The effects of differing
cytokinin and
auxin concentrations on resistance of tobacco (Nicotiana tabacum L.) tissue cultures to race 0 of Phytophthora parasitica var. nicotianae were examined. With 1 micromolar
kinetin and either 11.5 micromolar
indoleacetic acid or 1 micromolar 2,4-dichlorophen-oxyacetic
acid, 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 micromolar
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
indoleacetic acid levels reversed the effects of high
cytokinin concentrations. The balance of
phytohormones apparently controls the host response to the fungus; thus, in this system, resistance or susceptibility can be studied without changing either host or fungal genotype.