RNA silencing machinery contributes to inability of BSBV to establish infection in Nicotiana benthamiana: evidence from characterization of agroinfectious clones of Beet soil-borne virus

Beet soil-borne virus (BSBV) is a sugar beet pomovirus frequently associated with Beet necrotic yellow veins virus, the causal agent of the rhizomania disease. BSBV has been detected in most of the major beet-growing regions worldwide, yet its impact on this crop remains unclear. With the aim to understand the life cycle of this virus and clarify its putative pathogenicity, agroinfectious clones have been engineered for each segment of its tripartite genome. The biological properties of these clones were then studied on different plant species. Local infection was obtained on agroinfiltrated leaves of Beta macrocarpa. On leaves of Nicotiana benthamiana, similar results were obtained, but only when heterologous viral suppressors of RNA silencing were co-expressed or in a transgenic line down regulated for both dicer-like protein 2 and 4. On sugar beet, local infection following agroinoculation was obtained on cotyledons, but not on other tested plant parts. Nevertheless, leaf symptoms were observed on this host via sap inoculation. Likewise, roots were efficiently mechanically infected, highlighting low frequency of root necrosis and constriction, and enabling the demonstration of transmission by the vector Polymyxa betae. Altogether, the entire viral cycle was reproduced, validating the constructed agroclones as efficient inoculation tools, paving the way for further studies on BSBV and its related pathosystem.

RNAseq Analysis of Rhizomania-Infected Sugar Beet Provides the First Genome Sequence of Beet Necrotic Yellow Vein Virus from the USA and Identifies a Novel Alphanecrovirus and Putative Satellite Viruses

“Rhizomania” of sugar beet is a soilborne disease complex comprised of beet necrotic yellow vein virus (BNYVV) and its plasmodiophorid vector, Polymyxa betae. Although BNYVV is considered the causal agent of rhizomania, additional viruses frequently accompany BNYVV in diseased roots. In an effort to better understand the virus cohort present in sugar beet roots exhibiting rhizomania disease symptoms, five independent RNA samples prepared from diseased beet seedlings reared in a greenhouse or from field-grown adult sugar beet plants and enriched for virus particles were subjected to RNAseq. In all but a healthy control sample, the technique was successful at identifying BNYVV and provided sequence reads of sufficient quantity and overlap to assemble > 98% of the published genome of the virus. Utilizing the derived consensus sequence of BNYVV, infectious RNA was produced from cDNA clones of RNAs 1 and 2. The approach also enabled the detection of beet soilborne mosaic virus (BSBMV), beet soilborne virus (BSBV), beet black scorch virus (BBSV), and beet virus Q (BVQ), with near-complete genome assembly afforded to BSBMV and BBSV. In one field sample, a novel virus sequence of 3682 nt was assembled with significant sequence similarity and open reading frame (ORF) organization to members within the subgenus Alphanecrovirus (genus Necrovirus; family Tombusviridae). Construction of a DNA clone based on this sequence led to the production of the novel RNA genome in vitro that was capable of inducing local lesion formation on leaves of Chenopodium quinoa. Additionally, two previously unreported satellite viruses were revealed in the study; one possessing weak similarity to satellite maize white line mosaic virus and a second possessing moderate similarity to satellite tobacco necrosis virus C. Taken together, the approach provides an efficient pipeline to characterize variation in the BNYVV genome and to document the presence of other viruses potentially associated with disease severity or the ability to overcome resistance genes used for sugar beet rhizomania disease management.

First Draft Genome Sequence of a Polymyxa Genus Member, Polymyxa betae, the Protist Vector of Rhizomania

Polymyxa betae belongs to the Plasmodiophorida (Phytomyxea, Rhizaria). Here, we report the first draft genome sequence of a member of the Polymyxa genus, which includes two obligate root endoparasite species, vectors of important soilborne plant viruses.

Ability of Non-Pathogenic Fusarium oxysporum Strain Fo47 to Suppress Rhizomania Disease of Sugar Beets in Morocco

Rhizomania is one of the most devastating diseases of sugar beet worldwide. The disease poses a serious threat to Moroccan production and it is capable of significantly decreasing quality and yield of sugar beet plantations. The long-term survival of its fungal vector (Polymyxa betae) in soil makes it a very difficult disease to manage. Therefore, this study investigated the potential of a non-pathogenic fungal Fusarium oxysporum strain Fo47 to control Polymyxa betae. This biocontrol agent was applied as soil treatment, seed treatment, or a combination of the both treatments. A bio-test was performed on treated soil. After four weeks of culture, the roots of sugar beet seedlings were retrieved and analyzed by the DAS-ELISA test. Results indicated that F. oxysporium Fo47 reduced the activity and survival of P. betae when compared to a reference biocontrol agent Trichoderma harzianum, which only revealed significant in reducing the viral load of Beet Necrotic Yellow Vein Virus (BNYVV) as seed treatment. The non-pathogenic Fusarium oxysporum Fo47 was more effective as soil treatment and allowed almost the same reduction of BNYVV virus concentration as T. harzianum 908. Therefore, our findings emphasizes that the performance of the biocontrol agent depends on the method of application.

The efficiency for the decrease of soil’s inhabiting by fungi Polymyxa betae K. by the way of biological plant protection measures

There were proposed results by the effective usage of biological preparations during the beet’s growing. The preparation’s influence was studied by the Polymyxa betae K storage in the plants. There were determined norms and the ways of biological preparations usage.