The Phloem-Delivered RNA Pool Contains Small Noncoding RNAs and Interferes with Translation

In plants, the vascular tissue contains the enucleated sieve tubes facilitating long-distance transport of nutrients, hormones, and proteins. In addition, several mRNAs and small interfering RNAs/microRNAs were shown to be delivered via sieve tubes whose content is embodied by the phloem sap (PS). A number of these phloem transcripts are transported from source to sink tissues and function at targeted tissues. To gain additional insights into phloem-delivered RNAs and their potential role in signaling, we isolated and characterized PS RNA molecules distinct from microRNAs/small interfering RNAs with a size ranging from 30 to 90 bases. We detected a high number of full-length and phloem-specific fragments of noncoding RNAs such as tRNAs, ribosomal RNAs, and spliceosomal RNAs in the PS of pumpkin (Cucurbita maxima). In vitro assays show that small quantities of PS RNA molecules efficiently inhibit translation in an unspecific manner. Proof of concept that PS-specific tRNA fragments may interfere with ribosomal activity was obtained with artificially produced tRNA fragments. The results are discussed in terms of a functional role for long distance delivered noncoding PS RNAs.