23英文文献水稻镉art_0.86_939-8433-5-5.docx

《23英文文献水稻镉art_0.86_939-8433-5-5.docx》由会员分享，可在线阅读，更多相关《23英文文献水稻镉art_0.86_939-8433-5-5.docx（8页珍藏版）》请在得力文库 - 分享文档赚钱的网站上搜索。

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

25、gher in indica cultivars compared to japo- nica cultivars (Arao and Ae 2003; He et al., 2006b; Uraguchi et al. 2009b) and moreover, some specific cul- tivars among indica rice accumulate much higher Cd in vegetative tissues and grains (Uraguchi et al. 2009b). Recently, several transporters have been

26、 identified as a Cd transporter in rice by forward and reverse genetics and their functions in these processes will be reviewed in the following sections and summarized in Figure 1. Uptake by roots In plants and mammals, many of transporters for divalent transition metals have a Cd2+ uptake activity

27、. In mam- mals, ZIP8 and ZIP14, two transporters belonging to the Zinc-regulated transporter (ZRT) -like, Iron-regulated transporter (IRT) -like protein (ZIP) family, transport Cd2+ as well as Mn2+, Fe2+ and Zn2+ (He et al. 2006a; Himeno et al. 2009; Nebert et al. 2009). In plants, it has been demon

28、strated much earlier than in mammals that AtIRT1, a ZIP family transporter for Fe2+, Zn2+ and Mn2 + also mediates Cd uptake in roots of Arabidopsis thali- ana (Connolly et al. 2002; Vert et al. 2002). AtIRT1 is the primary transporter for the strategy-1 iron (Fe2+) uptake system in A. thaliana. In r

29、ice, iron is mainly absorbed by the strategy-2 system in the form of Fe-phytosiderophore (Romheld and Marschner 1986), but Fe2+ transporters have been also identified which may function in Fe2+ uptake by roots (Ishimaru et al. 2006). OsIRT1 and OsIRT2 have an influx activity of Cd2+ as well as Fe2+

30、in yeasts, suggesting that OsIRTs play some role in root Cd uptake especially after release of pounded water during intermittent water management (Ishimaru et al. 2006; Nakanishi et al. 2006). They suggested that under flooded paddy soils, OsIRTs might be induced by lower levels of available iron an

31、d after water release induced OsIRTs might contribute to uptake of Cd which was much avail- able in aerobic conditions. When OsIRT1 was overex- pressed, Cd accumulation in roots and shoots was increased under MS medium containing excess Cd, but this phenotype was not observed in the field condition

32、(Lee and An 2009). These suggest that OsIRT1 is poten- tially involved in root Cd uptake but its contribution is largely affected by the environmental (soil) conditions. Transporters of natural resistance-associated macro- phage protein (Nramp) family are also known to mediate Cd transport. In mamma

33、ls, DCT1/DMT1/Nramp2 func- tions in Cd2+ uptake as well as uptake of Zn, Mn, Fe, Co and Ni ions (Gunshin et al. 1997; Hosoyamada et al. 2003). In plants, at the same time, it has been demon- strated that AtNramp1, AtNramp3 and AtNramp4 mediate inward Cd transport in yeasts and overexpression of AtNr

34、amp3 resulted in increased sensitivity to Cd (Thomine et al. 2000). In rice, there are seven Nramp Uraguchi and Fujiwara Rice 2012, 5:5 http:/ Page 3 of 8 Figure 1 A schematic model of Cd transport processes from soil to grains in rice. Cd is absorbed from soils into roots. OsIRT1 and OsNramp1 are s

35、uggested to mediate this process. OsHMA3n (the functional allele of OsHMA3) play a critical role in Cd compartmentation into vacuoles in root cells and thus negatively regulates Cd xylem loading. OsHMA3a (the non-functional allele of OsHMA3) can not function in vacuolar Cd compartmentation in roots

36、and which results in high efficiency of root-to-shoot Cd translocation. OsLCT1 contributes to Cd remobilization from leaf blades via phloem and also is likely to play a part in intervascular Cd transfer at nodes. genes. OsNramp1, an iron transporter, has been estab- lished as a Cd-influx transporter

