Small tandem target mimic-mediated blockage of microRNA858 induces anthocyanin accumulation in tomato

Main conclusion: Our work strongly suggests that microRNA858 regulates anthocyanin biosynthesis in tomato by modulating the expression of two R2R3 MYB transcription factors, underscoring the importance of microRNAs in the gene regulatory network controlling specialized metabolism in plants. The biological functions of microRNA858 (miR858), a recently identified small RNA, are not well understood. Here, we identified miR858 as a negative regulator of anthocyanin biosynthesis in tomato (Solanum lycopersicum). RNA ligase-mediated 5′RACE cleavage assay showed that miR858 mediates the cleavage of SlMYB7-like and SlMYB48-like transcripts in tomato. Expression analysis revealed an inverse correlation between the accumulation of miR858 and its target SlMYB7-like mRNA, in different tissues of tomato. Subsequently, a small tandem target mimic construct for blocking miR858 (STTM858) was generated and transformed into tomato. The majority of endogenous miR858 was blocked in STTM858 over-expressing tomato plants, whereas SlMYB7-like transcripts increased significantly. Concomitantly, upregulated expression was detected for several anthocyanin biosynthetic genes, including PAL, CHS, DFR, ANS and 3GT. As a result, anthocyanins were highly accumulated in young seedlings, leaves, stems and leaf buds of the transgenic plants under normal growth conditions. In addition, over-expression of STTM858 in tomato activated another MYB transcription factor, SlMYB48, implicating the possible involvement of SlMYB48 in anthocyanin biosynthesis.