Biochemical basis of insect resistance in Vigna unguiculata

Abstract

The bruchid beetle Callosobruchus maculatus (F.) causes extensive damage to seeds of the cowpea, Vigna unguiculata (L.) (Walp.), when this important tropical foodstuff is stored. A variety of cowpea resistant to attack by this pest has been described. In the present work seeds of a number of cowpea varieties, including the resistant one, were tested for the presence of a physical resistance to C. maculatus, in terms of repulsion of oviposition or of failure of larvae to enter the seeds. No evidence to suggest the presence of a physical resistance was found. When seeds of cowpea varieties were tested for the presence of various antimetabolic secondary compounds, only inhibitory activity against trypsin and, to a much lesser extent, chymotrypsin, could be detected. The resistant variety of cowpea contained a significantly higher level of inhibitors, about twice as much as any other variety. A proteinase inhibitor active against trypsin was purified from cowpea varieties by affinity chromatography on trypsin-Sepharose. The purified inhibitor was shown to inhibit chymotrypsin also, in such proportions as to account for chymotrypsin inhibition by seed extracts. The inhibitor was shown to consist of a number of isoinhibitors by gel electrophoresis and isoelectric focusing, but no qualitative differences in the inhibitor between varieties could be detected. The antimetabolic nature of the cowpea trypsin inhibitor was confirmed by insect feeding trials in which various protein fractions were added to a basic meal and the effect on larval survival noted. The albumin proteins of cowpea (containing the trypsin inhibitors) at a level of 10% were toxic to larvae of C. maculatus whereas the globulin fractions were not. Further, if cowpea trypsin inhibitor was removed from the albumin proteins they ceased to be toxic. When purified cowpea trypsin inhibitor was added to the basic meal it was shown that a level slightly less than that found in the resistant variety of cowpea caused complete mortality of larvae, whereas lower levels had lesser or no effect. It is concluded that this example of insect resistance in the cowpea is due to an elevated level of trypsin inhibitor.

Variant vicilins from a resistant Vigna unguiculata lineage (IT81D-1053) accumulate inside Callosobruchus maculatus larval midgut epithelium

“…In the 1970's, screening of the cowpea germplasm collection at the International Institute of Tropical Agriculture (Ibadan, Nigeria), allowed the identification of the first genotype highly resistant to C. maculatus, the cultivar TVu 2027 (Gatehouse et al, 1979), from which several other lineages such as IT81D-1032, IT81D-1053, IT81D-1045, and IT81D-1064 show different levels of resistance to the cowpea beetle were bred. The observed antibiosis was firstly attributed to relatively high levels of trypsin inhibitors (Gatehouse et al, 1979), but subsequently it was demonstrated that the resistance was in fact caused by the presence of variant vicilins (Macedo et al, 1993).…”

“…The observed antibiosis was firstly attributed to relatively high levels of trypsin inhibitors (Gatehouse et al, 1979), but subsequently it was demonstrated that the resistance was in fact caused by the presence of variant vicilins (Macedo et al, 1993). The defensive role of variant vicilins against the cowpea beetle was first reported from the genotype IT81D-1045, that is derived from the cultivar TVu 2027 and is also resistant to C. maculatus (Macedo et al, 1993).…”

Resistência de genótipos de feijão-caupi ao ataque de Callosobruchus maculatus (Fabr., 1775) (Coleoptera: Chrysomelidae: Bruchinae)

“…No presente trabalho, a baixa viabilidade de ovos no genótipo TE96-290-12G, em teste sem chance de escolha, deve-se provavelmente à presença de substâncias inibidoras da alimentação de C. maculatus, como exemplo a presença de proteína arcelina nos grãos deste genótipo. A presença de substâncias inibidoras de alimentação em carunchos é relatada na literatura, a exemplo da arcelina que confere resistência a Z. subfasciatus em feijoeiro (oriAni; lArA, 2000) e inibidores de tripsina responsáveis pela antibiose em alguns genótipos de feijão-caupi (GAtehouSe et al, 1989).…”

The serine protease inhibitors and plant-insect interaction

“…In a similar study, Gatehouse et al (1979) suggested that the resistance in a crop of caupi (Vigna unguiculata) to the attack of the weevil Callosobruchus maculatus was due to the high concentration of inhibitors in this cultivar.…”