Cytoplasmic pH dynamics in maize pulvinal cells induced induced by gravityvector changes

In maize (Zea mays) and other grasses, changes in orientation of stems are perceived by pulvinal tissue, which responds to the stimulus by differentia l growth resulting in upward bending of the stem. The amyloplast-containing bundle sheath cells are the sites of gravity perception, although the init ial steps of gravity perception and transmission remain unclear. In columel la cells of Arabidopsis roots, we previously found that cytoplasmic pH (pH( c)) is a mediator in early gravitropic signaling (A.C. Scott, N.S. Allen [1 999] Plant Physiol 121: 1291-1298). The question arises whether pH(c) has a more general role in signaling gravity vector changes. Using confocal rati ometric imaging and the fluorescent pH indicator carboxy seminaphtorhodaflu or acetoxymethyl ester acetate, we measured pH, in the cells composing the maize pulvinus. When stem slices were gravistimulated and imaged on a horiz ontally mounted confocal microscope, pH(c) changes were only apparent withi n the bundle sheath cells, and not in the parenchyma cells. After turning, cytoplasmic acidification was observed at the sides of the cells, whereas t he cytoplasm at the base of the cells where plastids slowly accumulated bec ame more basic. These changes were most apparent in cells exhibiting net am yloplast sedimentation. Parenchyma cells and isolated bundle sheath cells d id not show any gravity-induced pH(c) changes although all cell types respo nded to external stimuli in the predicted way: Propionic. acid and auxin tr eatments induced acidification, whereas raising the external pH caused alka linization. The results suggest that pH(c) has an important role in the ear ly signaling pathways of maize stem gravitropism.