American Journal of Plant Sciences, 2011, 2, 156-164
doi:10.4236/ajps.2011.22017 Published Online June 2011 (http://www.SciRP.org/journal/ajps)
Copyright © 2011 SciRes. AJPS
No Statistic Correlation between Superoxide
Dismutase and Peroxidase Activities and
Aluminum-Induced Lipid Peroxidation in Maize,
Implying Limited Roles of Both Enzymes in
Prevention against Aluminum-Induced Lipid
Peroxidation
Liang Wang1,2 Jian-Yu Zhao1,2, San-Min Wu1,2, Jiang-Long Pan1,2, Zhang-Bao Huang1, Zi-Kai Wu3,
Xian-Wei Fan1,2, You-Zhi Li1,2
1State Key Laboratory for Conservation and Utilization of Subtropical Agro-bioresources, Nanning, China; 2College of Life Science
and Technology, Nanning, China; 3Agricultural College, Guangxi University, Nanning, China.
Email: liyouzhigxu@163.com
Received January 25th, 2011; revised April 13th, 2011; accepted April 20th, 2011.
ABSTRACT
Changes of and correlation am ong root tolera nce index (RTI), root Aluminum (Al) content, root/shoo t ratio (RSR), ro ot
malondialdehyde (MDA) content, and Superoxide dismutase (SOD) and peroxidase (POD) isoforms of maize YQ 7-96
were investigated under Al stress and removal of the stress (RS). Consequently, Al stress led to significan t decreases in
RTI, RSR, SOD and POD activities, but resulted in sign ifica nt increa se in ro ot MDA and , Al accumu lation in th e tissu es;
Root SOD and POD activities did not correlate with Al and MDA contents in roots; The activities of SOD and POD
were much lower in roots than in leaves , and some isoforms were differentia lly expressed in a tissu e-specific manner. It
could be concluded that 1) Al stress can lead to lipid peroxida tion; 2) there is a larger POD family composed of differ-
ent POD isoforms, some of which are of tissue-specific expression and play different roles in detoxification of Al in
maize; 3) for POD isoforms, POD 2 is root-specific. POD 6 and POD 7 are all leaf-specific, POD 5 is not only
root-specific but also RS-responsive; 4) high sensitivity of maize to Al is associated in part with much lower activities of
both SOD and POD in roots; and 5) more importantly, both SOD and POD are therefore hinted to be not key players in
prevention agains t Al-induced lipid peroxidation.
Keywords: Al Toxicity, Antioxidant Enzyme, Antioxidation, Lipid Degradation, Plant
1. Introduction
Aluminum (Al) toxicity is one of major environmental
factors that constrain crop production and quality in acid
soil. Acid soil accounts for 30% - 40% of the world’s
arable lands [1] and for 21% of the arable lands in China
[2]. Al toxicity can make a notable impact on plants, in-
cluding inhibition of plant root elongation and excessive
accumulation of superoxide radicals such as 2
O
at the
cell level [3]. So far, intensive efforts have been made to
understand Al-induced peroxidation in plants [4-8]. The
excessively accumulated superoxide radicals can induce
degradation of many cell constituents such as membrane
lipid, protein and DNA [3]. Membrane lipid is one of
major constituents forming the skeleton of cell mem-
branes. Degradation of membrane lipid results in loss of
integrity of cell membranes. The integrity of cell mem-
branes is very critical in many biochemical reactions in
cells [9-11]. In plants, mechanisms for detoxification of
Al include external avoidance that prevents uptake of Al
into plant roots as well as internal tolerance that detoxi-
fies Al internally [1,12]. Internal tolerance depends par-
tially on many antioxidant enzymes, such as superoxide
dismutase (SOD) and peroxidase (POD) [11,13-15].
SOD and POD constitute the first line of defence against
No Statistic Correlation between Superoxide Dismutase and Peroxidase Activities and Aluminum-Induced 157
Lipid Peroxidation in Maize, Implying Limited Roles of Both Enzymes in Prevention against
Aluminum-Induced Lipid Peroxidation
reactive oxygen species (ROS) under the stress [16], and
their activity and amounts are usually used as an indicator
of Al tolerance in plants since they were found to have
higher activity in the Al-tolerant plants than in the
Al-sensitive plants [17].
