Lectins as plant chemical defenses against insects

Joseph Edwin Huesing, Purdue University

Abstract

To determine the potential of plant lectins as chemical defenses against insects and to broaden our knowledge of the roles plant lectins play in nature, seventeen plant lectins were screened in an artificial seed system using the cowpea weevil, Callosobruchus maculatus, as a model insect. Five caused significant delays in cowpea weevil developmental time. These lectins were classed into one of two groups: lectins with specificity for N-acetylgalactosamine residues (GalNAc), which included osage orange lectin and peanut agglutinin, and lectins with specificity for N-acetylglucosamine residues (GlcNAc), a constituent of chitin. The results suggest that GlcNAc-specific plant lectins represent a class of biologically active proteins effective against the cowpea weevil. The GlcNAc-specific lectins studied were derived from wheat germ (WGA), rice (RL), tomato (TL), Jimson weed (DSA) and stinging nettle (UDA). Among the most active lectins is WGA, the isolectins of which were found to be equally detrimental to the cowpea weevil. To better understand which molecular properties, such as relative binding affinity, binding site number, and molecular size affect GlcNAc-specific lectin toxicity, the five above named lectins were analyzed in an in vivo structure-activity analysis. The relative toxicity of the GlcNAc-specific lectins on a molar basis is WGA$>$RL$>$TL${>>}$UDA${>>>}$DSA. On a binding site basis, the relative order is WGA=TL$>$RL$>$UDA$>$DSA. Toxicity appears to be a function of the number of GlcNAc-binding sites. The GlcNAc-specific lectins appear to be members of the chitin-binding protein family, which is characterized by stable proteins containing extensive disulfide cross linkages. Histological evidence suggests that WGA acts in the midgut to cause pathology. Good correlation exists between lectin dose, lesion intensity, and impact on insect growth and survival. It appears that there exists in cowpea weevil physiological/biochemical systems vulnerable to selected plant lectins. The genes coding for effective plant lectins could, in principle, serve as antibiosis factors to use in plant transformation to confer insect resistance. Previous claims that phytohemagglutinin (PHA) is toxic to the cowpea weevil were not supported in the present investigation. The toxic effects of a commercial PHA preparation are due to an $\alpha$-amylase inhibitor impurity, a lectin-like protein of the common bean lectin gene family.

Degree

Ph.D.

Advisors

Murdock, Purdue University.

Subject Area

Entomology|Botany|Botany

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