Article

Barrier function of the cell wall during uptake of nickel ions

Russian Journal of Plant Physiology (Impact Factor: 0.95). 05/2011; 58(3):409-414. DOI: 10.1134/S1021443711030137

ABSTRACT

Cell walls were isolated from roots of six plant species to study their ion-exchange capacity for nickel ions (S
Ni) at Ni2+ concentration of 10−3 M. The S
Ni values varied depending on the plant species from 50 to 150 μmol Ni2+ per gram dry wt; the sorption capacity increased in a row: Poaceae < Chenopodiaceae < Fabaceae. At pH 5 the sorption capacity
of cell walls for nickel ions was determined by the presence of carboxyl groups of polygalacturonic acid in the polymeric
cell-wall matrix. In all cases the ion-exchange capacity of cell walls was higher at pH 8 than at pH 5, indicating that Ni2+ binds also to a carboxyl group different from that of polygalacturonic acid. Irrespective of plant species, the presence
of EDTA in the solution diminished drastically the absorption capacity of cell walls for Ni2+. It is concluded that the presence of 10−3 M EDTA weakens the defense properties of cell walls. The sequestration of Ni2+ in the cell wall can be considered as an effective means of plant cell defense against elevated concentrations of nickel
ions in the external medium.

Keywordshigher plants–cell walls–nickel ions–carboxyl groups–EDTA

ISSN 10214437, Russian Journal of Plant Physiology, 2011, Vol. 58, No. 3, pp. 409–414. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © N.R. Meychik, Yu.I. Nikolaeva, O.V. Komarynets, I.P. Ermakov, 2011, published in Fiziologiya Rastenii, 2011, Vol. 58, No. 3, pp. 345–350.
409
INTRODUCTION
The cell wall compartment of plant roots is crucial
for primary absorption of mineral nutrients owing to
its physical and physicochemical characteristics. Dur
ing growth and development the plant needs a wide
range of elements, including heavy metals such as Cu,
Ni, and Zn that are constituents of many enzymes and
other proteins. However, elevated concentrations of
these elements in soil are toxic to most plants and
inhibit their growth.
In the presence of elevated concentrations of metal
ions (Me
2+
), symptoms of toxicity appear both at the
cell and molecular levels. For example, the binding of
Me
2+
with sulfhydryl groups of proteins was found to
inhibit activity and/or cause protein destruction,
while the replacement of an essential element with
heavy metal ions entering the plant entails the deficit
of essential nutrients and disrupts the operation of
metabolic systems [1]. The excess of Me
2+
may pro
mote the generation of free radicals and reactive oxy
gen species and, consequently, cause oxidative stress
[2, 3]. In response to stress initiated by Me
2+
, plants
develop a wide range of mechanisms aimed at prevent
ing the accumulation of toxic Me
2+
concentrations in
the cytoplasm. These mechanisms include: (i) immo
bilization of Me
2+
in the cell wall; (ii) the decreased
absorption and/or effective excretion of absorbed
Me
2+
by plasmamembrane transporters; (iii) chela
tion of Me
2+
in the cytoplasm by various ligands
(organic acids, amino acids, phytochelatins, metal
lothioneins, metalbinding proteins); (iv) repair of
stressdamaged proteins; and (v) sequestration of
Me
2+
in the vacuole by means of the tonoplast trans
porters [4].
In the first of the aforementioned mechanisms, the
root cell wall acts as a protective barrier, whose effec
tiveness is due to the capacity of absorbing Me
2+
.
Scarce data available in the literature concerning
immobilization of Me
2+
on the cell walls provide evi
dence for the important role of extracellular mecha
nism in plant resistance to this type of stress [5–11].
