Principal Research Results

Differences in Metal Accumulation Abilities among Rice Cultivars

Background
Plants showing a special ability in metal accumulation, such as hyper-accumulators and ultra-low-accumulators, are necessary for phytoremediation of contaminated soil with harmful metals like cadmium and cropping without contamination in at least in
edible parts of plants (i.e. contaminant-free cropping). On the other hand, rice plants are the most common crop in Japan, and themechanisms of some essential metal uptakes, which are highly related to harmful metal uptakes, are well analyzed at both the physiological and molecular levels. It is probable that comparisons of some metal uptake abilities between rice and the other plant species oramong rice cultivars provide useful information to breed hyper-accumulators and ultra-low-accumulators.

Objectives
This study aims to clarify tolerances and accumulation abilities for copper, zinc and cadmium in some cultivars of rice, andcompare the abilities with the other plant species. It also aims to clarify the cross effect of excess metal (i.e. iron, copper, zinc andcadmium) exposures on the abilities.

Principal Results
Totally, 36 families 81 genera 114 species/subspecies of herbal angiosperms commonly grown in Japan, and 12 and 7 cultivars of rice plants showing Japonica and Indica phenotypes, respectively, were provided for the experiments and gave the following
results:
1. Differences in copper, zinc and cadmium tolerance and accumulation ability between rice plants andthe other plant species
Tolerances and accumulation abilities for copper, zinc and cadmium were observed in each species of plants and the relative
statuses were compared between rice plants and the other plant species. The rice plants commonly showed relatively higher tolerances
against copper, zinc and cadmium than the other species of plants. On the other hand, every metal concentration in rice plants, whichwere utilized as an index of the metal accumulation ability, was limited in a middle- and narrow-ranged concentration in comparison
to the other species of plants (Fig. 1).
2. Differences in copper, zinc and cadmium tolerances among rice cultivars and a speculation of themechanisms
Each of the tolerances against copper, zinc and cadmium and correlations between two of these tolerances clearly indicated adifference between Japonica phenotype and Indica phenotype (Fig. 2). This suggested the existence of phenotype-specific mechanism
for each metal tolerance.
3. Differences in divalent metal accumulation abilities among rice cultivars and a speculation of themechanisms
The concentrations of iron, copper and cadmium in upper parts of the rice plants showing Japonica phenotype were higher
than those of the rice plants showing Indica phenotype at the control condition; however, this tendency was inverted under the excessiron exposure condition (Fig. 3). This indicated the possibility that cadmium mainly shared the same mechanism with iron and copper
to migrate from root to shoot. Also, it is possible that the mechanism was differently regulated in the rice plants showing Japonica
phenotype and Indica phenotype.
Of note, this research was supported, in part, by Grant-in-aid for Scientific Research from the Japan Society for thePromotion of Science (JSPS; 16380232) to TY.

Future Developments
Genes related to metal transport will be isolated using the knowledge obtained by this research. Subsequently, a hypothesisthat the metal tolerance and accumulation ability among rice cultivars is basically dependent on the mechanism of a specific metalaccumulation and/or the transport will be certified with the isolated gene expressions and/or the functions. In addition, the genes will
be utilized to breed hyper-accumulators and ultra-low-accumulators.
Main Researcher: Toshihiro Yoshihara, Ph. D.,
Senior Research Scientist, Bio-technology Sector, Environmental Science Research Laboratory
ReferenceT. Yoshihara, et.al., 2008, “Differences in tolerance and accumulation abilities for Copper, Zinc, and Cadmium between Japonica andIndica cultivars of Rice”, CRIEPI Report V07010 (in Japanese)
T. Yoshihara, et.al., 2008, “Tolerance and accumulation abilities for Copper and Zinc among major herbal angiosperms in Japan”,
CRIEPI Report V07012 (in Japanese)50

(B) Zn(C) Cd

1.0

1.6

1.2

0.8
0.6
0.4
0.2
0.0

Cd tolerance(A) CuZn tolerance

Cu tolerance

2. Environment

1.2
0.8
0.4

0.4
0.0

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1E-3 0.01 0.1
1Cu conc. in shoot (mg/g-DW)

0.8

1E-3 0.01 0.1
1
Cd conc. in shoot (mg/g-DW)

0.01
0.1
1Zn conc. in shoot (mg/g-DW)

Fig.1 Relative correlations between tolerance and accumulation ability for Cu, Zn and Cd among
common plant species
Correlations for Cu (A), Zn (B) and Cd (C) were indicated. Each dot shows the data for one plant species. Red dots show thedata for rice plants.
(A) Correlation between Cd and Zn tolerance (B) Correlation between Cu and Zn tolerance
1.4

1.0

Cu tolerance

Cd tolerance

1.2
1.0
0.8
0.6
0.4

0.0
0.0

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0.9

1.2

0.60.40.2Japonica
Indica0.20.81.5

Japonica
Indica0.00.0Zn tolerance

0.60.91.2

1.5

Zn tolerance

Fig.2 Differences in correlations of metal tolerances between Japonica and Indica phenotypes
Indica phenotypes (a dotted line) show higher tolerances in Zn and Cd than Japonica phenotype (a solid line). Correlation ofZn and Cd tolerances is positive in both the phenotypes (A). On the other hand, Cu tolerance is not different between thesephenotypes. Correlation of Zn and Cu tolerances is positive in Japonica phenotypes, but is negative in Indica phenotype (B).

(A) Rice plants grown underthe control condition(B) Rice plants grown underthe excess iron condition

800

Japon ica > Indica700(p=0.03)

600

Jp

In

500
400
300200100

Fe conc. in shoot (og/g-DW)

Fe conc. in shoot (og/g-DW)

the phenotype specific tendency of Fe accumulation isinverted by the growth condition

01000
800

Indica > Japonica(p=0.05)
In

Jp

600
400
200
0

Cultivars

Cultivars

The Fe concentration in shoots grown under the control condition(A) and the excess Fecondition (B) are shown for each cultivar (N=5-15). Average and S.D. are indicated asrectangles and bars.The rectangles colored by red, blue, and white are data for Indica
phenotype, Japonica phenotype and each average, respectively.

Fig.3 Iron concentration in shoots grown under the control and the excess Fe conditions
Cultivars belonging to Japonica phenotype have higher Fe concentration than those belonging to Indica phenotype under thecontrol condition (A), whereas the tendency is inverted under the excess Fe condition (B).

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