Maize (Zea mays L.) is more sensitive to Al toxicity
than other cereals [18]. Increased activities of SOD and
POD were observed in some maize lines exposed to ele-
vated Al concentrations [13]. However, Al-induced per-
oxidation of lipid in maize is still conflicting. For exam-
ple, Boscoloa et al. [13] found that Al treatment did not
induce lipid peroxidation in both sensitive and tolerant
maize lines. However, Giannakoula et al. [11] indicated
that Al treatment could trigger lipid peroxidation in the
sensitive maize line. In addition, it is unknown that
whether some tissue-specific SOD and POD isoforms in
maize response to Al stress exist. Therefore, further re-
searches such as statistic analysis of correlation among
the data are needed to have an insight into roles of SOD
and POD in prevention against Al-induced peroxidation
of lipid in maize.
YQ 7-96 is a maize inbred line of moderate Al toler-
ance as well as a short anthesis-silking interval of 0 - 1
day. This study focused on the relationship of SOD and
POD activities with lipid degradation in this maize line
exposed to Al to clarify roles of these two enzymes in
preventing Al-induced lipid peroxidation.
2. Materials and Methods
2.1. Plant Culture
The maize inbred line YQ 7-96 was used in this study,
which was bred by Professor Zi-Kai Wu from our re-
search group. Fully mature maize seeds of the same size
were soaked for 12 h at 28˚C in distilled water, sur-
face-sterilized for 2 min in 75% (v/v) ethanol, and then
fully rinsed with sterile water. The sterilized seeds were
germinated at 28˚C in sand moistened with sterile water.
The maize seedlings were trimmed for removal of resid-
ual endosperms, and mounted for growing in the lattice
of the plastic container (75 × 40 × 25 cm) containing 12
L of original Hoagland nutrient solution [19]. Before
three-leaf stage, the nutrient solution was vigorously aer-
ated for 15 min every 1 h; pH of the solution was ad-
justed every day to 7 ± 0.2. The seedlings were trans-
ferred at three-leaf stage for Al stress treatment into the
nutrient solution with pH 4.5 ± 0.2 and containing 0.5
mM AlCl3·7H2O, where toxic Al3+ concentration was
estimated to be 48 μΜ by using the Geochem 2.0 soft-
ware [20]. Following 72 h of Al stress, the stressed seed-
lings were treated for another 48 h by removal of the
stress (RS) in the nutrient solution without AlCl3·7H2O.
The control was in parallel conducted in the nutrient so-
lution without AlCl3·7H2O. All the experiments were
conducted at 25˚C under a 12 h photoperiod (120
μmol·m2·s1) in a growth chamber with 60% - 80% rela-
tive humidity. The nutrient solution was renewed once
every 3 days. The treatment duration was in detail indi-
cated in the text.
2.2. Plant Growth Assay
Root length from the node between roots and stems down
to the tips of taproots was measured at 10 a.m. to obtain
root elongation (RE) per day. Root growth under Al was
expressed in root tolerance index (RTI, %). RTI was
calculated as the formula:
RTI (%) = (REAl stress/REControl) × 100.
For assay of root/shoot ratio (RSR), the seedlings were
then immediately oven-dried for 1 h at 100˚C and then
dried for 8 h at 70˚C. The dried seedlings were separated
into two parts of roots and shoots, which were separately
weighed. The RSR was calculated as the formula: RSR =
dry weight (DW) of roots/DW of shoots.
2.3 Assay of Al Content
Al content in plant tissues was assayed as the method
described by He and Liang [21] but with slight modifica-
tion. Briefly, 0.1 g of dried tissues was digested for 24 h
in 1.5 mL of 2 M HNO3 solution; The resulting hydro-
lyzate was diluted 20 times with deionized water; An
aliquot (1 mL) of the diluted solution was immediately
transferred into a tube, and then the following reactants
were sequentially added to the tube: 1 mL of 0.1 M
HNO3 solution, 2 mL of 5 mM cetyltrimethyl ammonium
bromide solution, 2 mL of 50 mM EDTA-Zn solution, 2
mL of the solution with 0.05% (w/v) chromazurol-S, and
4 mL of 40% (w/v) ammonioformaldehyde solution; The
solution was then diluted up to 25 mL with deionized
water, placed for 20 min at 25˚C, and then assayed for
absorbance at 635 nm by using the 722 spectrophotome-
ter (Shanghai Leng Guang Technology Co. Ltd. China).
