The Role of Natural Calcium Oxalate Crystals in Plant Defense Against Chewing Insects

Inquiry: The University of Arkansas Undergraduate Research Journal Volume 4 Article 15 Fall 2003 The Role of Natural Calcium Oxalate Crystals in Plant Defense Against Chewing Insects Sarah J. Doege University of Arkansas, Fayetteville Follow this and additional works at: http://scholarworks.uark.edu/inquiry Part of the Plant Biology Commons, and the Plant Pathology Commons Recommended Citation Doege, Sarah J. (2003) "The Role of Natural Calcium Oxalate Crystals in Plant Defense Against Chewing Insects," Inquiry: The University of Arkansas Undergraduate Research Journal: Vol. 4 , Article 15. Available at: http://scholarworks.uark.edu/inquiry/vol4/iss1/15 This Article is brought to you for free and open access by ScholarWorks@UARK. It has been accepted for inclusion in Inquiry: The University of Arkansas Undergraduate Research Journal by an authorized editor of ScholarWorks@UARK. For more information, please contact , . Doege: The Role of Natural Calcium Oxalate Crystals in Plant Defense Aga 88 INQUIRY Volume 4 2003 THE ROLE OF NATURAL CALCIUM OXALATE CRYSTALS IN PLANT DEFENSE AGAINST CHEWING INSECTS By Sarah J. Doege Department of Biological Sciences Faculty Mentor: Kenneth L. Korth Department of Plant Pathology Abstract: Calcium oxalate is the most abundant insoluble mineral found in plants and it is common among many plant families. Calcium oxalate crystals in plants can appear as irregular rectangles, spiked balls, or needles. Theformidableappearance of these crystals has lead to speculation that they might serve as a form ofpest control by deterring chewing insects. We utilized mutant plant lines to assess the effects ofplant calcium oxalate crystals on the survival and feeding habits of chewing insects. We have taken advantage of calcium oxalate-deficient (cod) mutants of the barrel medic, Medicago truncatula Calcium oxalate crystals accumulate in wildtype M. truncatula leaves along the vascular strands of secondary veins. Results demonstrate that beet army-worm, Spodoptera exigua, larvae that feed on M. truncatula cod mutants with reduced levels of calcium oxalate crystals grow faster and larger than insects that feed on normal wildtype plants. Pupae formed by larvae raised on cod plants are significantly larger than those raised on the wildtype plants. The results oftwo-way choice tests indicate that older(4'h instar or later) S. exigua larvae prefer to feed on leaves lacking calcium oxalate, whereas young larvae (2nd instar or earlier) show no feeding preference. This development-specific feeding preference is perhaps due to the feeding habits of the herbivore; young larvae typically feed between secondary veins, awayfrom areas where the calcium oxalate crystals are localized. Accumulation of RNA transcripts encoding wound-inducible gene products is normal in the cod mutants, suggesting that these plants are not altered in their ability to sense or respond to wounding by insect herbivores. Because calcium oxalate crystals seem to serve as a feeding deterrent to insects, understanding how they are made and distributed could ultimately lead to novel, environmentally sound strategies for improving insect resistance in crop plant species. preformed and induced chemical and/or physical barriers. The tobacco plant, for example, uses the toxic chemical nicotine as a herbivore-defense. Thorns and trichomes are examples ofphysical defenses that plants might utilize as protection against herbivorous insects. Crystals of calcium oxalate are prevalent in nature and can be found in over 215 plant families (McNair, 1932). Calcium oxalate is the most common insoluble mineral in plants. Among those that produce the crystals, calcium oxalate can account for up to 3-80% of a plant's dry weight (Zindler-Frank, 1976; Libert and Franceschi, 1987). There are several hypotheses attempting to explain the role of calcium oxalate in plants- these include the regulation of bulk free-calcium, ion balance, light gathering and reflection, tissue support, and detoxification - but evidence supporting most of these hypotheses is insufficient (reviewed by Franceschi and Homer, 1980). Introduction: The placement, size, and shape of calcium oxalate crystals within some plant tissues suggest that they may play an important role in defense (Franceschi, 2001). In scanning electron microscope images, the more formidable crystals appear as multi-faceted spiked balls, or as needle-like with or without barbs. In some instances, the function of the crystals as defense mechanisms is clear, such as that of the stinging plant, Tragia ramosa (Thurston, 1976). The stinging hairs that cover its surface are conical cells, each containing a raphide (needleshaped) crystal of calcium oxalate. Upon contact, this cell will break open, allowing the crystal to puncture the skin and administer a dermal irritant by way of grooves in the crystal. Calcium oxalate crystals in daffodil and agave are also known dermal irritants (Julian and Bowers, 1997; Salinas, et al., 2001). Many edible aroids contain the crystals in their leaves and corms, and as a result may cause swollen lips, mouth, and throat pain if prepared improperly for consumption (Bradbury and Nixon, 1998). All plants need mechanisms of defense to limit the amount of damage by herbivores. A plant with poor defenses faces the possibility of severe foliage damage, rendering it incapable of performing an adequate amount of physiological or reproductive measures to survive. Plants defend themselves using a variety of Research more specifically related to calcium oxalate crystals as herbivore defense reports that the dorcas gazelle, larvae of a lepidopteran species (Polytella clines), and a land snail (Eremina desertorum) all avoid feeding on certain tissues of a Negev desert lily (Pancratium sickenbergeri) that contain Published by ScholarWorks@UARK, 2003 1 Inquiry: The University of Arkansas Undergraduate Research Journal, Vol. 4 [2003], Art. 15 PLANT PATHOLOGY: Sarah Doege. Natural Calcium Oxalate Crystals 89 calcium oxalate raphides (Ward, et al., 1997). In other research, accumulation of calcium oxalate around the mycelia of commercially grown mushrooms has been shown to act as a deterrent of fungal gnats (Binns, 1980). To date however, there is no solid evidence that plant-derived calcium oxalate crystals serve as a defense against insect herbivores, as unrelated physical and biochemical variations in the makeup of tissues previously tested might explain observed differences in feeding behavior of herbivores. on artificial diet. The wildtype M. truncatula Al7, which was derived from the commercial cultivar "Jemalong", was the parental line in the mutagenesis that gave rise to the cod mutants and is used as the control treatment of all 'of our experiments. Plant cuttings from individualS-week-old cod mutant lines were maintained hydroponically and placed into covered Petri dishes. At the initiation of the experiment to measure insect gro (...truncated)