Trophic microRNA: Precursor and mature microRNA ingestion downregulates the target transcripts and hampers larval development in Plutella xylostella

MicroRNAs (miRNAs) are post-transcriptional gene regulators. In the miRNA pathways cytoplasmic part, the miRNA is processed from a hairpin-structured precursor (pre-miRNA) to a double-stranded (ds) mature RNA and ultimately to a single-stranded mature miRNA. In insects, ingesting these two ds forms can regulate the target gene expression; this inspired the trophic miRNAs use as a functional genomics and pest management tool. However, systematic studies enabling comparisons of pre- and mature forms, dosages, administration times, and instar-wise effects on target transcripts and phenotypes, which can help develop a miRNA administration method, are unavailable due to the different focuses of the previous investigations. We investigated the impact of trophically delivered Px-let-7 miRNA on the Brassicaceae pest, Plutella xylostella, to compare the efficacies of its pre- and ds-mature forms. Continuous feeding on the miRNA-supplemented diet suppressed expressions of FTZ-F1 and E74, the target ecdysone pathway genes. Both the pre-let-7 and mature let-7 miRNA forms similarly downregulated the target transcripts in all four larval instars. Pre-let-7 and let-7 ingestions decreased P. xylostellas larval mass and instar duration and increased mortality in all instars, exhibiting adverse effects on larval growth and development. Pre-miRNA processing Dicer-1 was upregulated upon pre-let-7 ingestion, indicating the miRNA uptake by the midgut cells. The scrambled sequence controls did not affect the target transcripts, suggesting the sequence-specific targeting by the mature miRNA and hairpin cassettes non-involvement in the target downregulation. This work provides a framework for miRNA and target gene function analyses and potentiates the trophic miRNAs utility in pest management.