水稻的砷胁迫响应与耐性:生理、细胞和分子途径(英文)

时间:2018-09-11 作者:博学论文网
  Abstract:Arsenic (As) , a potentially toxic metalloid released in the soil environment as a result of natural as well as anthropogenic processes, is subsequently taken up by crop plants. In rice grains, As has been reported in Asia, North America and Europe, suggesting a future threat to food security and crop production. As3+ by dint of its availability, mobility and phytotoxicity, is the most harmful species of As for the rice crop. Specific transporters mediate the transport of different species of As from roots to the aboveground parts of the plant body. Accumulation of As leads to toxic reactions in plants, affecting its growth and productivity. Increase in As uptake leads to oxidative stress and production of antioxidants to counteract this stress. Cultivars tolerant to As stress are efficient in antioxidant metabolism compared to sensitive ones. Iron and selenium are found to have ameliorating effect on the oxidative stress caused by As. Microbes, even many indigenous ones, in the plant rhizosphere are also capable of utilizing As in their metabolism, both independently and in association. Some of these microbes impart tolerance to As-stress in plants grown in As contaminated sites.
  
  Keyword:arsenic; rice; phytotoxicity; hyperaccumulator; phytochelatin; antioxidant; mitigation;
    
  Arsenic (As) , a potentially toxic metalloid, is a naturally occurring element ubiquitous to all soils (Smedley and Kinniburgh, 2002;William et al, 2005) .It is the 20th most common element in the earth's crust (Mandal and Suzuki, 2002) .Soil contains1.5–3.0 mg/kg As.As is present in inorganic forms in various minerals in soil, the important ones being realgar, arsenopyrite, etc.From the minerals, As in both inorganic and organic forms, gets mobilized due to natural and human activities, and becomes more readily available to living organisms.
  
  Among the natural sources of mobilization of As, weathering of As-containing minerals (Woolson, 1977;Smedley, 2006) and the activities of microorganisms (Bentley and Chasteen, 2002;Turpeinen et al, 2002) are prominent.On methylation by microbes, As is released as monomethyl arsenate (MMA) or dimethyl arsenate (DMA) (Bentley and Chasteen, 2002) .Some microbes utilize As in their metabolism, and release the toxic trimethyl arsine oxide (TMAs O) gas which is subsequently released to the atmosphere (Mandal and Suzuki, 2002) .
  
  Human activities that release more As in bio-available forms include application of As pesticides, use of As in paints to be subsequently released by molds and bacteria, mining of metals and heavy extraction of groundwater.Insecticides[calcium arsenate (Ca3As2O8) and lead arsenate (Pb HAs O4) ], herbicide[sodium arsenite (Na As O2) ], rodenticides arsenious oxide (As2O3) and sodium arsenite (Na As O2) contain As (ICAR, 2009) .Their residues remain in the soil, get dissolved in the groundwater and become easily available to living organisms.As-containing wallpaper paints, fed upon by molds and bacteria, led to release of the'Gosio'gas, which was identified to be trimethyl arsine (Woolson, 1977) .Metal mining, processing of ores and related activities, has contributed to the release of As in the soil and groundwater (Smedley, 2006) in several countries including Thailand, Ghana, Turkey, England, Serbia, Bosnia, Poland, USA and Canada (Barringer and Reilly, 2013) .
  
  A very significant means of mobilization of As is the extraction of groundwater from shallow aquifers in various countries, where surface water is contaminated by disease causing microorganisms.Groundwater is generally more vulnerable to As contamination than surface water because of the interaction of groundwater with aquifer minerals and the increased potential in aquifers for the generation of the physicochemical conditions favourable for As release (Smedley, 2006) .Groundwater obtained from shallow aquifers is more likely to contain higher amount of As than that obtained from deep aquifers (Smedley, 2006) .As contaminated groundwater is found in many countries like Bangladesh, India, China, Argentina, Chile, Vietnam, Hungary and Mexico (Smedley and Kinniburgh, 2002;Smedley, 2006;Chakraborti et al, 2012) .Deep aquifers containing high As levels have also been reported in the Mekong Valley, Vietnam, where high amount of groundwater extraction has resulted in subsidence of the land level by almost 3 cm per year as measured by satellite based radar images from 2007to 2010 (Erban et al, 2013) , leading to speculations that similar deep aquifers of groundwater may not remain As free over the years to come.
  
  In soil, As exhibits a number of oxidation states, the common ones being As5+, As3+and As3-.In aerobic soils, As is mainly present in the oxidized form as arsenate (As5+) .While in anaerobic environments like paddy soil, it mainly exists in the reduced form as arsenite (As3+) (Takahashi et al, 2004) .
  
