Modification of Disease Resistance of Tobacco Callus Tissues by Cytokinins

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Abstract
The effects of differing cytokinin and auxin concentrations on resistance of tobacco (Nicotiana tabacum L.) tissue cultures to race O of Phytophthora parasitica var. nicotianae were examined. With 1 .mu.M kinetin and either 11.5 .mu.M IAA or 1 .mu.M 2,4-D tissues from resistant cultivars exhibited a hypersensitive reaction to zoospores of the fungus and subsequently were colonized only slightly. With susceptible cultivars or with tissues from resistant cultivars supplied with higher cytokinin levels (e.g., 10 .mu.M kinetin), this hypersensitive reaction did not occur and tissues were heavily colonized. Benzylaminopurine and kinetin were particularly effective in eliminating both the hypersensitive reaction and disease resistance. Zeatin and 6-(3-methyl-2-butenylamino)purine were less effective. Increases in IAA levels reversed the effects of high cytokinin concentrations. The balance of phytohormones apparently controls the host response to the fungus; thus resistance or susceptibility can be studied without changing either host or fungal genotype.