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 mu M) 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 mu M) 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 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 heir leads to an elevated [Ca2+](c) that may be required to sustain root hair elongation.