  With the use of As-contaminated water in irrigation and due to the various processes of its mobilization, the levels of As in soils have escalated, affecting the agricultural system and resulting in the uptake of the element by crop plants.The translocation factor (TF) for As is higher in rice (0.8) compared to the other crops like wheat (0.1) and barley (0.2) (Xu et al, 2008) .The amount of As in paddy soil and soil solution stand elevated (Brammer and Ravenscroft, 2009) .The scenario is the worst in rice fields in affected areas such as Bangladesh, India, China and Thailand, where rice is the staple food for most of the people (Abedin et al, 2002;Williams et al, 2006;Garnier et al, 2010) .Humans also have been exposed to As through drinking of As-contaminated water and consumption of As-contaminated crops.The contaminated groundwater used to cultivate vegetables and rice for human consumption may be an important pathway of As ingestion and exposure to chronic As (Chakraborti et al, 2004) .Food imported into the United Kingdom from Bangladesh has a 2-to 100-fold higher As concentration than vegetables cultivated in the United Kingdom and North America (Al Rmalli et al, 2005) .In the South Punjab and Sindh provinces of Pakistan, As is a major contaminant of soil, water and fodder, thereby making its way into the food chain of cattle and subsequently the humans (Zubair et al, 2017) .Thus, food is an important route of As exposure in some regions and these exposures can have long-term negative health effects in humans.Even, raised air-As concentration, caused by smelter contaminants in Yunnan (China) poisoning incident in 1993, has been found to contaminate crops grown in the area, where90%of the human As intake is from the food (rice and corn) , and only 10%or less from direct inhalation (Mandal and Suzuki, 2002) .
  
  The World Health Organization (WHO) certified that the maximum contaminant level (MCL) of As in drinking water is 10?g/L (reduced from 50?g/L) in1993.Regulations in different countries have recommended different values of the MCL (Smedley, 2006;Barringer and Reilly, 2013) .Countries like USA, New Zealand, Japan, Argentina, Brazil, Chile and Colombia place 10?g/L as MCL in drinking water.Australia uses a standard of 7?g/L.The New Jersey state in USA places 5?g/L as the MCL.Mexico has adopted a standard of 25?g/L, whereas some developing countries like Bangladesh have maintained the earlier 50μg/L MCL standard.In July2014, the WHO set worldwide guidelines for what it considers to be safe levels of As in rice, suggesting the maximum of 200?g/kg for white rice and 400μg/kg for brown rice (Sohn, 2014) .Safety measures can be taken for removing As from drinking water (Ghosh and Singh, 2009) by various methods even at household levels (Chaurasia et al, 2012) .However, removal of As from food or crop fields after its incorporation is not economically viable.
  
  As is considered to be one of the most highly toxicand carcinogenic elements.According to the US Environmental Protection Agency (EPA) and the International Agency for Research on Cancer (IARC) , As and its compounds have been ranked as a Group 1human carcinogen.As present in drinking water or contaminated crops can have severe impact on the health of human beings as well as other animals (Ng et al, 2003) .It has been found to be a fatal carcinogenic element in cases of extreme intake.Several deaths have been reported from areas of high contamination.As is a leading cause of serious health problems such as cancers of the skin, lung, bladder, liver and kidney, as well as adverse effects on cardiovascular, neurological, haematological, renal and respiratory systems (Barringer and Reilly, 2013) .As being taken up from soils, plant roots are the initial tissues to be exposed.The metalloid inhibits the extension and proliferation of roots.Upon its translocation to the shoot, As can inhibit plant growth severely by hindering the expansion and biomass accumulation as well as compromising the plant reproductive capacity through losses in fertility and yield (Rahman et al, 2007, 2008) .At sufficiently high concentrations, As interferes with critical physiological and metabolic processes such as cellular membrane damage, and increase in antioxidant mechanisms, which can even lead to the death of the plant (Garg and Singla, 2011) .
  
  Occurrence and distribution of arsenic in rice growing areas
  
  Rice, the dietary staple food for half of the world's population, is believed to have originated in Asia (Lahkar and Tanti, 2017) , and gradually spread to all the continents.It has an affinity of taking up As from contaminated soil and water and even air to some extent (Abedin et al, 2002;Mandal and Suzuki, 2002) .Thus, over the past few decades, with the increase in As content of soil and groundwater, and with the use of contaminated water in irrigation, subsequently, As content in rice grains has also been found to be rising in several parts of the world (Fig.1) .
  
  The rice fields of Bangladesh are heavily irrigated with As contaminated water obtained from millions of shallow tube wells installed by the country's farmers.As a result, in several areas of the country, As content in rice grain has been found to rise to alarming levels which bears threat to food safety and also crop production in future (Williams et al, 2006;Rahman and Hasegawa, 2011) .The highest mean As content in rice grains is found in Faridpur (0.95?g/g) , followed by Rajbari (0.76?g/g) , Golapganj (0.57?g/g) , Dinajpur (0.54?g/g) , Srinagar (0.48–0.53?g/g) and Sonargaon (0.46?g/g) districts of the country (Williams et al, 2006) .Of the overall mean of 0.13μg/g As in Bangladesh rice grains, about 80%is found to be inorganic, which is more toxic than the organic forms (Williams et al, 2006;Zavala et al, 2008) .
  
