Phloem and RNA: Identification and Characterization of Distinct Phloem-Allocated Non-coding RNA Molecules

• Applicant: Professorin Dr. Julia Kehr
• Subject Area: Plant Cell and Developmental Biology ; Plant Biochemistry and Biophysics ; Plant Physiology
• Term: from 2015 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 269649566

The general aim of the project is to study the biogenesis and function of tRNA halves allocated via the vasculature of flowering plants. In plants it is well established that the phloem serves as system-wide delivery pathway for nutrients and hormones. However, in the last decade this view was challenged by the discovery that the phloem also allocates specific RNA molecules. Such phloem delivered RNAs include messenger RNA (mRNA), silencing-induced RNA (siRNA) and micro RNA (miRNA). Furthermore, these phloem allocated RNA molecules were shown to act as important long distance signals effecting growth. In the course of our studies we identified a considerable number of full length and fragments of small non-coding RNAs (ncRNA) such as ribosomal RNA (rRNA), transfer RNA (tRNA), small nuclear RNA (snoRNA), and other unknown small ncRNA in the phloem. Although our study on the PS ncRNAs was not comprehensive with respect to the number of analyzed and identified transcripts, we uncovered an interesting bias in the tRNA pool found in the phloem exudate. A number of distinct and essential tRNAs are missing, which suggests a selective delivery into the phloem stream. In addition, we detected high amounts of specific tRNA fragments (tRNA halves) exclusively in the phloem. Emphasizing the significance of our finding we demonstrated that, in contrast to the native leaf RNA pool, the native phloem RNA pool inhibits in a non-specific fashion translation. In line with a later tRNA halves study performed on human cells, we could show that tRNA halves contribute to this inhibition. Such tRNA halves are also observed in other organisms such as yeast and human cells. A number of studies suggest that tRNA halves appear during developmental switches and under stress conditions. Apart from this a role for the tRNA halves remains to be shown in all organisms. In yeast a specialized RNase T2 and in mammalians an RNase A related enzyme, named angiogenin, cut specific tRNAs at their anticodon loop and produce tRNA halves. In plants angiogenin related RNase enzymes do not exist, however, genes encoding RNases T2 related enzymes are abundant (e.g. five in A. thaliana). Up to now these enzymes are not tested regarding their specificity towards tRNAs. Hence, the plant tRNA endonuclease(s) producing tRNA halves remains to be identified and characterized. The identification and analysis of the tRNA halves producing enzyme will provide the means to increase or decrease the production of tRNA halves in living plants and to unravel their potential role in plant growth.The proposed aims are: i) to study the function of tRNA halves allocated via the phloem, ii) to identify and functionally characterize the tRNA halves producing enzyme, and iii) to build a comprehensive database of the phloem small ncRNAs distinct from siRNA and miRNA.

• DFG Programme: Research Grants