2.4. Assay of Specific Activities SOD and POD
The fresh tissues of roots and leaves were rinsed with
sterile distilled water and then immediately frozen in
liquid nitrogen. The tissues (0.5 g) were ground into the
homogenate in 10 mL of pre-cooling buffer solution of
pH 7.4 and composed of 1 mM EDTA and 50 mM
H3PO4. The homogenate was centrifuged for 20 min at
16,000 × g at 4˚C. The supernate was collected and
stored at –80˚C as tissue extract for further use.
Specific activity of SOD was determined as p-nitro
Copyright © 2011 SciRes. AJPS
No Statistic Correlation between Superoxide Dismutase and Peroxidase Activities and Aluminum-Induced
158
Lipid Peroxidation in Maize, Implying Limited Roles of Both Enzymes in Prevention against
Aluminum-Induced Lipid Peroxidation
blue tetrazolium chloride (NBT) photoreduction [22]. In
brief, 0.1 mL of the tissue extract was added to a glass
tube containing the solution of pH 7.4 and composed of
the reagents of 1.5 mL of 50 mM H3PO4, 0.3 mL of 13
mM methionine, 0.3 mL of 75 μM NBT, 0.3 mL of 10
μM EDTA-Na2, 0.3 mL of 2 μM riboflavin, and 0.5 mL
sterile deionized water. The tube was positioned 30 cm
away from a fluorescent lamp with 3098 Lux units for 20
min at 25˚C. The resulting reaction mixture in the tube
was then analyzed for absorbance at 560 nm. One unit of
SOD is defined by the amount of enzyme that inhibits
NBT photoreduction by 50% [23]. The specific activity
of enzyme was expressed as unit mg–1 protein [24].
Specific activity of POD was assayed following the
guaiacol oxidation method [22]. Briefly, 20 mL of the
tissue extract were reacted with 3 mL solution containing
1% H2O2 (v/v), 0.5% (w/v) guaiacol and 50 mM H3PO4.
The reaction mixture was then analyzed for absorbance
at 470 nm. One unit of the enzyme was defined as an
optical density value of 0.01. The specific activity of the
enzyme was given as unit min–1·mg–1 protein.
2.5. Analysis of Isoforms of SOD and POD
The isoforms of enzymes were analyzed following non-
denaturing polyacrylamide gel electrophoresis (PAGE)
method [25]. For PAGE, 25 mL of the tissue extract per
gel slot were loaded. The electrophoresis was conducted
for 15 min at 70 v and then for 2 h at 220 v.
For SOD isoforms, the gel was stained for 20 min in
dark in the solution composed of 0.24 mM NBT and 50
mM phosphate buffer of pH 7.8, and then further stained
for 15 min in the solution containing 33.2 μM riboflavin,
0.2% (v/v) tetramethylethylenediamine and 50 mM phos-
phate buffer of pH 7.8. Following staining, the gel was
transferred into the phosphate buffer containing 1 mM
EDTA and positioned 30 cm away from the light source
of 3000 Lux units at room temperature until colored. The
SOD isoforms were distinguished by their sensitivity to 2
mM KCN or 5 mM H2O2 [25].
Analysis of POD isoform followed the method in the
literature [26] but with modification. In brief, following
PAGE, the gel was soaked for coloring for 5 min at room
temperature in the solution containing 1 mM benzidine
and 0.2% H2O2.
Following coloring, the gel was analyzed by using the
Gel-Pro analyzer software (Media Cybernetics)
2.6. Assay of Root Malondialdehyde (MDA)
Root MDA content was assayed as thiobarbituric acid
(TBA) method [22]. Briefly, an aliquot (1 mL) of root
extract was mixed with 2 mL of the solution composed
of 0.6% TBA and 10% trichloracetic acid. The reaction
mixture was incubated for 30 min in water of 100˚C,
quickly cooled up to 0˚C in ice water, and then centri-
fuged for 5 min at 11,600 × g at 4˚C. The supernate was
analyzed at 532 nm (A523) and 450 nm (A450), respec-
tively. The MDA content was calculated as the formula:
MDA (μm·g–1 fresh weight)
= [(6.45 × A523 – 0.56 × A450) × 10 mL]/0.5.
