Systemic Acquired Resistance (SAR)

Abstract

Nicotiana species and cultivars possessing the dominant N gene, e.g. Nicotiana glutinosa, N. tabacum “Samsun NN” and N. tabacum “Xanthi nc”, produce necrotic local lesions upon inoculation with the type strain of tobacco mosaic Tobamovirus (TMV). When one half of a leaf of such a N-gene-containing plant is inoculated with TMV, a high level of resistance to TMV is induced in the opposite virus-free half of that leaf. Challenge-inoculation with TMV of the latter leaf-half approx. 7 days after inoculation of the other half leads to the formation of lesions with reduced diameters, ranging from one-fifth to one-third the size of those in the appropriate control halves. In some cases, the number of lesions is also negatively affected. The resistance is maximal when the test plants are kept at 20 °C before and after challenge-inoculation and it lasts for approx. 20 days. SAR can also be brought about by nonnecrotic localised viral infections and even by chemical compounds, e.g. ethylene.