[Mannan sulfates--inducers of plant resistance to viral infection].

Mannan sulphates (MS) synthesized on the basis of extracellular linear mannan (LM) of Rhodotorula rubra induce resistance of Immune-580 tobacco and thornapple (Datura stramonium) to tobacco mosaic virus (TMV). The resistance is manifested in a decrease in the number and/or size of viral local lesions (LL) in MS-treated (mg/ml) leaf halves. The reduction in the LL size does not seem to be due to the direct inhibition of TMV multiplication by the polysaccharide as the virus accumulation in the tissues of the systemic host (Samsun tobacco) does not decrease when MS is used 15 min after infection. The induced resistance is partially inhibited by actinomycin D and completely by heating (32 degrees C). Unlike MS, neutral LM decreases only the number of LL on the treated parts of the tissue but does not influence their size. The antiviral activity of LM does not change in the presence of actinomycin D. The possible mechanisms of the LM and MS protective effect in plants are discussed.