37、 in the plasma-membrane (Takahashi et al. 2011). The cell-type specificity of OsN- ramp1 expression was not examined but the mRNA expression was much higher in roots than in shoots. OsN- ramp1-overexpressing rice plants accumulated less Cd in roots and much Cd in shoots compared to the wild-type pla

38、nts when grown in media containing 1 M or less Cd, suggesting that OsNramp1 is possibly involved in Cd transport into roots. Much interesting finding of their report is higher expression of OsNramp1 in indica culti- vars. Comparing Sasanishiki (a japonica cultivar) and Habataki (a indica cultivar),

39、the root-to-shoot Cd translo- cation ability is greater in Habataki (Uraguchi et al. 2009b). Following QTL analysis of a mapping population obtained by crossing the two cultivars, a major QTL for high Cd accumulation in shoots of Habataki was detected on the short arm of Chr. 7 (Ishikawa et al. 2010

40、). Although the responsible gene has not been identified, this QTL contains OsNramp1. There was no difference in the amino acid sequences of OsNramp1 between Habataki and Sasanishiki, but the gene expression was higher in Habataki and other indica cultivars probably caused by several insertion and d

41、eletion in the promoter region (Takahashi et al. 2011). This higher expression of OsN- ramp1 in indica cultivars may partly explain higher Cd accumulation in shoots of indica rice independent from the effect of OsHMA3, another Cd transporter described in the next section. Xylem loading and root-to-s

42、hoot translocation The ability of xylem-mediated Cd translocation into shoots is shown as a major determinant for shoot Cd accumulation in many plants including rice (Hart et al. 2006; Mori et al. 2009; Uraguchi et al. 2009b). In A. thaliana and A. halleri (a Cd/Zn hyperaccumulator), the key transpo

43、rters for xylem Cd transport have been first identified. In A. thaliana, the P1B-type ATPase Uraguchi and Fujiwara Rice 2012, 5:5 http:/ Page 4 of 8 AtHMA2 and AtHMA4 regulate root-to-shoot translo- cation of Cd and Zn (Hussain et al. 2004; Verret et al. 2004; Wong and Cobbett 2009). In A. halleri,

44、AhHMA4, a homolog of AtHMA4 plays a critical role in transloca- tion of Cd and Zn into shoots (Hanikenne et al. 2008). The enhanced promoter activity and increased gene copy number of AhHMA4 result in higher expression of AhHMA4 in root stele and contribute to hyperaccmula- tion and hypertolerance o

45、f Cd and Zn in A. halleri. Following the identification of the genes for xylem Cd transport in A. thaliana and A. halleri, OsHMA3 has been identified as a regulator for xylem Cd transport in rice by mediating vacuolar sequestration of Cd in root cells (Miyadate et al. 2011; Ueno et al. 2010). Compar

46、ed to AtHMA4 and AhHMA4, OsHMA3 has some unique fea- tures. All these HMAs mediate Cd efflux transport, but OsHMA3 reportedly does not transport other metals such as Zn, whereas AtHMA4 and AhHMA4 functions in both Zn and Cd transport. Subcellular localization also differs between OsHMA3 and others.

47、OsHMA3 is suggested to be localized to the vacuolar membrane, but AtHMA4 and AhHMA4 are localized to the plasma-membrane. The major difference of OsHMA3 and Arabidopsis HMA4s is the physiological function in plants. In the Nipponbare background, RNAi-mediated knock-down of OsHMA3 increased root-to-s

48、hoot Cd translocation and the overex- pression reduced shoot Cd accumulation (Ueno et al. 2010). They suggests that in Nipponbare, OsHMA3 func- tions in vacuolar compartmentation of Cd in root cells and hence reduces the xylem loading of Cd and subse- quent shoot Cd accumulation (Ueno et al., 2010),

49、 whereas AtHMA4 and AhHMA4 facilitate loading of Cd into the xylem. This finding reveals the mechanism for limiting Cd translocation into shoots in japonica rice. At the same time, they found a single amino acid substi- tution in OsHMA3 from some indica cultivars (Anjana Dhan, Cho-ko-koku and Jarjan) which results in the loss- of-function (Miyadate et al. 2011; Ueno et al. 2011; Ueno et al.