Grain Inorganic Arsenic Content in Rice Managed Through Targeted Introgressions and Irrigation Management

Effects of alternate wetting and drying on oxyanion-forming and cationic trace elements in rice paddy soils: impacts on arsenic, cadmium, and micronutrients in rice

Rice is a global dietary staple and its traditional cultivation under flooded soil conditions leads to accumulation of arsenic (As) in rice grains. Alternate wetting and drying (AWD) is a widely advocated water management practice to achieve lower As concentrations in rice, water savings, and decreased methane emissions. It is not yet clear whether AWD leads to tradeoffs between concentrations of As and micronutrient elements (e.g., zinc, manganese, molybdenum) in rice grain. We analyzed pore water chemistry and rice grain composition data from a field experiment conducted in Arkansas, USA, in 2017 and 2018 to test the hypothesis that AWD will have diverging effects on oxyanion-forming (arsenic, molybdenum) vs. cationic (cadmium, zinc, manganese, copper) trace elements. This was hypothesized to occur via decreases in soil pH and/or precipitation of iron oxide minerals during oxidizing conditions under AWD. Solubility of all trace elements, except zinc, increased in more reducing conditions. Consistent with our hypothesis, AWD tended to increase grain concentrations of cationic elements while decreasing grain concentrations of oxyanionic elements. Decreases in total As in rice grains under AWD were mainly driven by changes in dimethylarsinic concentrations, with negligible changes in inorganic As. Linear mixed-effects modeling showed that effects of AWD on grain composition were more significant in 2017 compared to 2018. These differences may be related to the timing of dry-downs in the developmental stage of rice plants, with dry-downs during the heading stage of rice development leading to larger impacts on grain composition of certain elements. We also observed significant interannual variability in grain elemental composition from continuously-flooded fields and postulate the warmer temperatures in 2018 may have played a role in these differences.

Mitigating the accumulation of arsenic and cadmium in rice grain: A quantitative review of the role of water management

Arsenic exposure through rice consumption is a growing concern. Compared to Continuous Flooding (CF), irrigation practices that dry the soil at least once during the growing season [referred to here as Alternate Wetting and Drying (AWD)] can decrease As accumulation in grain; however, this can simultaneously increase grain Cd to potentially unsafe levels. We modelled grain As and Cd from field studies comparing AWD and CF to identify optimal AWD practices to minimize the accumulation of As and Cd in grain. The severity of soil drying during AWD drying event(s), quantified as soil water potential (SWP), was the main factor leading to a reduction in grain total As and inorganic As, compared to CF. However, lower SWP levels were necessary to decrease grain inorganic As, compared to total As. Therefore, if the goal is to decrease grain inorganic As, the soil needs to be dried further than it would for decreasing total As alone. The main factor driving grain Cd accumulation was when AWD was practiced during the season. Higher grain Cd levels were observed when AWD occurred during the early reproductive stage. Further, higher Cd levels were observed when AWD spanned multiple rice growth stages, compared to one stage. If Cd levels are concerning, the minimum trade-off between total As and Cd accumulation in rice grain occurred when AWD was implemented at a SWP of -47 kPa during one stage other than the early reproductive. While these results are not meant to be comprehensive of all the interactions affecting the As and Cd dynamics in rice systems, they can be used as a first guide for implementing AWD practices with the goal of minimizing the accumulation of As and Cd in rice grain.

Recent advances in arsenic mitigation in rice through biotechnological approaches

Arsenic (As) is a major threat to the environment and human health due to its toxicity and carcinogenicity. Occurrence of alarming concentrations of As in water and soil leads to its bioaccumulation in crops which is a major health concern globally. Rice (Oryza sativa) is a staple food for a large population staying in As contaminated areas so, it is of utmost importance to reduce As levels in rice, especially grains. Amongst several strategies in practice, biotechnology may provide an effective option to reduce As accumulation in rice grains. Genetic engineering can be a viable approach to exploit potential genes playing roles in As metabolism pathway in plants. Besides, developing low As accumulating rice varieties through breeding is also an important area. Identifying genotypic variation in rice is a crucial step toward the development of a safe rice cultivar for growing in As-affected areas. Significant genotypic variation has been found in rice varieties for As accumulation in grains and that is attributable to differential expression of transporters, radial oxygen loss, and other regulators of As stress. This review provides recent updates on the research advances leading to transgenic and breeding approaches adopted to reduce As levels in rice, especially grains.

