Effects of cytokinins on fungus development and host response in powdery mildew of barley

Epidermal tissues partially dissected from colcoptiles of barley (Hordeum vulgare L.) were inoculated with Erysiphe graminis (DC) Merat f. sp. hordei Em. Marchai to test the effects of cytokinins on fungus development and the hypersensitive reaction (HR). Kinetin at 10⁻⁵to 10⁻⁴M applied 16h after inoculation accelerated HR in an incompatible host-parasite combination and doubled the number of cells that died at each infection site, suggesting that kinetin had increased the spread of killing factors beyond the cells that contained primary haustoria. With a compatible host-parasite combination, kinetin applied at 16 h after inoculation decreased initiation of hyphae at 10⁻⁵ to 10⁻⁴ m and decreased hyphal growth at 10⁻⁶ to 10⁻⁴M. Kinetin applied at inoculation slowed haustorium development, interfered with haustorium differentiation and decreased the number of secondary haustoria formed by hyphal colonies. The central bodies of haustoria were enlarged and spherical, indicating that kinetin had interfered with the normal elongation processes of the bodies. Zeatin had no effects on either fungus development or HR.. Kinetin inhibited appressorium formation apart from the host on nitrocellulose membranes, suggesting that the inhibitory effects of kinetin on fungus development were direct rather than through the host.