  In India, As-contaminated rice has been reported in the states of Chattisgarh (Patel et al, 2005) and WestBengal (Santra et al, 2013) , and more will potentially be reported in future, with groundwater contamination by As reported in a number of states (Singh, 2004;Chakraborti et al, 2012) .In West Bengal, Boro rice varieties (0.451μg/g) contain more As than Aman ones (0.334μg/g) , while improved high-yielding rice varieties also contain more As than the local ones (Bhattacharya et al, 2010b;Santra et al, 2013) .Moreover, on dietary basis, daily As intake is 560 and393μg for adults and children, respectively, and the people having poor nutrition are affected more from As toxicity than those having adequate nutrition (Santra et al, 2013) .In Chhattisgarh, the concentration of total As in rice samples is 0.018 to 0.446?g/g (Patel et al, 2005) .
  
  Fig.1.Areas showing arsenic contaminated groundwater and rice fields.

  
  In China, mining industries contribute a great deal to As contamination of rice, and the standard threshold for As concertration in food is 0.2?g/g.As level in brown rice in Guangdong Province of China, which is susceptible to As poisoning from an abandoned tungsten mine (Lianhuashan) , is 1.09?g/g (Liu et al, 2010) , which is several times higher than the national safety limit.The total As content in polished rice grains from different production regions of the country ranges from 0.065 to 0.274?g/g, with an average value of 0.114?g/g, where inorganic As is predominant, accounting for approximately 72%of the total As with a mean concentration of 0.082?g/g (Liang et al, 2010) .The major role (60%) in exposing the Chinese population to inorganic As is played by rice.Moreover, people from southern China are more prone to acquire cancer from As poisoning than those in the northern part of the country (Li et al, 2011) .
  
  Rice from USA contains higher total grain As content compared to rice from counties of Asia and Europe, although the amount of inorganic As (42%±5%) is lesser (Williams et al, 2006) .Different reports suggest that rice from USA predominantly contains the organic form of As (Meharg et al, 2009) , with DMA being a major species (Zavala et al, 2008) .Besides these, presence of As in rice has also been reported in several other countries as mentioned in Table 1.
  
  Uptake of arsenic by rice plants
  
  As is available to rice plants in various forms from the soil water.Among the different species of As in soil, the concentration of one often dominates the rest depending upon the various physicochemical parameters in the soil environment.Inorganic As is dominant in soil water over organic (methylated) forms.Under oxidising conditions, the concentration of As5+is the highest, while microbial activity may also lead to accumulation of MMA, DMA or TMAs O, whereas under reducing conditions, As3+is the dominant form in soil (Mandal and Suzuki, 2002) .Paddy is grown under waterlogging conditions, which is anaerobic and reducing in nature.The soil water in a paddy field goes on from being As5+dominant to As3+dominant with the passing of time during the cropping season as more and more irrigation water is provided.During the important stages of cropping from panicle development to grain filling, As content of soil water remains high, with the major species being As3+, which is the more toxic form.Therefore, As3+is the most important species of As in soil concerning rice crop growth and yield.Later on, as waterlogging conditions are over, near harvesting period, the soil conditions become oxidising and As5+becomes the major form in the soil (Panaullah et al, 2008) .
  
  The uptake of the different species of As from the soil water by rice plants takes place at different rates.The amount of As taken up by rice plants followed the following trend:As3+>MMA>As5+>DMA (Marin et al, 1992) .As3+being very easily taken up by plants and also being the dominant one (Mandal and Suzuki, 2002) and one of the most phytotoxic forms (Marin et al, 1992) , it is a major hazard for the rice growing and consuming communities of the world.
  
  The transport rates of different As species within the rice plant vary considerably, which leads to variation in their accumulation in different organs of the plant (Fig.2) .Organic As species are more easily transported to the shoots in rice plants than the inorganic species (Marin et al, 1992;Raab et al, 2007;Carey et al, 2010) .DMA is readily translocated to the shoots.However, the accumulation of the inorganic forms of As do not significantly increase with increase in As concentration in the growth medium (Marin et al, 1992) .The majority of the As in rice grains is dominated by As3+and DMA, and the former is retained only in the ovular vascular trace whereas the latter is dispersed throughout the external parts of the grain and up to the endosperm (Carey et al, 2010) .This difference in As translocation and accumulation is prevalent in a number of other plant species belonging to several families of both monocots and dicots, suggesting a similarity in the basic mechanism of transport of the respective As species in different plants (Raab et al, 2007) .
  
  Table 1.Arsenic (As) found in sampled rice.

  
  a Samples were obtained from markets, and samples in China were from Taiwan Province.
  
