Many fungi develop specialized infection structures, termed appressoria, which are necessary for successful penetration of their hosts. Formation of these appressoria, a cell differentiation event, occurs when the fungus perceives specific signals from the host. In the bean rust fungus, Uromyces appendiculatus, such signals are entirely topographical and are located at stomata, the only sites at which the fungus can successfully penetrate the leaf of the host plant. Previous research with this fungus has shown that the cytoskeleton is involved in mediating the signal for cell differentiation. More recent data suggest that the plasma membrane is involved in initial reception of the signal. In the proposed continuation of this research the role of the plasma membrane in signal reception will be examined in greater detail. Electrophysiological studies of the membrane during signal reception will be done to determine whether stretch-activated ion channels or ATPase pump activities are primarily involved in the reception process. Possible changes in the distribution of intramembrane particles during signal reception will also be characterized. Finally, biochemical pathways known as membrane signal transducers in other systems will be sought in the plasma membrane of Uromyces. These studies will advance our knowledge of the mechanisms involved in the sensing responses that result in the accurate placement of the invading fungal appressorium over the host leaf stomate during colonization of the host plant by the fungus. The significance of these experiments extends beyond the fungus-host plant interaction, however. Little is yet known about the mechanisms by which cells, in general, sense and respond to the physical features of their environment. Stretch-activated ion channels in cell membranes have only recently been discovered, and their function in cellular sensing of mechanical stimuli has been proposed, but not clearly established. Studies in this interesting system, could therefore shed light on the general problem of the mechanism of mechanical sensitivity of cells, and the possible role of stretch-activated ion channels in this process.
