Identification of Phloem-Mobile mRNA in Arabidopsis

Signaling between cells/tissues/organs is essential for multicellular organisms to coordinate and adapt their growth and development to internal and environmental changes. Plants have evolved a long-distance signaling pathway between organs via a vascular tissue, called phloem, and achieved efficient photosynthate allocation as well as intercellular communications. Phloem delivers not only small molecules such as metabolites and phytohormones but also proteins and RNAs. Recent studies on the phloem revealed that phloem proteins and small RNAs have crucial roles in physiological processes including flowering, systemic silencing and nutrient allocation. However, the role of mRNAs transported in the phloem pathway is not yet clear, even the mobility over long-distance has been well evidenced. To gain our knowledge, it is important to collect further information on the profile of mRNAs in the phloem translocation stream. Here, we identified 138 Arabidopsis transcripts as mobile mRNAs, termed the mRNA mobilome, by conducting a RNA-Seq analysis on a hetero-graft system with Nicotiana and Arabidopsis. In this system, if some of Arabidopsis transcripts move to the Nicotiana plant part across the graft union, they can be identified as distinct transcripts from those of Nicotiana based on their sequence differences. The transcripts which we identified includes potential signaling factors and, unexpectedly, more general factors. In our investigations, we found neither preferred transcript length nor previously known sequence motifs in promoter/transcript sequences. This finding would increase the understanding of the mechanisms behind long-distance signaling in plants and on how plants adapt their growth and development to the surrounding environment.