For example, it was found with histochemical meth
ods that
Pb
2+
binds strongly to functional groups in
maize root cell walls, where it accumulates in substan
tial quantities [6]. The cell walls of
Chrysanthemum
coronarium
and
Sorghum sudanense
were found to
accumulate copper ions in amounts of 60 and 76%,
respectively, of the total content of copper ions perme
ated into the root cells [11]. The significant role of cell
walls in compartmentalization of Me
2+
was established
for the roots of the fern
Athyrium
yokoscense
inhabiting
RESEARCH
PAPERS
Barrier Function of the Cell Wall during Uptake
of Nickel Ions
N. R. Meychik, Yu. I. Nikolaeva, O. V. Komarynets, and I. P. Ermakov
Department of Plant Physiology, Faculty of Biology, Moscow State University, Moscow, 119899 Russia;
fax: 7 (495) 9394309; email: meychik@mail.ru
Received August 31, 2010
Abstract
—Cell walls were isolated from roots of six plant species to study their ionexchange capacity for
nickel ions (
S
Ni
) at Ni
2+
concentration of 10
–3
M. The
S
Ni
values varied depending on the plant species from
50 to 150
µ
mol Ni
2+
per gram dry wt; the sorption capacity increased in a row: Poaceae < Chenopodiaceae <
Fabaceae. At pH 5 the sorption capacity of cell walls for nickel ions was determined by the presence of car
boxyl groups of polygalacturonic acid in the polymeric cellwall matrix. In all cases the ionexchange capac
ity of cell walls was higher at pH 8 than at pH 5, indicating that Ni
2+
binds also to a carboxyl group different
from that of polygalacturonic acid. Irrespective of plant species, the presence of EDTA in the solution dimin
ished drastically the absorption capacity of cell walls for Ni
2+
. It is concluded that the presence of 10
–3
M
EDTA weakens the defense properties of cell walls. The sequestration of Ni
2+
in the cell wall can be consid
ered as an effective means of plant cell defense against elevated concentrations of nickel ions in the external
medium.
Keywords:
higher plants, cell walls, nickel ions, carboxyl groups, EDTA.
DOI:
10.1134/S1021443711030137
Abbreviations
: PGA—polygalacturonic acid;
S
Ni
—ionexchange
capacity of cell walls for Ni
2+
.
Page 1
410
RUSSIAN JOURNAL OF PLANT PHYSIOLOGY Vol. 58 No. 3 2011
MEYCHIK
et al.
serpentine (metalcontaminated) soils. The cell walls
of this species contained up to 70–90% of copper,
zinc, and cadmium of the total amount of metals
detected in the root cells [5].
Analysis of literature data shows that, under ele
vated concentrations of Me
2+
in the environment, the
deposition of metals in the cell wall can be considered
as a mechanism protecting the cells from detrimental
action of Me
2+
. However, the described experimental
conditions, procedures for evaluating the Me
2+
sorp
tion capacity of cell walls, and techniques for identifi
cation of functional groups responsible for this capac
ity, rely on destructive and basically different methods,
which allow only semiquantitative estimates (e.g.,
[10, 11]). It is therefore impossible to compare the
Me
2+
adsorption capacities of cell walls in various
plant species under different conditions. Among vari
ous heavy metals, nickel is an essential nutrient in
minute doses but acts as a toxicant at concentrations
above
10
–6
M. Biological effects of Ni
2+
are less char
acterized than those of other Me
2+
, while interactions
of Ni
2+
with the cell wall have not been yet investi
gated.
Using the previously developed method [12] and
roots of various plant species, we quantified the Ni
2+
sorption capacity of cell walls isolated without
destructive physicochemical procedures and deter
mined the dependence of this parameter on the num
ber of ionexchange groups in cell walls and on the ion
composition of the solution.
MATERIALS AND METHODS
We used 21dayold wheat plants (
Triticum aesti
vum
L., family Poaceae) grown on a modified Prya
nishnikov nutrient medium; 50dayold plants of
Suaeda altissima
(L.) Pall. and spinach (
Spinacia oler
acea
L., cv. Matador) from the family Chenopodi
aceae grown on Robinson nutrient solution [13]; and
20dayold plants from the family Fabaceae—mung
bean
Vigna radiata
L.; chickpea
Cicer arietinum
L., cv.
Bivanij; and vetch
Vicia narbonesis
L., line Sel2384—
grown on Pryanishnikov nutrient medium.
The polymeric matrix of cell walls was isolated as
previously described [14]. Plant tissues were placed
into an ionexchange glass column (
V
= 250 ml) and
washed in a dynamic regime sequentially with 1%
NaOH (~0.5 l), H
2
O (~2 l), 1% HCl (0.5 l), and finally
with distilled water until the disappearance of chloride
in the eluted water. Chloride content was assayed by
titration with mercury nitrate. Next, the material was
dried at
55–60
°
C
in the presence of moisture absor
bent CaCl
2
to a constant weight. The described proce
dure of material standardization, i.e., the method for
converting all cationexchange groups within the cell
wall structure into the H
+
form and all anion
exchange groups into the form of free amine, is suit
able for comparative investigation of sorption proper
ties of natural ionexchange materials, which structure
contains different functional groups [12].
The extent of purification of isolated cell walls from
the protoplast components was controlled with fluo
rescence microscopy as previously described [14].
