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Plant Information Superhighway
RNA Molecules May Carry Signals Long Distances
By Rosalind Schrempf
Plants may have their own "information superhighway" that uses RNA molecules to send long-distance signals throughout the organism. Researchers have discovered a protein in plants that can apparently carry large RNA molecules throughout the plant via specialized conducting cells that form a complex set of pipelines, called the phloem. The narrow networks that feed the phloem were previously thought incapable of accommodating large protein molecules. The new finding, by William Lucas, University of California, Davis, and colleagues, provides insight into a plant transport system that might control gene expression in distant cells and could also provide new clues about how information travels among plant parts such as leaves and flowers.
The researchers made their discovery by extrapolating from the behavior of plant viruses. To spread infection throughout a plant, viruses must be able to enter the phloem as large nucleic acid molecules. The viruses somehow widen the narrow access channels to the phloem using special "viral movement proteins." The team tracked a large plant protein, CmPP16, and the RNA that encodes it by applying an antibody against a viral movement protein to pumpkin phloem sap. The CmPP16 and the RNA molecules appeared to move from pumpkin cells through the accessory networks into the phloem, and later were found in a cucumber plant that had been grafted to the pumpkin plant.
These findings suggest that CmPP16 acts like a viral movement protein, carrying RNA into the phloem. Lucas and his colleagues speculate that the RNA might be part of a messenger system within the plant used by different plant parts to communicate with each other. For example, it has long been known that some substance travels from leaves to buds to signal a plant to flower in response to certain cues such as length of day. The exact identity of this messenger is unknown, and this research finding suggests it could be RNA or another large molecule.
This research was funded by the Office of Basic Energy Sciences, Energy Biosciences Division. Contact: Dr. William J. Lucas, University of California at Davis, wjlucas@ucdavis.edu.
Reference: Science, January 1, 1999, Vol. 283, No. 5398.
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 Tracking the movement of CmPP16 and RNA molecules through pumpkin phloem sap
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