The Rice Aquaporin Lsi1 Mediates Uptake of Methylated Arsenic Species

Pentavalent methylated arsenic (As) species such as monomethylarsonic acid [MMA(V)] and dimethylarsinic acid [DMA(V)] are used as herbicides or pesticides, and can also be synthesized by soil microorganisms or algae through As methylation. The mechanism of MMA(V) and DMA(V) uptake remains unknown. Recent studies have shown that arsenite is taken up by rice (Oryza sativa) roots through two silicon transporters, Lsi1 (the aquaporin NIP2;1) and Lsi2 (an efflux carrier). Here we investigated whether these two transporters also mediate the uptake of MMA(V) and DMA(V). MMA(V) was partly reduced to trivalent MMA(III) in rice roots, but only MMA(V) was translocated to shoots. DMA(V) was stable in plants. The rice lsi1 mutant lost about 80% and 50% of the uptake capacity for MMA(V) and DMA(V), respectively, compared with the wild-type rice, whereas Lsi2 mutation had little effect. The short-term uptake kinetics of MMA(V) can be described by a Michaelis-Menten plus linear model, with the wild type having 3.5-fold higher V_max than the lsi1 mutant. The uptake kinetics of DMA(V) were linear with the slope being 2.8-fold higher in the wild type than the lsi1 mutant. Heterologous expression of Lsi1 in Xenopus laevis oocytes significantly increased the uptake of MMA(V) but not DMA(V), possibly because of a very limited uptake of the latter. Uptake of MMA(V) and DMA(V) by wild-type rice was increased as the pH of the medium decreased, consistent with an increasing proportion of the undissociated species. The results demonstrate that Lsi1 mediates the uptake of undissociated methylated As in rice roots.Arsenic (As) contamination affects millions of people worldwide, particularly in South Asia where As-contaminated groundwater has been extracted for drinking (Chakraborti et al., 2002; Nordstrom, 2002). Recent studies have shown that foods, especially rice (Oryza sativa), are an important source of inorganic As to populations dependent on a rice diet (Kile et al., 2007; Ohno et al., 2007; Mondal and Polya, 2008). Paddy rice is more efficient than other cereal crops in accumulating As (Williams et al., 2007). This is because anaerobic conditions in submerged paddy soils lead to mobilization of arsenite [As(III); Takahashi et al., 2004; Xu et al., 2008], which is then taken up by rice roots mainly through the highly efficient transport pathway for silicon (Si; Ma et al., 2008). The relatively high accumulation of As in rice is of concern, as it may pose a significant health risk (Zhu et al., 2008; Meharg et al., 2009).A number of As species may be present in soil depending on soil conditions and the history of As contamination. These include arsenate [As(V)], As(III), and methylated As species such as monomethylarsonic acid [MMA(V): CH₃AsO(OH)₂] and dimethylarsinic acid [DMA(V): (CH₃)₂AsO(OH)]. As(V) is the main species in aerobic soils, while As(III) dominates in anaerobic environments such as flooded paddy soils. Both MMA(V) and DMA(V) have been found in paddy soils (Takamatsu et al., 1982), which may have been derived from microbial and algal biomethylation and/or past uses of methylated As compounds. MMA(V), as sodium or calcium salt, and DMA(V), as sodium salt or free acid (also called cacodylic acid), are herbicides widely used for weed control on cotton (Gossypium hirsutum), orchards, and lawns, or as a defoliant of cotton (U.S. Environmental Protection Agency, 2006). Conversion of cotton fields for the production of paddy rice in the United States may be a reason for the high levels of methylated As reported for the U.S. rice (Meharg et al., 2009).The mechanism of As(V) uptake by plants through the phosphate transport system has been well established (for review, see Zhao et al., 2009). In contrast, As(III) is taken up into the cells by aquaglyceroporins in Escherichia coli, yeast (Saccharomyces cerevisiae), and mammalian tissues (for review, see Bhattacharjee and Rosen, 2007). Recent studies have shown that several plant aquaporin channels belonging to the Nodulin 26-like Intrinsic Protein (NIP) subfamily are permeable to As(III) when