Exogenous application of non-mature miRNA-encoded miPEP164c inhibits proanthocyanidin synthesis and stimulates anthocyanin accumulation in grape berry cells
Secondary metabolic pathways in grape berries are tightly regulated by an array of molecular mechanisms, including microRNA-mediated post-transcriptional regulation. As recently discovered, before being processed into mature miRNAs, the primary transcripts of miRNAs (pri-miRNAs) can encode for small miRNA-encoded peptides (micropeptides - miPEPs) that ultimately led to an accentuated downregulation of the respective miRNA-targeted genes. Although few studies about miPEPs are available, the discovery of miPEPs reveals a new layer of gene regulation at the post-transcriptional level and may present a key advantage in agronomy. Here, we identified a miPEP encoded in non-mature miR164c putatively targeting grapevines transcription factor VvMYBPA1 (miPEP164c/miPEP-MYBPA1), a positive regulator of key genes in the proanthocyanidin-biosynthetic pathway, one that competes directly for substrate with the anthocyanin-biosynthetic pathway. Thus, the objective of this work was to test the hypothesis that the exogenous application of miPEP164c (miPEP-MYBPA1) can modulate the secondary metabolism of grape berry cells by inhibiting PA biosynthetic pathway while simultaneously stimulating anthocyanin synthesis. The exogenous application of miPEP164c to suspension-cultured cells from grape berry (cv. Gamay) enhanced the transcription of its corresponding pri-miR164c, thus leading to a more pronounced post-transcriptional silencing of its target VvMYBPA1. This led to a significant inhibition of the proanthocyanidin pathway, mostly via inhibition of leucoanthocyanidin reductase and anthocyanidin reductase enzymatic activities and VvLAR1 downregulation. In parallel, the anthocyanin-biosynthetic route was stimulated. Anthocyanin content was 31 % higher in miPEP164c-treated cells, in agreement with the higher activity of VvUFGT and the corresponding VvUFGT1 transcripts.