Relationships Among Arsenic-Related Traits, Including Rice Grain Arsenic Concentration and Straighthead Resistance, as Revealed by Genome-Wide Association

There is global concern that rice grains and foods can contain harmful amounts of arsenic (As), motivating breeders to produce cultivars that restrict As accumulation in grains to protect human health. Arsenic is also toxic to plants, with straighthead disorder (StHD), causing panicle sterility, being observed in rice. The genetic variation in StHD resistance suggests that plants have evolved mechanisms that reduce As toxicity, possibly via regulation of As uptake, transport, or detoxification/sequestration. Because these mechanisms could also underlie the wide (3- to 100-fold) differences in grain As concentration (grain-As) observed among diverse rice genotypes, it was hypothesized that some genes reduce both grain-As content and StHD susceptibility and may be detectable as co-located StDH and As quantitative trait loci (QTL). We used a machine-learning Bayesian network approach plus high-resolution genome-wide association study (GWAS) to identify QTL for grain-As and StHD resistance within the USDA Rice Minicore Collection (RMC). Arsenic enters roots through phosphorus (P) and silica (Si) transporters, As detoxification involves sulfur (S), and cell signaling to activate stress tolerance mechanisms is impacted by Si, calcium (Ca), and copper (Cu). Therefore, concentrations of Si, P, S, Ca, and Cu were included in this study to elucidate physiological mechanisms underlying grain-As and StHD QTL. Multiple QTL (from 9 to 33) were identified for each of the investigated As-associated traits. Although the QTL for StHD, Si, and grain-As did not overlap as heavily as our hypothesis predicted (4/33 StHD and 4/15 As QTL co-located), they do provide useful guidance to future research. Furthermore, these are the first StHD and Si QTL to be identified using high-density mapping, resulting in their being mapped to shorter, more precise genomic regions than previously reported QTL. The candidate genes identified provide guidance for future research, such as gene editing or mutation studies to further investigate the role of antioxidants and ROS scavenging to StHD resistance, as indicated by candidate genes around the commonly reported qStHD8-2 QTL. Other genes indicated for future study for improving grain-As and StHD include several multidrug and toxic compound extrusion (MATE) genes, F-box genes, and NIPs not documented to date to transport As.

Identification of Promising Genotypes Through Systematic Evaluation for Arsenic Tolerance and Exclusion in Rice (Oryza sativa L.)

Rice remains a major staple food source for the rapidly growing world population. However, regular occurrences of carcinogenic arsenic (As) minerals in waterlogged paddy topsoil pose a great threat to rice production and consumers across the globe. Although As contamination in rice has been well recognized over the past two decades, no suitable rice germplasm had been identified to exploit in adaptive breeding programs. Therefore, this current study identified suitable rice germplasm for As tolerance and exclusion based on a variety of traits and investigated the interlinkages of favorable traits during different growth stages. Fifty-three different genotypes were systematically evaluated for As tolerance and accumulation. A germination screening assay was carried out to identify the ability of individual germplasm to germinate under varying As stress. Seedling-stage screening was conducted in hydroponics under varying As stress to identify tolerant and excluder genotypes, and a field experiment was carried out to identify genotypes accumulating less As in grain. Irrespective of the rice genotypes, plant health declined significantly with increasing As in the treatment. However, genotype-dependent variation in germination, tolerance, and As accumulation was observed among the genotypes. Some genotypes (WTR1-BRRI dhan69, NPT-IR68552-55-3-2, OM997, and GSR IR1-5-Y4-S1-Y1) showed high tolerance by excluding As in the shoot system. Arsenic content in grain ranged from 0.12 mg kg-1 in Huang-Hua-Zhan (indica) from China to 0.48 mg kg-1 in IRAT 109 (japonica) from Brazil. This current study provides novel insights into the performance of rice genotypes under varying As stress during different growth stages for further use in ongoing breeding programs for the development of As-excluding rice varieties for As-polluted environments.