Isolde M. Francis

The Cellobiose Sensor CebR Is the Gatekeeper of Streptomyces scabies Pathogenicity

ABSTRACT A relatively small number of species in the large genus Streptomyces are pathogenic; the best characterized of these is Streptomyces scabies. The pathogenicity of S. scabies strains is dependent on the production of the nitrated diketopiperazine thaxtomin A, which is a potent plant cellulose synthesis inhibitor. Much is known about the genetic loci associated with plant virulence; however, the molecular mechanisms by which S. scabies triggers expression of thaxtomin biosynthetic genes, beyond the pathway-specific activator TxtR, are not well understood. In this study, we demonstrate that binding sites for the cellulose utilization repressor CebR occur and function within the thaxtomin biosynthetic cluster. This was an unexpected result, as CebR is devoted to primary metabolism and nutritive functions in nonpathogenic streptomycetes. In S. scabies, cellobiose and cellotriose inhibit the DNA-binding ability of CebR, leading to an increased expression of the thaxtomin biosynthetic and regulatory genes txtA, txtB, and txtR. Deletion of cebR results in constitutive thaxtomin A production and hypervirulence of S. scabies. The pathogenicity of S. scabies is thus under dual direct positive and negative transcriptional control where CebR is the cellobiose-sensing key that locks the expression of txtR, the key necessary to unlock the production of the phytotoxin. Interestingly, CebR-binding sites also lie upstream of and within the thaxtomin biosynthetic clusters in Streptomyces turgidiscabies and Streptomyces acidiscabies, suggesting that CebR is most likely an important regulator of virulence in these plant-pathogenic species as well. IMPORTANCE What makes a microorganism pathogenic is not limited to the genes acquired for virulence. Using the main causative agent of scab lesions on root and tuber crops as an example, our work identified the subtle but essential genetic changes that generate the cis-acting elements necessary for proper timing of the expression of the cluster of genes responsible for the biosynthesis of thaxtomin A, the primary virulence factor in plant-pathogenic streptomycetes. These data illustrate a situation in which a regulator associated with primary metabolism in nonpathogens, CebR, has been coopted as a master regulator of virulence in pathogenic species. Furthermore, the manipulation of CebR-mediated control of thaxtomin production will facilitate overproduction of this natural and biodegradable herbicide for commercial purposes. Our work thus provides a concrete example of how a strictly theoretical and computational work was able to elucidate a regulatory mechanism associated with the virulence of a plant pathogen and to generate solutions to purely agro-industrial concerns. What makes a microorganism pathogenic is not limited to the genes acquired for virulence. Using the main causative agent of scab lesions on root and tuber crops as an example, our work identified the subtle but essential genetic changes that generate the cis-acting elements necessary for proper timing of the expression of the cluster of genes responsible for the biosynthesis of thaxtomin A, the primary virulence factor in plant-pathogenic streptomycetes. These data illustrate a situation in which a regulator associated with primary metabolism in nonpathogens, CebR, has been coopted as a master regulator of virulence in pathogenic species. Furthermore, the manipulation of CebR-mediated control of thaxtomin production will facilitate overproduction of this natural and biodegradable herbicide for commercial purposes. Our work thus provides a concrete example of how a strictly theoretical and computational work was able to elucidate a regulatory mechanism associated with the virulence of a plant pathogen and to generate solutions to purely agro-industrial concerns.

The vicious cycle of lettuce corky root disease: effects of farming system, nitrogen fertilizer and herbicide

AimsThis study was aimed at testing the hypothesis that lettuce corky root (CR) disease caused by Rhizorhapis suberifaciens was less severe in organic than conventional farms, due to the absence of herbicide and fertilizer, and greater soil microbial activity in organic farms.MethodsCR severity and soil quality were assessed in pairs of conventional and organic farms in California. To determine factors contributing to CR, effects of N fertilizer and pronamide herbicide were assessed on CR severity and plant weight in separate field experiments.ResultsCR was significantly more severe in conventional than organic farms, and there was a negative exponential relationship between CR severity and microbial activity. Split applications of soluble N fertilizer enhanced susceptibility to CR compared to pre-plant application of slow release N fertilizer. Pronamide increased disease severity on seedlings compared to untreated controls and reduced the dry weights of seedlings and mature heads.ConclusionsConventional practices, like fertilizer and herbicide use, increase plant susceptibility to and reduce microbial competition or antibiosis against R. suberifaciens in conventional lettuce production farms, potentially leading to enhanced environmental pollution due to a decrease in nutrient use efficiency and an increased need for fertilizer and water for diseased plants.

