Calcium oxalate formation in specialized cells of Pistia stratiotes L.: Regulation of deposition and dissolution during changing calcium availability

Calcium is important to many physiological processes in plants. However, uptake is often not regulated and calcium concentrations can reach levels that overwhelm normal cellular regulation processes. Pistia stratiotes L. was used as a model system for the study of calcium oxalate crystallization because it produces large amounts of two forms of crystal idioblasts, raphide and druse. Crystal idioblasts have been referred to as calcium sinks because of their ability to sequester large amounts of calcium in this physiologically inactive form. Recently it has been shown that crystallization can occur rapidly and is that it is reversible. However, the mechanisms involved in formation and dissolution of crystals have received little attention.; The use of various staining techniques combined with transmission electron microscopy, scanning electron microscopy and light microscopy provided valuable insights to ultrastructural features of young and mature idioblasts. Concentrations of calcium sequestering organelles where found to be greatly increased in crystal idioblasts compared to mesophyll cells. Flocculent material, parallel and precipitation membranes were observed in the vacuole prior to raphide crystal precipitation. These membranes persisted throughout crystal development and were still present on mature crystals. The core, and all exterior surfaces of druse crystals were also covered with membranes throughout development. Minifacets were observed on the larger crystal faces of young crystals, but not on mature crystals. Involvement of membranes and minifacets as a possible control sites for crystal growth is an area identified as a direction for more detailed research in the future.; Immunolocalization studies determined that the shape and rate of crystal growth is influenced by a Matrix protein with a high calcium binding ability and several other biochemical features common to crystal proteins in animal systems. Antibodies were used to localize this protein and were found specifically in idioblasts in close association with crystals.; Dissolution of crystals allows for the remobilization of calcium yet increases the amount of oxalic acid in the vacuole. Oxalate oxidase was shown for the first time to be involved in the metabolism of oxalic acid.