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Induction of Systemic Resistance in Plants Against Viruses by a Basic Protein from Clerodendrum aculeatum Leaves. H. N. Verma, Department of Botany, Lucknow University, Lucknow-226007, India; Shalini Srivastava(2), Varsha(3), and Dhirendra Kumar(4). (2)(3)Department of Botany, Lucknow University, Lucknow-226007, India; (4)International Centre for Genetic Engineering and Biotechnology, Aruna Asaf Ali Marg, New Delhi-110067, India. Phytopathology 86:485-492. Accepted for publication 16 January 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/Phyto-86-485.

A nonphytotoxic, systemic resistance-inducing agent present in Clerodendrum aculeatum leaves was purified. A specific basic protein (C. aculeatum-systemic resistance inducing [CA-SRI]) with a molecular mass of 34 kDa was observed consistently in leaf extracts by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Treatment of plants with the purified protein preparation induced a very high level of systemic resistance against virus infection. Resistance was detectable a few hours after challenge-inoculation with virus and resulted in lesions that were fewer in number or totally absent. The minimum time required for induction of systemic resistance in nontreated leaves of susceptible host plants was 5 to 30 min depending on the host. The resistance-inducing activity of CA-SRI was not affected by protease treatment. After digestion of CA-SRI with endoproteinase Arg-C, the eluted protein fragments from SDS-PAGE were biologically active. An antiserum to the 34-kDa protein was highly specific for CA-SRI, and Western blots of the purified protein recognized the 34-kDa protein band. The isoelectric point of the protein was 8.65. Treatment of susceptible healthy test hosts with purified CA-SRI consistently resulted in the accumulation of a virus inhibitory agent in the resistant leaves. An extract prepared from resistant leaves reduced the infectivity of added virus, with an average reduction in the number of lesions by more than 90%. The specific 34-kDa protein was observed consistently in the leaves of plants with induced resistance and was highly active in reducing the infectivity of the virus. There seems to be a causal relationship between induced resistance and accumulation of the 34-kDa protein based on gel electrophoresis. The protein was present naturally in very low amounts in nontreated healthy plants.