Apparent calcium mediation of resistance of an ml-o barley mutant to powdery mildew

Resistance to Erysiphe graminis f. sp. hordei conditioned by the ml-o gene in barley was inhibited with treatments that were expected to lower the concentration of cytoplasmic, ionized calcium in the host cells. When partially dissected coleoptiles were incubated on 10 m Ca(NO₃)₂ or CaCl₂, resistance was very high, whereas lowering the exogenous calcium concentration to c. 0.25 m decreased resistance by 50%. Adding indoleacetic acid to the low levels of calcium enhanced this effect, and resistance was almost totally inhibited. Treatments with the calcium ion chelators, chlortetracycline and anhydrochlortetracycline, also inhibited resistance, whereas treatment with the nonchelating analogue, isochlortetracycline, did not. This inhibition could be overcome by adding calcium to the tetracycline solution. These results lead us to hypothesize that calcium is required for activation of the resistance mechanism and that the ml-o mutation affects calcium regulation in the cell, resulting in an elevated cytosolic calcium ion level in the resistant isoline.