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PLANT PHYSIOLOGY , Vol 114, Issue 1 39-45, Copyright © 1997 by American Society of Plant Biologists
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DEVELOPMENT AND GROWTH REGULATION |
The Cytosolic Ca2+ Concentration Gradient of Sinapis alba Root Hairs as Revealed by Ca2+-Selective Microelectrode Tests and Fura-Dextran Ratio Imaging
H. H. Felle and P. K. Hepler
Botanisches Institut l, Universitat Giessen, Senckenbergstrasse 17, D-35390 Giessen, Germany (H.H.F.)
Using Ca2+-selective microelectrodes and fura 2-dextran ratio imaging, the
cytosolic free [Ca2+] was measured in Sinapis alba root hair cells. Both
methods yielded comparable results, i.e. values between 158 to 251 nM for
the basal [Ca2+] of the cells and an elevated [Ca2+] of 446 to 707 nM in
the tip region. The zone of elevated [Ca2+] reaches 40 to 60 [mu]m into the
cell and is congruent with the region of inwardly directed Ca2+ net
currents measured with an external Ca2+- selective vibrating electrode. The
channel-blocker La3+ eliminates these currents, stops growth, and almost
completely eliminates the cytosolic [Ca2+] gradient without affecting the
basal level of the ion. Growth is also inhibited by pressure-injected
dibromo-1,2-bis(o-aminophenoxy)ethane-N,N,N[prime],N[prime]-tetraacetic
acid, which causes a decrease in the [Ca2+] in the tip in a
concentration-dependent manner. Indole-3-acetic acid, used as a model
stimulus, decreases cytosolic free [Ca2+] by 0.2 to 0.3 pCa units in the
tip, but only by about 0.1 pCa unit in the shank. Nongrowing root hairs may
or may not display a [Ca2+] gradient, but still reversibly respond to
external stimuli such as La3+, Ca2+, or indole-3-acetic acid with changes
in cytosolic free [Ca2+]. During short time periods,
dicyclohexylcarbodiimide inhibition of the plasma membrane H+-ATPase, which
stops growth, does not abolish the [Ca2+] gradient, nor does it change
significantly the basal [Ca2+] level. We conclude that the cytosolic [Ca2+]
gradient and an elevated [Ca2+] in the tip, as in other tip-growing cells,
is essential for tip growth in root hairs; however, its presence does not
indicate growth under all circumstances. We argue that with respect to
Ca2+, tip growth regulation and responses to external signals may not
interfere with each other. Finally, we suggest that the combination of the
methods applied adds considerably to our understanding of the role of
cytosolic free [Ca2+] in signal transduction and cellular growth.
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