ARS | Publication request: Physical characterizations of Medicago truncatula calcium oxalate crystals determine their effectiveness in insect defense

Submitted to: Entomologia Experimentalis et Applicata
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 1, 2008
Publication Date: N/A
Publisher's URL: http://www.blackwellpublishing.com
Reprint URL: http://www.blackwellpublishing.com

Interpretive Summary: Many plants make crystals made up of calcium and oxalate. These crystals often reside in various plant tissues such as leaves, stems, flowers, and seeds. We have previously shown that plants such as Medicago truncatula (model legume) make these crystals as a part of their defense mechanisms to protect themselves against chewing insects. In this study we show by adding various amounts of commercial calcium oxalate crystals (chemically the same but smaller than the crystals produced by the plant) to plant tissues that the total amount of crystals is not the only important factor in deterring insect from eating the plants but also the size of the crystals plays a major determining role. Microscopic examination of the insect's teeth and gut suggests that these hard crystals exert their deterring effect by causing wear on the teeth rather than affecting the gut of the insects. On the human size scale it may be similar to having sand (commercial calcium oxalate crystals) in your green salad versus having gravel (plant produced calcium oxalate crystals) in your green salad. Such insights are important for us to know so that we may apply this knowledge in the design of strategies to utilize calcium oxalate formation to protect crop plants from insect damage. If successfully implemented this type of strategy could reduce the need for using expensive and harmful chemical pesticides in the production of much of our food.

Technical Abstract: Plant structural traits often act as defenses against herbivorous insects, causing them to avoid feeding on a given plant or tissue. Mineral crystals of calcium oxalate in Medicago truncatula Gaertn. leaves have previously been shown to be effective deterrents of lepidopteran insect feeding, but also as inhibitors of conversion of plant material into insect body mass during or after consumption. Analyses of the effects of calcium oxalate crystals of differing sizes on insect growth indicate that the size and shape of the crystals are the primary determinants of their effects on insect performance. Growth of Spodoptera exigua Hübner, beet armyworm, larvae was correspondingly greater on calcium oxalate defective (cod) mutants of M. truncatula with lower levels of crystal accumulation. The larger calcium oxalate crystals found in M. truncatula leaves have greater effects on insect growth and mandible wear than do smaller amorphous crystals found in commercial preparations. Commercial calcium oxalate can be added to insect artificial diet at levels up to 7.5-fold higher than levels found in wildtype M. truncatula leaves with minimal effects on insect growth or lepidopteran mandibles. In contrast to the dramatic physical effects that M. truncatula-derived crystals have on insect mandibles, we could detect no damage to insect peritrophic gut membranes due to consumption of these crystals. Taken together, the data clearly indicate that the size and shape of prismatic M. truncatula oxalate crystals are important factors in determining effects on insect growth. If manipulation of calcium oxalate is to be used in developing improved insect resistance in plants, then our findings suggest that controlling not only the overall amount, but also the size and shape of crystals, could be valuable traits in selecting desirable plant lines.