Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/2/2017
Publication Date: 9/14/2017
Citation: Heuschele, D.J., Pinson, S.R., Smith, A.P. 2017. Metabolic responses to arsenic in rice seedlings that differed in grain arsenite concentration. Crop Science. https://doi.org:10.2135/cropsci2016.06.0493.
Interpretive Summary: There is public concern over arsenic (As) levels in rice grains and food products. Because of the flooded conditions under which most rice is produced, rice plants readily uptake arsenic and transport a portion of it into their grain. Our research objective was to identify genetically controlled physiological/biochemical mechanisms breeders can select for to produce rice varieties that limit the accumulation of As in their grains. Because As is also toxic to plants, they have evolved mechanisms to reduce toxicity or accumulation of arsenic. For example, a rice plant might reduce the uptake of As into its roots, or might regulate root-to-shoot transport, or sequestration of the As in roots or leaves. Or the plants might metabolize other As-induced toxic compounds in order to better tolerate or survive As exposure. Because many of these mechanisms that enhance plant tolerance of As could also lead to less As being transported into the grains of those plants, this study started with rice varieties known to be either high or low in grain arsenic concentrations (a.k.a. As grain-accumulators or grain-excluders, respectively) and asked if their seedlings exhibited any differences in metabolic responses to arsenite [As(III)]. The study used three As grain-accumulators and three grain-excluders identified in previous study of 1700 rice accessions from around the world. Interestingly, two of the grain-excluders were US rice varieties released a decade or more ago. When the grain-accumulators and grain-excluders were compared for rates of As uptake, transport, sequestration, and detoxification of secondary stress compounds, the most consistent difference found between the two types of cultivars was seen in the leaves. All three grain-excluders showed 2-fold increased production of glutathione (GSH) by or before 72 hours after As exposure. Because it is hypothesized that binding of As to either GSH or to a GSH-containing phytochelatin molecule is required before the As can be rendered harmless by transferring it into a cell vacuole, the boost in GSH production we detected in grain-excluders suggests that grain-excluders are more efficient than grain-accumulators at sequestering As into their leaf vacuoles. Activation of leaf As sequestration only among the grain-excluders suggests that leaf As sequestration is likely also preventing larger amounts of free-As from being transported to grains in field-grown plants. While other factors were found to reduce upward movement of As in this study (e.g., most arsenic was retained in the roots and not transferred into shoots) they limited As movement equally among the grain-accumulators and grain-excluders, and thus, it appears that these other mechanisms are not the cause of the wide differences in grain-As concentrations. Further study is now underway to verify that increased production of GSH in seedling and later leaves has a causative relationship with low grain-As concentrations. If proven true, then this metabolic trait could be used by breeders to select for As grain-excluders among seedlings, accomplishing this selection with less labor, expense, and time than that required for production and analysis of grains from those same plants.
Technical Abstract: Arsenic (As) occurs naturally in air, water and soil and being ubiquitous in the environment, is also present in all edible and non-edible plant tissues. Because As becomes more available for plant uptake when soils are flooded, there is more concern about As in rice than other grain crops. Arsenic, like other heavy metals, is also detrimental to plant health. Plants may regulate uptake, transport, sequestration/tolerance or a combination of all three to prevent toxicity of heavy metals to the plant; these same mechanisms could also be used to reduce accumulation of As in crop grains. Previous study of approximately 1700 international rice accessions identified some as having high grain-As concentrations (As grain-accumulators) and identified others for low grain-As (grain-excluders). This study investigated which physiological responses, uptake, transport, or sequestration/tolerance explained the grain-As difference between these two groups. Hydroponically grown seedlings were treated with 0 (controls) or 100 micro Molar As(III) at the three-leaf stage, destructively sampled at 0, 24, 48, and 72h, and analyzed for concentrations of As plus key compounds known or suspected to be involved in As metabolism. As(III) treated versus control data were compared to determine if observed changes were As-induced. Both grain-accumulators and grain-excluders actively concentrated As within their roots (27-fold higher concentration than hydroponic solution), and both groups limited root-to-leaf transport, with roots having 10-fold higher As concentrations than leaves. At 72 h grain-accumulators and grain-excluders contained similar concentrations of As in roots and leaves, suggesting their grain-As differences were not due to differences in As uptake or transport. However, for both grain-accumulators and grain-excluders, leaves and roots acted differently in their production of As-stress related compounds. In response to As, roots of both grain-excluders and grain-accumulators increased in cysteine and total phytochelatin (PC) production which supports a PC sequestration of As hypothesis. Glutathione (GSH) production in leaves in response to As(III) distinguished grain-accumulators from grain-excluders; only grain-excluders doubled in GSH concentration by 72h. However, the additional leaf GSH did not appear to be utilized in the formation of additional PC because leaf PC concentrations remained constant. The GSH may be instead used to form As-adducts which also aid in sequestration. Studies to verify that increased leaf GSH production in the presence of high environmental As(III) causes reduced grain-As concentrations are underway. If proven true, this could then be used by breeders to select for As grain-excluders in the seedling stage, saving labor and time compared to selections based on grain-As data.