2.7. Statistical Data Analysis
The statistical data analysis was conducted with the soft-
ware SPSS 13.0 (http://www.spss.com/).
3. Results
3.1. RTI and Root Al Content of Maize Inbred
Line YQ 7-96
RTI of maize YQ 7-96 seedlings sharply decreased with
Al stress, dropped to 42% at 72 h, but was up to 79% after
a 48 h treatment of RS (Figure 1), agreeing with previ-
ous studies that Al stress can inhibit plant root elongation
[9,13,27-31].
Al content in roots of the seedlings increased with Al
stress and reached 752 μg·g–1 DW at 72 h, decreased to
622 μg·g–1 DW after a 48 h RS treatment (Figure 1 ).
The above results indicated that in maize YQ 7-96 Al
Figure 1. Changes in RTI and Al content in maize inbred
line YQ 7-96 roots with Al stress. Treatments of maize seed-
lings began at the three-leaf stage in the original Hoagland
nutrient solution with or without addition of AlCl3·7H2O.
During each treatment period, lengths from the node be-
tween roots and stems down to the tips of taproots were
measured at 10 a.m. every day. RS was conducted on
72-h-stressed seedlings in the nutrient solution without
AlCl3·7H2O. Each datum is the mean ±SD (n = 10 - 15 for
RTI; n = 3 - 5 for Al content). RE, Root elongation. RS,
Removal of the stress. RTI, Root tolerance index.
Copyright © 2011 SciRes. AJPS
No Statistic Correlation between Superoxide Dismutase and Peroxidase Activities and Aluminum-Induced 159
Lipid Peroxidation in Maize, Implying Limited Roles of Both Enzymes in Prevention against
Aluminum-Induced Lipid Peroxidation
stress-caused inhibition of root elongation was coupled
with increased Al content in the roots, consisting with pre-
vious results observed in Al-stressed maize [13,28,29,31].
3.2. RSR of Maize YQ 7-96
The RSR of Al-stressed maize seedlings significantly
declined in comparison with that of control seedlings.
After the 48 h RS treatment, RSR obviously increased
but was still lower than that of control seedlings at the
same growth stage (Figure 2). The results underpinned
the previous conclusion that Al stress effect on plants lies
mainly in inhibiting the root growth [9,15].
3.3. SOD and POD Activities
Change in SOD and POD activities is a common feature
of higher plants in response to abiotic stresses [3,32].
Specific activities of SOD and POD significantly (p <
0.05) decreased in Al-stressed YQ 7-96 roots in com-
parison with those in control seedlings (Figure 3). How-
ever, enzyme activities in the stressed roots fluctuated
with the stress, which decreased at 24 h, increased at 48 h
and then significantly dropped at 72 h. After the 48 h RS
treatment, specific activities of the enzymes in stressed
roots obviously increased in comparison with those in
72-h-stressed seedlings (Figure 3). Alteration in en-
zymes’ activities during Al stress was similar to results in
Al-stressed emerging roots of barley seeds [6], but dif-
fered from results in both Al-tolerant and Al-sensitive
Figure 2. Change in RSR of maize inbred line YQ 7-96 with
Al stress. Treatments of maize seedlings began at the
three-leaf stage. RS was conducted on 72-h-stressed seed-
lings in the nutrient solution without AlCl3·7H2O. The har-
vested seedlings were immediately dried, and the roots and
leaves of the dried seedlings were weighed, respectively.
Each datum is the mean ±SD (n = 10 - 15). RS, Removal of
the stress. RSR, root/shoot ratio.
(a)
(b)
Figure 3. Changes in activities of SOD and POD in maize
inbred line YQ 7-96 roots with treatment time. (a): Specific
activity of SOD. (b): Specific activity of POD. Treatments of
maize seedlings began at the three-leaf stage. RS was con-
ducted on 72-h-stressed seedlings in the nutrient solution
without addition of AlCl3·7H2O. Specific activities of SOD
and POD were determined as methods described by Tang
[22]. SOD was assayed based on inhibition of NBT pho-
toreduction; POD was assayed following the guaiacol oxida-
tion method. Measurement of enzyme activity was biologi-
cally repeated, one seedling was designed for one biological
repeat. Each datum is mean ±SD (n = 3). NBT, p-nitro blue
tetrazolium chloride. RS, Removal of the stress.
maize lines [13]. Anyway, enzyme’s activities correlated
with declines in root elongation (Figure 1) as well as
RSR (Figure 2).
