| Baculoviruses are arthropod-specific, double-stranded DNA viruses, with potential for use in insect pest management. Modern baculovirology is driven by the genetic enhancement of their insecticidal properties. A recombinant baculovirus (AcMLF9.ScathL) that expresses a cathepsin L-like protease, ScathL, kills larvae of the tobacco budworm Heliothis virescens (Fabricius) significantly faster than the wild type Autographa californica multiple nucleopolyhedrovirus (AcMNPV C6). AcMLF9.ScathL triggers melanization and tissue fragmentation shortly before death of infected larvae. To investigate the tissue specificity of ScathL expressed by AcMLF9.ScathL, we used light microscopy, transmission electron microscopy and scanning electron microscopy to examine the tissues of insects infected with AcMLF9.ScathL, with a virus expressing a catalytically inactive form of ScathL, AcMLF9.ScathL.C146A, or wild type virus AcMNPV C6 as control treatments. We found damage to the basement membrane overlaying the midgut, fat body and muscle fibers in larvae infected with AcMLF9.ScathL, but not in larvae infected with the control virus AcMLF9.ScathL.C146A, or the wild type virus AcMNPV C6. We injected yeast-expressed ScathL and can conclude that ScathL results in damage to the basement membrane and subsequent loss of tissue integrity. At high concentrations, ScathL results in complete loss of the gut. Loss of the gut may be an indirect effect resulting from lysis of cells that have lost their overlaying basement membrane. Because AcMLF9.ScathL triggers melanization shortly before death of the host insect, an alternative hypothesis is that larval death results from production of cytotoxic free radicals produced during the melanization process. |
