In vitro and in vivo effects of commercial tannic acid and geranium tannin on tobacco mosaic virus

Abstract

A water extract of common geranium ( Pelargonium hortorum Bailey) leaves inhibited development of starch lesions in cucumber cotyledons when applied by vacuum infiltration 24 hours after inoculation with tobacco mosaic virus (TMV). The active principle in the geranium extract was related to tannic acid. Tannic acid, had an LD 50 of 1.4 × 10 −4 M. Complete inhibition of starch lesions was obtained by infiltrating an effective dosafe at any time between 1 hour and 33 hours after virus inoculation; between 33 and 72 hours, inhibition decreased progressively. Infiltration at 24 hours after inoculation of the systemic host Physalis floridana resulted in reduced virus titer. Tannic acid infiltration reduced the total host ribonucleic acid (RNA) in cucumber cotyledons during the first few hours after infiltration, but the cotyledons recovered within 24 hours. Virus titer assays on treated cucumber cotyledons revealed a linear increase in virus replication to higher levels than in untreated leaves, even though the number of starch lesions was reduced. In the cucumber-TMV system, tannic acid prevented the development of starch lesions and interfered with the host defense mechanism, thus permitting systemic virus invasion. Tannic acid inactivated both TMV and TMV RNA in vitro by forming noninfectious complexes. TMV RNA was much more sensitive to inactivation than was whole TMV. TMV appeared to have three in vitro reactions with tannic acid: an instantaneous reaction that was reversible with caffeine: a slow first-order reaction partially reversible with caffeine: and a denaturing reaction, at high TMV concentrations, not reversible with caffeine. The tannic acid reaction with TMV RNA was caffeine reversible. Part of the inactivation of whole TMV appeared to be due to a reaction of tannic acid with the RNA core. A brown coloration resulted when tannic acid was allowed to react with TMV, TMV RNA, uridylic acid, or guanylic acid. This reaction did not occur with TMV protein, other nucleotides, nucleotides, or purine or pyrimidine bases. The reaction was thus specific for uridylic and guanylic acids.