Assessment of the importance of α-amylase inhibitor-2 in bruchid resistance of wild common bean

Both alpha-amylase inhibitor-2 (alpha AI-2) and arcelin have been implicated in resistance of wild common bean (Phaseolus vulgaris L.) to the Mexican bean weevil (Zabrotes subfasciatus Boheman). Near isogenic lines (NILs) for arcelin 1-5 were generated by backcrossing wild common bean accessions with a cultivated variety. Whereas seeds of a wild accession (G12953) containing both alpha AI-2 and arcelin 4 were completely resistant to Z. subfasciatus, those of the corresponding NIL were susceptible to infestation, suggesting that the principal determinant of resistance was lost during backcrossing. Three independent lines of transgenic azuki bean [Vigna angularis (Willd.) Ohwi and Ohashi] expressing alpha AI-2 accumulated high levels of this protein in seeds. The expression of alpha AI-2 in these lines conferred protection against the azuki bean weevil (Callosobruchus chinensis L.), likely through inhibition of larval digestive alpha-amylase. However, although the seed content of alpha AI-2 in these transgenic lines was similar to that in a wild accession of common bean (G12953), it did not confer a level of resistance to Z. subfasciatus similar to that of the wild accession. These results suggest that alpha AI-2 alone does not provide a high level of resistance to Z. subfasciatus. However, alpha AI-2 is an effective insecticidal protein with a spectrum of activity distinct from that of alpha AI-1, and it may prove beneficial in genetic engineering of insect resistance in legumes.