Determination of cellwall sorption capacity for
nickel ions.
Weighed amounts of dried cell walls (
40 ±
0.1
mg) were placed into 50ml glass vials with tightly
fit glass caps, and 12.5 ml of 10
–3
M
NiCl
2
×
6H
2
O
solution was added. The samples were allowed to stay
for 7 days, and then they were removed from the solu
tion. Before and after the incubation of samples, the
pH of the solutions was measured with a 3320 Jenway
pH Meter (United Kingdom), and the concentration
of nickel ions was determined by spectrophotometric
measurements.
Based on changes in
Ni
2+
concentration in the
solution, the ionexchange capacity of cell walls was
calculated from the formula:
where
S
Ni
is the sorption capacity of cell wall samples
for nickel ions expressed in
µ
mol/g dry wt;
C
in
and
C
fin
are initial and final concentrations of Ni
2+
in the solu
tion, mM;
V
is the solution volume, ml; and
g
is the
sample weight, g.
Spectrophotometric assay of nickel concentration in
solutions.
The known aliquot of Ni
2+
containing solu
tion was first transferred into a 25ml flask and then
supplemented sequentially with 1 ml of 1% dimethyl
glyoxime aqueous solution, 2 ml of freshly prepared
4% ammonium persulfate, and 5 ml of concentrated
ammonia. The solution volume in the flask was
adjusted with water to 25 ml and stirred. After 10min
incubation, the optical density of colored solution was
measured at 445 nm (blue filter) using a KFK 2MP
photocolorimeter (Russia). The reference solution
was prepared in the same way but in the absence of
Ni
2+
. In preliminary experiments the photocolorime
ter was calibrated using
Ni
2+
solutions containing 5–
100
µ
g
Ni
2+
per sample; these solutions were prepared
according to the spectrophotometric assay of Ni
2+
determination.
Desorption of nickel ions from cell walls.
After sorp
tion of nickel ions, the cell wall preparations were
transferred onto a funnel with a paper filter and
washed with distilled water until disappearance of Ni
2+
in washing water (the presence of Ni
2+
in washing
waters was checked from the reaction with dimethyl
glyoxime). Next, the cell walls preparations were
wiped with filter paper, transferred into the flasks (
V
=
5 ml), and poured with 2 ml of 0.1 M HCl solution.
After 7day incubation, the cell walls were separated
S
Ni
C
in
C
fin
()V×
g
,=
Page 2
RUSSIAN JOURNAL OF PLANT PHYSIOLOGY Vol. 58 No. 3 2011
BARRIER FUNCTION OF THE CELL WALL DURING UPTAKE 411
from the solution. An aliquot of the solution was trans
ferred into a flask (25 ml), and the content of nickel
ions was determined spectrophotometrically as
described above. The sorption capacity of cell walls for
nickel ions (
S
Ni
) was calculated from the formula:
where
S
Ni
is the sorption capacity of cell walls for
nickel ions,
µ
mol/g dry wt;
C
Ni
is nickel content in the
aliquot with volume
V
а
,
µ
g;
V
а
is the volume of solution
sampled for
Ni
2+
analysis, ml;
V
р
is volume of 0.1 M
HCl used for desorption, ml;
g
is the weighed amount
of cell walls, g; and Mw
Ni
is the nickel atomic mass.
Statistical analysis was performed using the Excel
program. The diagrams show mean values and stan
dard deviations determined from 3–5 assays or repli
cate experiments.
RESULTS AND DISCUSSION
The sorption capacity of cell walls for Ni
2+
(
S
Ni
)
was determined from ion concentration changes in the
solution after its equilibration with the cell walls, as
well as from Ni
2+
quantities eluted from cell walls in
the presence of 0.1 M HCl. As can be seen in Fig. 1,
both methods gave similar estimates of
S
Ni
within 5%
relative error, indicating that any of these methods is
applicable for determination of ionexchange capacity
of isolated cell walls for Ni
2+
.
The ability of cell walls to adsorb
Ni
2+
from aque
ous solutions depended significantly on plant species,
ranging from 50 to 150
µ
mol (3500–9000
µ
g) Ni per
gram dry wt (Fig. 1). The lowest
S
Ni
value was noted for
cell walls of wheat roots, and the largest values were
observed for isolated cell walls from mung bean roots.
However, it should be noted that high
Ni
2+
absorbing
capacity of cell walls in all plant species examined was
only evident with pure aqueous solutions containing
Ni
2+
as the only cation species.
