Positive and Negative Regulation of the Virulence-Associated Coronafacoyl Phytotoxin in the Potato Common Scab Pathogen Streptomyces scabies

Regulation of virulence mechanisms in plant-pathogenic Streptomyces

Streptomyces have a uniquely complex developmental life cycle that involves the coordination of morphological differentiation with the production of numerous bioactive specialized metabolites. The majority of Streptomyces spp. are soil-dwelling saprophytes, while plant pathogenicity is a rare attribute among members of this genus. Phytopathogenic Streptomyces are responsible for economically important diseases such as common scab, which affects potato and other root crops. Following the acquisition of genes encoding virulence factors, Streptomyces pathogens are expected to have specifically adapted their regulatory pathways to enable transition from a primarily saprophytic to a pathogenic lifestyle. Investigations of the regulation of pathogenesis have primarily focused on Streptomyces scabiei and the principal pathogenicity determinant thaxtomin A. The coordination of growth and thaxtomin A production in this species is controlled in a hierarchical manner by cluster-situated regulators, pleiotropic regulators, signalling and plant-derived molecules, and nutrients. Although the majority of phytopathogenic Streptomyces produce thaxtomins, many also produce additional virulence factors, and there are scab-causing pathogens that do not produce thaxtomins. The development of effective control strategies for common scab and other Streptomyces plant diseases requires a more in-depth understanding of the genetic and environmental factors that modulate the plant pathogenic lifestyle of these organisms.

Bacillus velezensis K-9 as a Potential Biocontrol Agent for Managing Potato Scab

Crop pathogen infections can lead to substantial economic losses, but biocontrol, an environmentally friendly approach, can be used to control infections. For the biological management of potato scab disease, we assessed the potential use of Bacillus velezensis as a biocontrol agent. B. velezensis K-9 inhibited up to 44.90% of the infection caused by Streptomyces scabies, the causative agent of potato scab. Treatment of the S. scabies-infected potato plants with B. velezensis K-9 resulted in a significant reduction in the depth of the disease lesions compared with the untreated infected potato plants. In a radish seedling test, the B. velezensis K-9 culture and cell-free filtrate significantly reduced (P < 0.05) potato scab disease symptoms, suggesting that the strain K-9 was able to reduce S. scabies pathogenesis on potatoes. In a field test, the disease and scab indexes for B. velezensis K-9 against potato scab were significantly different from the control. In 2021, the potato yield for the B. velezensis K-9-treated plants was 12.44% higher than that for the control plants. In 2022, the potato yield following B. velezensis K-9 treatment increased by 12.65% compared with the control. In conclusion, B. velezensis K-9 prevented potato scab and increased potato yield. Thus, B. velezensis K-9 substantially reduced the occurrence of potato scab and could be used as a potential biocontrol agent for the management of potato scab.

Streptomyces can be an excellent plant growth manager

Streptomyces, the most abundant and arguably the most important genus of actinomycetes, is an important source of biologically active compounds such as antibiotics, and extracellular hydrolytic enzymes. Since Streptomyces can have a beneficial symbiotic relationship with plants they can contribute to nutrition, health and fitness of the latter. This review article summarizes recent research contributions on the ability of Streptomyces to promote plant growth and improve plant tolerance to biotic and abiotic stress responses, as well as on the consequences, on plant health, of the enrichment of rhizospheric soils in Streptomyces species. This review summarizes the most recent reports of the contribution of Streptomyces to plant growth, health and fitness and suggests future research directions to promote the use of these bacteria for the development of a cleaner agriculture.

Isolation and identification of an endophytic bacteria Bacillus sp. K-9 exhibiting biocontrol activity against potato common scab

Potato scab is an important soil-borne disease that can significantly reduce the quality and economic value of potatoes. The purpose of this study was to isolate, screen and identify endophytic bacteria that have antagonistic and control effects on potato scab disease, and to determine the control effect and yield traits of the selected strains on potato scab disease in field conditions. A bacterial strain K-9 was isolated from the junction between scab spot and healthy epidermis of potato tuber. The K-9 strain was identified as Bacillus sp. through morphological, physiological and biochemical characterization, and 16S rDNA and gyrB gene sequence analysis. The diameter of the inhibition zone of strain K-9 against Streptomyces scabies on the YME plate was 3.82 cm. The K-9 strain could inhibit eight types of crop pathogens, with the highest inhibition rate (70.39%) against another soil-borne potato disease (potato black scurf). In the field test, the control effect of K-9 strain against potato scab was not significantly different from that of mixed bacteria or chemical agents, but the disease index and the scab index in the K-9 treatment were significantly lower than in the control. The potato yield in the K-9 treatment was 12.44% higher than the control. In summary, the K-9 strain can prevent not only potato scab, but also increase potato yield. Therefore, the endophytic bacterial K-9 strain may be a potential biological control agent.

