Wound-healing as a factor in limiting the size of lesions in Nicotiana glutinosa leaves infected by the very mild strain of tobacco mosaic virus (TMV-VM)

Abstract

In the temperature range 20–25°C, the very mild strain of tobacco mosaic virus (TMV-VM) produces necrotic lesions of limited size on leaves of N. glutinosa. The lesions stop enlarging 2–3 days after inoculation. Factors involved in this limited lesion expansion have been investigated. In immature growing leaves a lesion consists of a white halo zone surrounding a necrotic brown center; in mature leaves the necrotic brown spot is directly surrounded by green color without a white halo zone. The white halo results from cell division, similar to wound-periderm formation. The nonhalo lesions in older leaves are like TMV lesions in pinto bean primary leaves; the necrotic area is surrounded by nonnecrotic cells with callose deposited in their cell walls. The effectiveness of these structures (wound periderm and callosed wall) as a barrier against expansion of necrosis was tested by transferring the VM inoculated plants from 21°C to 30°C. When infected plants were transferred to 30°C at 0 or 24 hr after inoculation, lesion appearance was delayed for 24–30 hr compared to that incubated continuously in 21°C, and when the lesions appeared they were large and expanding. Such expanding lesions showed no wound periderm and only a small amount of callose deposition in the walls of normal cells adjacent to the necrotic cells. However, those lesions that were allowed to form for 3–4 days at 21°C remained small and nonexpanding when subsequently transferred to 30°C. Histological changes similar to that induced by VM necrotic lesions at 21°C could also be induced by simple mechanical or chemical injury. Needle punctures induced wound-periderm formation, whereas localized application of chemicals (such as 10 μg/ml cycloheximide or 0.01 M HCl) induced callose deposition in walls of cells around the treated area in all ages of leaves tested. Thus, the limited expansion of VM lesions at 21°C may be attributed to nonspecific wound-healing processes induced by rapid necrosis of infected cells.