Calcium oxalate in plants.

Calcium oxalate is a common biomineral in plants, occurring as crystals of various shapes. It can be found in any tissue or organ in plants and is often formed in the vacuoles of specialized cells called crystal idioblasts. Recent work indicates that calcium oxalate formation is generally a mechanism for regulating bulk-free calcium levels in tissues and organs. However, various other functions might have evolved secondarily. A function in physical protection against grazing animals is implicated by the size, shape and placement of crystals in some tissues and organs, as illustrated in Claoxylon leaves (Fig. 1Fig. 1).Fig. 1Cross-section of a mature Claoxylon sandwicense leaf. A large calcium oxalate crystal idioblast has grown by intrusive growth in between palisade and spongy mesophyll cells and now spans the entire cross section of the leaf. The idioblast has a single large styloid crystal with sharply pointed ends. A role in defense of the plant is obvious because any animal trying to chew on this leaf will have the equivalent of a 2–3-mm long, double-pointed needle to contend with. Many plants have specialized cells, called crystal idioblasts, that contain single or multiple needle-like crystals that appear to serve a primary function in bulk regulation of calcium in tissues and a secondary function in defense against grazing animals. The needles of calcium oxalate can even have grooves and barbs, which are thought to be responsible for channeling toxins or anchoring the needles in the wound. Scale bar=400 μm.View Large Image | Download PowerPoint SlideA novel function in light regulation during photosynthesis is indicated in the shade plant, Peperomia (Fig. 2Fig. 2). Several Peperomia species have a leaf anatomy with a thick, clear multiple epidermis layer (window tissue) overlying a single layer of palisade parenchyma cells that form the major photosynthetic layer of the leaf. Unlike most typical dicot species with a palisade layer(s), each palisade cell in Peperomia contains a druse crystal within the vacuole. My observations of six different species indicate that the crystals are restricted to the palisade cells and that they are not related to photosynthesis type because both C3 and CAM species contain them. The druse crystal is a multifaceted spherical conglomerate with facets radiating in all directions from a central core. The observation that calcium oxalate crystals in Peperomia are specifically produced in the photosynthetic palisade cells rather than in specialized idioblasts indicates a function other than calcium regulation in these species. Ling-Long Kuo-Huang (National Taiwan University) and I hypothesize that the crystals help to evenly distribute light to the chloroplasts lining the radial wall and surrounding the vacuole in this low-light-adapted plant. The crystals might also help to dissipate excess light during intermittent sunflecks by reflecting light back up to the window tissue. If it were shown that calcium oxalate crystals are produced primarily to participate in the photosynthetic process, this would represent a novel structural variation that had evolved to influence photosynthesis.Fig. 2Peperomia leaf. (a) Scanning electron micrograph of a cross section in the region of the palisade layer. Each columnar palisade cell, most of which are cleaved open in this sample, has a spherical-looking druse crystal near the top of the cell. Scale bar=20 μm. (b) High-magnification image of druse crystals. The crystal is a conglomerate of multiple facets that radiate out from a central core. Scale bar=5 μm.View Large Image | Download PowerPoint Slide