ARS | Publication request: MEDICAGO TRUNCATULA MUTANTS DEMONSTRATE THE ROLE OF PLANT CALCIUM OXALATE CRYSTALS AS AN EFFECTIVE DEFENSE AGAINST CHEWING INSECTS

Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 25, 2006
Publication Date: March 2, 2006
Citation: Korth, K.L., Doege, S.J., Park S.H., Goggin, F.L., Wang, Q., Gomez, S.K., Liu, G., Jia, L., Nakata, P.A. 2006. Medicago truncatula mutants demonstrate the role of plant calcium oxalate crystals as an effective defense against chewing insects. Plant Physiology. 141(1):188-195.

Interpretive Summary: Many plants form hard crystals made of calcium and oxalate. Why plants make these crystals has remained a point of debate. In this study, the authors provide evidence that these crystals play a role in protecting the plant from chewing insects. The conducted studies compared leaves with normal and lower amounts of calcium oxalate and showed that the chewing insects avoid eating plant tissues containing the hard crystals. Insects forced to eat the tissue with the hard crystals showed the effects through the damage to their teeth. A defensive role for crystal formation is hypothesized. In addition, the ability to form crystals may be of great potential value in breeding and engineering efforts to make more insect-resistant plants, and thereby permitting a decrease in the use of chemical pesticides.

Technical Abstract: Calcium oxalate is the most abundant insoluble mineral found in plants and its crystals have been reported in over 200 plant families. In the barrel medic, Medicago truncatula Gaertn., these crystals accumulate predominantly in a sheath surrounding secondary veins of leaves. Mutants of M. truncatula with decreased levels of calcium oxalate crystals were used to assess the defensive role of this mineral against insects. Caterpillar larvae of the beet armyworm, Spodoptera exigua (Hubner), show a clear feeding preference for tissue from calcium oxalate defective mutant lines cod5 and cod6 in choice-test comparisons with wildtype M. truncatula. Compared to their performance on mutant lines, larvae feeding on wildtype plants with abundant calcium oxalate crystals suffer significantly reduced growth and increased mortality. Induction of wound-responsive genes appears to be normal in cod5 and cod6, indicating that these lines are not deficient in induced insect defenses. Electron micrographs of insect mouthparts indicate that the prismatic crystals in M. truncatula leaves act as physical abrasives during feeding. Food utilization measurements show that after consumption, calcium oxalate also interferes with the conversion of plant material into insect biomass during digestion. In contrast to their detrimental effects on a chewing insect, calcium oxalate crystals do not negatively affect the performance of the pea aphid, Acyrthosiphon pisum (Harris), a sap-feeding insect with piercing-sucking mouthparts. The results confirm a long-held hypothesis for the defensive function of these crystals, and point to the potential value of genes controlling crystal formation and localization in crop plants.