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Today there are no antiviral drugs of chemical nature that can completely cure virus-infected plants. The fact that their effect is limited to minimizing the pathogenic effect of viruses motivates many researchers to look for alternatives. In recent years it has been shown that lipopolysaccharides (LPS) of some bacteria, in particular representatives of the Pseudomonas genus were active against Tobacco mosaic virus (TMV). Therefore, we were interested in the additional study of LPS of phytopathogenic bacteria Pantoea agglomerans as a possible drug acting as antiviral agent. The aim of current study was to evaluate the antiviral activities of LPS obtained from phytopathogenic bacteria P. agglomerans against TMV in vitro. Methods. The antiviral activity of LPS preparations was investigated in vitro and assessed according to the inhibition percentage towards the number of local lesions in Datura stramonium leaves. P. agglomerans LPS was isolated from dry bacterial mass by phenol-water method. LPS mild acid degradation allowed to separate O-specific polysaccharide (OPS) and lipid A, which structures were identified by us earlier. The analysis of TMV and LPS interactions was carried out using a JEM 1400 transmission electron microscope (Jeol, Japan) at an accelerating voltage of 80 kV. Results. The most active were LPS preparations from P. agglomerans P324 and 8488. In vitro inhibitory efficacies of TMV infection by these LPS preparations was 59 and 60% respectively. LPS preparations of P. agglomerans 7969, 7604 and 9637, on the contrary, were inactive. Comparative analysis of the antiviral activity of LPS structural components of two P. agglomerans P324 and 7604 strains showed that the greatest inhibitory effect on the infectivity of TMV was exhibited by P. agglomerans P324 lipid A, the antiviral activity of which practically did not differ from the activity of the LPS molecule (it was lower by 7%). At the same time, the inhibitory effect of P. agglomerans 7604 core oligosaccharide (OG-core) against TMV was slightly higher compared to the effect of the whole LPS molecule. It can be assumed that the OG-core stimulated the defense mechanisms of plants and prevented the development of viral infection. Electron microscopic dates have shown that P. agglomerans P324 LPS at the concentration of 1 mg/ml influenced on freely located virions in the control causing “sticking” thus forming dense clusters, complexes or “bundles” of the virus. The individual structural components of P. agglomerans P324 LPS (lipid A and OG-core) did not have the same effect as a whole molecule. Conclusions. The study of the antiviral activity of LPS in the model system TMV – Datura stramonium L. plants showed that the most active were LPS preparations of only two strains of P. agglomerans (P324 and 8488) while the other seven strains were inactive. Individual structural components: lipid A from P. agglomerans P324 and OG-core from P. agglomerans 7604 decreased the infectivity of TMV by 7 and 15% higher than the initial LPS molecule. According to electron microscopy data the virions sticked together forming the dense clusters in case of the direct LPS-virus contacting in vitro whereas in the control it was observed just a single free virus particles. A more detailed study of the effect of individual structural components will help to understand the regularities of the LPS structure effect on TMV infectivity.

Anti-TMV Activities of Pantoea agglomerans Lipopolysaccharides in vitro