  Different transporters are responsible for the uptake and translocation of different As species in the rice plants (Fig.2) .The uptake and translocation of As3+are mediated by the rice silicic acid transporters Lsi1 (Os NIP2;1) , belonging to nodulin26-like intrinsic protein (NIP) family of major intrinsic proteins and Lsi2 belonging to putative anion transporters (Ma et al, 2008) .Lsi1 provides the ability to uptake As3+from the medium, whereas Lsi2 helps in translocating As3+to the shoots by regulating its efflux to the xylem.Transporters belonging to the plasma membrane intrinsic proteins (PIP) of rice, viz.Os PIP2;4, Os PIP2;6 and Os PIP2;7, are also observed to play a role in the transport of As3+in heterologous systems (Arabidopsis thaliana and Xenopus laevis oocytes) by increasing membrane permeability.Among these transporters, Os PIP2;6 has also been demonstrated to efflux As3+to the growth medium in transgenic A.thaliana overexpressing Os PIP2;6 during long term experiments, thus providing As3+tolerance to the plant (Mosa et al, 2012) .As5+transport is regulated by phosphate transporters in several plants including rice.As5+is an analogue of P, with its uptake being reduced by the presence of higher concentrations of P in the growth medium as determined in the As hyperaccumulator Pteris vittata (Wang et al, 2002) , or Brassica carinata (Irtelli and Navari-Izzo, 2008) .In case of rice, Os PHF1 (for phosphate transporter traffic facilitator 1) defective rice mutants uptake less Pi and As5+.Moreover, transgenic rice overexpressing the Pi transporter Os Pht1;8 (Os PT8) or those overexpressing the transcription factor Os PHR2 (for phosphate starvation response 2) uptake more Pi and As5+in hydroponic culture, thus raising the maximum influx by three to five times (Carey et al, 2010) suggests strong affinity of Pi transporters for the uptake and translocation of As5+in rice.Among the organic species of As, the methylated forms, MMA and DMA, are taken up by the rice aquaporin Lsi1 (Li et al, 2009) .
  
  Fig.2.Transport of arsenic (As) from roots to different parts of rice plants by means of various transporters.DMA, Dimethyl arsenate;MMA, Monomethyl arsenate.

  
  The translocation of DMA within the rice plant takes place at the highest rate among all the species of As (Marin et al, 1992) .Along with As3+, DMA forms the major part of As in the rice grains (Carey et al, 2010) .However, in reports of As content in grains obtained from various affected countries, rice varieties varying in speciation of grain As content have been found, based on which two major groups of rice varieties can be identified, viz.one with more inorganic As in grains, and the other with more organic As in grains (Zavala et al, 2008) .It has been recently shown that higher plants, including rice, are unable to methylate inorganic As (Lomax et al, 2012) .However, there is evidence suggesting that organic As can be partially converted to inorganic forms by Sinapis alba (Jedynak et al, 2010) .The exact mechanisms responsible for variation of the major As species in rice grains remain to be worked out.
  
  Arsenic accumulation and toxicity
  
  The amount of As accumulated in different parts of rice plants follows the order:straw>husk>grain (Abedin et al, 2002;Bhattacharya et al, 2010c) .Abedin et al (2002) also showed that the concentrations of As in root and straw do not vary too much, suggesting that translocation from roots to straw takes place readily.However, its concentration decreases considerably in husk and yet further in grains.
  
  From what evidence is present regarding transportation of different As species in rice plants, it can be seen that As3+is the most easily taken up by roots, and DMA is the most easily translocated within the rice plants.As3+and DMA are the two most prominent species of As that are translocated to the grains.However, a considerable amount of As5+is also being taken up from the soil by the rice roots, and its concentration in the aboveground tissues of the plant is affected by mechanisms associated with As-tolerance response such as the presence of different arsenate reductases in rice, like Os HAC1;1, Os HAC1;2 and Os HAC4, which regulate the conversion of arsenate to arsenite.These genes are expressed mainly in the roots, and catalyse the reaction in the outer cell layer of root, thereby, facilitating arsenite efflux from the root to soil (Xu et al, 2017) .A significant amount of As5+is reduced to As3+in root tissues of plants such as Indian mustard (Pickering et al, 2000) , tomato and rice (Xu et al, 2007) .Two ACR2-like genes, Os ACR2.1 and Os ACR2.2, similar to the yeast ACR2 genes responsible for arsenate reduction are identified in the rice genome, with the former being much active than the latter (Duan et al, 2007) .Mutation of such genes led to decreased arsenate reduction in root, decreased arsenite efflux and increased As accumulation in root and grain.However, Chao et al (2014) demonstrated that ACR2-like genes are only found to reduce As5+to As3+in vitro and not in living plants.Moreover, the sequences of the yeast ACR2 and HAC1 genes found in plants are similar, and it is only the HAC1 gene rather than the ACR2-like genes that takes part in arsenate reduction and arsenite efflux in plants.HAC1can therefore be an important candidate gene, which can reduce the accumulation of As in aboveground tissues due to its role in As3+efflux mechanism.
  
  Accumulation of As in rice varies according to the genotype of the plant (Norton et al, 2010) and its mobilization and intake is controlled by different factors such as As speciation, soil texture, p H and organic matter present (Mitra et al, 2017) .The variation in accumulation may be due to the difference in presence of As in irrigation water, soils and the genetic makeup.The root characteristics also play a major role in As accumulation, which includes the root aeration, porosity (Wu et al, 2011) , rhizosphere interactions (Dasgupta et al, 2004) and Fe-plaque formation on the root surfaces (Liu et al, 2004b) .The solubility of As in the soils and its bioavailability to the rice plants is controlled by the soil texture and p H (Quazi et al, 2011) .Silt and clayey soils with the fine texture provide large surface area for As scavenging compared to the sandy and loamy soils mainly based on the presence of Fe-oxides.Soil p H[both low (<5.0) and high (>8.5) ]greatly affects As uptake and accumulation in the rice plants.The soils with low p H (<5) provide higher As binding species such as Fe-oxyhydroxide compounds and enhance As uptake by plants.Under high p H, the negative surface charge is increased, giving rise to increase in hydroxyl ions, thus facilitating desorption of As from Fe-oxides and resulting in As transport in root vicinity and accumulation in the plant (Bhattacharya et al, 2010a;Ahmed et al, 2011) .Organic matter has a greater affinity for As adsorption due to the formation of an organo-As complex, thereby reducing As availability to plants (Mitra et al, 2017) .
  
