Calcium Biochemistry
167
inhibition of bone resorption accompanied
by a reduction in the secretion of resorptive
enzymes.
7.3
The Role of Calcium in Plants
Most Ca
2
+
dependent signal transduction
pathways as described for animal cells
are also true for plant cells. So it is not
surprising that most systems described
before controlling calcium homeostasis
found their counterparts in plants also.
Nevertheless, there are some peculiar
Ca
2
+
-dependent processes or properties,
only found in plants, which should be
described in more detail.
7.3.1
Touch Genes
Plants show some speciFc reactions that
seem to be calcium-dependent; that is, they
sense and actively respond to many en-
vironmental stimuli such as rain, wind,
gravity, changes in osmotic conditions,
and touch. The latter is demonstrated very
impressively by the Venus fly trap (
Dion-
aea muscipula
) using a sensitive touch
response to catch preys, or by the
Mi-
mosa pudica
closing the leaflets upon touch
stimuli. It was discovered by Braam and
Davies that the plant
Arabidopsis
demon-
strated a rapid and strong induction of
gene expression following touch stimu-
lation. Several of these genes revealed
either virtual identity or close relationship
to calmodulin, indicating that these gene
produc
tsmayp
layacen
tra
lro
leinCa
2
+
-
dependent signal transduction pathways
involved in touch response. The response
is relatively fast; for example, all of the
responding genes showed up to 100-fold
increased expression after 10 min, but
maximal expression varied for the different
genes. ±urthermore, these genes not only
demonstrated a time-dependent but also a
dose-dependent response to touch stimuli;
that is, the level of transcripts of the touch
genes was proportional to the strength
of the stimuli. These Fndings indicated
that there is an immediate, but transient,
requirement for Ca
2
+
binding proteins
following mechanical stimulation, result-
ing in growth inhibition. It is possible
that these proteins act as Ca
2
+
-dependent
modulators of target enzymes, but other
explanations are also possible such as in-
volvement in the regulation of cellular
Ca
2
+
homeostasis or changing microtubu-
lar arrangements. The latter is particularly
attractive since this activity could influ-
ence the orientation of the axes of cell
division and elongation as a consequence
of mechanical stimulation.
7.3.2
Calmodulin-like Domain Protein
Kinase (CDPK)
In
plants,
a
new
class
of
calcium-
dependent kinases has been described.
These enzymes require micromolar Ca
2
+
concentrations for activity, but they do not
seem to need the presence of calcium effec-
tor molecules such as calmodulin, phos-
phatidylserine, or diacylglycerol. It could
be demonstrated that they bind Ca
2
+
di-
rectly through a regulatory domain on the
same polypeptide chain containing the cat-
alytic domain. Cloning of the correspond-
ing cDNA encoding a calmodulin-like
domain protein kinase (CDPK) revealed
characteristic regions of serine/threonine
kinases. In addition, the regulatory region
contains a domain with a high degree of
homology to calmodulin including four
Ca
2
+
binding sites of the E±-hand-type
family. Thus, the CDPK family can be dis-
tinguished from other calcium-regulated
protein kinases by fusion of a regulatory,
calcium binding, calmodulin-like domain
to a protein kinase catalytic domain. Inves-
tigation of the substrate speciFcity of this
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