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Big Freight in the Phloem

Plants shuttle their nutrients and messenger molecules through a vast network of pipes, called the phloem. Because this highway system has narrow onramps, biologists had expected the traffic to be limited to small and simple signaling molecules. But now they've found a carrier protein that can apparently truck large RNA molecules into the phloem. The finding, reported in tomorrow's Science, could help resolve some long-standing mysteries about how information travels among plant parts such as leaves and flowers.

A clue to the discovery came from viruses, which spread infection throughout a plant via large nucleic acids. To get these bulky molecules into the phloem, viruses use special "movement" proteins to widen access channels in so-called sieve cells that line the phloem tube. So a team led by plant cell biologist William Lucas of the University of California, Davis, decided to look for plant counterparts. When they applied an antibody against a viral movement protein to pumpkin phloem sap, it bound to a protein called CmPP16. The researchers then detected this protein and the RNA that encodes it in the sieve elements, which themselves have no nuclei--and thus don't produce RNA--or machinery to make proteins. That suggests that the protein and RNA had moved in from adjacent cells.

Then, by grafting a piece of a cucumber plant onto a pumpkin plant, the researchers showed that CmPP16 and its messenger RNA (mRNA) can move long distances. In phloem sap from the cucumber graft, they found the pumpkin CmPP16 and its mRNA, indicating that these large molecules had traveled into the graft. "There's a possibility that what we're looking at, in fact, is a very sophisticated situation where regulation occurs by macromolecules over very long distances," says Robert Turgeon, a plant physiologist at Cornell University.

Several caveats remain, however. The work doesn't actually show that CmPP16 and its mRNA traverse the channels that enter the phloem, and so doesn't rule out the possibility that the protein and mRNA found in the sieve elements are remnants from immature sieve elements, which do contain nuclei, notes Turgeon. And even if CmPP16 is actually trucking RNAs into the phloem for long journeys, at the moment no one is quite sure what those RNAs are doing. "What we need are good functional tests to assign significance to these proteins and nucleic acids in the phloem," says James Carrington of Washington State University, Pullman.