Submitted to: Plant Physiology and Biochemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/29/2007
Publication Date: 3/1/2007
Citation: Nakata, P.A., McConn, M. 2007. Isolated medicago truncatula mutants with increased calcium oxalate crystal accumulation have decreased ascorbic acid levels. Plant Physiology and Biochemistry. 45:216-220. Interpretive Summary: We are interested in learning how plants produce crystals made of calcium and oxalate. To begin deciphering this process the scientists visually looked at the leaves from a population of plants that contained random genetic mutations. Within this population of plants the scientists found several mutant plants that were able to make more calcium oxalate crystals than normal plants. The scientists found these mutants by examining the leaves from each plant using a microscope. Further examination of the mutant leaves revealed that the increase in amount of crystals in these mutant plants was due to the plant's ability to accumulate additional crystals in a different portion of the leaves. Using the genetic techniques of complementation and mapping, the researchers determined that each mutant plant was able to accumulate more crystals because of a mutation in a different gene. Having different mutations that all result in the same outcome, increased crystal accumulation, indicated that this process of plant crystal formation is quite complex. Biochemical measurements of the putative substrate for oxalate biosynthesis, along with substrate induction assays, indicated that ascorbic acid is the substrate in the biosynthesis of these additional crystals.
Technical Abstract: The mechanisms controlling oxalate biosynthesis and calcium oxalate formation in plants remains largely unknown. As an initial step toward gaining insight into these regulatory mechanisms we initiated a mutant screen to identify plants that over-accumulate crystals of calcium oxalate. Four new mutants were identified, from an ethyl methanesulfonate (EMS)-mutagenized Medicago truncatula (cv. Jemalong genotype A17) population, that over-accumulated calcium oxalate crystals. The increased calcium oxalate content of these new mutants, as with the previously isolated mutant cod4, resulted from an increase in druse crystals accumulated within the mesophyll cells of leaves. Complementation and segregation analysis revealed that each mutant was affected at a different locus. This was confirmed through the genetic mapping of each mutation to different linkage groups. Together, these findings emphasize the complexity of factors that can contribute to oxalate biosynthesis and crystal formation in these plants. In addition, each mutant showed an inverse relationship between ascorbic acid and oxalate content providing genetic support for an ascorbic acid to oxalate biosynthetic pathway in druse crystal formation. Further support was obtained by the ability of an exogenous supply of ascorbate to induce druse crystal formation while other organic acids such as citrate did not induce crystal production.