expressed heterologously in yeast (Bienert et al., 2008; Isayenkov and Maathuis, 2008; Ma et al., 2008). The rice Si transporter Lsi1 (OsNIP2;1; Ma et al., 2006) is also permeable to As(III) when expressed in yeast or Xenopus laevis oocytes (Ma et al., 2008). Furthermore, the lsi1 rice mutant lost 57% of the influx capacity for As(III) compared to the wild type in short-term assays, suggesting that Lsi1 is an important entry route for As(III) (Ma et al., 2008). In rice roots, a second Si transporter, Lsi2, functions as an efflux carrier to transport Si efflux from the exodermis and endodermis cells toward the stele for xylem loading (Ma et al., 2007). This transporter also mediates As(III) efflux; two independent lsi2 mutants had 73% to 91% lower concentrations of As(III) in the xylem sap than their wild types (Ma et al., 2008). The shared uptake pathway between Si (silicic acid) and As(III) (arsenous acid) is consistent with their physiochemical properties; both are present predominantly as undissociated neutral molecules at the normal environmental and physiological pH range (pK_a = 9.2, >99% undissociated at pH ≤ 7.0), and the two molecules have similar sizes.The uptake mechanisms of methylated As species by plant roots are not known. Previous studies showed that both MMA(V) and DMA(V) can be taken up by roots and translocated to shoots in a number of plant species (Marin et al., 1992; Carbonell-Barrachina et al., 1998, 1999; Burló et al., 1999). Marin et al. (1992) found that uptake by rice followed the order of As(III) > MMA(V) > As(V) > DMA (V), although DMA(V) was more efficiently translocated from roots to shoots than other As species. Raab et al. (2007) reported large variations in the absorption and translocation efficiencies for As(V), MMA(V), and DMA(V) among the 46 plant species tested. On average, root absorption of As(V) was 2.5- and 5-times higher than MMA(V) and DMA(V), respectively. The translocation efficiency, defined as the shoot-to-root concentration ratio after 24-h exposure, was highest for DMA(V) (0.8), followed by MMA(V) (0.3) and As(V) (0.09). The concentration-dependent uptake kinetics of MMA(V) in rice roots could be described by the Michaelis-Menten equation, whereas the limited uptake of DMA(V) appeared to be linear in relation to the increasing concentration in the uptake medium (Abedin et al., 2002). Abbas and Meharg (2008) showed that DMA(V) uptake by maize (Zea mays) seedlings was enhanced by more than 10-fold by a pretreatment of phosphorus starvation; this compared with only 2-fold increase in As(V) uptake. They thought that DMA(V) might be taken up by the phosphate transporters, or that phosphorus starvation altered expression of a range of membrane transporters or even membrane permeability itself.In addition to the root uptake of methylated As species, some plants appear to be able to biomethylate As, but the pathway and enzymology remains unclear (Wu et al., 2002; Zhao et al., 2009). In microbes, As methylation follows the Challenger pathway involving repeated steps of As reduction and oxidative methylation (Bentley and Chasteen, 2002). As(V) is first reduced to As(III), which is methylated by S-adenosylmethyltransferase using S-adenosyl-l-Met as the methyl donor. The product of this reaction is pentavalent MMA(V), which is reduced by a reductase to trivalent MMA(III) with thiols (e.g. glutathione). Methylation and reduction steps continue to produce di- and trimethyl As compounds. MMA(III) and DMA(III) are intermediates in the As methylation pathway, which is not very stable (Gong et al., 2001). In rice grain, DMA(V) is the main form of methylated As, and can account for up to 80% of the total As (Zavala et al., 2008; Meharg et al., 2009). In light of the significant presence of methylated As in rice, it is important to elucidate the transport and assimilation pathways of these As species in plants.In this study, we present evidence that MMA(V) and DMA(V) are taken up by rice roots, at least partly, through the NIP aquaporin channel Lsi1, and that this process is strongly pH dependent. We also show that MMA(V) can be reduced to MMA(III) in planta.     