SOD isoforms were not detectable in both control and
Al-stressed roots (Figure 4(a)). Four POD isoforms were
detected in roots, which were named POD 1, POD 2,
POD 3, and POD 4 according to migration rate on the gel
(Figure 4(b)). Based on analysis of Gel-Pro analyzer
software (data not shown) POD 1 and POD 2 activities
significantly decreased, but POD 3 and POD 4 activities
significantly increased in Al-stressed roots in comparison
with control roots; However, total activity of detected
Copyright © 2011 SciRes. AJPS
No Statistic Correlation between Superoxide Dismutase and Peroxidase Activities and Aluminum-Induced
Lipid Peroxidation in Maize, Implying Limited Roles of Both Enzymes in Prevention against
Aluminum-Induced Lipid Peroxidation
Copyright © 2011 SciRes. AJPS
160
Figure 4. Time-course analysis of isoforms of SOD and POD in maize inbred line YQ 7-96. (a): Activities of root SOD iso-
forms; (b): Activities of root POD isoforms; (c): Activities of leaf SOD; (d): Activities of leaf POD; Analyses of SOD and POD
isoforms were based on non-denaturing P AGE. SOD isoforms were distinguished by their sensitivity to 2 mM KCN or 5 mM
H2O2 [25]. POD isoforms were analyzed by coloring [26]. Quantitative analysis of enzyme activities was conducted by using
the Gel-Pro analyzer softw are (Media Cybernetics). Thr ee independent expe riments were conducte d. PAGE, polyacry lamide
gel electrophoresis. RS, Re moval of the str ess.
POD isoforms significantly decreased in Al-stressed
roots throughout the stress; Even after RS treatment, total
activity of POD isoforms in stressed roots was only 96%
of that in control roots. Interestingly, after RS treatment,
a novel POD isoform, POD 5, occurred in Al-stressed
roots (Figure 4(b)), suggesting that it is an enzyme of
RS-specific response.
Two SOD isoforms, MnSOD and CuZnSOD, were
observed in maize leaves (Figure 4(c)). MnSOD activity
decreased by about 50% when compared to that in con-
trol leaves. CuZnSOD activity significantly declined at
24 h, increased at 48 h, and decreased again at 72 h (Fig-
ure 4(c)). Interestingly, the profile of POD isoforms in
leaves (Figure 4(d)) differed from that in roots (Figure
4(b)). RS-responsive POD 5 and Al stress-responsive
POD 2 in roots (Figure 4(b)) did not appear in leaves
(Figure 4(d)). Leaf-specific POD isoforms included
POD 6 and POD 7 (Figure 4(d)). In stressed leaves,
POD 1 activity significantly increased at 48 h and de-
creased at 72 h; POD 3 activity increased at 24 and 48 h,
but decreased at 72 h; POD 4 activity decreased at 24 and
48 h; Activities of both POD 6 and POD 7 decreased
throughout the stress.
3.4. MDA Content in YQ 7-96 Roots
MDA content in maize YQ 7-96 roots was analyzed be-
cause MDA is an indicator as lipid degradation [3,11].
As a result, MDA content in stressed roots significantly
(p < 0.05) increased upon Al stress; After RS treatment,
MDA content in roots of seedlings stressed for 72 h sig-
nificantly decreased in comparison with that in roots of
the seedlings before RS treatment, but it was still higher
than that in control roots at the same stage (Figure 5). All
these results suggest that lipid degradation occurred dur-
ing Al stress, solidifying a previous viewpoint that lipid
is an important target for Al toxicity [33].
3.5. Comprehensive Analysis of the Data
To conclude whether there was a correlation between the
data from stressed roots, a comprehensive statistic analy-
sis of the data was conducted by suing the software SPSS
13.0 (Table 1). In stressed roots, Al content correlated
negatively with both RTI and RSR but positively with
MDA content. Al content did not show significant corre-
lation with changes in both SOD and POD activities.
Both RTI and RSR had a significant negative correlation
with root MDA content. Taken together, it can be at least
partly suggested that that growth of maize seedlings
when exposed to Al depends on the integrity of cell
membranes.