There are no published data concerning the
Ni
2+
content in cell walls of the plant roots examined. How
ever, the content of copper ions is known for cell walls
of
C. coronarium
and
S. sudanense
roots [11]. The root
cell walls of these plants were found to accumulate
0.1–0.5
µ
mol
Cu
2+
per gram dry wt at
Cu
2+
concentra
tion in the medium ranging from
10
–6
to
5
×
10
–5
M.
Nishizono et al. [5] showed that the root cell walls of
A. yokoscence
plants inhabiting coppercontaminated
soils are characterized by high Cu content, up to
56
µ
mol/g root dry wt. The authors noted that the
ability of cell walls from pteridophyte roots to adsorb
heavy metal ions is independent of growth conditions,
because the walls of many fern species inhabiting soils
with low
Cu
2+
content had also high ionexchange
capacity for copper ions. Under our experimental
S
Ni
C
Ni
V
p
V
a
gМм
Ni
,=
conditions (
10
–3
M Ni
2+
, proportion of solution vol
ume and cell wall dry weight ~312.5 ml/g), the cell
walls of all plants examined absorbed larger amounts
of Ni
2+
, as compared to absorption of copper ions by
cell walls of chrysanthemums, Sudan grass, and ferns.
The capacity of cell walls to adsorb Ni
2+
in exam
ined plant families increased in the following row:
Poaceae < Chenopodiaceae < Fabaceae. We assume
that this series reflects the abundance of pectins in the
cell walls. The role of cell wall pectins in binding of
heavy metal ions has been widely discussed in the lit
erature. For example, the roots of
Lygodium japonicum
accumulated copper in cell walls owing to copper–
pectin interactions, and at least 66% of
Cu
2+
was
tightly bound to homogalacturonans [10]. The in vitro
pectin from the seagrass
Zostera marina
was shown to
bind lead, cadmium, and copper; its binding capacity
was on the whole weaker than that of chelating agents
and thiolcontaining substances but it was markedly
higher than that of activated charcoal, microcrystal
line cellulose, and other sorbents [15].
We have found previously that the content of car
boxyl groups of polygalacturonic acid (PGA) in cell
walls depends largely on plant species [16]. The cell
walls of leguminous plants contained the highest con
centration of PGA carboxyl groups, while the walls of
gramineous plants had the lowest content of such
groups. Our results are fully consistent with the sup
posed significance of pectin polymers in binding heavy
metal ions at high metal concentrations in the envi
ronmental medium. Indeed, the legumes featuring the
highest content of PGA carboxyl groups were charac
200
150
100
50
0
1
2
S
Ni
,
µ
mol/g dry wt
Wheat
Spinach
Suaeda Vicia Cicer Vigna
1
Vigna
2
Roots Stem Roots
Fig. 1.
Sorption capacity for Ni
2+
(
S
Ni
) in isolated cell
walls from various plant species.
Nickel was absorbed from aqueous solutions containing
10
–3
M Ni
2+
. Data on Ni
2+
sorption in cell walls were
obtained by two methods based on determination of: (
1
)
decline of Ni
2+
content in the solution and (
2
) Ni
2+
des
orption from cell walls in the presence of 0.1 M HCl. Data
represent mean values and standard deviations obtained
from 3–5 replicate experiments.
Page 3
412
RUSSIAN JOURNAL OF PLANT PHYSIOLOGY Vol. 58 No. 3 2011
MEYCHIK
et al.
terized by the highest sorption capacity for
Ni
2+
. The
results showed that, under experimental conditions
adopted in this study, the ionexchange capacity of cell
walls for Ni
2+
and the amount of PGA carboxyl groups
are interrelated parameters (Fig. 2). This is evident
from the correlation coefficient of 0.93. Thus, we can
state that the ability of plant cell wall to adsorb Ni
2+
is
mainly due to the presence of PGA carboxyl groups in
its polymeric matrix.
Calculations showed that, depending on plant spe
cies, the nickel binding can engage 30 to 80% of all
available PGA carboxyl groups in the cell wall. It
should be pointed out that divalent nickel ions bind to
the cellwall active sites by interacting with two PGA
carboxyl groups. Under our experimental conditions,
not all carboxyl groups of uronic acids were involved in
ionexchange reactions; this was probably due to the
deficiency of groups with an appropriate spatial
arrangement required for the efficient binding of diva
lent ions.