Ceramide in Rice Grain

A rice lamina inclination test that is simple and specific for brassinosteroids was used as a micro-quantitative bioassay for brassinolide 1 and its 6-keto congener, castasterone 2, in the concentration range of 5 x 10^–5 /ig/ml to 5 x 10^–3μg/ml, when uniform seedlings of the rice cultivars Arborio J-l and Nihonbare were selected. A phytohormone, indole-3-acetic acid (IAA), showed similar activity in this bioassay. Its lowest effective concentration, however, was 50 /ig/μl, about five orders of magnitude greater than that of brassinolide. Other phytohormones, abscisic acid (ABA) and the cytokinins kinetin and A⁶-benzyladenine, inhibited the lamina inclination of rice seedlings. The addition of a cytokinin reduced the promoting effect of brassinolide. Thus, the rice lamina inclination test can be used both as a micro-quantitative bioassay for brassinosteroids and as a method for detecting antibrassinolide compouds.     

水稻对砷的吸收及代谢机制研究进展

砷（As）是一种广泛存在的致癌的微量元素。环境中日益严重的As污染问题,影响了水稻的生长和品质,并通过食物链进一步威胁着人类健康。为降低食物链中As的污染并提高水稻对As的耐性,需要深入了解水稻对As的吸收及As在水稻体内转运、代谢过程的生理及分子生物学机制。本文就以上几个问题综述了近些年来国内外的研究结果,并对今后深入研究提出建议。     

超富集植物蜈蚣草中砷化学形态的EXAFS研究

采用同步辐射扩展X射线吸收精细结构(SREXAFS)技术研究了超富集植物蜈蚣草(PterisvittataL.)中As的化学形态及其在转运过程中的变化。结果表明,蜈蚣草中的As主要以As(Ⅲ)与O配位的形态存在。As(V)被植物吸收后,很快转化为As(Ⅲ),其转化过程主要发生在根部。As(Ⅲ)向地上部转运的过程中价态基本不变。在植物的根部和部分叶柄中存在少量与As-GSH相似的As-S结合方式,但是在As含量最高的羽叶中基本上未发现这种结合方式。与需要提取和分离过程的化学方法相比,采用EXAFS方法研究植物中的砷形态不需经过预分离或化学预处理就可以直接测定植物样品中元素的化学形态,因此可以避免样品预处理过程对As形态的干扰,并获得可靠的砷化学形态方面的信息。     

超富集植物蜈蚣草中砷化学形态的EXAFS研究

采用同步辐射扩展X射线吸收精细结构(SR EXAFS)技术研究了超富集植物蜈蚣草(Pteris vittata L.)中As的化学形态及其在转运过程中的变化.结果表明,蜈蚣草中的As主要以As(Ⅲ)与O配位的形态存在.As(Ⅴ)被植物吸收后,很快转化为As(Ⅲ),其转化过程主要发生在根部.As(Ⅲ)向地上部转运的过程中价态基本不变.在植物的根部和部分叶柄中存在少量与As-GSH相似的As-S结合方式,但是在As含量最高的羽叶中基本上未发现这种结合方式.与需要提取和分离过程的化学方法相比,采用EXAFS方法研究植物中的砷形态不需经过预分离或化学预处理就可以直接测定植物样品中元素的化学形态,因此可以避免样品预处理过程对As形态的干扰,并获得可靠的砷化学形态方面的信息.     

水稻籽粒中的淀粉合成关键酶及其与籽粒灌浆和稻米品质的关系

文章介绍水稻籽粒中淀粉合成关键酶及其在籽粒灌浆和制约稻米品质中的作用,以及影响它们的外界环境因素的研究进展。     

Phytoprotective Effect of Arbuscular Mycorrhizal Fungi Species Against Arsenic Toxicity in Tropical Leguminous Species

Arbuscular mycorrhizal fungi (AMF) improve the tolerance of hosting plants to arsenic (As) in contaminated soils. This work assessed the phytoprotective effect of Glomus etunicatum, Acaulospora morrowiae, Gigaspora gigantea, and Acaulospora sp. on four leguminous species (Acacia mangium, Crotalaria juncea, Enterolobium contortisiliquum, and Stizolobium aterrimum) in an As-contaminated soil from a gold mining area. AMF root colonization, biomass production, As and P accumulation, as well as arsenic translocation index (TI) from roots to shoots were measured. The AMF phytoprotective effect was assessed by the P/As ratio and the activity of plant antioxidant enzymes. The AMF colonization ranged from 24 to 28%. In general, all leguminous species had low As TI when inoculated with AMF species. Inoculation of C. juncea with Acaulospora sp. improved significantly As accumulation in roots, and decreased the activity of ascorbate peroxidase (APX) and superoxide dismutase (SOD), highlighting its phytoprotective effect and the potential use of this symbiosis for phytoremediation of As-contaminated soils. However, S. aterrimum has also shown a potential for phytoremediation irrespectively of AMF inoculation. APX was a good indicator of the phytoprotective effect against As contamination in C. juncea and A. mangium. In general P/As ratio in shoots was the best indicator of the phytoprotective effect of all AMF species in all plant species.     