  Accumulation of As by rice plants results in various toxic reactions and affects the growth, morphological and physiological processes of the plant (Abbas et al, 2018) .Increasing As content of growing medium or increased accumulation reduces chlorophyll content of the rice plants, the extent of reduction varying slightly among different varieties.This results in reduction in the rate of photosynthesis, leading to reduction in root and shoot growth and grain yield (Rahman et al, 2007;Halim et al, 2014) .Yield reduction has also been observed with the increase in As content in soils in subsequent cropping years due to retention of As from previous years'contaminated irrigation water in the soils (Panaullah et al, 2008) .A physiological disease called straight head is also attributed to As toxicity (Rahman et al, 2008) .The symptoms of this disease include reduction in the sterility of florets/spikelets, desreased grain yield, and in severe cases, nonformation of the panicles or heads.
  
  The level of sensitivity to As toxicity varies in different rice genotypes (Chaturvedi, 2013) .Moreover, all the As species cause dissimilar levels of toxicity in rice plants.DMA is the least toxic form of As as determined by its effect on plant dry mass, and its application in low concentrations has only resulted in increased dry mass, while high concentration leads to no changes at all.Application of As3+causes no significant change in dry mass.However, As3+and MMA are highly toxic to plants, with the effect of MMA being greater than As3+ (Marin et al, 1992) .
  
  Arsenic hyperaccumulation and tolerance
  
  It is well established that As accumulation leads to various toxic reactions in plants.However, some plants are tolerant to elevated levels of As and accumulate high levels of the metalloid in their bodies.The fern Pteris vittata is the first reported As hyperaccumulator whose fronds contain high amounts of mostly the toxic inorganic forms of As (Ma et al, 2001) .Sarcosphaera coronaria, an ectomycorrhizal ascomycetous fungus, also accumulates very high amounts of As (mostly in the form of methylarsonic acid) in its fruit bodies (Falandysz and Borovi?ka, 2013) .The yeast ACR3 arsenite effluxer provides As tolerance on its heterologous expression in Arabidopsis thaliana (Ali et al, 2012) .Two genes from P.vittata, Pv ACR3 and Pv ACR3;1, similar to the yeast ACR3arsenite effluxer, have been isolated.Of these, Pv ACR3 has a definite role in As tolerance in the plant (Indriolo et al, 2010) .ACR3 is a membrane protein.Once taken up by the roots, As5+gets reduced to As3+.Pv ACR3 effluxes As3+into the vacuole for sequestration.ACR3 gene is absent in angiosperms.However, when Pv ACR3 is expressed in transgenic A.thaliana, its tolerance to As is increased.Pv ACR3 transgenic seeds of A.thaliana could even germinate in very high concentrations of As that are potentially lethal for wild type plants (Chen et al, 2013) .P.vittata has been demonstrated to help in phytoremediation by removing As from soil.Rice cultivated in soils where P.vittata is grown previously has been found to accumulate less As in straw and grains (Ye et al, 2011) .
  
  QTLs for As accumulation in shoots have been identified in a doubled haploid population obtained from anther culture of F1 plants generated from a cross between rice cultivars CJ06 (japonica) and TN1 (indica) .The QTLs, As S, As Se1 and As Se2, are located on chromosomes 2, 6 and 8, respectively (Zhang et al, 2008) .As Se2 collocates with the QTL for P accumulation in shoots at the seedling stage, having LOD value of 4.97.
  
  Dasgupta et al (2004) found that in general indica rice varieties are more tolerant to As stress than japonica ones.In their experiments, CO39 is the most tolerant whereas Dawn is the most susceptible.They identified an As tolerance gene As Tol on chromosome6 between markers RZ516 and RG213.Three tolerance genes in the rice genome play a part in As susceptibility or tolerance, and two are located on chromosome 6 whereas the other on chromosome 10.The genes, As Tol6.1, As Tol6.2 and As Tol10.1, are epistatic in action with tolerance resulting in plants having inherited a minimum of two of the three genes from a tolerant parent (Norton et al, 2008) .Plasma membrane intrinsic proteins from rice, Os PIP2;4, Os PIP2;6 and Os PIP2;7, when overexpressed in transgenic Arabidopsis, are found to increase As3+tolerance in the plants during short term exposure (Mosa et al, 2012) .These transporters might as well play important roles in rice tolerance to As.Arabidopsis plants mutant for auxin transporters have been found to be more sensitive to As3+stress.Providing exogenous auxin improves As3+tolerance in such mutants (Krishnamurthy and Rathinasabapathi, 2013) .This hints at a possible role of auxin in ameliorating As stress in rice, however, the molecular basis of the mechanism need to be tested.
  
