Functional analysis of dopa decarboxylase in the larval pupation and immunity of the diamondback moth, Plutella xylostella

The diamondback moth (Plutella xylostella L.), a notorious pest infesting cruciferous vegetables worldwide, has developed a high level of resistance to various commonly used chemical pesticides. In this paper, we explore whether dopa decarboxylase (DDC), which is essential for survival and development in insects, could be used as a potential target for the control of P. xylostella. Here, the full-length cDNA (PxDDC) of P. xylostella was identified, with a complete open reading frame of 1434 bp in length, encoding a protein of 477 amino acids. The temporal and spatial expression analysis showed a periodical expression pattern of PxDDC during molting, reaching a peak during the process of pupation, and it was found to be highly expressed in the epidermis of prepupal stage, indicating a crucial role of PxDDC in larval-pupal metamorphosis of P. xylostella. Subsequently, there was a significant decreasing in pupation and eclosion rates, and less production of melanin in P. xylostella after the disruption of PxDDC function by the injection of dsPxDDC (RNAi, RNA interference) or feeding a larval diet supplemented with L-α-methyl-DOPA (L-α-M-D) as DDC inhibitor. In addition, we found four antimicrobial peptide genes were significantly inhibited after feeding P. xylostella with L-α-M-D, and the injection of Escherichia coli could significantly increase insect mortality of enzyme inhibitor treated P. xylostella, suggesting PxDDC was involved in immune responses as well. In summary, these results confirm that PxDDC is required for larval-pupal metamorphosis and immunity of P. xylostella, suggesting a critical potential future novel insecticide target for RNAi based pest control.