Protective Mechanism of the Mexican Bean Weevil against High Levels of α--Amylase Inhibitor in the Common Bean

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Abstract
α--Amylase inhibitor (α-AI) protects seeds of the common bean (Phaseolus vulgaris) against predation by certain species of bruchids such as the cowpea weevil (Callosobruchus maculatus) and the azuki bean weevil (Callosobruchus chinensis), but not against predation by the bean weevil (Acanthoscelides obtectus) or the Mexican bean weevil (Zabrotes subfasciatus), insects that are common in the Americas. We characterized the interaction of α-AI-1 present in seeds of the common bean, of a different isoform, α-AI-2, present in seeds of wild common bean accessions, and of two homologs, α-AI-Pa present in seeds of the tepary bean (Phaseolus acutifolius) and (α-AI-Pc in seeds of the scarlet runner bean (Phaseolus coccineus), with the midgut extracts of several bruchids. The extract of the Z. subfasciatus larvae rapidly digests and inactivates α-AI-1 and α-AI-Pc, but not α-AI-2 or [alphha]AI-Pa. The digestion is caused by a serine protease. A single proteolytic cleavage in the [beta] subunit of α-AI-1 occurs at the active site of the protein. When degradation is prevented, α-AI-1 and α-AI-Pc do not inhibit the α--amylase of Z. subfasciatus, although they are effective against the α--amylase of C. chinensis. α-AI-2 and α-AI-Pa, on the other hand, do inhibit the α--amylase of Z. subfasciatus, suggesting that they are good candidates for genetic engineering to achieve resistance to Z. subfasciatus.