Differential Effects of Viral Coat Protein on Induction of Hypersensitive Response and Systemic Movement of Tobacco Mosaic Virus and Tobacco Mild Green Mosaic Virus in Nicotiana megalosiphon

Abstract

Tobacco mosaic virus (TMV) and Tobacco mild green mosaic virus (TMGMV) are both known to induce hypersensitive response (HR) local lesions in Nicotiana megalosiphon, a hybrid plant from a cross between N. suaveolens and N. fragrans, but only TMV is capable of systemic movement. Therefore, the determinants of induction of hypersensitive response and systemic movement of TMV and TMGMV in N. megalosiphon were further analyzed. HR was shown to be independent of the temperature indicating that the resistance responses are different from N gene responses induced in tobacco (N. tabacum cv. Xanthi-nc.) to TMV. Comparison of lesion growth between wild-type and transgenic tobacco plants encoding salicylate hydroxylase (nahG) indicated that TMV spread similarly in N. megalosiphon and Xanthi-nc./nahG plants. In addition, exogenous application of SA did not prevent systemic movement of TMV. Coat protein-deficient TMV failed to induce HR and move systemically in N. megalosiphon indicating that CP is the inducer of HR and determinant for systemic movement. However, single epidermal cells expressing TMV-CP did not undergo cell death suggesting that formation of HR cell death requires viral movement out of the initially infected epidermal cells or the presence of intact virus particles. Furthermore, comparison of different TMV strains, including virus vector carrying the CP of TMGMV-U5 in place of TMV-CP, showed no differences in the induction of HR and timing of systemic virus movement suggesting that CP is not the determinant for differential invasion of TMV and TMGMV.