Submitted to: Plant Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/19/2003
Publication Date: 3/19/2003
Citation: NAKATA, P.A., MCCONN, M.M. CALCIUM OXALATE CRYSTAL FORMATION IS NOT ESSENTIAL FOR GROWTH OF MEDICAGO TRUNCATULA. PLANT SCIENCE. 2003. v. 164. p. 901-909. Interpretive Summary: In most plants, one may find calcium oxalate crystals, which are crystals that contain calcium and oxalate salts. These crystals form in the vacuoles of certain cells. The crystals create a problem in that they prevent the calcium in edible plants from being absorbed by the people and animals that eat the plants. Some plants, like spinach, contain a great deal of calcium. Scientists would like to find a way to release that calcium so people could realize a major bone-building benefit by eating these calcium-rich plant vegetables in their daily diets. They hope to achieve that goal in light of the fact that many growing children, especially girls, are not getting sufficient calcium to build strong bones for life from the amount of dairy products they are consuming (or in some cases, don't consume at all). Researchers have long labored under a dilemma in trying to figure out ways to get that calcium out of the plant and made bioavailable, because the scientific community traditionally has thought that these crystals were essential for the plant's growth and development. However, we developed a mutant plant called Medicago truncatula (a forage plant eaten by livestock) that can't form crystals, but, nonetheless, grows and develops just as well as wild-type plants. This is a big scientific coup, because this finding requires a major overhaul of existing hypotheses. Our flourishing mutant plant shows that it appears to be possible to increase calcium bioavailability by removing the oxalate from a plant. Our mutant Medicago plants have the same amount of calcium as their wild-type peers. So we now think that it could be possible to manipulate similarly calcium-rich plants, like spinach, which so many people enjoy in salads. This type of manipulation could provide a way to get a lot more calcium into people's bones so they could stave off osteoporosis and other bone-thinning diseases, and thus be able to enjoy a much higher-quality life for a longer period of time.
Technical Abstract: Plants invest a considerable amount of resources and energy into the formation of calcium oxalate crystals. A number of roles for crystal formation in plant growth and development have been assigned based on the prevalence of crystals, their spatial distribution, and the variety of crystal shapes. As a step toward determining whether crystal formation plays a critical role in plant growth and development, we characterized the growth, oxalate content, and mineral content of the calcium oxalate defective mutant, cod5. Examination of control plants, using light microscopy, revealed the accumulation of prismatic crystals along the vascular strands in all the different plant tissues with the exception of roots, in which no crystals were observed. In contrast, no prismatic crystals were detected in any of the different tissues of the cod5 mutant. Crystals of calcium oxalate were observed in the pods of cod5, but they were of a different morphology. Measurements of the oxalate content in the different tissues confirmed the cod5 crystal phenotype by exhibiting low oxalate levels compared to those of controls. The cod5 pods did contain measurable oxalate levels, but at levels several times lower than controls. Although compromised in its ability to accumulate crystals of calcium oxalate, cod5 exhibited growth, which was similar to that of controls. Moreover, cod5 and controls contained similar amounts of calcium, sodium, and potassium. Our findings suggest that calcium oxalate crystal formation is not essential for plant growth or development in the case of Medicago truncatula.