Translocated wound stimuli affecting plant virus infections

Abstract

Heating one primary Pinto bean leaf for 20 seconds at 70°C increased the number of lesions induced by the sweetpotato strain of tobacco mosaic virus in the opposite inoculated leaf to as many as 100 times the number in control plants. This translocated heat stimulus (THS) was greatest when the heat was applied within 1 hour after inoculation, but was clearly apparent when applied as much as 10 hours before or after inoculation. Manifestation of the THS was inhibited by high humidity, and this inhibition was complete when the heated and inoculated plants were held at high humidity for 10 hours after inoculation. The THS was greatest in plants held at about 26°, was similar for plants held for 48 hours in darkness after inoculation and heating and for plants in natural light but was less for plants held in darkness for 48 hours before inoculation and heating. When the heated leaf was removed within 2 minutes of heating, no translocation of the stimulus was noted, but when the heated leaf was removed at 5 to 1440 minutes after heating a progressive and possibly pulsating increase in lesion numbers resulted. The stimulus moved through a 1-mm column of water between the heated leaf and the stem. The rate of movement of the stimulus was estimated to be 4 mm per minute. When lesions manifested by starch staining, rather than necrotic lesions visible in unstained leaves, were the criterion of infection, the THS was much less, and it is believed that the stimulus is primarily on the necrotic reaction of the host, rather than on the establishment of infection. A similar increase in infection resulted from freezing the noninoculated leaf or treating the noninoculated leaf with certain chemicals which caused severe injury.