Title: Primary germ tubes and host cell penetrations from appressoria of Erysiphe graminis hordei.
Abstract: Conidia of Erysiphe graminis hordei usually produce multiple germ tubes on epidermal cells of barley coleoptiles. Primary germ tubes precede appressorial germ tubes by several hours and ordinarily send penetration pegs into the host cell walls, eliciting papilla formation. We investigated by interference contrast microscopy the in vivo time courses of selected events from 0 to 24hr after inoculation to determine whether or not host responses to primary germ tubes might influence the penetration efficiency of the appressoria that subsequently develop from appressorial germ tubes. Cytoplasmic aggregates were associated with 86% of the primary germ tubes, and appressorial germ tubes emerged from 88% of those conidia that had primary germ tubes. In only 16.5% of the conidia with mature appressoria did the appressoria produce haustoria in the absence of cytoplasmic aggregates below the primary germ tubes. Whenever cytoplasmic aggregates were associated with primary germ tubes, these aggregates always preceded the development of appressoria from appressorial germ tubes of the same conidia. For time course analyses, conidia that attempted penetrations from appressoria were divided into groups defined by the timing of primary germ tube-associated cytoplasmic aggregate initiation and termination, appressoria emergence, and whether or not haustorial central bodies were formed. The interval from primary germ tube-associated cytoplasmic aggregate initiation to appressorial germ tube emergence remained almost constant. The interval between primary germ tube-associated cytoplasmic aggregate initiation and termination was longer when appressoria failed to produce haustoria than when they succeeded. Late termination of primary germ tube-associated cytoplasmic aggregates correlated with a reduction in subsequent haustorial formation.