Concentrations of Heavy Metals and Arsenic in Market Rice Grain and Their Potential Health Risks to the Population of Fuzhou,China

Heavy metals and As in rice are of increasing concern in China. In this study, concentrations of Pb, Cd, Cu, Cr, Sb, Ni, and As in rice collected from markets in Fuzhou, China, were investigated by ICP-MS and AFS, and their potential health risk to inhabitants were estimated by target hazard quotient (THQ), hazard index (HI), and target cancer risk (TR). The results showed that the concentrations of the seven studied elements in rice grain were all below the permissible limits of China's national standards for foodstuffs (NY/T 419–2007 and GB 2726–2012). For non-carcinogenic risk, the THQ values of individual elements were within the safe interval. However, without considering the bio-accessibility and speciation of toxic elements, the HI values suggest inhabitants in Fuzhou may experience potential health effects due to rice consumption. Cadmium is the major contributor to HI, followed by As. The TR values of As for adults and children were both higher than the acceptable range. The results of this study suggest that more attention should be paid toward monitoring toxic substances (especially Cd and As) in rice in order to assure the food safety for consumers in Fuzhou, China.     

利用HPLC法测定水稻子粒主要黄酮类化合物

通过建立水稻儿茶素、杨梅素、槲皮素和山奈酚含量的HPLC测定方法,研究水稻黄酮类化合物含量及组成差异,为高黄酮水稻种质资源的定向改良提供理论依据.本研究建立了利用HPLC法测定水稻黄酮类化合物含量的测定方法;水稻糙米的黄酮化合物组成主要以儿茶素和山奈酚的形式存在,不含杨梅素和槲皮素,不同品种儿茶素和山奈酚组成比例及含量存在差异;粳稻含有更丰富的儿茶素,有色稻山奈酚和儿茶素含量大于无色稻;糙米中含有较高的黄酮类化合物,而精米中不含或少含黄酮类化合物.     

杂交水稻直链淀粉含量遗传分析

杂交水稻稻谷是从Ｆ１代杂合体收获的Ｆ２代种子,由于Ｆ２代属分离群体,直链淀粉含量等稻米品质会发生分离现象．本研究选择直链淀粉含量低、中、高的５个亲本配组的５个杂交组合,按单粒分析法分析双亲亲本、Ｆ１和Ｆ２代的种子的直链淀粉含量．结果表明,Ｆ２代呈现１（低）：３（中＋高）孟德尔分离规律．在５个组合的单粒分析中发现分离的Ｆ２代群体呈双峰的连续分布,该现象表明高直链淀粉含量对低直链淀粉含量为不完全显性,除单一基因控制外,还受微效多基因修饰．因此杂交水稻育种工作中,尤其是优质杂交稻新组合选育时,必须注意选择双亲的直链淀粉含量基本一致,以防分离现象影响米质．     

Biochemical Changes in the Rice Grain during Germination

Changes in the content of starch, protein, and RNA and in the activity of their hydrolases in the rice endosperm (Oryza sativa L., variety IR8) were determined during the first week of germination without added nutrient both in the dark and in the light. Changes were generally more rapid in the dark than in the light. Oxygen uptake and RNase activity started to increase and the root protruded on the second day, followed by the coleoptile on the third day, and the primary leaf on the fourth day. ATP level was at a maximum on the fourth day. The activity of amylases and R enzyme increased progressively, but that of phosphorylase tended to decrease during starch degradation. A new α amylase isozyme band appeared during germination. Glucose was the major product of starch degradation. Sucrose, maltose, maltotriose, raffinose, and fructose were also detected. Protease activity reached a maximum on the fifth or sixth day and closely paralleled the increase in soluble amino N and soluble protein.     

