1021-4437/01/4804- $25.00 © 2001
Russian Journal of Plant Physiology, Vol. 48, No. 4, 2001, pp. 426–430. Translated from Fiziologiya Rastenii, Vol. 48, No. 4, 2001, pp. 498–502.
Original Russian Text Copyright © 2001 by Maksimov, Darginaviciené,ˇ Jurkoniene, Merkys.
The plasma membrane (plasmalemma) is known to
be a multifunctional structure. Its involvement in the
life of the cell and the entire organism is surely not lim-
ited only by its barrier function and by the generation
and maintenance of the electrochemical potential. Due
to the presence of various ion channels, carriers, and
the active systems of receptors and ion transport, the
plasmalemma is able to perform various speciﬁc regu-
latory functions, in particular, the regulation of growth
processes [1–3]. Hence, it can be assumed that the plas-
malemma could also be involved in the control of the
plant gravitropic response, since the gravity vector, a
physical factor of the environment, induces lateral
growth polarization, that is, the difference in the growth
rates of the upper and lower halves of the horizontally
arranged axial organs (coleoptiles, roots, etc.) [4, 5].
This problem has been poorly investigated.
A number of environmental factors, such as light,
low and high temperatures, water deﬁcit, change in the
gravity vector force, etc., are known to induce one of
the primary responses of plants, that is, an increase in
the free calcium concentration in the cytosol [6–8].
We proceeded from the assumption that the earliest
cell responses to gravitropic stimulation (GS) should
occur in the plasmalemma. They include, ﬁrst of all, a
change in the membrane permeability for ions and, in
particular, for calcium ions. This results in an increase
in the level of free (ionic) calcium with the subsequent
modiﬁcation of the intracellular regulatory systems,
including the hormonal ones [6, 9]. The experimental
veriﬁcation of this logical, but largely hypothetical,
suggestion is the objective of this work.
MATERIALS AND METHODS
The work was carried out using coleoptile segments
from actively growing etiolated three- to four-day-old
seedlings of spring wheat (
Selpek). The seedlings grew vertically upwards in moist
in darkness. The 20–25-mm-long
coleoptiles were decapitated by cutting off the 3-mm-long
apices, and the ﬁrst leaves were removed. The segments
were horizontally placed, thus being exposed to GS of
for the various periods of time indicated in the ﬁg-
ure captions. Control coleoptiles constantly remained
in the vertical position.
The membrane fraction enriched with plasmalem-
mal vesicles was isolated from wheat coleoptiles by
differential centrifugation and puriﬁcation in the
sucrose density gradient with some modiﬁcations [10–
12]. All steps of the isolation were performed at 0–4
A speciﬁc binding of IAA by the plasmalemmal
preparations was determined as described in  and
expressed as a difference between the total binding of
H-IAA, 962 GBq/mmol) and the
residual amount of label. The plasmalemmal fraction
was incubated with 5
H-IAA and 5
of cold IAA in a medium of the following composition:
1 mM Tris–Mes, pH 7.2; 50 mM KCl, 3 mM MgCl
and 4 mM 2-mercaptoethanol. The membrane fraction
collected from the sucrose gradient was incubated in
the IAA-containing medium (about 50
g of protein
l of the medium) at 25
C for 30 min. Excess
free IAA was removed by equilibrium dialysis .
The total binding of
H-IAA in the absence of cold IAA
was equal to 30000–40 000 cpm. Speciﬁc binding was
about 10% of the total.
Role of the Plasmalemma
in Controlling the Gravitropic Response of Wheat Coleoptiles
G. Maksimov, Yu. Darginaviciené,
S. Jurkoniene, and A. Merkys
Institute of Botany, Zhalyuyu Ezhyaru, 49, Vilnius, LT-2021 Lithuania; fax: 8 (0122) 72-9950; e-mail: firstname.lastname@example.org
Received April 18, 2000
—The changes in the speciﬁc binding of
H-IAA to the plasmalemma from segments of wheat (
L.) coleoptiles and the physiological activity of the IAA–protein complexes thus formed in
dependence on the duration of gravitational stimulation (GS) (1
) were studied. The overall inhibition of the
formation of IAA–protein complexes was accompanied by a transverse polarization of their functional activity
occurring as early as within two minutes after the onset of GS. The pretreatment of plasmalemmal vesicles with
IAA–protein complex formation in the plasmalemma. It is suggested that
the GS results in an increase in the plasmalemma permeability for Ca
, which reduces the capacity of the plas-
malemma to bind IAA at the early stages of the gravitropic response.
Key words: Triticum aestivum - gravitropic response - plasmalemmal vesicles - speciﬁc binding of IAA - Ca
: GS—gravitropic stimulation.