Cytoplasmic Free Calcium Distributions During the Development of Root Hairs of Arabidopsis thaliana
Carol L. Wymer1,2, Tatiana N. Bibikova2 and Simon Gilroy
Department of Biology, 208 Mueller Lab, The Pennsylvania State University, University Park, PA 16802
SUMMARY
We have used confocal ratio-analysis to image the relationship between cytoplasmic free calcium concentration ([Ca2+]c) and the development of root hairs of Arabidopsis thaliana. Although a localized change in [Ca2+]c did not precede or predict the site of root hair initiation, once initiated the majority of emerging root hairs showed an elevated [Ca2+]c (>1 µM) in their apical cytoplasm, compared to 100nM in the rest of the cell. These emerging root hairs then moved into a 3-5 h phase of sustained elongation during which they showed variable growth rates. Root hairs that were rapidly elongating also exhibited a highly localized, elevated [Ca2+]c at the tip. Non-growing root hairs did not exhibit the [Ca2+]c gradient. The rhd-2 mutant which is defective in sustained root hair growth showed an altered [Ca2+]c distribution, as compared to wild-type. These results implicate [Ca2+]c in regulating the tip growth process. Treatment of elongating wild-type root hairs with the Ca2+-channel blocker verapamil (50 µM) caused dissipation of the elevated [Ca2+]c at the tip and cessation of growth, suggesting a requirement for Ca2+-channel activity at the root hair tip to maintain growth. Manganese treatment also preferentially quenched Indo-1 fluorescence in the apical cytoplasm of the root hair. As manganese is thought to enter cells through Ca2+-permeable channels this result also implicates increased Ca2+-channel activity at the tip of the growing hair. Taken together, these data suggest that although Ca2+ does not trigger the initiation of root hairs, Ca2+ influx at the tip of the root hair leads to an elevated [Ca2+]c that may be required to sustain root hair elongation.
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