Accumulation of BEL1-like transcripts in solanaceous species

Although numerous RNAs have been detected in the phloem, only a few have been confirmed to move long distances. In potato, full-length mRNA of the BEL1-like transcription factor, StBEL5, moves from leaf veins through the phloem to stolon tips to activate tuber formation. BEL1-like transcription factors are ubiquitous in plants and interact with KNOTTED1-types to regulate numerous developmental processes. To explore the range of KNOTTED1- and BEL1-like mRNAs present in phloem, an analysis of the transcript profile of phloem sap was undertaken. Using a modified technique for the collection of phloem-enriched exudate from excised stems, numerous RNAs encoding these transcription factors were detected in the phloem sap from several solanceous species. All seven known BEL1-like RNAs of potato were detected in the phloem-enriched exudates of stem, whereas several stolon-abundant RNAs were not. After refining the technique to minimize the contamination from RNA arising from wounded cells, KNOTTED1-like RNAs were detected in phloem-enriched sap of potato and BEL5 RNA was detected in the sap collected from two closely related nontuber-bearing potato species and tomato. BEL5 RNA was also detected in RNA extracted from leaf veins of tobacco. The detection of these full-length mRNAs from the KNOTTED1- and BEL1-like families in phloem sap indicates that their potential role as long-distance signals seems to be much more extensive than previously known.