Eicosanoid biosynthesis is activated via Toll, but not Imd signal pathway in response to fungal infection

Phospholipase A(2) (PLA(2)) catalyzes hydrolysis of phospholipids at sn-2 position and usually releases arachidonic acid, which is oxygenated into various eicosanoids that mediate innate immune responses in insects. PLA(2) activities were measured in both immune-associated tissues of hemocyte and fat body in the beet armyworm, Spodoptera exigua. Upon challenge of an entomopathogenic fungus, Beauveria bassiana, the PLA(2)s were significantly activated in both hemocyte and fat body. The fungal infection also induced gene expression of antimicrobial peptides (AMPs), such as two attacins, cecropin, gallerimycin, gloverin, hemolin, and transferrin of S. exigua. RNA interference of Toll or Imd signal pathway using double-stranded RNAs (dsRNAs) specific to SeToll or SeRelish suppressed specific AMP gene expressions, in which dsRNA specific to SeToll suppressed two attacins, cecropin, gallerimycin, gloverin, hemolin, and transferrin I, while dsRNA specific to SeRelish suppressed only cecropin. Interestingly, dsRNA specific to SeToll also significantly inhibited the activation of PLA(2) in response to the fungal infection, but dsRNA specific to SeRelish did not. Eicosanoid-dependent hemocyte nodulation was inhibited by dsRNA specific to SeToll but was not by dsRNA specific to SeRelish. These results suggest that eicosanoid biosynthesis is activated via Toll, but not Imd signal pathway in response to fungal infection in S. exigua.