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Prevention of the Hypersensitive Reaction in Tobacco Leaves by Heat-Killed Bacterial Cells. J. Carlos Lozano, Graduate Student, Department of Plant Pathology, University of Wisconsin, Madison 53706; Luis Sequeira, Professor, Department of Plant Pathology, University of Wisconsin, Madison 53706. Phytopathology 60:875-879. Accepted for publication 22 December 1969. DOI: 10.1094/Phyto-60-875.

The hypersensitive reaction (HR) induced by infiltration of tobacco leaves with cell suspensions (5 × 109 cells/ml) of race 2 isolates of Pseudomonas solanacearum was prevented by prior infiltration (18 hr) with a similar suspension of heat-killed cells of either race 2 or race 1. A similar protective effect was induced by heat-killed cells of P. lachrymans and Xanthomonas axonopodis, but not by those of Escherichia coli. In leaf areas infiltrated with heat-killed cells, protection was first evident when leaves were challenged with live cells 18 hr later; thereafter, the protective effect spread to neighboring intercostal areas not previously infiltrated with heat-killed cells. By 48 hr after infiltration, inhibition of the HR was also evident in leaves immediately above the infiltrated one. In leaves infiltrated with heat-killed cells and subsequently challenged with live cells of compatible (K60) or incompatible (S210) isolates, the populations of both bacteria decreased. Populations of K60 decreased very gradually (100-fold in 48 hr), but did not induce the typical compatible, necrotic reaction. Populations of S210 decreased sharply (100-fold in 12 hr) in the protected tissues at a rate that was more rapid than during the course of the HR, but no symptoms other than slight yellowing were present. Prevention of the HR was light-dependent. In leaves infiltrated with heat-killed cells and incubated in the dark for 24 hr before infiltrating with live S210 cells, the HR was not prevented. Increasing the exposure to light resulted in progressively better protection against the HR. In leaves infiltrated with heat-killed cells in alternate intercostal areas, a 24-hr exposure to light before challenging with live cells completely prevented the HR over the entire laminae.