Resistance of Vigna unguiculata (cowpea) seeds to Callosobruchus maculatus is restricted to cotyledonary tissues

Abstract

We have investigated the survival of Callosobruchus maculatus larvae when reared on resistant IT81D 1045 Vigna unguiculata seeds, whose resistance has been associated with variant forms of vicilins. Here, we present data which show that larvae of C. maculatus feeding on embryonic axis of resistant cowpea reach a mass of around 28 times higher than those feeding on cotyledonary tissues. Additionally, incorporation of 5-10% of embryonic axis flour on artificial seeds made of resistant flour restores toxicity of seeds to the bruchid. Vicilins purified from both susceptible and resistant embryonic axis had no deleterious effects either on insect development or on insect survival until a level of 4% of incorporation. In contrast, vicilins from resistant cotyledons show an LD 50 (50% lethal dose) and WD 50 (half weight dose) of 2%. Total vicilin contents of embryonic axes were around two times lower than in cotyledonary tissues, while proteolytic activities of all four proteinase classes were always higher in the former tissues. By 2D-PAGE we visualized eight protein spots, which seem to be exclusively found on resistant cotyledons.

The damage caused by Callosobruchus maculatus on cowpea grains is dependent on the plant genotype

“…The remaining tested cultivars supported insect emergence and may provide favorable conditions for insect development. The decreased insect emergence may result from high larvae mortality caused by proteins with insecticidal potential, such as vicilin, which is present in some legumes . Higher emergence of adult insects results in higher quantitative damage, loss of bean nutritional quality, and negative effects on bean appearance, which makes them unsuitable for commercialization and consumption.…”

“…Furthermore, arcelin was observed to inhibit the development of Zabrotes subfasciatus (Boh. ), and vicilin was observed to interfere in the development of C. maculatus …”

“…Weevil resistance has been studied in several cowpea cultivars . These studies have mostly focused on the identification of cultivars that have developed C. maculatus resistance due to trypsin and β ‐amylase inhibitors .…”

Characterization of cowpea genotype resistance to Callosobruchus maculatus

“…The IT81 D-1045 genotype descends from TVu 2027, which was selected from the germplasm bank of the International Institute of Tropical Agriculture, used by improvement programs to obtain resistant varieties. The resistance of these genotypes has been associated with variant forms of the reserve protein, vicilin, which cannot be metabolized by the insect's midgut proteinases, limiting the larvae's food supply and interfering with the development of C. maculatus (Domingues et al, 2006). This may have been the case in the present study in (Table 2), despite the high percentage of viable eggs, is the fact that this genotype has lower oviposition than the other ones evaluated (although the differences observed were not significant).…”

“…The insects that fed normally did not gain more weight than the ones with low intake. Domingues et al (2006) studied the survival of C. maculatus larvae when grown on IT81 D-1045 resistant seeds and found that the larvae feeding on resistant cowpea embryos developed better than the ones feeding on the cotyledons. Using computed axial scanning (tomography), Tarver et al (2006) concluded that C. maculatus feeds differently when grown on resistant or susceptible cowpea seeds, and that there is a region in the core of the resistant seeds that has a negative effect on the insect, which avoids this area.…”