Breeding Phaseolus vulgaris (common bean) for resistance to the major pest bruchids Zabrotes subfasciatus and Acanthoscelides obtectus : biochemical bases for seed resistance in wild lines
Phaseolus vulgaris (common bean) is an important source of protein for human and animal consumption. Economic losses post-harvest in storage are primarily due to the bruchid beetles Acanthoscelides obtectus (Bean Weevil) and Zabrotes subfasciatus (Mexican Bean Weevil). Wild-lines exhibiting resistance to these two species have been found and the mechanisms of resistance to each species are investigated. The mechanisms of resistance to the two species are found to be multiple, and different for each species. In the case of Z. subfasciatus, the presence of a novel storage protein and absence of the conventional storage protein constitutes the primary mechanism. The novel protein ("arcelin") is antimetabolic when included in artificial diets. In vitro digestibility studies indicate that it is indigestible to Z. subfasciatus larval gut proteases, and since arcelin constitutes the major protein of the seed the larvae starve. Arcelin has a similar amino- acid sequence to PHA. Also present is WBAI, a highly specific inhibitor of larval amylase of the two bruchid pests Z. subfasciatus and Callosobruchus maculatus, whilst having virtually no inhibitory activity on the mammalian amylase, nor on other insect, bacterial nor fungal enzymes. WBAI is similar in gross structure to the conventional amylase inhibitor obtainable from RKB (commercial P. vulgaris), and both are also immunologically similar to PHA. Both of these mechanisms are suitable for incorporation into commercial seed, and the former has already been tested, using meal from seeds, in feeding trials using rats, confirming absence of mammalian toxicity. Resistance to A. obtectus damage is accompanied by reduced starch content, and high content of an acidic polysaccharide (whose structure has not been elucidated). No protein cause for resistance was found. Inheritance of resistance to A. obtectus is recessive. Since the factor responsible for resistance is not a primary gene product and is expressed recessively, this factor is unsuitable for incorporation into breeding lines to be used for developing commercial cultivars.