Jan. 5, 1999 UC Davis biologists report the discovery of an important element in the complicated internal communication and transportation systems of plants: a previously unknown "movement protein" that carries information-bearing RNA from stems and leaves to faraway roots and flowers.
The findings should provide basic insight into the evolutionary processes underlying complex plants and could lead to better defenses against crop diseases.
The Davis team, led by professor of plant biology William Lucas, is one of only a few worldwide that are unraveling how plants transport many important internal cargoes, including genetic messages that govern growth and flowering.
"This new study is very important," says Richard Jorgensen, an associate professor of plant sciences at the University of Arizona and also an expert in the field. "What they've identified is probably a component in a radically new system for communication between cells and between organs of the plant."
The current picture of the plant's transportation, or phloem, system looks something like a bustling subway. The tube- shaped sieve elements of the phloem are the subway lines, the companion cells of the sieve elements are the stations, and connecting tunnels called plasmodesmata allow cargoes to move from the stations into the subway lines.
In the Jan. 1 issue of the journal Science, the UC Davis study introduces the new factor, the movement protein.
In the cells of leaves and stems, the movement protein binds to an informative segment of genetic code called messenger RNA (mRNA). Like a subway ticket, the movement protein lets the mRNA enter the plasmodesmal tunnel to the subway line, or phloem translocation stream. Once in the subway line, the complex of movement protein and mRNA travels very rapidly to distant stations located in roots and flowers.
At its destination, the report suggests, the messenger RNA probably influences the level of some other protein. That level conveys information to local tissues about, for instance, the overall physical condition of the plant, the season of the year or the presence of an invading pathogen.
"The large, structurally complex plants we see today evolved an elaborate vascular system to carry water and the products of photosynthesis all over the organism," says Lucas.
"A parallel communication system also had to evolve, to permit such large plants to integrate events happening in distant organs, such as sugar production in leaves, reproduction in flowers, and nutrient acquisition in roots.
"Our finding supports the hypothesis that a critical element of this communication system is the transport of RNA molecules through the plant's vascular system to those distant tissues."
The new protein was named CmPP16 because it is a phloem protein, 16 kilodaltons in size, first found in the Halloween pumpkin, Cucurbita maxima.
Another interesting feature of CmPP16 is that its genetic sequence and its behavior are very much like those of a movement protein used by viruses.
"Plant viruses appear to have acquired the ability to use plant communication pathways to infect an entire plant," Lucas says. "The parallels between viral movement proteins and CmPP16 provide the first strong evidence that viruses may have acquired that ability by stealing it from plant genes."
The gene that makes the CmPP16 protein in pumpkins is also found in a wide range of other crop plants, says Lucas, and it probably functions in the same way. The Davis researchers are now trying to backtrack through plant evolution to learn when the gene first began to assist whole-plant communication.
The lead authors of the Science paper are three postdoctoral researchers in the Lucas lab: Beatriz Xoconostle-Cazares, Yu Xiang and Roberto Ruiz-Medrano. Their co-authors are three other UC Davis postdoctoral researchers -- Hong-Li Wang, Jan Monzer and Byung-Chun Yoo -- Lucas and staff research associate K.C. McFarland. Vincent R. Franceschi, professor of plant biology at Washington State University, is also an author.
The research was funded by the U.S. Department of Energy and the National Science Foundation. Ruiz-Medrano received some funding from CONACyT, the Mexican national council on science and technology.
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