Phloem Long-Distance Trafficking of Proteins and RNAs

Abstract Phloem sap contains a wide range of macromolecules, including ribonucleic acids (RNAs) and proteins. Recent advances in genomic and proteomic technologies have enabled the identification of thousands of these macromolecules. However, technical limitations are still the main obstacle to the complete characterization of macromolecule profiles in the sieve tube. Grafting experiments have established that numerous macromolecules are capable of moving long distances between tissues, providing the foundation for the hypothesis of an interorgan information superhighway in higher plants involving the phloem. This chapter presents the current knowledge on proteins and messenger RNA (mRNA) molecules present in the sieve tube. The challenge now is to characterize the biological role of trafficking macromolecules, such as their involvement in the signal-transduction pathway(s) elicited by environmental cues and in orchestrating developmental processes at the whole-plant level. Key concepts Phloem sap contains macromolecules such as proteins and RNAs. Protein profiles in the sieve tube are tissue specific. Phloem proteins act as long-distance signalling molecules. Plant viruses move long distance via the phloem. Endogenous RNA molecules are capable of trafficking long distance via the phloem. mRNA molecules may act as long-distance signalling molecules.