Calcium oxalate crystal formation is not essential for growth of Medicago truncatula

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.