Analysis of the N gene hypersensitive response induced by a fluorescently tagged tobacco mosaic virus

The hypersensitive response (HR) triggered on Nicotiana edwardsonii by toba cco mosaic virus was studied using a modified viral genome that directed ex pression of the green fluorescent protein. Inoculated plants were initially incubated at 32 degrees C to inhibit the N gene-mediated HR. Transfer to 2 0 degrees C initiated the HR, and fluorescent infection foci were monitored for early HR-associated events. Membrane damage, which preceded visible ce ll collapse by more than 3 h, was accompanied by a transient restriction of the xylem within infection sites. Following cell collapse and the rapid de siccation of tissue undergoing the HR, isolated, infected cells were detect ed at the margin of necrotic lesions. These virus-infected cells were able to reinitiate infection on transfer to 32 degrees C, however, if maintained at 20 degrees C they eventually died. The results indicate that the tobacc o mosaic virus-induced HR is a two-phase process with an early stage culmin ating in rapid cell collapse and tissue desiccation followed by a more exte nded period during which the remaining infected cells are eliminated.