SCIENCE & TECHNOLOGY P H O T O
ESSAY
THE SECRET LIFE OF PLANT CRYSTALS Microscopic view of plant tissue reveals a hidden world of calcium oxalate crystals of diverse shapes IVAN AMATO, C & E N
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of a cafeteria in Chicago contacted the city's D e p a r t m e n t of Health to report a spate of illnesses in people who had eaten at the facil ity three days earlier. Symptoms included burning and stinging in the mouth, difficulty in swallowing, a n d facial swelling. O n e person had enough trouble breathing t o be admitted to an intensive care unit. For some, symptoms lasted for weeks. I n t h e e n s u i n g investigation, h e a l t h d e p a r t m e n t researchers interviewed t h e cafeteria's staff and inspected the kitchen. Later, scientists at t h e F o o d & D r u g Ad ministration's Forensic Chemistry Center
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in C i n c i n n a t i c o n d u c t e d analyses of t h e "Chinese braised vegetable" entrée that all of t h e 10 affected people h a d eaten. Last year, in the journal Clinical Toxicology (2005, 1,17), t h e t e a m reported w h a t they found: T h e entrée contained tiny javelin-shaped crystals known as raphides, and it was these crystals that most likely caused the troubling symptoms. "To our knowledge, this is the first reported foodborne disease outbreak associated with exposure to raphides," the researchers stated. Raphides? Little k n o w n beyond a cadre of botanists, chemists, forensic investigators, and others interested in the interface of biology and crystals, raphides are one of
S T A G E
Views Of Druse And Their Crystal Kin
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hen biologist Gary Cote of Virginia's Radford University used a light microscope to view a piece of a petallike plant stannen through crossed polarizing filters, he observed this tissue's exuberant distribution of idioblast cells, each harboring a tiny druse crystal aggregate made of calcium oxalate (top right). In another preparation, this one of a single capsule-shaped cell with two breakable ends, a bundle of raphide crystals, poised
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perhaps to be expelled into the mouth of a would-be plant eater, shows results of biological crystal-making (bottom right). In the hands of forensic analysts tasked with identifying plant materials, micrographs like this one of cells containing prismatic calcium oxalate crystals (left) can provide distinctive clues. In this case, Élan Sudberg of Alkemists Pharmaceuticals could tell that the cells are from Tinospora cordifoiia, an ingredient of some herbal medicines.
several crystal forms of calcium oxalate found in plants. T h e biological functions of t h e s e crystals, w h i c h typically g r o w w i t h i n individual cells, are still n o t completely understood. "Seventy-five percent of flowering plants make one or more kinds of the crystals, and they form in specific places in tissues and organs," says Iowa State University's H a r r y T. (Jack) Horner, w h o has b e e n studying calcium oxalate crystallization in plants for decades. Some plants m a k e prismatic crystals, akin to miniaturized sugar grains. Others make "crystal sand" with less regular particle shapes that are shaped like little tetrahedrons. By far the most spectacular crystals are the ones known as druses. They look like microscopic carnations, but their rigid petals probably are sharp enough to lacerate the mouths of marauding insects. T h a t possibility strongly suggests to some researchers that one evolutionary valuable trait of these crystals is to make it unpleasant, even downright dangerous, for a wouldb e herbivore to eat t h e plants. O n e plant known for its abundance of calcium oxalate crystals is dumbcane. T h o s e w h o chew on it find that their mouth and tongue swell so much that it is painful and hard to talk. SOME RAPHIDES form within pressurized capsulelike cells that, w h e n pierced or bitt e n into, forcibly expel t h e raphides a n d cell contents. I n some plants, these raphides even have grooved faces t h a t help to channel toxins m a d e by t h e cell into tiny w o u n d s created by t h e expelled crystals, notes molecular biologist Paul A. Nakata of the Department of Agriculture's Children's Nutrition Research Center, housed at Baylor College of Medicine in Houston. Last J u n e in the AnnualReview of Plant Biology, ( 2 0 0 5 , 5 6 , 41), he and t h e late Vincent R. Franceschi of Washington State University provided an extensive status report on what is and is not k n o w n about calcium oxalate in plants. Although the presence of calcium oxalate crystals was first described in plants by pioneer microscopist Antonie van Leeuwenhoek in the late 1600s, those few who study these crystals are quick to note that the structures' roles in plants, how plants manufacture the oxalate molecules that end u p in the crystals, and how the crystals form remain open questions. "Most of us are interested in what factors control crystal cell development, and that gets down to the cellular, genetic, and molecular levels," Horner says. M a n y researchers are convinced t h a t calcium oxalate crystallization, which usually occurs within plant cells called crystal idioblasts, is t h e primary means for m a n y WWW.CEN-0NLINE.ORG
MICROSCOPY
Botanic Crystal Fashion Show
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any plants have specialized cells, called crystal idioblasts, in which calcium and oxalate ions assemble in to crystals. Scientists have ascribed various roles to these crystals, among them calcium concentration control in tis sues, defense against herbivores, and light gathering for photo synthesis. When viewed with a variety of microscopes, either in preparations that isolate the crystals or ones that show the crys tals in their tissue venues, a visually striking and appealing diver sity of botanic crystallization becomes apparent. The major mor phological classes of crystals include prismatic ones like those in the micrograph of a coat of an Oxalis corniculata seed (a); thornlike
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plants to manhandle the calcium they often can't help but absorb from the soil into the low, nontoxic concentrations that their cells need. One line of evidence for this comes fromsmdiesmwhichcrystal-formingplants grown in calcium-rich or calcium-poor en vironments produce more or fewer crystals, respectively Researchers have proposed, with vary ing degrees of experimental support, that calcium oxalate crystals, besides being in volved in defense and calcium regulation, also sequester poisons including heavy met als; provide mechanical support; regulate ionic balances; and even, Horner says, help WWW.CEN-0NLINE.ORG
styloids as represented by this dried preparation of petiole tissue from Eichornia crassipes lb); "crystal sand" like these particles iso lated from a leaf of Nicotiana glauca (c); javelin-like raphides rep resented here by a bundle of the crystals isolated from the flow er bud of a Psychotria punctata plant (d); and the most spectacular type, flowerlike druse crystals (sometimes also called rosettes) represented here by images of crystals isolated from the leaf of a Peperomia astrid plant (e) and a variety of Peperomia rotundifolia (f). All of the images were taken by Harry T. (Jack) Horner of Iowa State University. The dimensions of the crystals range from about 1μηη to tens of micrometers.
the plant out by gathering and focusing light onto photosynthetic pigments enclosed in chloroplasts. Compared with tooth, bone, and shell formation, calcium oxalate crystallization is an unsung example of biomineralization. Yet it plays roles in everyday plant life and survival and in human health. Calcium oxa late, after all, is one of the compounds that accrete into kidney stones in people. In the hands of Elan Sudberg, chief op erating officer of a contract analysis firm called Alkemists Pharmaceuticals in Costa Mesa, Calif, calcium oxalate crystals even serve for making taxonomic and forensic
identifications, such as of adulterants in dietary supplements. "The combination of the particular crystal types, their location in the plant, as well as other cellular char acteristics are meaningful in identifying the genus and species of many botanicals," Sudberg says. Displayed on these pages and at www. cen-online.org is a gallery of optical and electron microscope images that reveal the diverse ways in which plants sculpt calcium oxalate crystals into functional forms that are visually stunning and, at times, physically stunning to those they pierce, whether they be caterpillars or cafeteria diners. • C&EN / FEBRUARY
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