DIFFERENTIAL INVASION BY TOBAMOVIRUSES OF NICOTIANA MEGALOSIPHON FOLLOWING THE HYPERSENSITIVE RESPONSE

The tobamoviruses tobacco mild green mosaic virus (TMGMV) and tobacco mosaic virus (TMV) differ in their behavior in Nicotiana megalosiphon. Both viruses induce necrotic lesion formation in the inoculated leave s, but only TMV moves from the initially infected (necrotic) areas and is able to spread systemically, overcoming the hypersensitive respons e. The appearance and growth of necrotic local lesions induced by TMV or TMGMV in N. megalosiphon were shown to follow similar kinetics; how ever, virus accumulation in the local lesions was about five times gre ater for TMV than for TMGMV. In accordance with this, TMV replicated t o much higher levels than TMGMV did in N. megalosiphon protoplasts. In N. tabacum 'Xanthi-nc,' a hypersensitive host for both viruses that w as used as a control, no difference was observed in the kinetics of lo cal lesion development nor in virus accumulation, either in local lesi ons or in protoplasts, for TMV or TMGMV. Thus, the ability of TMV, but not of TMGMV, to overcome the hypersensitive response of N. megalosip hon does not appear to be related to differences in the elicitation of the hypersensitive response but rather to the differential effect of a second resistance factor over the multiplication of TMGMV but not of TMV.