RECOGNITION OF A PATHOGEN AND A NONPATHOGEN BY BARLEY COLEOPTILE CELLS .1. CYTOPLASMIC RESPONSES TO THE NONPATHOGEN, ERYSIPHE-PISI, PRIOR TO ITS PENETRATION

Cytoplasmic responses of barley coleoptile cells to Erysiphe pisi, a nonpathogen for barley, were examined during the prepenetration stages of fungal infection using a time-lapse video system and oil droplets microinjected into cells. Conspicuous cytoplasmic strands were seen underneath and extending away from appressoria of the fungus. These strands became visible when a circular structure appeared in one of the appressorial lobes, a feature which was used to define appressorial maturity. Thereafter, active cytoplasmic streaming occurred near the appressoria. These cytoplasmic responses were evident 4-5 h prior to the actual penetration of the host cell by the fungus. To monitor the cytoplasmic activity of coleoptile cells, a droplet of silicon oil was injected into individual coleoptile cells before the fungus was inoculated, and the movement of the oil droplet was monitored with a video system. The droplet moved at 2-3 μm min-1 soon after injection but its velocity increased to 4-5 μm min-1 after a germling of the fungus was transferred onto the individual coleoptile cell. When the appressoria matured on the cells, the velocity increased dramatically to 73 μm min-1. High velocities were then maintained, except for intermittent periods of slowing, until cytoplasmic aggregates were initiated beneath appressoria, when the velocity was reduced to 4-5 μm min-1 and the oil droplet remained near the appressorium. Although movement of the oil droplet did not correlate closely with rate of cytoplasmic streaming, the changes in droplet movement clearly indicated that cell cytoplasm responded to the presence of the fungus. These results suggest that appressoria of the fungus release a signal(s) upon maturation, 4-5 h prior to penetration, which induces physiological changes in the cytoplasm of the coleoptile cell. © 1990.