Abstract: The presence ofRNA molecules in plant phloem sieve elements which can do long-distance transport is an important discovery in plant kingdom. And it has been discovered that there are three kinds of RNA molecules in phloem sap including exogenous viroids or virus RNA，plant endogenous mRNA and small non-protein-coding RNA（snRNA）. Long-distance transport of RNAs in phloem sap can function as signal molecules in mediating plant growth and development. In this paper，the methods of how to collect phloem sap，the RNA molecules in plant phloem sap and the signaling pathways as well as the mechanisms by which how they are transported are discussed and summarized.

Key words: long-distance transport, phloem sap, virus, mRNA, miRNA, siRNA