Cytoplasmic free calcium distributions during the development of root hairs of Arabidopsis thaliana

In this study, confocal ratio analysis was used 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 that preceded or predicted the site of root hair initiation could not be detected, once initiated the majority of emerging root hairs showed an elevated Ca2+c (> 1 microM) in their apical cytoplasm, compared with 100-200 nM 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 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 compared with 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 microM) 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 Ca(2+)-permeable channels, this result also suggests 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.