Incompatibility Conditioned by theMlaGene in Powdery Mildew of Barley: The Halt in Cytoplasmic Streaming

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Abstract
Cytoplasmic streaming stopped in host epidermal cells of barley under attack by E. graminis f. sp. hordei when host-parasite incompatibility was conditioned by the Mla gene. The cytoplasm halted 1-3 h before the host cells collapsed, hypersensitively. These events in tissues partially isolated from coleoptiles were viewed at 1-4 h intervals by direct observation, or continuously by time-lapse cinephotomicrography. The hypersensitive collapse of cells usually occurred 18-26 h after inoculation when the primary haustorium was partly formed. The halt in cytoplasmic streaming that preceded this collapse was, in turn, preceded by a 0.5-h period in which host cytoplasm accumulated in small amounts near the haustorium. Organelles within this cytoplasm moved in a localized, restricted fashion. An early metabolic or structural change associated with gene-for-gene incompatibility interferes with cytoplasmic movement.