Rice tissue accumulation of particular elements is dependent on the plant’s physiology

Authors: TARPLEY, LEE - Texas A&M Agrilife; CHITTOORI, RATNAPRABHA - Boise State University; Pinson, Shannon; SALT, DAVID - University Of Aberdeen

Submitted to: Meeting Abstract
Publication Type: Experiment Station
Publication Acceptance Date: 6/2/2015
Publication Date: 6/23/2015
Citation: Tarpley, L., Chittoori, R., Pinson, S.R., Salt, D.E. 2015. Rice tissue accumulation of particular elements is dependent on the plant’s physiology. Texas Rice, Highlights in Research Special Edition 2015 [Meeting Abstract]. p. 8-9. Availiable: http://beaumont.tamu.edu/eLibrary/Newsletter/2015_Highlights_in_Research.pdf

Interpretive Summary: The objective of this analysis was to gain knowledge as to what plant physiological factors most likely play a role in regulating the amount or concentration of arsenic ([As]) that is accumulated in rice grains, so that future studies can then be targeted toward altering those regulatory mechanisms with the goal of producing rice with low levels of [As]. The study was based on the fact that some elements are known to share uptake and regulatory mechanisms such that a change in the concentration of one element can be predictive of a change in the concentration of the other. Examples of element pairs known to share uptake and regulatory mechanisms are calcium (Ca) and strontium, and rubidium and potassium. This study first evaluated the concentrations of 15 elements in addition to As to determine which individual elements (e.g., [Ca]) or 2-element combinations (e.g. [Ca]/[Cu]) were most strongly associated with grain [As]. Knowledge on how those associated elements are mobilized in the plant was then interpreted relative as to how those mechanisms might also be affecting [As].

Technical Abstract: The leaves, grain and other parts of a rice plant require mineral nutrients for various metabolic and other physiological functions. Breeders sometimes want to manipulate the element composition of the grain. Nutrient-dense grain, which is primarily seen as a means for improving nutrition in some countries, could also enhance the appeal of rice as a nutritious food in our domestic market. However there are some concerns about the levels of undesirable elements, especially arsenic, in rice grain. High concentrations of undesirable elements can also negatively affect plant health and yield. Both agronomic and genetic approaches have been proposed to manipulate tissue element composition towards desired endpoints, but it is unclear how this may impact plant function. Elemental concentrations are known to affect plant health and physiology which, in turn, impact rates of element uptake, transport, and metabolism. It is known that some elements are taken up by and transported through plants using the same uptake and transport mechanisms, such that as one element goes up in concentration within a plant, so does the other. By identifying the association between the concentration of arsenic ([As]) and other elements, we can better understand what plant physiological factors are most likely involved with regulating [As] in rice leaves and grains. This first study considered the concentrations of 16 different elements, including arsenic, in the seedling leaves and mature grains of 40 diverse rice cultivars. Tips of leaves (4th to 6th fully expanded) from an outdoor potted plant study were evaluated for concentrations of the 16 elements. Grains from a separate field study of the same 40 diverse genotypes were screened for concentrations of the same 16 elements. Data for all individual element concentrations (e.g., calcium, [Ca]), and all 2-element ratios of concentrations (e.g., ratio of calcium and copper, [Ca]/[Cu]) in the leaves and grains were tested for association with [As] in either leaves or grains. One of the strongest associations observed was between leaf [As] and the ratio of [Ca]/[Cu] also in the seedling leaves, with r = 0.48. Though the associations were less strong, both leaf [Ca]/[Cu] and grain [Ca]/[Cu] were also found associated with grain [As]. The associations detected between [As] and [Ca]/[Cu] suggest that factors known to affect [Ca] and [Cu] in both or either leaves or grains may also be affecting [As] in rice grains. The Ca to Cu ratio is not likely regulating arsenic, just associated. Calcium and As are both initially transported in the plant in the water transport system called the xylem, which is partly driven by the pull of water by the transpiring leaves. During seed filling, the transport of substances, including elements, to the filling grain is mostly through the “food” distribution system called the phloem, which is partly driven by positive pressures created by sugars, potassium and other solutes attracting water into the phloem. Thus the seed is filled by a combination of “food” being pushed into it followed by the sugars being removed (e.g., into starch) to maintain the gradient. Ca is selectively excluded from the phloem, so very little gets into the seed. Cu must be bound to compounds known as “carriers” for it to be transported through a plant. Thus accumulation of Cu in leaves and grain also depends on the biosynthesis of the “carrier” compounds. Based on our observations, arsenic accumulation in grain appears to be related to Ca which is mobilized in the plant through the xylem transpiration stream. This suggests that, in addition to cultivar selection, development of agronomic practices that alter transpiration rates might be a complementary means to manipulate elemental compositions of rice tissues with the goal of reducing or limiting [As] in rice grains.