Two entomopathogenic bacteria, Xenorhabdus and Photorhabdus, are known to be able to synthesize and secrete eicosanoid biosynthesis inhibitors (EIBs) that can enhance pathogenicity of Bacillus thuringiensis (Bt) against different target insects. Such enhancements can be explained by the suppression of immune responses in the hemocoel by EIBs. However, little is known about the role of EIBs in the defense against Bt pathogenicity in the gut. This study was focused on the role of insect gut immunity in the defense against Bt pathogenicity, in which the cooperative effect of bacterial metabolites was assessed. Screening 14 different bacterial strains, bacterial culture broth of Photorhabdus temperata subsp. temperata ANU101 (Ptt) gave the highest cooperative effect on Bt virulence along with significant inhibitory activity against phospholipase A 2 (PLA 2) of Plutella xylostella. In gut lumen, Ptt culture broth suppressed the generation of reactive oxygen species (ROS) induced by Bt treatment and facilitated bacterial growth, similar to vitamin E, an antioxidant. To analyze the ROS source, dual oxidase (Px-Duox) and NADPH-dependent oxidase (Px-Nox) genes were predicted from P. xylostella genome and their expressions were confirmed in larval gut. RNA interference (RNAi) of Px-Duox expression reduced ROS levels in both gut epithelium and lumen while RNAi of Px-Nox expression reduced ROS levels only in gut epithelium. Ptt extract significantly suppressed gene expression levels of Px-Duox and Px-Nox, leading to lower ROS concentrations in the gut lumen. Three commercial PLA 2 inhibitors significantly increased the insecticidal activity of Bt by suppressing ROS levels in the gut lumen. These results indicate that Ptt extract containing EBIs can prevent up-regulation of ROS level in the midgut in response to Bt infection and enhance the virulence of Bt against P. xylostella.

Vatanparast M., Hosseininaveh V., Ghadamyari M., Minoo Sajjadian S. (2014): Plant cell wall degrading enzymes, pectinase and cellulase, in the digestive system of the red palm weevil, Rhynchophorus ferrugineus (Coleoptera: Curculionidae). Plant Protect. Sci., In digestion, the red palm weevil, Rhynchophorus ferrugineus, has been adapted to overcome the plant cell wall barrier, specially lignocellulosic and pectic compounds, by producing cellulase and pectinase enzymes. Partial biochemical characterisations of cellulase and pectinase were determined in the larval digestive system of the pest. Larval midgut extract showed an optimum activity for cellulase and pectinase against carboxyl methyl cellulose and pectin at pH 6.0 and 7.0, respectively. Larval midgut cellulase and pectinase were more stable at pH 4.0-8.0 and pH 6.0-8.0 than in highly acidic and alkaline condition, respectively. However, cellulase and pectinase showed to be more stable at pH 6.0 and 7.0, respectively, when the incubation time increased. Maximum activity for cellulase and pectinase incubated at different temperatures was observed at 50°C. Cellulase and pectinase activity significantly decreased in the presence of EDTA and SDS. On the contrary, Ca 2+ , Mg 2+, and Na + significantly affect pectinase activity and K + did not affect the enzyme activities. Ca 2+ and Mg 2+ increased cellulase activity as well. K M and V max for pectinase activity were 0.92 mg/ml and 290 units/mg. Zymogram analyses revealed the presence of one form of pectin methyl esterase and one form of cellulase in the larval digestive system.

Phospholipase A 2 (PLA 2 ) hydrolyzes fatty acids from phospholipids at the sn-2 position. Two intracellular PLA 2 s, iPLA 2 A and iPLA 2 B, have been found in Spodoptera exigua.

No abstract

No abstract