INHIBITION OF THE GRAVITROPIC RESPONSE OF SNAPDRAGON SPIKES BY THE CALCIUM-CHANNEL BLOCKER LANTHANUM CHLORIDE

The putative Ca2+-channel blocker LaCl3 prevented the gravitropic bend ing of cut snapdragon (Antirrhinum majus L.) spikes (S. Philosoph-Hada s, S. Meir, I. Rosenberger, A.H. Halevy [1996] Plant Physiol 110: 301- 310) and inhibited stem curvature to a greater extent than vertical an d horizontal stem elongation at the bending zone. This might indicate that LaCl3, which modulates cytosolic Ca2+, does not influence general stem-growth processes but may specifically affect other gravity-assoc iated processes occurring at the stem-bending zone. Two such specific gravity-dependent events were found to occur in the bending zone of sn apdragon spikes: sedimentation of starch-containing chloroplasts at th e bottom of stem cortex cells, as seen in cross-sections, and establis hment of an ethylene gradient across the stem. Our results show that t he lateral sedimentation of chloroplasts associated with gravity sensi ng was prevented in cross-sections taken from the bending zone of LaCl 3-treated and subsequently gravistimulated spikes and that LaCl3 compl etely prevented the gravity-induced, asymmetric ethylene production es tablished across the stem-bending zone. These data indicate that LaCl3 inhibits stem curvature of snapdragon spikes by preventing several gr avity-dependent processes. Therefore, we propose that the gravitropic response of shoots could be mediated through a Ca2+-dependent pathway involving modulation of cytosolic Ca2+ at various stages.