4. Discussion
Mechanisms of plant Al tolerance include prevention of
Al uptake, and detoxification of internal Al in the cell
[12]. Inhibition of root elongation of maize inbred line
No Statistic Correlation between Superoxide Dismutase and Peroxidase Activities and Aluminum-Induced 161
Lipid Peroxidation in Maize, Implying Limited Roles of Both Enzymes in Prevention against
Aluminum-Induced Lipid Peroxidation
Figure 5. Change in MDA content in maize inbred line YQ
7-96 roots with treatment time. Treatments of maize seed-
lings began at the three-leaf stage. RS was conducted on
72-h-stressed seedlings in the nutrient solution without ad-
dition of AlCl3·7H2O. Root MDA was assayed by using TBA
method [22]. Measurement of enzyme activity was biologi-
cally repeated, one seedling were designed for one biological
repeat. The presented datum for each time point is the
mean ± SD (n = 5). MDA, malondialdehyde. RS, Removal of
the stress. TBA, Thiobarbituric acid.
YQ 7-96 grown in the presence of Al was likely due to
loss of extensibility of the root cell wall [30] because of
degradation of membrane lipid (Figure 5). After 48 h AS
treatment, recovery of root elongation (Figure 1) was
likely because of decrease in Al bound to cell wall [34].
Even if RS treatment time was extended, root elongation
of the stressed seedlings could not reach the control level
at the same growth stage (data not shown) mainly be-
cause Al-affected roots are inefficient in absorbing both
nutrients and water [9].
Activities of plant SOD and POD were found to vary
greatly during Al stress. For example, activities of both
the enzymes changed in an opposite manner in Al-
stressed rice: SOD activity decreased while POD activity
increased only after long-term treatment [35]. Also in
rice, Al-induced increase in POD activity was found in
roots of the Al-resistant cultivar, but increment of the
enzyme activity was only half of the Al-sensitive cultivar
[26]. Activities of SOD and POD indeed increased in the
root tips of soybean exposed to Al [4]. Diverse patterns
of SOD and POD activities were also found in different
maize lines grown under Al. For example, Boscolo et al.
[13] found that changes in changes in activities of both
SOD and POD were the same, either transient increase at
the later stress in the Al-sensitive maize or little change
over Al stress in Al-tolerant maize line. More recently,
Giannakoula et al. [36] found that Al stress resulted in
increased activities of SOD and POD in the Al-tolerant
maize line but not in the Al-sensitive line. Unlike these
results, our results clearly indicated that there was no
statistic correlation between changes in SOD and POD
specific activities, RTI, and Al accumulation in the
stressed YQ 7-96 roots (Table 1), agreeing with that
Al-caused oxidative stress is not the primary cause of
maize root growth inhibition [13]. Discrepancy among
existing results are maybe associated with that the abiotic
stresses are often of species or location specific [37].
Both SOD and POD play role in antioxidation [38-40].
According to growth phenotype (Figures 1 and 2), de-
creased activities of both enzymes at 24 h of the stress
were likely due to quick decline in cell viability because
even exposed for short-term (5 min) to Al maize root cell
division can also be inhibited [18]. Increased activities of
the enzymes at 48 h of the stress were likely associated
with enhancement of Al-induced oxidative stress. De-
creased activities of the enzymes after long- term stress
treatment (Figure 3) may be ascribed to overproduction
of ROS and/or a build-up of a protection against oxida-
tive damage [25], and partially to cell damage and death
Table 1. Analysis of correlation between the data from Al-stressed YQ 7-96 roots exposed to Al.
Items tested Root Al content RTI RSR Root MDA content Root SOD activity Root POD activity
Root Al content 1
RTI 0.968* 1
RSR 0.997** 0.974* 1
Root MDA content 0.964* 0.992** 0.978*1
Root SOD activity 0.653 0.710 0.708 0.788 1
Root POD activity 0.835 0.741 0.855 0.800 0.792 1
*p < 0.05; **p < 0.01. The analysis was based on the full set of raw data from all Al-treated time points instead of at one time point. The figures indicate r2
values between two sets of analyzed data. Analysis was conducted by using the software SPSS 13.0 (http://www.spss.com/). The r2 value of either p < 0.05 or p
0.01 means that there is a significant correlation between the data. <
Copyright © 2011 SciRes. AJPS
No Statistic Correlation between Superoxide Dismutase and Peroxidase Activities and Aluminum-Induced
162
Lipid Peroxidation in Maize, Implying Limited Roles of Both Enzymes in Prevention against
Aluminum-Induced Lipid Peroxidation
due to overall expression of Al toxicity and Al-induced
secondary stress such as drought [8].
