Physical characteristics of calcium oxalate crystals as determinants in structural defense against chewing insects in Medicago truncatula

Authors: KORTH, KENNETH - University Of Arkansas; Nakata, Paul; MCGEHEE, ROBERT, JR. - University Arkansas For Medical Sciences (UAMS); NAGARAJAN, R. - University Arkansas For Medical Sciences (UAMS)

Submitted to: Biology of Plant Microbe Interactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/1/2009
Publication Date: N/A
Citation: N/A

Interpretive Summary: We have been able to show that some plants can generate hard rock-like crystals, made of calcium oxalate, that deter insects from eating them. Microscopic examination of the stomachs, teeth, and waste products of the insects that were forced to eat the plant food containing these hard rock-like crystals was conducted. This examination allowed us to conclude that these crystals exerted their effect by making the plant food unpalatable to the insects by physically grinding on the insect’s teeth when they chew. An analogy for humans would be eating a green salad that contained sand or soil. The sand/soil would make the salad unpalatable to eat by grinding on our teeth when we chew. As a step toward determining how the plant makes these crystals, we attempted to identify the genes that were expressed in a plant that makes crystals and not expressed in a plant that does not make crystals. The idea here is that the genes required to make the crystals would be expressed in the plant that makes crystals and not in the plant, which does not make crystals. Several candidate genes were identified by this method. We hypothesize that it is through the expression of these candidate genes that allow certain plants to make crystals. It is our hope that the identified calcium oxalate genes can be used in future studies to make crop plants more resistant to insects. Such a feat should aid efforts to reduce the use of chemical pesticides in the production of plant foods.

Technical Abstract: In addition to the numerous chemical defenses that plants employ to fend off insect herbivores, simple structural components can also play important roles in effective protection. Our investigations have shown that plant crystals of calcium oxalate can function in insect defense. The isolation of calcium oxalate defective mutant lines in Medicago truncatula has provided a set of valuable tools to elucidate the biological function(s) of calcium oxalate formation, especially as it pertains to defense against pests. Microscopic and biochemical characterization of the insects fed a diet of these mutants containing different crystal concentrations, showed that these crystals do not appear to act as a deterrent by causing damage to the gut, or through oxalate toxicity, but rather as an abrasive factor that deters insect feeding. The size of the oxalate crystal was also shown to be an important factor in this abrasive effect. Gene expression studies, using the Affymetrix Medicago Gene Chip arrays, showed that several genes were differentially expressed in two of the reduce crystal mutants compared to controls. Future studies will focus on the role of differentially expressed genes in formation of calcium oxalate crystals.