Role of carbohydrates in insect resistance in Phaseolus vulgaris

The bruchid beetles Acanthoscelides obtectus (Say) and Zabrotes subfasciatus Boheman, are major storage pests of the economically important legume Phaseolus vulgaris (haricot bean). In the present study, resistance of a wild line of P. vulgaris G12953, towards these two insects is confirmed. Mature seeds from both resistant and susceptible lines were fractionated and tested in feeding trials in an attempt to elucidate the basis of resistance. The heteropolysaccharide fraction isolated from the resistant line and a susceptible line were each incorporated into artificial beans over a concentration range up to 10% dry wt. At a concentration of 4%, the approximate physiological concentration within the seed, the heteropolysaccharide fraction from the resistant line was very toxic resulting in 80–85% larval mortality of A. obtectus with an LC₅₀ of 2.5%. Furthermore, surviving larvae showed a marked increase in their development period. The corresponding fraction from a susceptible line, on the other hand, even at twice the physiological concentration, had a negligible effect upon larval development both in terms of adult emergence and development period. The effects of albumin proteins and globulin proteins on larval development of A. obtectus were also tested by incorporation into artificial beans at a range of concentrations up to 10%. Whilst those from the susceptible line had little effect upon development both the albumins and globulins from the resistant line were found to cause a slight decrease in adult emergence and a small increase in the median development period. These effects, however, are not sufficient to account for the observed seed resistance of G12953. Mature seeds from the resistant line and nine susceptible lines were also screened for the following secondary plant compounds: proteinase inhibitors, α-amylase inhibitors, lectins (phytohaemagglutinins), saponins, tannins and alkaloids. Although some of these antimetabolites were found to be present none, except for α-amylase inhibitors, were thought to play any significant role in conferring seed resistance. It is concluded that this example of insect resistance in seeds of P. vulgaris, line G12953, is due, at least in part, to the presence of a heteropolysaccharide which has an unusually high arabinose and fucose content. In the absence of other antimetabolic compounds the levels of this heteropolysaccharide present in G12953 are sufficiently high to account for the observed resistance. However, it is possible regarding seed resistance towards Z. subfasciatus, that α-amylase inhibitors may be having a synergistic effect.