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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. The formidable appearance of these crystals has lead to speculation that they might serve as a form of pest control by deterring chewing insects. We utilized mutant plant lines to assess the effects of plant 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 of two-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, away from 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.

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