A key protein in the "information superhighway" of higher plants has been identified by researchers at the University of California, Davis. The work opens a window into understanding how plants function as supercellular organisms, said William Lucas, a professor of plant biology at UC Davis and senior author on the paper.

Sap, or phloem, carries water and nutrients between leaves, roots and other parts of a plant, much like blood circulation in animals. But it also carries information in the form of proteins and pieces of RNA. Those messages control activity in different parts of the plant, switching genes on and off and telling flowers and leaves when and how to grow. Proteins and RNA move from cells to the phloem and back through specialized channels called plasmodesmata.

Jung-Youn Lee, Byung-Chun Yoo, Marla Rojas and Lucas at UC Davis with Natalie Gomez-Ospina and L. Andrew Staehelin at the University of Colorado, Boulder, isolated the protein called Non-Cell Autonomous Pathway Protein 1 (NCAPP-1) by using a common sap protein to trap it from cell abstracts. They then showed that NCAPP-1 bound to about a third of the 600 proteins commonly found in the phloem.

NCAPP-1 is found inside cells and shuttles proteins made in the cell to the plasmodesmata, where they enter the main transport system.

Plants treated with a defective version of NCAPP-1 or with an altered NCAPP-1 gene did not grow properly, with flowers, leaves and other structures misshaped or missing altogether. The defective protein apparently competed with the normal NCAPP-1 for binding to phloem proteins but was then unable to transport them.

The results open a window for further study of this transport system, Lucas said.

"Long distance movement of information in plants is key to understanding how plant physiology is regulated," he said.

The paper is published in the Jan. 17 issue of Science.