5. RISK MANAGEMENT OF CADMIUM IN RICE
5.3. Low Cd-Accumulating Rice Cultivars
5.3.1. Genotypic Variation in Grain Cd Concentration in Rice
Selection and breeding of low Cd-accumulating cultivars is the most cost-effective and environmentally friendly method for reducing the risk of contamination from Cd in food (Grant et al., 2008). Natural variations in the concentrations of Cd among cultivars have been well documented in staple crops including rice. Figure 4.4 shows the genotypic variation in grain Cd concentration in 35 rice cultivars grown in two types of Cd- polluted paddy soils (Arao and Ae, 2003; Arao and Ishikawa, 2006). Cadmium con- centrations in brown rice ranged from 0.13 to 4.31 mg kg−1 in Fluvisols and 0.79–7.65 mg kg−1 in Andosols. The ranking of rice cultivars was maintained across the soils, suggesting that Cd concentration of rice grains could be controlled by genetic factors rather than environmental conditions.
Generally, Cd concentrations are higher in Indica-type rice varieties than in Japonica ones. The representative Japanese Japonica cultivars, Nipponbare, Koshihikari, and Sasanishiki, were categorized as the rice group with low grain Cd concentrations. The lowest grain Cd concentrations were found in two varieties, LAC23 and HU-LO-TAO. These rice varieties can be used as good materials to develop varieties with lower Cd levels than those in the varieties currently under cultivation. Indica rice variety, IR-8, which led to the green revolution in the Asia in 1960s, was categorized as the highest group in grain Cd concentrations. Milyang 23 and Habataki were produced from the common ancestor, IR-8, and high Cd concentrations of these Indica
varieties must be inherited from IR-8. These high Cd- accumulating rice varieties can be used as the “cleaning plants” of Cd-contaminated paddy- field soil (see Section 5.5). Thus, large differences in Cd accumulation among rice cultivars enable us to develop phytotechnologies, such as breeding of low Cd-accumulating varieties and phytoextraction of Cd by using high Cd-accumulating varieties for reducing the Cd levels in rice grains.
5.3.2. Physiological and Genetic Mechanisms
Understanding of the physiological and genetic aspects underlying Cd trans- port in rice is important to control Cd transfer into grains. The level of Cd in rice grains may be influenced by any of several physiological processes:
1) root Cd uptake, 2) sequestration of Cd into root vacuoles, 3) transfer from roots to shoots via the xylem, 4) transfer from xylem to phloem, and 5) phloem transport into grains. Uraguchi et al. (2009) characterized the physiological properties involved in the differences in shoot and grain Cd accumulation between the low Cd-accumulating variety Sasanishiki and high Cd-accumulating variety Habataki. The activity of root Cd uptake was higher for Sasanishiki than for Habataki. However, Cd levels of xylem sap were well correlated with the shoot-Cd concentration in the two cultivars.
Figure 4.4 Genotypic variation in grain Cd concentration in rice. Thirty-five rice cul- tivars were cultivated in a container filled with two types of Cd-polluted soils under upland conditions (soil A, soil B). Koshikikari, Nipponbare, Sananishiki and Hu-Lo-Tao are Japonica and IR-8, Habataki and Milyang 23 are Indica subspecies. LAC23 is tropical Japonica (Javanica) variety (modified data from Arao and Ishikawa (2006)). A, Fluvisols (0.5 mg Cd kg−1 of dry soil); B, Andosols (5.1 mg Cd kg−1 of dry soil). For color version of this figure, the reader is referred to the online version of this book.
A positive and strong correlation between Cd concentrations in the xylem sap and subsequent shoot-Cd accumulation was also observed in a world rice core collection consisting of 69 accessions, which covers the genetic diversity of almost 32,000 accessions of cultivated rice (Fig. 4.5). These find- ings suggest that root-to-shoot Cd translocation via the xylem is the major and common physiological process determining shoot-Cd accumulation among rice cultivars.
Rice-phloem sap can be collected using cut stylets of the brown plant hopper (Kawabe et al., 1980). Using this method, Cd levels in the phloem sap were measured and it was found that more than 90% of Cd present in grains is translocated via phloem (Tanaka et al., 2007). Moreover, Cd concentration of the phloem sap of LAC23, the variety with lowest Cd accumulation in grains, was significantly lower than that of Koshihikari, the Japanese elite variety, despite similar levels of Cd in xylem sap in these cultivars (Kato et al., 2010). Thus, differences in grain Cd concentrations in rice cultivars may be in part explained by a different ability of phloem to transport Cd to grains.
