Researchers have found hints that the channels by which such molecules move between plant cells may also be mirrored in animal cells. Thus, discoveries about plant development may be more broadly applicable to understanding the fundamental processes of how complex tissues develop from a few cells -- one of the central mysteries in biology.
Duke University biologist Philip Benfey and his colleagues published their latest findings in the Oct. 26, 2004, issue of the journal Current Biology. Besides Benfey, other co-authors are Kimberly Gallagher of Duke, Alice Paquette of New York University and Keiji Nakajima of the Nara Institute of Science and Technology in Japan. Their research was supported by the National Institutes of Health.
In their studies, the researchers sought to understand details of how a protein made by a gene called Short-Root travels from one cell to another in the developing plant root. In previous studies, Benfey and his colleagues made the surprising finding that the Short-Root protein is one means by which one root cell "talks" to another to instruct it to develop in a certain way.
Short-Root is so named because genetic mutations that generate a non-functional protein produce plants with stunted roots. The Short-Root protein is a transcription factor, a protein that acts as a master controller of a multitude of genes.
The Arabidopsis plant on which they experimented is a widely used model in plant biology research, and its genetics and biology have been thoroughly studied. The plant's root is an excellent model for studying tissue development because -- unlike the impossibly intricate convolutions and migrations of developing animal bodies -- each new Arabidopsis root cell arises conveniently from its neighbor and maintains the same position throughout development.
"Until now, our hypothesis that there was movement of the Short-Root protein between cells was based on a discordance between where the protein acted and where it was originally made, which was in a different cell," said Benfey. "However, very little was known about how or whether this movement was controlled."
The scientists theorized that the protein moved from cell to cell through channels called plasmodesmata that exist between plant cells. Once it reaches its target cell, the protein migrates to the cell's nucleus -- the site of the cell's genetic material -- where it exerts its effects. Similar channels, called "nanotubes," have only recently been discovered in animal cells.
The question, said Benfey, was whether the movement of the Short-Root protein was simple random diffusion between the two cells' cytoplasm -- their liquid interior -- or whether transport occurred via a controlled process.
To follow the transport of the protein, Benfey and his colleagues used a tracer molecule that enabled them to pinpoint the Short-Root protein in living root cells. These studies indicated that in the cells from which the protein moves, it exists in the cytoplasm not associated with any molecular complex. However, the tracer did not reveal that the protein was moving from one cell to another.
The researchers also found that they could disrupt normal movement of the protein by genetically mutating its gene at only a single point. This discovery implied that the protein required a transport machinery to move from one cell to the other. The mutation presumably disrupted the protein's ability to dock with that machinery.
"So, we believe we've shown that there is an active process that recognizes signals, and it's not just a matter of the protein being cytoplasmically localized," said Benfey. "We've shown that, while cytoplasmic localization is essential for movement, it's not sufficient. We're left with this conclusion that the mutated protein can't move because there's a disruption in some interaction that facilitates movement."
While the latest finding represents only the earliest hints of a mechanism by which the protein moves from cell to cell, it offers a promising pathway for further exploring that machinery, said Benfey.
"Next, we're systematically cutting up and changing the Short-Root protein, to identify those regions that are just required for movement," said Benfey. Thus, he said, the researchers hope ultimately to reveal the machinery by which the developing cells talk to one another in the critical process of generating a complex tissue from individual cells.
More broadly, said Benfey, there are parallels between plants and animals in such signaling. "While the process we are studying could be a highly specialized, unique process to plants, there are indications that similar processes could be occurring in developing animal cells. Such similarities could extend the significance of our work beyond plants," he said.