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Legumes produce insecticidal toxins

It was already known that an albumin found in pea seeds had specific insecticidal properties. However, proteins from the same family recently identified in Medicago truncatula may be much more potent entomotoxins and represent a promising avenue of research for designing new biopesticides.

Tychius aureolus, a weevil that consumes alfalfa seeds. © INRA, CARRE Serge
By Pascale Mollier, translated by Jessica Pearce
Updated on 07/04/2014
Published on 05/28/2014

A natural insecticidal toxin in the pea

Pea albumin 1 subunit b (PA1b) is a toxin present in pea seeds that protects them from harmful granivorous insects in general and weevils in particular. A true “venom”—albeit of plant origin—this toxin has a unique mode of action that is insect specific: it inhibits a v-ATPase found in the insect digestive tract, an enzyme that acts as the main protein pump for cells and that is crucial for nutrient absorption. This type of toxic compound, which has a minor or even non-existent impact on the environment and human health, can serve as an alternative to the chemical pesticides, such as pirimiphos or deltamethrin, that are currently being used to protect grain stores.

A peptide ten times more toxic found in Medicago truncatula

Peptides belonging to the same family as PA1b have recently been discovered in the legume Medicago truncatula (barrel clover), thanks to the sequencing of this model organism’s genome. This process revealed an unprecedented variety of genes that code for these proteins. This degree of diversity can be explained by the assiduous work of duplication mechanisms over the course of evolution. While exploring this variation, researchers identified a few potentially functional peptides, which they synthesized and refolded in vitro. One of them is ten times more toxic to insects than is PA1b. This discovery was patented in 2013.

In contrast to the pea protein, which is found in seeds, the M. truncatula albumin 1b proteins are found in roots and root nodules.

A unique mechanism

Research advances have also been made with regards to the enzyme targeted by these albumin 1b peptides: the v-ATPase found in insects. The two enzyme subunits to which the toxin binds have been identified. Studies employing comparative genomics should allow researchers to examine the co-evolutionary relationship between the albumin 1b family and its target, v-ATPase. The results of this work should reveal the components that constitute this efficient ligand-receptor system and thus allow us to understand why certain insects and all vertebrates are impervious to A1b peptides.

This original research, which relies on some fundamental genomic tools—phylogenetic and transcriptome analyses, has thus been validated by the discovery of new insecticidal compounds. It has also allowed us to learn more about the evolutionary history of this protein family, which is found both in M. truncatula in particular and in legumes in general.

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Plant Health and Environment
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