Identification of genes and physiological factors that reduce accumulation of arsenic in rice grain

Authors: Pinson, Shannon; Heuschele, Deborah - Jo; SALT, DAVID - University Of Aberdeen; SMITH, AARON - Louisiana State University; TARPLEY, LEE - Texas Agrilife Research

Submitted to: Rice Technical Working Group Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 3/25/2016
Publication Date: 9/12/2017
Citation: Pinson, S.R.M., Heuschele, D.J., Salt, D.E., Smith, A., Tarpley, L. 2017. Identification of genes and physiological factors that reduce accumulation of arsenic in rice grain. Rice Technical Working Group Meeting Proceedings. p. 172. March 1-4, 2016. CDROM.

Interpretive Summary:

Technical Abstract: The arsenic (As) levels in rice grains and food products can reach toxic levels when produced under certain growing conditions. The World Health Organization (WHO) recently set a CODEX limit of 0.2 ppm inorganic As in milled white rice, and lower limits may be set for baby food products. While studies have shown that sufficient genetic variation for grain-As exists to support breeding efforts, it is not known what biochemical and physiological mechanisms are used by these rice cultivars to limit the accumulation of As in their grains. Possibilities include reduced As-uptake by roots, reduced transfer from roots to shoots, and increased sequestration in roots or leaves rendering the As immobilized in plant tissues and unable to enter grains. The present study aimed at identifying genes and physiological mechanisms that can be used to develop cultivars that limit the concentration of As in their rice grains. The c. 1800 rice accessions in the USDA Core collection, originating from 114 countries, were in a replicated field study that included both flooded and unflooded (aerated) plots. A 200-fold difference was observed for concentration of total-As in the grain; grain-As was 10x higher under flooded conditions than unflooded. Under flooded conditions, accessions in the indica ancestral lineage showed higher grain-As concentrations than in tropical japonicas; lower yet were the temperate japonicas. Within each ancestral group, mean and maximum grain-As were lower among early maturing accessions than among later maturing accessions. It appears that US rice breeding efforts have inadvertently decreased grain-As in that US cultivars released more than 30 years ago accumulated more grain-As than more recent US cultivars. Compared to global varieties with a similar intermediate maturity, modern USA cultivars have lower grain-As. Across the range in heading time among the USDA Core accessions (50–130 days), accessions that accumulated significantly higher-than-average grain-As (aka “grain-accumulators”) and others with significantly lower grain-As (aka “grain-excluders”) concentrations were identified. A set of 16 rice accessions (8 grain-accumulators and 8 excluders, including ‘Lemont’ and ‘Jefferson’) were selected from among the USDA Core accessions, and grown in soil amended with arsenic (monosodium methyl arsenate herbicide, MSMA) as well as in native or non-amended flooded field plots. Application of MSMA has been used for decades to select rice cultivars resistant to As-induced straighthead disease. Association was found between grain-As and straighthead response, with none of the grain-As accumulators being resistant to straighthead. Reduced As uptake and/or increased As detoxification (e.g. sequestration) in vegetative tissues might be contributing to both straighthead resistance and grain-As exclusion. Only accessions resistant to straighthead set seed in the As-amended plots; both leaf-As and, grain-As concentrations were higher when grown in in As-amended soil than in native soil. Seven of the 8 grain-accumulators had higher As concentrations in their flag leaves before grain fill than at grain maturity, consistent with As being translocated out of their flag leaves during grain fill. In contrast, all 8 of the grain-excluders continued to accumulate As in their flag leaves during grain fill, suggesting that sequestration of As in leaves contributes to grain-exclusion. To identify and study the genes that control grain-As, grain-excluders were crossed with grain-accumulators to create 15 different segregating progeny populations. Mendelian segregation patterns (i.e. 1:2:1 or 3:1) concentration of total As in grain among F2 progeny, indicative of single major-gene control of grain-exclusion of As, were seen in five F2 populations, three of which have been selectively genotyped to date. Interestingly, although increased g