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 micr oautoradiographic, immunological, and ultra-structural techniques, the proc ess of raphide crystal growth, and how crystal growth is coordinated with c ell growth, was studied in idioblasts of Pistia stratiotes. Incorporation o f Ca-45(2+) directly demonstrated that, relative to surrounding mesophyll c ells, crystal idioblasts act as high-capacity Ca2+ sinks, accumulating larg e amounts of Ca2+ within the vacuole as crystals. The pattern of addition o f Ca2+ during crystal growth indicates a highly regulated process with bidi rectional crystal growth. In very young idioblasts, Ca-45(2+) is incorporat ed along the entire length of the needle-shaped raphide crystals, but as th ey 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 pr ocesses at the plasma membrane. In situ hybridization for ribosomal RNA sho ws young idioblasts are extremely active cells, are more active than older idioblasts, and have higher general activity than surrounding mesophyll cel ls. Polarizing and scanning electron microscopy demonstrate that the crysta l morphology changes as crystals develop and includes morphological polarit y 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 cont rolling the idioblast cell shape, but the regulation of crystal growth and morphology is under a different control mechanism.