Sugar-induced enhancement of tobacco mosaic virus ribonucleic acid infectivity

Abstract

The addition of sugars to tobacco mosaic virus ribonucleic acid (TMV-RNA) inocula increased their infectivity over that of control inocula. Sucrose increased infectivity on Phaseolus vulgaris var. Pinto, while the same inoculum on Nicotiana glutinosa L., N. tabacum L. var. Xanthi-nc and Chenopodium amaranticolor Coste & Reyn produced no significant increase. Enhancement by sucrose was greatest when the TMV-RNA was in acetate buffer at pH 7.0 (133% increase) and distilled H 2O (116% increase). No significant increase in infectivity occurred when citrate, Tris, and phosphate buffers were used at this pH. Maximum enhancement was obtained when sucrose and TMV-RNA were applied to the leaf simultaneously. Treatments which increased the infectivity of TMV-RNA or the susceptibility of leaves decreased enhancement by sucrose. Conversely, enhancement occurred only under conditions where relatively few lesions were induced by the control inoculum. Preinoculation heat-dark and dark treatment reduced enhancement by sucrose 80 and 77%, respectively. Other sugars, disaccharides and monosaccharides, and polyalcohols increased infectivity in a comparable fashion to sucrose. The molar concentrations for optimum enhancement by sucrose and glucose were similar, hence, the effect seemed dependent on the colligative properties of the sugars. No interaction of sugar with the RNA was detectable by ultraviolet spectrophotometry. Sucrose did not stabilize the TMV-RNA, but increased its rate of inactivation, especially if the sucrose had been autoclaved. The argument is advanced that enhancement is brought about by the action of sucrose on the cell penetration phase of the infection process. Sucrose does not enhance the infectivity of the virion, and for this reason the mechanism of penetration by the viral RNA is thought to be different.