According to previous findings, two types of car
boxyl groups in the polymeric matrix of higher plant
cell walls participate in cation binding at respective pH
of the external solution [12, 14, 16]. These are the
PGA carboxyl groups with
рК
а
~ 5
and the carboxyl
groups with
рК
а
~ 7
, possibly attributed to amino acid
residues of oxyamino acids or hydroxycinnamic acids
in the composition of cellwall polymeric matrix [12,
14, 16, 17]. The sorption of Ni
2+
by isolated cell walls
led to the pH decrease in solutions to 3.2–4.8 depend
ing on plant species (data not shown), in accordance
with the reaction:
2RCOOH + Ni
2+
(RCOO
)
2
Ni + 2H
+
,
where R designates the cell wall polymeric matrix.
Since carboxyl groups with pK ~ 7 are competent
in ion exchange only at pH > 6, their possible involve
ment in ionexchange reactions was evaluated by
assaying the ability of cell walls to adsorb Ni
2+
from
1 mM solution of ammonia buffer at pH 8 (Fig. 3). In
all cases the ionexchange capacity of cell walls was
higher at pH 8 than at pH 5, indicating the involve
ment of a second carboxyl group of the cellwall poly
meric matrix in Ni
2+
binding. These results confirm
our previous conclusion that the cell walls contain, in
addition to PGA carboxyl groups, the second type of
carboxyl groups that are involved in the exchange of
cations with an external medium under physiological
conditions [16]. It should be noted that in gramineous
plants, whose cell walls possess much smaller content
of PGA carboxyl groups compared to those of
legumes, the contribution of carboxyl groups with
pK ~ 7 to binding of nickel ions is considerably higher
than in leguminous plants (Fig. 3).
Presently, the absorption of heavy metals by plants
in the presence of complexing agents is being actively
investigated [18–20]. It is known that metal absorp
tion by cells is enhanced in the presence of ligands,
e.g., a “synthetic” complexon EDTA and “natural
ligands,” such as citrate and malate that are excreted
from the root to soil under certain conditions. One
hypothesis to explain the increased absorption of met
als in the presence of ligands is that the ligand–metal
complex is directly absorbed by the cells owing to acti
vation of absorption mechanisms, including specific
membranebound transport systems [18]. However,
the cell wall response to the presence of metalcom
plexing agents in the medium remained unknown.
200
100
0 200 400 600 800
1
2
3
4
5
6
y =
0.339
x
– 13.45
R
2
= 0.932
S
Ni
,
µ
mol/g dry wt
S
PGA
,
µ
mol/g dry wt
Fig. 2.
Relation between Ni
+
exchange capacity of iso
lated root cell walls (
S
Ni
) and the content of carboxyl
groups of polygalacturonic acid (
S
PGA
).
(
1
) Maize, (
2
) wheat, (
3
)
Suaeda
, (
4
)
Cicer
, (
5
)
Vigna
,
(
6
)
Vicia.
Values for
S
Ni
and
S
PGA
were obtained in this
study and in earlier work [12], respectively. The straight
line corresponds to the equation for trend dependence.
Wheat
SuaedaViciaVigna
S
Ni
,
µ
mol/g dry wt
300
250
200
150
100
50
0
1
2
Cucum Maize
Fig. 3.
Dependence of ionexchange capacity for Ni
2+
in
isolated root cell walls (
S
Ni
,
µ
mol/g dry wt) on pH and
plant species.
(
1
) pH 8 (10
–3
M ammonium buffer); (
2
) pH 5 (10
–3
M
acetate buffer). The initial solutions contained 10
–3
M
Ni
2+
. Data represent mean values and standard deviations
obtained in 3–5 replicate experiments.
ber
Page 4
RUSSIAN JOURNAL OF PLANT PHYSIOLOGY Vol. 58 No. 3 2011
BARRIER FUNCTION OF THE CELL WALL DURING UPTAKE 413
Our results show that, regardless of plant species,
the presence of EDTA in the solution dramatically
reduces the sorption capacity of cell walls for nickel
ions (Fig. 4). The cell walls of chickpea roots were
characterized by the highest value of this parameter
(16
µ
moles per gram dry wt), while this parameter for cell
walls of other plant species did not exceed 7
µ
moles.
Obviously, the low sorption capacity under these con
ditions was due to the reduced capacity of complex
formation by PGA carboxyl groups compared with
EDTA. Based on these results, we suppose that the
presence of EDTA in the medium weakens the protec
tive functions of the cell wall, which could lead to the
increased absorption by plant cells of Me
2+
in the form
of Me–EDTA.
Thus, the cell wall of higher plant roots has a high
ability to adsorb nickel ions from aqueous solutions.