Comparative Genomics of Potato Common Scab-Causing Streptomyces spp. Displaying Varying Virulence

Common scab of potato causes important economic losses worldwide following the development of necrotic lesions on tubers. In this study, the genomes of 14 prevalent scab-causing Streptomyces spp. isolated from Prince Edward Island, one of the most important Canadian potato production areas, were sequenced and annotated. Their phylogenomic affiliation was determined, their pan-genome was characterized, and pathogenic determinants involved in their virulence, ranging from weak to aggressive, were compared. 13 out of 14 strains clustered with Streptomyces scabiei, while the last strain clustered with Streptomyces acidiscabies. The toxicogenic and colonization genomic regions were compared, and while some atypical gene organizations were observed, no clear correlation with virulence was observed. The production of the phytotoxin thaxtomin A was also quantified and again, contrary to previous reports in the literature, no clear correlation was found between the amount of thaxtomin A secreted, and the virulence observed. Although no significant differences were observed when comparing the presence/absence of the main virulence factors among the strains of S. scabiei, a distinct profile was observed for S. acidiscabies. Several mutations predicted to affect the functionality of some virulence factors were identified, including one in the bldA gene that correlates with the absence of thaxtomin A production despite the presence of the corresponding biosynthetic gene cluster in S. scabiei LBUM 1485. These novel findings obtained using a large number of scab-causing Streptomyces strains are challenging some assumptions made so far on Streptomyces’ virulence and suggest that other factors, yet to be characterized, are also key contributors.

Genomic and Metabolomic Analysis of the Potato Common Scab Pathogen Streptomyces scabiei

Streptomyces scabiei is a key causative agent of common scab disease, which causes significant economic losses to potato growers worldwide. This organism produces several phytotoxins that are known or suspected to contribute to host-pathogen interactions and disease development; however, the full metabolic potential of S. scabiei has not been previously investigated. In this study, we used a combined metabolomic and genomic approach to investigate the metabolites that are produced by S. scabiei. The genome sequence was analyzed using antiSMASH and DeepBGC to identify specialized metabolite biosynthetic gene clusters. Using untargeted liquid chromatography-coupled tandem mass spectrometry (LC-MS2), the metabolic profile of S. scabiei was compared after cultivation on three different growth media. MS2 data were analyzed using Feature-Based Molecular Networking and hierarchical clustering in BioDendro. Metabolites were annotated by performing a Global Natural Products Social Molecular Networking (GNPS) spectral library search or using Network Annotation Propagation, SIRIUS, MetWork, or Competitive Fragmentation Modeling for Metabolite Identification. Using this approach, we were able to putatively identify new analogues of known metabolites as well as molecules that were not previously known to be produced by S. scabiei. To our knowledge, this study represents the first global analysis of specialized metabolites that are produced by this important plant pathogen.

Virulence mechanisms of plant-pathogenic Streptomyces species: an updated review

Gram-positive Actinobacteria from the genus Streptomyces are best known for their morphological complexity and for their ability to produce numerous bioactive specialized metabolites with useful applications in human and veterinary medicine and in agriculture. In contrast, the ability to infect living plant tissues and to cause diseases of root and tuber crops such as potato common scab (CS) is a rare attribute among members of this genus. Research on the virulence mechanisms of plant-pathogenic Streptomyces spp. has revealed the importance of the thaxtomin phytotoxins as key pathogenicity determinants produced by several species. In addition, other phytotoxic specialized metabolites may contribute to the development or severity of disease caused by Streptomyces spp., along with the production of phytohormones and secreted proteins. A thorough understanding of the molecular mechanisms of plant pathogenicity will enable the development of better management procedures for controlling CS and other plant diseases caused by the Streptomyces.