Moreover, PPAEs have been reported to carry out the proteolysis
of the proPO precursor [39]. The dsRNA-mediated gene knockdown
of the proPO and PPAE transcripts significantly reduced total PO
activity and enhanced shrimp susceptibility to infection [39e41]. In
Mallotus japonicus, dsRNA-mediated gene knockdown of the proPO
reduced hemocyte count and expression of antimicrobial peptide
genes, increased bacterial load in the hemolymph and caused high
shrimp mortality [42]. In L. vannamei, PPAE gene knockdown
reduced the survival of shrimp after V. harveyi infection [43]. Our
study demonstrated that PPAE1 was down-regulated in the multiple
infection group. The down-regulation of PPAE1 would benefit
the secondary infection by bacteria for shrimp with low PO activity
or cause high shrimp mortality. Stimulation of PPAE1 or PPAE2
would lead to the activation of proPO1 and 2 (to PO1 and 2) and
result in the production of melanin and reactive oxygen compounds.
Down-regulation of PPAE1 or PPAE2 genes would significantly
decrease the expression levels of other genes related with