ARS | Publication request: Influence of calcium oxalate crystal accumulation on the calcium content of seeds from Medicago truncatula

Submitted to: Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 3, 2011
Publication Date: April 1, 2012
Citation: Nakata, P.A. 2012. Influence of calcium oxalate crystal accumulation on the calcium content of seeds from Medicago truncatula. Plant Science. 185-186:246-249.

Interpretive Summary: Legume seeds are an essential food source especially in developing countries where plant foods provide a significant portion of the daily dietary nutrients. One such dietary nutrient important for bone development is calcium. The amount of calcium in seeds, fruit, and tubers is low, making it difficult for many to meet the daily dietary requirement consuming a diet based primarily on these plant foods. Thus, there has been an interest in increasing the calcium content of various edible seeds. A major hurdle hindering these efforts is our incomplete understanding of the factors determining the amount of calcium that is deposited in seeds. This study investigates whether calcium oxalate crystal formation is one such determining factor. A microscopic and biochemical comparison of a crystal-forming plant line to a non-crystal-forming plant line revealed that calcium oxalate formation is a factor that restricts the amount of calcium that accumulates in seeds. Regulating the amount of calcium the plant uptakes from the soil also appears to be another mechanism involved in determining seed calcium content. Such findings provide new insights into our understanding of the factors that determine the calcium content of seeds. Hopefully such information will be useful in the design of strategies to enhance the calcium content of edible seeds.

Technical Abstract: Crystals of calcium oxalate often form in cells adjacent to the vascular bundles in the tissues along the xylem stream. This spatial crystal pattern suggests a role for calcium oxalate formation in regulating calcium transport and partitioning to edible organs such as seeds. To investigate this potential role, microscopic and biochemical comparisons were conducted on the different tissues of Medicago truncatula wild-type and the calcium oxalate defective (cod) 5 which lacks the ability to accumulate prismatic crystals in the cells adjacent to the vascular bundles. Calcium measurements showed that cod5 seeds had more calcium and cod5 pods contained less calcium than the corresponding wild-type tissues. Roots, stems, and leaves from cod5 and wild-type had similar calcium content. Although cod5 was devoid of prismatic crystals, cod5 pods were observed to form druse crystals of calcium oxalate not found in wild-type pods. Taken together these findings suggest a functional role for calcium oxalate formation in regulating calcium transport to the seeds. Regulating calcium uptake at the roots also appeared to be another point of control in determining seed calcium content. Overall, regulating the long distance transport and partitioning of calcium to the seeds appears to be a complex process with multiple points of control.