Cell and calcium oxalate crystal growth is coordinated to achieve high-capacity calcium regulation in plants

Crystal idioblasts are cells which are specialized for accumulation of Ca2+ as a physiologically inactive, crystalline salt of oxalic acid. Using microautoradiographic, immunological, and ultra-structural techniques, the process of raphide crystal growth, and how crystal growth is coordinated with cell growth, was studied in idioblasts of Pistia stratiotes. Incorporation of Ca-45(2+) directly demonstrated that, relative to surrounding mesophyll cells, crystal idioblasts act as high-capacity Ca2+ sinks, accumulating large amounts of Ca2+ within the vacuole as crystals. The pattern of addition of Ca2+ during crystal growth indicates a highly regulated process with bidirectional crystal growth. In very young idioblasts, Ca-45(2+) is incorporated along the entire length of the needle-shaped raphide crystals, but as they mature incorporation only occurs at crystal tips in a bidirectional mode. At full maturity, the idioblast stops Ca2+ uptake, although the cells are still alive, demonstrating an ability to strictly regulate Ca transport processes at the plasma membrane. In situ hybridization for ribosomal RNA shows young idioblasts are extremely active cells, are more active than older idioblasts, and have higher general activity than surrounding mesophyll cells. Polarizing and scanning electron microscopy demonstrate that the crystal morphology changes as crystals develop and includes morphological polarity and an apparent nucleation point from which crystals grow bidirectionally. These results indicate a carefully regulated process of biomineralization in the vacuole. Finally we show that the cytoskeleton is important in controlling the idioblast cell shape, but the regulation of crystal growth and morphology is under a different control mechanism.