Structural features of fungal beta-D-glucans for the efficient inhibition of the initiation of virus infection on Nicotiana tabacum.

Glucans of fungal origin have been shown to inhibit the early stages of infection of Nicotiana by numerous viruses of different taxonomic groups. Several glucans were isolated from the cell walls of Phytophthora parasitica, Phytophthora megasperma f. sp. glycinea (Pmg) and Fusarium oxysporum, and their antiviral activity compared on tobacco leaves inoculated with tobacco mosaic virus. These polysaccharides consist of a mixture of (1-->3)(1-->6)-beta-D-glucans with M(r) varying from 1.1 x 10(3) to 2 x 10(6). Requirements for a prominent antiviral activity of the fungal polysaccharides are a beta-(1-->3)(1-->6)-D-glucan structure with mono-, di-, tri- or tetra-glucosidic side branches attached to a linear main chain of beta-(1-->3)-linked-D-glucose residues. Very high activity is correlated with a high degree of branching at position 6 and with the size and glycosidic nature of the side chains. The molecular masses and the organized structure of fungal beta-D-glucans are not essential for their antiviral activity. The structural motif for antiviral activity in Nicotiana is distinct from that required for elicitation of phytoalexins in soybean cotyledons.