Cytosolic Calcium changes in response to touch but not gravity

Valerie Legue, Elison Blancaflor, Carol Wymer2, Dennis Fantin, Gérald Perbal and Simon Gilroy

University P. and M. Curie, Lab. CEMV, 4 Place Jussieu 75252 Paris cedex 05, France (V.L., G.P); Biology Department, The Pennsylvania State University, 208 Mueller Lab, University Park, PA 16802, USA (E.B., C.W., S.G.); Plant Biology Department, University of California, 111 Koshland Hall, Berkeley, CA 94720, USA (D.F)

Abstract
Changes in cytosolic calcium [Ca2+] have been proposed to be involved in signal transduction pathways in response to a number of stimuli including gravity and touch. The current hypothesis proposes that the development of gravitropic bending is correlated with a redistribution of [Ca2+] in the gravistimulated root. However no study has demonstrated clearly the development of an asymmetry of this ion during the establisment of root curvature. In this work we tested this hypothesis by quantifying the temporal and spatial changes in [Ca2+ ] in living seedling roots of Arabidopsis thaliana..We used indo-1, a dual-wavelength fluorescent Ca2+ indicator coupled with UV-confocal Ca2+-ratio imaging and vertical stage fluorescence microscopy to visualize [Ca2+]. Roots were loaded with Ca2+ imaging fluorescent indicators and followed for up to 12 h after gravistimulation. There were no changes in [Ca2+ ] observed to be associated with the graviresponse whether monitored at the whole organ level or in individual cells in the mature, elongating, meristematic or cap regions of the root. However, touch stimulation led to transient increases in [Ca2+ ] in all cell types monitored. The transients induced in the cap cells were larger and longer lived than those in the meristematic or elongation zone cells. These transients propagated from cell to cell for a range of approximately 150 µm from the point of stimulus with a speed of approximately 50 µm min-1. One mM La3+ and 100 µM verapamil did not prevent these responses whereas 5 mM EGTA or ruthenium red inhibited the transients, indicating an intracellular origin of the Ca2+ increase. These results suggest that although touch responses of roots may be mediated through a calcium-dependent pathway, the gravitropic response is not associated with detectable changes in [Ca2+].

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