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Plant Physiology Preview Published on June 26, 2008; 10.1104/pp.108.123638
OPEN ACCESS ARTICLE
Received May 28, 2008 Imaging of the Yellow Cameleon 3.6 Indicator Reveals that Elevations in Cytosolic Ca2+ Follow Oscillating Increases in Growth in Root Hairs of Arabidopsis thaliana
Department of Botany, University of Wisconsin, Madison WI 53706, USA; BioCurrents Research Center, Marine Biological Laboratory, Woods Hole, MA 02543, USA * Corresponding author; email: sgilroy{at}wisc.edu.
In tip growing cells, the tip-high Ca2+ gradient is thought to regulate the activity of components of the growth machinery including the cytoskeleton, Ca2+-dependent regulatory proteins and the secretory apparatus. In pollen tubes, both the Ca2+ gradient and cell elongation show oscillatory behavior reinforcing the link between the two. We report that in growing root hairs of Arabidopsis thaliana, an oscillating tip-focused Ca2+ gradient can be resolved through imaging of a cytosolically expressed Yellow Cameleon (YC) 3.6 FRET-based Ca2+ sensor. Both elongation of the root hairs and the associated tip-focused Ca2+ gradient show a similar dynamic character, oscillating with a frequency of 2-4 min-1. Cross-correlation analysis indicates that the Ca2+ oscillations lag the growth oscillations by 5.3 ± 0.3 s. However, growth never completely stops, even during the slow cycle of an oscillation, and the concomitant tip Ca2+ level is always slightly elevated compared to the resting Ca2+ concentration along the distal shaft, behind the growing tip. Artificially increasing Ca2+ using the Ca2+ ionophore A23187 leads to immediate cessation of elongation and thickening of the apical cell wall. In contrast, dissipating the Ca2+ gradient using either the Ca2+ channel blocker La3+ or the Ca2+ chelator EGTA is accompanied by an increase in the rate of cell expansion and eventual bursting of the root hair tip. These observations are consistent with a model where the maximal oscillatory increase in cytosolic Ca2+ is triggered by cell expansion associated with tip growth and plays a role in the subsequent restriction of growth.
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