The total activities of isoforms of both SOD and POD
were much lower in maize roots than in maize leaves
(Figure 4). No activities of SOD were detected by gel
analysis in maize roots grown under control and Al stress
conditions (Figures 4(a) and (c)). One of the obvious
reasons for this is that the enzyme is of much lower ac-
tivity in roots (Figure 4(a)) than that in leaves (Figure
4(c)). Anyway, high sensitivity of maize roots to Al [18]
may be explained at least partially by low SOD activity
as indicated by maize inbred line YQ 7-96. It is reported
in sunflower that different POD isoforms have different
functions, of which some work directly as oxygen spe-
cies scavenger and others could play a role in polyphe-
nols metabolism to increase the antioxidant capacity or
cross-linking UV-absorbing phenolics [37]. Occurrence
of POD 5 only in stressed roots after RS (Figure 4(b)) is
probably associated with requirement for damage repair
during RS treatment. All these results strongly suggest
that there exists a larger POD gene family composed of
different isoforms in maize, which are differentially ex-
pressed in the tissues of Al-stressed maize and have
functional difference in detoxification of Al.
Although activities of different isoforms of SOD and
POD varied greatly, the total activities of both the en-
zymes changed in the same manner with Al stress (Fig-
ure 3). This reflects synergistic roles of SOD and POD in
antioxidation. Usually, the superoxide radical (2
O
) is
first degraded by SOD into O2 and H2O2; Resulting H2O2
is then degraded by POD [13].
Al stress-caused peroxidation of lipid has been found
in other plant species such as soybean [4] and rice [5,35],
but was controversial in maize [11,13,41]. Al stress did
lead to lipid peroxidation of the stressed YQ 7-96 roots
because of a significant relationship between Al and
MDA contents (Table 1). According to all these results,
Al-caused peroxidation of lipid seems to depend greatly
on plant genotypes. In maize, one of likely reasons for
this is associated with differences between maize geno-
types in the negative charge on the cell wall. The nega-
tive charge on the cell wall is a major determinant of the
initial Al accumulation [34].
Statistical analysis clearly indicated that MDA content
in Al-stressed YQ 7-96 roots did not correlate with ac-
tivities of root SOD and POD of this maize line (Table
1). This suggests that roles of both the enzymes in pre-
vention of Al-caused peroxidation of lipid are very lim-
ited. It has been indicated that Al does not directly result
in peroxidation in membrane lipid because Al is a
non-transition metal and cannot catalyze the peroxidation
reaction [3]. Al bound to the membranes can directly
cause membrane rigidification that can therefore facili-
tate the iron catalyzed lipid peroxidation in the mem-
brane [42].
Taken together, Al stress can lead to lipid peroxidation;
there is a larger POD family composed of different POD
isoforms, some of which are of tissue-specific expression
and play different roles in detoxification of Al in maize;
For POD isoforms, POD 2 is root-specific. POD 6 and
POD 7 are all leaf-specific, POD 5 is not only root-spe-
cific but also RS-responsive; High sensitivity of maize to
Al is in part associated with much lower activities of
both SOD and POD in roots; More importantly, both
SOD and POD are therefore hinted to be not key players
in prevention against Al-induced lipid peroxidation,
suggesting that their activity and amounts are not used as
a reliable indicator for selection of Al-tolerant maize.
5. Acknowledgements
This work was supported by the National Basic Research
Program of China No. 2011CB100100, The 948 Program
of Introduction of Advanced Science and Technology of
International Agriculture from the Ministry of Agricul-
ture of the People’s Republic of China (2001-205), the
Development Program for Guangxi Science and Tech-
nology Research (Guikegong 10100005-4 and 0228019-
6), the Key Laboratory of Ministry of Education for Mi-
crobial and Plant Genetic Engineering (Director’s grant-
06-11), and The Opening Project of Guangxi Key Labo-
ratory of Subtropical Bioresource Conservation and Uti-
lization (SB0601).
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