It is necessary to understand the genetic aspects of Cd accumulation in order to devise a breeding plan for reducing Cd levels in rice grains.
Quantitative trait loci (QTL) analysis is a powerful tool for understand- ing the genetic control underlying agronomic and physiological traits in
0 10 20 30 40 50 60
0 200 400 600 800
Cd concentration in shoots (mg kg–1dry wt.)
Cd concentration in xylem sap (àg L–1) Extra high Cd- accumulating rice cultivars
Figure 4.5 Relationship between Cd concentration in shoots and that of xylem sap in diverse rice germplasms (Uraguchi et al., 2009 with permission).
rice ( Yamamoto et al., 2009). Using backcross inbred lines and advanced- backcross progenies derived from a cross between the low-Cd culti- var Sasanishiki and the high-Cd cultivar Habataki, Ishikawa et al. (2010) reported a major-effect QTL (named as qGCd7) controlling Cd concentra- tion in rice grains without affecting concentrations of essential trace metals (Cu, Fe, Mn, and Zn), and it is located on the short-arm chromosome 7.
Moreover, this QTL had no significant effect on important rice agronomic traits, such as grain yield, grain weight, and days to heading. Using other mapping populations derived from crosses between low-Cd Japanese rice cultivars and high-Cd Indica ones, the QTL with a major effect related to Cd-translocating ability from roots to shoots at the seedling stage has also been detected on the short arm of chromosome 7 (Tezuka et al., 2010;
Ueno et al., 2009). It also has been revealed that OsHMA3, a P1B-type ATPase, is a gene that controls root-to-shoot Cd translocation (Miyadate et al., 2011; Ueno et al., 2010). Functional analyses of the OsHMA3 gene in yeast showed that low-Cd cultivars contain a functional version of this gene, which is involved in Cd storage in root vacuoles. The high-Cd culti- vars have lost this function; consequently, a much higher amount of Cd was loaded into the xylem. Overexpression of the functional OsHMA3 gene from the low-Cd cultivar drastically decreased Cd accumulation, not only in shoots but also grains in rice. Thus, this gene can be used to develop the phytotechnologies for controlling Cd accumulation in rice.
5.3.3. Breeding of Low Cd-Accumulating Cultivars
Although Japanese rice cultivars are categorized as the low-Cd group and possess the functional OsHMA3 gene, some Cd-contaminated areas in Japan produce rice grains that exceed the maximum allowable limit of Cd.
Thus, a breeding program has been initiated to produce rice varieties that have lower grain Cd concentration than the elite cultivars currently grown in Japan (Yamaguchi, 2006). LAC23, the tropical Japonica rice variety, was selected as a donor of the low-Cd trait because this variety has lower grain Cd concentration than other Japanese rice cultivars. LAC23 is not a practi- cal variety in Japan because of late heading, long culms, long grains, and low yields. So crossing was undertaken with the Japanese rice cultivar Fuku- hibiki, which has a good plant shape and offers stable high yields. This way, one could develop lines with low Cd concentrations but also with improved cultivation characteristics. By analysis across three to five self-fertilized gen- erations (F3–F5), five promising lines were selected which, in comparison with Fukuhibiki and the elite cultivar Hitomebore, had 40–50% lower Cd
concentrations in brown rice (Fig. 4.6), headed sooner than LAC23, and became comparatively shorter in plant height (Plate 4.2). These five lines were assigned the local numbers Ukei1118 through Ukei1122 based on the place where they were raised (National Agricultural Research Center for Tohoku Region, Daisen City, Akita Prefecture, Japan). For other trace metal concentrations such as Cu, Fe, Mn, and Zn, newly developed lines were nearly equal to those of Fukuhibiki and Hitomebore. Thus, it was possible to develop lines in which only the concentrations of Cd were reduced in the brown rice. However, further improvement should be done to incor- porate high grain yield and good taste into promising lines. To develop practical low-Cd cultivars efficiently, attempts are being made to identify the QTL for the low-Cd trait controlled by LAC23 allele and to develop the DNA marker linked to the QTL for the screening process.