The Ni
2+
concentration in cell walls is determined by
the number of carboxyl groups in the cellwall poly
meric matrix. The selectivity of carboxyl groups is
independent of the plant species, while their cation
exchange capacity is solely determined by the number
of carboxyl groups capable of exchange reactions.
Based on our results we conclude that, among the
mechanisms preventing accumulation of heavy metal
ions in the cytoplasm to concentrations toxic for bio
chemical reactions, sequestering of Ni
2+
into the cell
wall is an effective means to protect plant cells from
the impact of elevated concentrations of nickel ions in
the environment.
ACKNOWLEDGMENTS
This work was supported by the Russian Founda
tion for Basic Research (project nos. 040449379a
and 080401398a) and by the Federal targeted pro
gram Scientific and Scientific–Pedagogical Personnel
of Innovative Russia (subprogram Cell Technologies,
State Contract no. P403).
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M
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Page 6
  • Source
    • "The cell walls can work as a barrier and effectively sequestrate heavy metal ions (Macfie and Welbourn 2000; Latha et al. 2005; Meychik et al. 2011). Therefore, the cell walls could be a key factor in heavy metal exclusion mechanisms for fungi. "
    [Show abstract] [Hide abstract] ABSTRACT: Our objective was to understand the cadmium (Cd) tolerance mechanisms by investigating the subcellular distribution, chemical forms of Cd and adsorptive groups in the mycelia of Exophiala pisciphila. We grew E. pisciphila in the liquid media with increasing Cd concentrations (0, 25, 50, 100, 200, and 400 mg L(-1)). Increased Cd in the media caused a proportional increase in the Cd uptake by E. pisciphila. Subcellular distribution indicated that 81 to 97 % of Cd was associated with the cell walls. The largest amount and proportion (45-86 %) of Cd was extracted with 2 % acetic acid, and a concentration-dependent extraction was observed, both of which suggest that Cd-phosphate complexes were the major chemical form in E. pisciphila. A large distribution of phosphate and Cd on the mycelia surface was observed by scanning electron microscopy-energy dispersive spectrometer (SEM-EDS). The precipitates associated with the mycelia were observed to contain Cd by transmission electron microscopy-energy dispersive X-ray spectroscopy (TEM-EDX). Fourier transform infrared (FTIR) identified that hydroxyl, amine, carboxyl, and phosphate groups were responsible for binding Cd. We conclude that Cd associated with cell walls and integrated with phosphate might be responsible for the tolerance of E. pisciphila to Cd.
    Full-text · Article · Jul 2015 · Environmental Science and Pollution Research
  • Source
    • "In the intine putative binding sites for Ni are mostly the carboxyl groups of uronic acids; in the exine—the carboxyl groups of hydroxycinnamic acids (Meychik et al. 2006). These data suggest that the massive wall of the pollen grains may, like the walls of somatic cells (Meychik et al. 2011), perform a barrier function to protect the protoplast from the toxic effect of Ni 2? . On the other hand, it may act as a target. "
    [Show abstract] [Hide abstract] ABSTRACT: To investigate the mechanisms of Ni(2+) effects on initiation and maintenance of polar cell growth, we used a well-studied model system-germination of angiosperm pollen grains. In liquid medium tobacco pollen grain forms a long tube, where the growth is restricted to the very tip. Ni(2+) did not prevent the formation of pollen tube initials, but inhibited their subsequent growth with IC(50) = 550 μM. 1 mM Ni(2+) completely blocked the polar growth, but all pollen grains remained viable, their respiration was slightly affected and ROS production did not increase. Addition of Ni(2+) after the onset of germination had a bidirectional effect on the tubes development: there was a considerable amount of extra-long tubes, which appeared to be rapidly growing, but the growth of many tubes was impaired. Studying the localization of possible targets of Ni(2+) influence, we found that they may occur both in the wall and in the cytoplasm, as confirmed by specific staining. Ni(2+) disturbed the segregation of transport vesicles in the tips of these tubes and significantly reduced the relative content of calcium in the aperture area of pollen grains, as measured by X-ray microanalysis. These factors are considered being critical for normal polar cell growth. Ni(2+) also causes the deposition of callose in the tips of the tube initials and the pollen tubes that had stopped their growth. We can assume that Ni(2+)-induced disruption of calcium homeostasis can lead to vesicle traffic impairment and abnormal callose deposition and, consequently, block the polar growth.
    Full-text · Article · Sep 2012 · Biology of Metals