Localization of Induced Resistance and Susceptibility in Barley Leaves Inoculated with the Powdery Mildew Fungus

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Abstract
Localization of induced susceptibility and resistance was studied with genetically defined barley (Hordeum vulgare) cultivars inoculated with races of Erysiphe graminis hordei and E. graminis tritici by a double inoculation method. Establishment of a rejection reaction required 6 h; inhibition of secondary hyphae became detectable after a 9 h interval. Both the rejection and growth inhibition activities elicited by a nonpathogenic race of E. graminis were restricted to the site of primary incompatible interaction, suggesting that the primary recognition and subsequent physiological conditioning toward resistance is strictly localized. Barley leaves exposed to a compatible race became susceptible to nonpathogenic Sphaerotheca fuliginea. The degree of susceptibility of cells was dependent on their distance from the cells that harbored the primary compatible haustorium. The nonpathogen established infection at a high frequency (75%) in cells that harbored compatible haustoria, but at low frequency in cells located 4 cell-rows away from the primarily induced cells. The induced susceptibility also was localized. These results suggest that the primary recognition becomes irreversible once the host cells are physiologically conditioned and that information required for these cellular conditionings is not transferred a long distace, at least not in powdery mildews.

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