Inhibition of the Gravitropic Response of Snapdragon Spikes by the Calcium-Channel Blocker Lanthanum Chloride

The putative Ca(2+)-channel blocker LaCl(3) prevented the gravitropic bending of cut snapdragon (Antirrhinum majus L.) spikes (S. Philosoph-Hadas, S. Meir, I. Rosenberger, A.H. Halevy [1996] Plant Physiol 110: 301–310) and inhibited stem curvature to a greater extent than vertical and horizontal stem elongation at the bending zone. This might indicate that LaCl(3), which modulates cytosolic Ca(2+), does not influence general stem-growth processes but may specifically affect other gravity-associated processes occurring at the stem-bending zone. Two such specific gravity-dependent events were found to occur in the bending zone of snapdragon spikes: sedimentation of starch-containing chloroplasts at the bottom of stem cortex cells, as seen in cross-sections, and establishment of an ethylene gradient across the stem. Our results show that the lateral sedimentation of chloroplasts associated with gravity sensing was prevented in cross-sections taken from the bending zone of LaCl(3)-treated and subsequently gravistimulated spikes and that LaCl(3) completely prevented the gravity-induced, asymmetric ethylene production established across the stem-bending zone. These data indicate that LaCl(3) inhibits stem curvature of snapdragon spikes by preventing several gravity-dependent processes. Therefore, we propose that the gravitropic response of shoots could be mediated through a Ca(2+)-dependent pathway involving modulation of cytosolic Ca(2+) at various stages.