Mining the genome of Rhodococcus fascians, a plant growth-promoting bacterium gone astray

Rhodococcus fascians is a phytopathogenic Gram-positive Actinomycete with a very broad host range encompassing especially dicotyledonous herbaceous perennials, but also some monocots, such as the Liliaceae and, recently, the woody crop pistachio. The pathogenicity of R. fascians strain D188 is known to be encoded by the linear plasmid pFiD188 and to be dictated by its capacity to produce a mixture of cytokinins. Here, we show that D188-5, the nonpathogenic plasmid-free derivative of the wild-type strain D188 actually has a plant growth-promoting effect. With the availability of the genome sequence of R. fascians, the chromosome of strain D188 was mined for putative plant growth-promoting functions and the functionality of some of these activities was tested. This analysis together with previous results suggests that the plant growth-promoting activity of R. fascians is due to production of plant growth modulators, such as auxin and cytokinin, combined with degradation of ethylene through 1-amino-cyclopropane-1-carboxylic acid deaminase. Moreover, R. fascians has several functions that could contribute to efficient colonization and competitiveness, but there is little evidence for a strong impact on plant nutrition. Possibly, the plant growth promotion encoded by the D188 chromosome is imperative for the epiphytic phase of the life cycle of R. fascians and prepares the plant to host the bacteria, thus ensuring proper continuation into the pathogenic phase.

Comment on “Evolutionary transitions between beneficial and phytopathogenic Rhodococcus challenge disease management”

We would like to address a number of concerns regarding this paper (Savory et al., 2017)

Tracking the Subtle Mutations Driving Host Sensing by the Plant Pathogen Streptomyces scabies.

ABSTRACT The acquisition of genetic material conferring the arsenal necessary for host virulence is a prerequisite on the path to becoming a plant pathogen. More subtle mutations are also required for the perception of cues signifying the presence of the target host and optimal conditions for colonization. The decision to activate the pathogenic lifestyle is not “taken lightly” and involves efficient systems monitoring environmental conditions. But how can a pathogen trigger the expression of virulence genes in a timely manner if the main signal inducing its pathogenic behavior originates from cellulose, the most abundant polysaccharide on earth? This situation is encountered by Streptomyces scabies, which is responsible for common scab disease on tuber and root crops. We propose here a series of hypotheses of how S. scabies could optimally distinguish whether cello-oligosaccharides originate from decomposing lignocellulose (nutrient sources, saprophyte) or, instead, emanate from living and expanding plant tissue (virulence signals, pathogen) and accordingly adapt its physiological response.

Proteomic Response to Thaxtomin Phytotoxin Elicitor Cellobiose and to Deletion of Cellulose Utilization Regulator CebR in Streptomyces scabies

Streptomyces scabies is responsible for common scab disease on root and tuber vegetables. Production of its main phytotoxin thaxtomin A is triggered upon transport of cellulose byproducts cellotriose and cellobiose, which disable the repression of the thaxtomin biosynthesis activator gene txtR by the cellulose utilization regulator CebR. To assess the intracellular response under conditions where S. scabies develops a virulent behavior, we performed a comparative proteomic analysis of wild-type S. scabies 87-22 and its cebR null mutant (hyper-virulent phenotype) grown in the absence or presence of cellobiose. Our study revealed significant changes in abundance of proteins belonging to metabolic pathways known or predicted to be involved in pathogenicity of S. scabies. Among these, we identified proteins of the cello-oligosaccharide-mediated induction of thaxtomin production, the starch utilization system required for utilization of the carbohydrate stored in S. scabiess hosts, and siderophore synthesis utilization systems, which are key features of pathogens to acquire iron once they colonized the host. Thus, proteomic analysis supported by targeted mass spectrometry-based metabolite quantitative analysis revealed the central role of CebR as a regulator of virulence of S. scabies.