ABSTRACT
Pseudomonas syringae pv. aptata is a member of the sugar beet pathobiome and the causative agent of leaf spot disease. Like many pathogenic bacteria, P. syringae relies on the secretion of toxins, which manipulate host-pathogen interactions, to establish and maintain an infection. This study analyzes the secretome of six pathogenic P. syringae pv. aptata strains with different defined virulence capacities in order to identify common and strain-specific features, and correlate the secretome with disease outcome. All strains show a high type III secretion system (T3SS) and type VI secretion system (T6SS) activity under apoplast-like conditions mimicking the infection. Surprisingly, we found that low pathogenic strains show a higher secretion of most T3SS substrates, whereas a distinct subgroup of four effectors was exclusively secreted in medium and high pathogenic strains. Similarly, we detected two T6SS secretion patterns: while one set of proteins was highly secreted in all strains, another subset consisting of known T6SS substrates and previously uncharacterized proteins was exclusively secreted in medium and high virulence strains. Taken together, our data show that P. syringae pathogenicity is correlated with the repertoire and fine-tuning of effector secretion and indicate distinct strategies for establishing virulence of P. syringae pv. aptata in plants.
ABSTRACT
Members of the Pseudomonas syringae species complex are heterogeneous bacteria that are the most abundant bacterial plant pathogens in the plant phyllosphere, with strong abilities to exist on and infect different plant hosts and survive in/outside agroecosystems. In this study, the draft genome sequences of two pathogenic P. syringae pv. aptata strains with different in planta virulence capacities isolated from the phyllosphere of infected sugar beet were analyzed to evaluate putative features of survival strategies and to determine the pathogenic potential of the strains. The draft genomes of P. syringae pv. aptata strains P16 and P21 are 5,974,057 bp and 6,353,752 bp in size, have GC contents of 59.03% and 58.77%, respectively, and contain 3,439 and 3,536 protein-coding sequences, respectively. For both average nucleotide identity and pangenome analysis, P16 and P21 largely clustered with other pv. aptata strains from the same isolation source. We found differences in the repertoire of effectors of the type III secretion system among all 102 selected strains, suggesting that the type III secretion system is a critical factor in the different virulent phenotypes of P. syringae pv. aptata. During genome analysis of the highly virulent strain P21, we discovered genes for T3SS effectors (AvrRpm1, HopAW1, and HopAU1) that were not previously found in genomes of P. syringae pv. aptata. We also identified coding sequences for pantothenate kinase, VapC endonuclease, phospholipase, and pectate lyase in both genomes, which may represent novel effectors of the type III secretion system. IMPORTANCE Genome analysis has an enormous effect on understanding the life strategies of plant pathogens. Comparing similarities with pathogens involved in other epidemics could elucidate the pathogen life cycle when a new outbreak happens. This study represents the first in-depth genome analysis of Pseudomonas syringae pv. aptata, the causative agent of leaf spot disease of sugar beet. Despite the increasing number of disease reports in recent years worldwide, there is still a lack of information about the genomic features, epidemiology, and pathogenic life strategies of this particular pathogen. Our findings provide advances in disease etiology (especially T3SS effector repertoire) and elucidate the role of environmental adaptations required for prevalence in the pathobiome of the sugar beet. From the perspective of the very heterogeneous P. syringae species complex, this type of analysis has specific importance in reporting the characteristics of individual strains.
ABSTRACT
Bacillus species are among the most researched and frequently applied biocontrol agents. To estimate their potential as environmentally friendly microbial-based products, reliable and rapid plant colonization monitoring methods are essential. We evaluated repetitive element-based (rep) and Random Amplified Polymorphic DNA (RAPD) PCR (Polymerase Chain Reaction) genotyping in a diversity assessment of 251 strains from bulk soil, straw, and manure samples across Serbia, highlighting their discriminative force and the presence of unique bands. RAPD 272, OPG 5, and (GTG)5 primers were most potent in revealing the high diversity of a sizable environmental Bacillus spp. collection. RAPD 272 also amplified a unique band for a proven biocontrol strain, opening the possibility of Sequence Characterized Amplified Region (SCAR) marker design. That will enable colonization studies using the SCAR marker for its specific detection. This study provides a guide for primer selection for diversity and monitoring studies of environmental Bacillus spp. isolates.