  Phytochelatins in arsenic metabolism
  
  Phytochelatins (PC) are thiol-rich peptides having the general formulaγ (Glu-Cys) n-Gly, where n ranges from 2 to 11 (called accordingly as PC2, PC3, PC4, etc.) , produced in plants in response to stress from heavy metals and metalloids including As (HartleyWhitaker et al, 2001a) .These are synthesized from glutathione by a specific y-glutamylcysteine dipeptidyl transpeptidase known as phytochelatin synthase (PCS) (Grill et al, 1989) .PCs help in As detoxification by sequestrating it in the vacuoles.PCs bind As3+, forming As-PC complexes which are then transported to vacuoles (Verbruggen et al, 2009) .As5+in plants is also reduced to As3+ (Pickering et al, 2000;Xu et al, 2007) , which can be subsequently bound by PC and transported to the vacuoles.The transport of As-PC complexes to the vacuoles is mediated by the At ABCC1 and At ABCC2 transporters in A.thaliana (Song et al, 2010) .Such ABC type transporters are supposed to help in transport of As-PC complexes in different plant species.
  
  PC production in response to As stress has been demonstrated in plants like Holcus lanatus, Agropyron repens, Glecoma hederacea, Leonurus marrubiAstrum, Lolium perenne, Urtica dioica and Zea mays (Hartley-Whitaker et al, 2001a, b;Schulz et al, 2007) .In A.thaliana, overexpressing the phytochelatin synthase At PCS1, tolerance to As stress, is increased (Li et al, 2004) .In H.lanatus, concentration of PC increases with increasing As concentration in medium in case of tolerant and partially tolerant clones.However, in non-tolerant clones, PC production decreases at higher As5+concentration in medium (Hartley-Whitaker et al, 2001a, b) .The findings of Schulz et al (2007) and Hartley-Whitaker et al (2001b) agree on the dominant forms of PC in tolerant and non-tolerant plants.The dominant form of PC found in tolerant plants is PC2 while in the non-tolerant plants it is PC3.In intermediate tolerant clones of H.lanatus, PC2 and PC3 concentrations are approximately equal, while PC4 is in lesser amounts in all clones.However, Schulz et al (2007) and Srivastava et al (2007) have shown that not all As is bound by PCs.After 4 d of treatment, the amount of As chelates by PCs in As3+treated plants is 39%, while in As5+treated plants, it is 35% (Srivastava et al, 2007) .In the As hyperaccumulator fern P.vittata, only PC2 is detected on As treatment.The concentration of PCs correlates significantly with As concentration.However, only a small portion of the As (1%–3%) is chelated with the PCs (Zhao et al, 2003) .This suggest that PCs have a limited role in the tolerance of this fern to As stress.
  
  In rice plants exposed to As stress, PCs belonging to the groups:PC[γ (Glu-Cys) n-Gly, n ranges from 2to 4], Ser-PCs[γ (Glu-Cys) n-Ser, n ranges from 2 to 4], Des-PCs[γ (Glu-Cys) n, n ranges from 2 to 3]and Glu-PCs, [γ (Glu-Cys) n-Glu, n ranges from 2 to 3]are produced.The concentration of PCs correlates with the concentration of i-As in the roots (Batista et al, 2014) .As-PC complexation in rice leaves leads to reduction in the translocation of As3+to grains (Duan et al, 2007) .However, the role of As-PC complexation in reducing grain As content is specific to certain varieties, while in others, different mechanisms such as As-PC release and transport seem to be decisive (Batista et al, 2014) .
  
  Oxidative response to arsenic stress
  
  Intake of toxic metals leads to synthesis of reactive oxygen species (ROS) such as H2O2 in plants.These ROS cause oxidative injury in plants (Sarma et al, 2016) , and plants resist this injury by various antioxidant metabolism reactions (Chutia et al, 2012) .Plants may be tolerant or sensitive to metal toxicity.Tolerant plants often demonstrate a much competent antioxidant defence system than sensitive ones (Fig.3) .
  