水稻砷的吸收机理及阻控对策

水稻积累砷的能力较其他农作物强,稻米是我国人群从食品中摄入无机砷的主要来源,降低稻米砷含量对增进农产品质量安全有重要意义。本文阐述了稻田砷的生物地球化学转化特征及水稻对不同形态砷的吸收、运输与储存机理,提出阻控稻米砷积累的对策,并讨论了需要进一步研究的问题。     

水稻糙米高蛋白基因的QTL定位

利用由糙米蛋白含量高达14．55％的广东省农家品种三春种配制的BC1群体进行糙米高蛋白基因的QTL定位,定位到了6个糙米蛋白基因的QTLs,其中有3个新的QTLs。有1个增效基因qCP1-2为主效基因,解释的表型变异高达44．2％,有可能是与谷蛋白基因Glu-1紧密连锁的新调控序列。本研究表明,所利用的BC1群体的糙米蛋白可能主要是由1个主效QTL所控制。     

水稻粒形遗传的研究进展

水稻（Oryza sativa）粒形性状包括粒长、粒宽、粒厚、粒重和长宽比等,是构成水稻产量的重要因素之一。因此,阐明水稻粒形遗传控制的机理,对提高水稻产量具有十分重要的意义。水稻粒形的遗传是多基因共同作用的数量性状遗传,相对于单一基因控制的性状,研究要相对复杂。该文综述了水稻粒形的遗传特点、QTL定位和基因克隆,并展望了粒形遗传的研究前景。     

水稻SL基因调控水稻的粒形

水稻产量和稻米品质的提高是水稻研究的中心问题。水稻产量主要取决于单株穗数、每穗粒数和粒重;粒重作为一个非常重要的产量性状,由粒长、粒宽和粒厚所决定。影响粒重和粒形的基因多为数量性状基因,精细定位并克隆到的较少。本研究中,我们克隆到一个影响粒形的基因SL,超表达（SL-OE）转基因植株表现出粒长增加、粒宽减小、叶宽减小的表型;同时,SL-RNAi的转基因植株呈现出粒长缩短、叶宽增加的表型。颖壳表面细胞在超表达转基因植株中伸长,而在RNAi转基因植株中缩短。叶片横向细胞数目在转基因植株中发生变化,推测乩基因可能与细胞分裂相关。SL-OE转基因植株中G嗽因被明显上调,说明盟基因可能通过调节GW2的表达对水稻粒宽造成影响。另外,观基因影响稻米的品质。     

8个野生稻种的叶片硅体研究

用强酸溶液分离8个野生稻种叶片中的硅体并用光学显微镜和电子扫描显微镜进行观测研究.结果显示,(1)叶片含有3种类型的硅体,即不规则硅体、哑铃形硅体和扇形硅体,前2种硅体不能作为属内分类的依据,但短药野生稻哑铃形硅体的表面纹饰独特,可作为识别该物种的特征.(2)同一个物种的扇形硅体变异很大,统计学表明它总以其中的某一种类型占有优势,紧穗野生稻和短舌野生稻中小型硅体占优势,药用野生稻和普通野生稻中约有一半为中型硅体,另一半为小型硅体,其余4种野生稻包括斑点野生稻、澳洲野生稻、展颖野生稻和短药野生稻中绝大多数为中型硅体,即野生稻的扇形硅体以中、小型为主.(3)根据扇面长/扇柄长的比值,将扇形硅体划分为长柄型、中间形和短柄型3种形态,依此对8个稻种的硅体分类.结果表明,斑点野生稻、药用野生稻、紧穗野生稻和澳洲野生稻以长柄型为优势;普通野生稻、短舌野生稻、展颖野生稻和短药野生稻则以中间型为主,且扇形硅体的扇面长/扇柄长的比值的大小与野生稻染色体组型或原产地有关.     

水稻花色苷含量的遗传研究进展

花色苷作为水稻重要的生物活性物质,已成为当前功能性水稻研究开发的热点之一。本文从水稻花色苷的生物合成及其组成成分入手,着重介绍了水稻花色苷含量的影响因素、遗传及分子机理的研究现状以及富集花色苷水稻种质鉴定、筛选与创新现状,并探讨了今后以提高水稻花色苷含量为目标的功能性水稻研究内容和方向。     

引进巨胚稻与普通稻的米质和营养成分分析比较

对引进的巨胚稻进行引种栽培并对其米质和营养成分进行全面分析.结果表明,巨胚稻的胚重是普通水稻胚重的3～5倍,多项营养成分如蛋白质、钙、磷、钾、钠、锰、VE、VB6和膳食纤维等的含量比普通稻米要高得多,这个结果表明巨胚稻是加工婴幼儿食品非常好的原料.     

Moderate Salinity Stress Reduces Rice Grain Yield by Influencing Expression of Grain Number- and Grain Filling-Associated Genes

Journal of Plant Growth Regulation - Soil salinity is an environmental stress severely impacting on rice grain yield. However, limited information is available on how salinity affects expression...