ARS | Publication request: Leaf calcium oxalate crystal structure and its role in defense against a chewing insect in Medicago truncatula

Submitted to: Plant Biology
Publication Type: Abstract Only
Publication Acceptance Date: March 1, 2007
Publication Date: May 1, 2007
Repository URL: http://www.aspb.org
Citation: Korth, K.L., Park, S., Nakata, P. 2007. Leaf calcium oxalate crystal structure and its role in defense against a chewing insect in Medicago truncatula [abstract]. Plant Biology. Abstract No. P14003.

Technical Abstract: Crystals of calcium oxalate are common in plants and widely distributed among many plant families. These hard and largely insoluble crystals take on many shapes and sizes depending on the tissue and species. In Medicago truncatula, calcium oxalate crystals are abundant in leaves and accumulate in sheaths around secondary veins. We have used calcium oxalate defective (cod) lines of M. truncatula to measure the effects of this mineral on insects. Larvae of beet armyworm, Spodoptera exigua, have a strong feeding preference for tissue lacking the crystals. The calcium oxalate causes deterioration of insect mandibles, interferes with digestion, and leads to higher mortality and lower growth rates. Measurements of wound-induced transcripts suggest that cod mutants perceive and react to damage much the same as wildtype lines. Unlike the obvious negative effects on insect mandibles, M. truncatula calcium oxalate does not have any visible effect on the peritrophic membranes of insect midguts. Calcium oxalate in M. truncatula accumulates as oblong, prismatic crystals about 10 microns in length, whereas commercial preparations are a mixture of amorphous, small crystals from 1-4 microns in diameter. Commercial crystals in artificial diets, at levels up to four-fold higher than in M. truncatula leaves, have no negative influence on insect growth. Even at levels 7.5-fold higher, there is no visible effect of commercial crystals on S. exigua mandibles. The data provide strong evidence that the impact of M. truncatula calcium oxalate on insects is due to the size and shape of the crystals. Because of this strong negative effect on insects, identifying genes and conditions that control crystal formation could lead to new strategies for improving insect resistance in crop plants.