Dicistronic tRNA-mRNA transcripts in grapevine (Vitis vinifera) display distinct, regional expression patterns that correlate with tRNA expression.

Transfer RNAs (tRNA) are crucial adaptor molecules between messenger RNA (mRNA) and amino acids. Recent evidence in plants suggests that dicistronic tRNA-like structures can also act as mobile signals for mRNA transcripts to move between distant tissues. Co-transcription is not a common feature in the plant nuclear genome and, in the few cases where polycistronic transcripts have been found, they include the expression of non-coding RNA species such as small nucleolar RNAs and microRNA clusters. It is not known, however, the extent to which dicistronic transcripts of tRNA and mRNAs are expressed in field-grown plants, or the factors contributing to their expression. To address these questions, we analysed tRNA-mRNA dicistronic transcripts in the major horticultural crop grapevine (Vitis vinifera) using a novel pipeline developed to identify dicistronic transcripts from high-throughput RNA sequencing data. We identified dicistronic tRNA-mRNA in grapevine leaf and berry samples from 22 commercial vineyards covering six sub-regions of the Barossa wine growing region, Australia. Of the 124 tRNA genes that were expressed in both tissues, 18 tRNA were expressed forming part of 19 dicistronic tRNA-mRNA molecules. The presence and the abundance of dicistronic molecules was tissue and geographic sub-region specific. In leaf tissue, the expression patterns of dicistronic tRNA-mRNAs significantly correlated with tRNA expression, suggesting that transcriptional regulation of their expression might be linked. We also found evidence of evolutionary conservation of dicistronic candidates in grapevine, and previously reported dicistronic transcripts in Arabidopsis, indicating a syntenic genomic arrangement of tRNAs and protein coding genes between species.