23英文文献水稻镉art_0.86_939-8433-5-5.docx
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1、Uraguchi and Fujiwara Rice 2012, 5:5 http:/ R E V I E W Open Access Cadmium transport and tolerance in rice: perspectives for reducing grain cadmium accumulation Shimpei Uraguchi* and Toru Fujiwara Abstract Cadmium (Cd) is a toxic heavy metal which harms human health. In Japan, a major source of hum
2、an Cd-intake is rice grains and contamination of paddy soils by Cd and accumulation of Cd in rice grains are the serious agricultural issues. There also exist Cd contamination of rice and its toxicity in several populations in countries including China and Thailand. Understanding the Cd transport me
3、chanisms in rice can be a basis for regulating rice Cd transport and accumulation by molecular engineering and marker-assisted breeding. Recently, a number of studies have revealed the behavior of Cd in rice, genetic diversity of Cd accumulation, quantitative trait loci controlling Cd accumulation a
4、nd transporter molecules regulating Cd accumulation and distribution in rice. In this article, we summarize recent advances in the field and discuss perspectives to reduce grain Cd contents. Introduction Cadmium (Cd) is a toxic heavy metal and is also known as one of the major environmental pollutan
5、ts. Moderate Cd contamination of arable soils can result in considerable Cd accumulation in edible parts of crops (Arao and Ae 2003; Arao et al. 2003; Wolnik et al. 1983). Such levels of Cd in plants are not toxic to crops but can contribute to sub- stantial Cd dietary intake by humans (Wagner 1993)
6、. In the case of “Itai-itai disease”, Cd-polluted rice was the major source of Cd intake in the patients (Yamagata and Shigematsu 1970). This is the early case of chronic Cd toxicity in general populations without specific industrial exposure. Even in recent general populations in Japan, the interna
7、l Cd level is higher than those of other countries and this is largely because of daily consumption of Japanese rice which contains relatively high Cd (Watanabe et al., 1996; Watanabe et al. 2000; Tsukahara et al. 2003). Cd concentrations of recent Japanese rice have been con- stantly higher compare
8、d to those of other countries (Watanabe et al., 1996; Shimbo et al., 2001), although the values are much lower than the limit established by the Codex Alimentarius Commission of FAO/WHO (0.4 mg/kg). In some areas in China and Thailand, production of highly Cd-polluted rice and renal disfunc- tions a
9、mong populations were reported (Nordberg et al., 1997; Jin et al., 2002; Honda et al., 2010). In the United States, increased consumption of rice and other cereals contributes to the recent increase of the dietary Cd intake (Egan et al. 2007). Many reports suggest importance to consider chronic effe
10、cts of Cd exposure through foods (Jarup and Akesson 2009). In Japanese populations, the average dietary Cd intake (3.0 g Cd/kg body weight/ week) exceeds the tolerable weekly intake (2.5 g Cd/kg body weight) set by the European Food Safety Authority (EFSA) and is about 50% of a provisional tolerable
11、 monthly intake (25 g Cd/kg body weight/month) estab- lished by the Joint Food and Agriculture Organization/ World Health Organization (FAO/WHO) Expert Commit- tee on Food Additives and Contaminants (JECFA). Reeves and Chaney (2008) suggested to consider the high Cd availability of rice for humans b
12、ecause of relatively low iron and zinc contents in rice-based foods (Reeves and Chaney, 2008). These suggest the importance of reducing grain Cd accumulation in rice and other cereals for better human health. Recently, as a model plant of cereals, physiological and molecular understanding of Cd tran
13、sport in rice plants have been advanced. In this review, we describe current * Correspondence: shimbu.iij4u.or.jp Graduate School of Agricunltural and Life Sciences, The University of Tokyo, Tokyo 113-8657, Japan knowledge of rice Cd transporters and their (possible) 2012 Uraguchi and Fujiwara; lice
14、nsee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http:/creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Uraguchi and
15、Fujiwara Rice 2012, 5:5 http:/ Page 2 of 8 roles in Cd accumulation. Several trials to generate “low-Cd-rice” based on these findings are also described. Physiology of rice Cd accumulation The average Cd concentration in Japanese soils is 0.2 - 0.3 mg/kg DW (Takeda et al. 2004), which is somewhat hi
16、gher than those of agricultural soils in China, Indone- sia and the United States (Holmgren et al., 1993; Hera- wati et al., 2000). In paddy soils largely affected by industrial activities like mining and smelter, the Cd con- centrations are much higher than the average (Xian, 1988; Asami et al., 19
17、95). In agricultural soils, atmo- spheric deposition (Keller and Schulin, 2003) is known as a major source of Cd input. In paddy fields, irrigation water is another Cd source which continuously loads Cd into soils (Kikuchi et al. 2007). Rice absorbs Cd2+ in soils, and after several processes of tran
18、sport Cd finally accumulates into grains. Cd is rapidly transported from roots to shoots by the xylem after absorption (Uraguchi et al. 2009b). Substantial Cd is detected in the xylem sap and shoot tissues 1 h after Cd treatment to roots, and this activity of root-to-shoot translocation by the xylem
19、 is the determinant for shoot Cd accumulation level. On the other hand, in the panicle neck, phloem is the major Cd transport route into grains (Tanaka et al. 2007). In phloem sap, Cd binds to an unknown 13 kDa protein and SH-compounds (Kato et al. 2010). The real-time live-imaging technique using a
20、 posi- tron emitting radio isotopes called PETIS revealed the detailed behavior of Cd in rice after absorption (Fujimaki et al. 2010). They demonstrated that Cd is rapidly trans- located from roots to shoots through culms and Cd tends to be retained in nodes. And after 7 h of Cd treatment, Cd is pre
21、ferentially deposited into panicles rather than into leaf blades. These suggest that nodes are the important tissue for redirecting Cd transport from roots probably by transferring Cd from xylem to phloem. In addition to Cd absorbed from roots, remobilization of Cd in leaf blades is also likely to c
22、ontribute to grain Cd accu- mulation (Rodda et al. 2011). They suggest that a sub- stantial amount of Cd accumulated in leaf blades before heading is remobilized and transported into grains during the ripening stage. These physiological studies indicate four major trans- port processes for rice Cd a
23、ccumulation: (1) root Cd uptake, (2) root-to-shoot translocation by xylem flow, (3) redirection at nodes and (4) remobilization from leaves (Figure 1). After the first report by Ishikawa et al. (2005), several studies conducted QTL analyses to iden- tify the responsible transporter gene for these pr
24、ocesses (Ishikawa et al. 2010; Ishikawa et al. 2005; Tezuka et al. 2010; Ueno et al. 2009). QTL analysis is a very useful approach because there is a clear genotypic difference in Cd accumulation in shoots and grains among cultivars. Generally, Cd accumulation in shoots and grains are potentially hi
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