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cytoplasm di¡er in response to stresses induced by wind and
cold, respectively.
A pertinent question in Ca2 signalling research at the mo-
ment is whether nuclear Ca2 changes are capable of directly
in£uencing gene expression. A study by Hardingham et al.
[12] using nuclear microinjection of non-di¡usible Ca2 che-
lators suggests that the answer to this question is a¤rmative.
Further support for a role of Ca2 in gene transcription
comes from a recent study by Carrion et al. [28] who identi¢ed
a new human transcriptional repressor, DREAM (prodynor-
phin downstream regulatory element antagonist modulator).
DREAM is capable of binding speci¢cally to DNA and rep-
resents the ¢rst Ca2-binding protein to function as a direct
repressor of transcription. The ¢nding that Ca2 can reduce
the DREAM DNA a¤nity is the ¢rst evidence for a direct
mechanism of di¡erential regulation of gene expression not
depending on changes in the activity of other transcriptional
e¡ectors or direct protein^protein interactions. That similar
scenarios might exist in plant cells is not unlikely, but as yet
the necessary experiments to test this hypothesis remain to be
carried out.
Although we primarily have focused on the plant nucleus as
a Ca2 accumulating organelle, the possibility should not be
overlooked that the nucleus also could act as a locus for the
generation of cytoplasmic Ca2 signals. Ehrhardt et al. [15]
studying nod-factor signalling in root hairs thus observed that
Ca2-spiking could originate within the nuclear region and
our own studies of Ca2 waves in pollen tubes suggest that
a nuclear origin of Ca2 waves is a distinct possibility [1].
Nuclear membranes are also known to act as inositol(1,4,5)-
trisphosphate and inositol(1,3,4,5)-tetrakisphosphate releas-
able Ca2 stores in mammalian cells [29,30].
We think that our ¢ndings may provide a new starting
point for research into how extracellular stimuli, transduced
via intracellular Ca2-dependent pathways, regulate signal-in-
duced gene expression in plants.
Acknowledgements: T.D.B. and J.P.T. gratefully acknowledge support
from the John Innes Foundation and the B.B.S.R.C. We wish to
thank N. Stacey for valuable assistance in maintaining the cell cul-
tures. Work in the labs of B.K.D. and P.J.S. was supported by the
B.B.S.R.C. Intracellular Signalling Initiative.
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