  As stress has also been found to induce oxidative response in plants.Polyamines involved in protection of plant structures against damage by ROS have been detected in red clover shoots after As treatment (Mascher et al, 2002) .Moreover, the activities of superoxide dismutase (SOD) and peroxidase (POD) are detected in As-treated red clover shoots.The presence of three different SOD isoenzymes, Mn-SOD, Cu/Zn-SOD I and Cu/Zn-SOD II, have been determined.There is a tendency of antioxidant activity to increase with increasing concentration of As in culture when As concentration is low.However, their activities get inhibited at high metal concentration (Mascher et al, 2002) .Malondialdehyde (MDA) is an indicator of the occurrence of lipid peroxidation due to oxidative damage, which is detected in spinach leaves when grown in presence of As (Srivastava and Sharma, 2013;Hore et al, 2017;Nahar et al, 2018) .Variation in the level of antioxidant activity is detected on comparison of tolerant and susceptible plant species.The As hyperaccumulator Pteris vittata, which is tolerant to much higher concentration of As, shows better resistance to oxidative injury than the nonhyperaccumulator Pteris ensiformis.P.vittata shows much less H2O2 concentration and lipid peroxidation as indicated by thiobarbituric acid-reactive substances (TBARS) formation as compared to P.ensiformis.Moreover, antioxidant defence is more competent in P.vittata than in P.ensiformis as indicated by ascorbate and glutathione pools.The ratios of reduced to oxidized forms of both ascorbate (As A/DAs A) and glutathione (GSH/GSSG) are higher in P.vittata than in P.ensiformis under all treatments (Singh et al, 2006) .In comparison of Pisum sativum and Pennisetum typhoides, it has been found that H2O2 concentration and lipid peroxidation are much higher in P.sativum than in P.typhoides.Moreover, concentrations of thiols, SOD activity, catalase (CAT) activity and ascorbate peroxidase (APX) activity indicate a much better antioxidant defence mechanism in P.typhoides than in P.sativum.This suggests that P.typhoides is the tolerant species whereas P.sativum is the sensitive one in response to As induced oxidative stress (Sharma, 2013) .In tolerant plants, antioxidant activity is enhanced at high metal concentration and with the passing of time.However, it has been observed that, although antioxidant activity is enhanced with increasing metal accumulation initially, it gradually gets inhibited after a few days and the plants succumb to oxidative damage.
  
  Fig.3.Roles of antioxidants SOD, CAT, APX, GSH and GR in plants response to arsenic stress induced reactive oxygen species formation.

  
  As, Arsenic;SOD, Superoxide dismutase;CAT, Catalase;APX, Ascorbate peroxidase;GSH, Glutathione;GR, Glutathione reductase;GSSG, Glutathione disulphide;MDA, Malondialdehyde;DHA, Dehydroascorbate;DHAR, Dehydroascorbate reductase;HAC1, High As content 1.
  
  In rice, H2O2 concentration and lipid peroxidation level as indicated by MDA concentration increase with the accumulation of As.However, iron supplemented with As in the growth medium results in decrease of ROS concentration in the rice plants.Antioxidants CAT, SOD, As A and glutathione (GSH) levels are also considerably high in only As5+treated rice plants as compared to Fe supplemented ones (Nath et al, 2014) .Fe plaque formation on root surface acts as a buffer, leading to the lowering of As translocation to shoots (Liu et al, 2004a, b) .Thus, reducing As stress leads to reduction of ROS concentration and levels of antioxidants in rice plants.This suggests the activity of an antioxidant defence mechanism against As stress in rice plants.The variation of this defence mechanism in different varieties of rice plants need to be worked out in order to identify varieties tolerant to As in the growth medium.
  
  Selenium (Se) has an ameliorating effect on As-induced oxidative stress measured in terms of H2O2 concentration, lipoxygenase (LOX) activity, lipid peroxidation as determined by estimation of MDA, and ion leakage measured in terms of electrical conductivity.Oxidative damage is found to decrease on supplementation of Se with As in the medium.The activities of CAT, APX and glutathione peroxidase (GPX) decrease under As treatment, but increase when As in the medium is supplemented with Se, indicating that Se boosts antioxidant activities and prevents any damage from oxidative stress (Kumar et al, 2015) .
  
  Role of rhizosphere microorganisms in arsenic stress
  
  Soil rhizospheric microbial activity can affect As adsorption/desorption, solubility, bioavailability, mobility, and soil-plant transfer by altering the chemical speciation of As in soil (Ayangbenro and Babalola, 2017) .Microorganisms can interconvert As3+and As5+and thus are capable of either solubilizing or immobilizing As in the soil-plant system (Suhadolnik et al, 2017) .These microbially induced biotransformations of As play important roles in the biogeochemical behavior of As and are key in risk assessment and remediation studies (Anguita et al, 2017) .
  
  Arbuscular mycorrhizal fungi (AMF) associations can enhance resistance to As stress in plants.AMF such as Glomus mosseae and G.caledonium have been shown to reduce the uptake of As5+by host Holcus lanatus plants, which are resistant to As (Gonzalez-Chavez et al, 2002) .Moreover, AMF from contaminated sites are resistant to As stress compared to those from non-contaminated sites.However, both resistant and non-resistant AMF reduce the uptake of As by host plants.In a number of non-resistant plants including soybean and maize, mycorrhizal associations have proven to be beneficial for the host plants in resisting stress due to As (Bai et al, 2008;Spagnoletti and Lavado, 2015;Nahar et al, 2016) .
  
