterial cells were unable to proliferate in treated tomato plants. The effect of sodium selenate against Pst was also assessed in vitro, demonstrating that the growth of the bacterium was affected in a dose-dependent manner (EC50 = 42 ppm). It is notable that in tomato plants treated with sodium selenate at the above reported dose, a marked callose deposition was observed as well as the expression of the salicylic-acid-responsive tomato ‘pathogenesis-related protein 1b1’ (PR1b1) but not of the jasmonate-mediated ‘proteinase inhibitor 2’ (PIN2) genes. Induced defence responses and direct antimicrobial activity protect treated tomato plants against Pst attacks, suggesting the potential of sodium selanate as an environmentally friendly and effective bacterial control means. Moreover, the increased Se content in treated tomatoes offers an effective approach to reduce Se deficiency problems in human diets.

Pseudomonas syringae pv. tomato is the causal agent of bacterial speck of tomato, an important disease that results in severe crop production losses worldwide. Currently, two races within phylogroup 01a (PG01a) are described for this pathogen. Race 0 strains have avirulence genes for the expression of type III system-associated effectors AvrPto1 and AvrPtoB, that are recognized and targeted by the effector-triggered immunity in tomato cultivars having the pto race-specific resistance gene. Race 1 strains instead lack the avrPto1 and avrPtoB genes and are therefore capable to aggressively attack all tomato cultivars. Here, we have performed the complete genome sequencing and the analysis of P. syringae pv. tomato strain DAPP-PG 215, which was described as a race 0 strain in 1996. Our analysis revealed that its genome comprises a 6.2 Mb circular chromosome and two plasmids (107 kb and 81 kb). The results indicate that the strain is phylogenetically closely related to strains Max13, K40, T1 and NYS-T1, all known race 1 strains. The chromosome of DAPP-PG 215 encodes race 1-associated genes like avrA and hopW1 and lacks race 0-associated genes like hopN1, giving it a race 1 genetic background. However, the genome harbors a complete ortholog of avrPto1, which allows the strain to display a race 0 phenotype. Comparative genomics with several PG01a genomes revealed that mobile DNA elements are rather involved in the evolution of the two different races.