Abstract
Studies investigating the resistance-susceptibility of crop insects to
proteins found in latex fluids have been reported. However,
latex-bearing plants also host insects. In this study, the gut
proteolytic system of Pseudosphinx tetrio, which feeds on Plumeria rubra
leaves, was characterized and further challenged against the latex
proteolytic system of its own host plant and those of other
latex-bearing plants. The gut proteolytic system of Danaus plexippus
(monarch) and the latex proteolytic system of its host plant (Calotropis
procera) were also studied. The latex proteins underwent extensive
hydrolysis when mixed with the corresponding gut homogenates of the
hosted insects. The gut homogenates partially digested the latex
proteins of foreign plants. The fifth instar of D. plexippus that were
fed diets containing foreign latex developed as well as those
individuals who were fed diets containing latex proteins from their host
plant. In vitro assays detected serine and cysteine peptidase inhibitors
in both the gut homogenates and the latex fluids. Curiously, the
peptidase inhibitors of caterpillars did not inhibit the latex
peptidases of their host plants. However, the peptidase inhibitors of
laticifer origin inhibited the proteolysis of gut homogenates. In vivo
analyses of the peritrophic membrane proteins of D. plexippus
demonstrate resistance against latex peptidases. Only discrete changes
were observed when the peritrophic membrane was directly treated with
purified latex peptidases in vitro. This study concludes that peptidase
inhibitors are involved in the defensive systems of both caterpillars
and their host plants. Although latex peptidase inhibitors inhibit gut
peptidases (in vitro), the ability of gut peptidases to digest latex
proteins (in vivo) regardless of their origin seems to be important in
governing the resistance-susceptibility of caterpillars.
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