  Other microbes in the rhizosphere soil such as some fungi and bacteria also play important roles in determining the level of As toxicity on plants.The ectomychorrizal fungus Sarchosphaera coronaria accumulates high levels of As in its fruiting bodies (Falandysz and Borovi?ka, 2013) .Other microbes have the capacity to transform As species in the soil, with great potential for increasing plant tolerance to As stress, and playing roles in bioremediation of Ascontaminated soil and groundwater.Many rhizospheric N2 fixing symbiotic microbes (eg.rhizobacteria) tolerate high metal concentration (Jaiswal, 2011) .NT-26 strain of Agrobacterium/Rhizobium branch ofα-Proteobacteria, strains MNZ1 (homologous to Enterobacter sp.) , strains MNZ4 and MNZ6 (both homologous to Klebsiella pneumoniae) have been reported to oxidize As3+to As5+ (Santini et al, 2000;Abbas et al, 2014) .In natural environments a variety of As3+oxidizers exist some autotrophic and others heterotrophic.They may further be aerobic and anaerobic.Mechanisms of As3+oxidization varies among different groups, as indicated by the variation in gene and amino acid sequences of the arsenite oxidases (Rhine et al, 2007) .This variation can be useful in isolation of useful microbial species that confers As stress resistance to plants in a variety of environments.Of particular importance in paddy cultivation can be the three strains, DAO1 (Azoarcus sp.) , DAO10 (Sinorhizobium sp.) and MLHE-1 (Alkalilimnicola ehrlichei) , which can oxidize As3+in anaerobic conditions similar to those in submerged paddy cultivation.Furthermore, there are also strains that can reduce As5+to As3+such as Desulfovibrio strain Ben-RA and Desulfomicrobium strain Ben-RB (Macy et al, 2000) .Indigenous microbes found in As contaminated environments are capable of transforming As.Both As5+reducing and As3+oxidising microbes often coexist and are supposed to be common in As contaminated sites (Macy et al, 2000) .As3+oxidising bacteria are also found to be effective in improving the bioremediation of As3+contaminated sites by Pteris vittata.These microbes can potentially reduce the toxic effect of As on the growth of plants.In case of rice plants, microbes capable of transforming As3+in anaerobic environment might be useful in promoting plant growth in As contaminated sites.
  
  Agronomic mitigation of arsenic accumulation in rice
  
  Different agronomic methods may be adopted to mitigate the effects of As accumulation in rice, which includes soil aeration by water management and preventing the reduction of As;creating the condition that forms the precipitation of insoluble As in soil;and decreasing As uptake and translocation in rice plants by augmenting mineral nutrients in soils that competes with As uptake (Fig.4) .
  
  Fig.4.Aspects of arsenic (As) stress in rice, and possible ways to deal with the problem.

  
  Supplementation of soil with mineral nutrients (S, P, Fe and Si) can significantly decrease the accumulation of As in the food grains by minimizing its uptake and transport in food crops.Fe plaque plays an important role in diminution of As absorption around the roots of rice plants (Bakhat et al, 2017) .P and Si which are complementary in nature to As competes for the intake by the rice plant through the transporter, thus the external application of P and Si reduces the chances of As intake from the soil.
  
  Management of irrigation system or development of water saving mechanism maintains the redox level of soil and aggrandises the oxidation state that consequently hinders the reduction of As5+to As3+:which has prominently higher solubility, plant availability and toxicity (Takahashi et al, 2004) .Thus, a water-saving regime has been reported to be an immediate and sustainable solution to decrease As content in rice (Arao et al, 2009) .It has been reported that under aerobic water management practices, rice takes up less As (0.23–0.26 mg/kg) than under anaerobic practices (0.60–0.67 mg/kg) (Talukder et al, 2011) .
  
  Bioremediation strategies such as increase in number of soil microbes improved the mineral content of the soil through immobilization and mineralization thus affecting the availability and transport of As in the environment (Huang, 2014) .Controlling the expression of transport mediator gene like Os Lsi1 and Os Lsi2 by AMF (Chen et al, 2012) also helps in mitigation of As toxicity.
  
  CONCLUSIONS
  
  The problem of As in rice has two dimensions, human health hazard due to incorporation of As in the food chain, and threat to the crop and sustainable agriculture due to the resulting stress.
  
  Different transporters are involved in the uptake and translocation of As in rice plants.With the translocation rates of different As species varying with their respective transporters, rate of uptake and translocation of As in rice plants could be reduced by working out the underlying mechanisms.The cause of variation of the major As species in American and Asian rice need to be identified.The reason could be either genotypic, or due to difference in soil conditions and agricultural practices.If any difference at a molecular level exists between these varieties, it could be worked out to ensure incorporation of lesser inorganic As in rice grains.As efflux genes such as HAC1 can be important in developing rice containing low grain As content.
  
  Rice varieties have shown difference in tolerance to As, with the identification of mechanisms of tolerance such as As3+efflux, vacuole sequestration or thiol complexation, and also genes for tolerance being detected, there is scope for traditional breeding approaches to develop tolerance to As among high yielding or traditional varieties.With phytohormone auxin being connected to As tolerance in Arabidopsis plants, such phenomenon in rice can be investigated.Hormone treatment of rice seedlings can be tried out to assess any change in tolerance to As stress.
  
  Phytoremediation and land reclamation processes can make use of As-hyperaccumulators such as Pteris vittata.Bioremediation using As accumulating rhizosphere bacteria is also a feasible way to reduce the impact of As stress on rice.Elements that ameliorate the impact of As stress are useful when present in As contaminated soils.The formulation of optimum levels of these elements for extrogenous application, depending upon the soil type can alleviate As stress in rice plants.Moreover, management of irrigation is also important in limiting the soil arsenic levels, thereby reducing As uptake and accumulation in rice plants.
  
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