CYTOPLASMIC FREE CA2+ IN ARABIDOPSIS ROOTS CHANGES IN RESPONSE TO TOUCH BUT NOT GRAVITY

Changes in cytoplasmic Ca2+ concentration ([Ca2+](i)) have been propos ed to be involved in signal transduction pathways in response to a num ber of stimuli, including gravity and touch. The current hypothesis pr oposes that the development of gravitropic bending is correlated with a redistribution of [Ca2+](i) in gravistimulated roots. However, no st udy has demonstrated clearly the development of an asymmetry of this i on during root curvature. We tested this hypothesis by quantifying the temporal and spatial changes in [Ca2+](i) in roots of living Arabidop sis seedlings using ultraviolet-confocal Ca2+-ratio imaging and vertic al stage fluorescence microscopy to visualize root [Ca2+](i). We obser ved no changes in [Ca2+](i) associated with the graviresponse whether monitored at the whole organ level or in individual cells in different regions of the root for up to 12 h after gravistimulation. However, t ouch stimulation led to transient increases in [Ca2+](i) in all cell t ypes monitored. The increases induced in the cap cells were larger and longer-lived than in cells in the meristematic or elongation zone. On e millimolar La3+ and 100 mu M verapamil did not prevent these respons es, whereas 5 mM EGTA or 50 mu M ruthenium red inhibited the transient s, indicating an intracellular origin of the Ca2+ increase. These resu lts suggest that, although touch responses of roots may be mediated th rough a Ca2+-dependent pathway, the gravitropic response is not associ ated with detectable changes in [Ca2+](i).