The phloem's role as a tissue responsible for the distribution of photoassimilates and nutrients among the various organs of higher plants has long been recognized. Recent studies have established that numerous proteins and mRNA molecules are also present in the phloem translocation stream; however, limited information is available on the identity of transcripts present within the phloem sap. In this study, a genomic approach was taken to produce a transcription profile of melon phloem sap. A cDNA library was constructed from mRNAs extracted from melon phloem sap and 1900 clones were randomly selected for sequencing. Selection of high-quality sequences resulted in 986 unique transcripts corresponding to 1830 ESTs. A comparison between the phloem-sap library and publicly available libraries from leaves and fruits indicated that the transcript profile of phloem sap is unique, with a substantially higher proportion of genes associated with biotic stimulus, response to stress, and metal-ion binding. Manual functional analyses revealed that over 40% of the transcripts are related to stress and defence responses, while over 15% of them are related to signal transduction. Out of the 1830 ESTs, only three were characterized as coding for chlorophyll-binding protein or ribulose bisphosphate carboxylase. Heterografting experiments established that six out of 43 examined transcripts are capable of long-distance trafficking from melon stocks to pumpkin scions. Annotation of these six transcripts revealed that three of them are associated with auxin-signal transduction while the other three were not identified. The potential role of the expressed transcripts in the phloem sap is discussed.
Bibliographical noteFunding Information:
This paper is a contribution from the Uri Kinamon Laboratory. AO was supported by a scholarship from the Kinamon Foundation. DL was supported by grant No. 1424 from the Israeli Ministry of Science to COBI (Center of Knowledge Bioinformatics Infrastructure), as part of the Bioinformatics Unit of the Hebrew University of Jerusalem. This research was supported by the German Federal Ministry of Education and Research (BMBF) within the framework of German–Israeli Project Cooperation (DIP grant number E.3.1) and by the Israel Science Foundation (ISF grant number 386/06).
- Companion cells
- Cucumis melo
- cDNA library