Plants, animals share signaling system

For the first time, scientists have provided concrete evidence that endosome-mediated signaling occurs in plants, not just in animals, according to a new report in Genes and Development.

"The fact that both [plants and animals] share some similarities in the endosomal signaling system means that this system is either much older than we could have ever assumed, or that plants have independently evolved the same solution to the same problem, a scenario that I favor," study author Niko Geldner of The Salk Institute in La Jolla, California, told The Scientist.

Plants and animals evolved very different receptor sets since their split more than a billion years ago. In the past, researchers considered receptor endocytosis as purely a mechanism of signal inactivation and down regulation. However, research in animals in the last decade has demonstrated that some ligand-bound receptors actually need to be internalized and travel in endosomes for signaling, Sacco de Vries, from Wageningen University in the Netherlands, who was not involved in this study, told The Scientist.

One benefit of endosomes is that the signal travels in compartments that can reach more specific targets, said Geldner.

More recent studies using protoplasts -- cells whose walls have been digested -- have hinted that endosome-mediated signaling also exists in plants, but the limitations of the protoplast system prevented researchers from confirming their results. "In the protoplast studies, [researchers] could image the receptor, but they couldn't activate it, or inactivate it, or show that signaling was actually happening," Geldner said.

In the current study, Geldner and his colleagues studied the growth-promoting brassinosteroid receptor (BRI1) in the plant Arabidopsis thaliana. They followed the ligand-bound receptor from the plasma membrane as it was internalized, and looked at how the receptor became activated in different localizations in the cytoplasm. "We found the receptor not only in the plasma membrane but also in endosomes, and it was always residing in endosomes, no matter if it was activated or not," Geldner said.

The researchers tagged the receptor with fluorescent protein and followed it in the root meristem cells using live-imaging without dissection. In several assays, they connected receptor localization to signaling in the intact root. "I was able to do it only because the receptor's signals and transduction pathways had been rigorously studied by Joanne Chory, the senior author of the paper," Geldner said. "I was able to take the ligand away, ask where the receptor was, give the ligand back, and ask where the receptor was then," he explained.

When they increased the localization of the receptor in endosomes, the activation of the pathway and the genomic responses were enhanced, which showed that the receptor localized in endosomes was active and signaling.

"This is a very important demonstration of the functionality of endocytosis in plants," de Vries said. "It's the first piece of evidence that endocytosis is functional in the transduction chain in which the brassinosteroid receptor is participating," he added.

"The nicest thing about this paper is that it shows that there are certain similarities in what happens to receptors when they are internalized in plant and in animal systems," David Robinson of the University of Heidelberg, Germany, who did not participate in this research, told The Scientist.

In recent years the idea of endosomal signaling was met with skepticism by classic plant physiologists, Robinson said. Endosome mediated signaling "is an emerging field in plant cell biology," he added. "The only trouble now is putting the pieces together."

Graciela Flores
mail@the-scientist.com

Links within this article

N. Geldner et al., "Endosomal signaling of plant steroid BRI1 receptor kinase BRI1, Genes and Development, July 1, 2007 (ahead of print).
http://www.genesdev.org/

A.V, Vieira et al., "Control of EGF receptor signaling by clathrin-mediated endocytosis," Science 274(5295): 2086-2089, 1996.
ttp://www.the-scientist.com/pubmed/8953040

W. Wunderlich et al., "A novel 14-kilodalton protein interacts with the mitogen-activated protein kinase scaffold mp1 on a late endosomal/lysosomal compartment," J Cell Biol 152(4): 765-776, 2001.
http://www.the-scientist.com/pubmed/11266467

Sacco de Vries
http://biochemistry.wur.nl/Biochem/research/sigtrans.htm

E. Russinova et al., "Heterodimerization and endocytosis of Arabidopsis brassinosteroid receptors BRI1 and AtSERK3 (BAK1)," Plant Cell, 16: 3216-3229, 2004
http://www.the-scientist.com/pubmed/15548744

S. Pincock, "Arabidopsis in space," The Scientist, November 1st, 2006.
http://www.the-scientist.com/article/display/25140/

C. Bishop, "Auxin regulated root growth," The Scientist, February 13th 2003.
http://www.the-scientist.com/article/display/21103/

Joanne Chory
http://www.hhmi.org/research/investigators/chory_bio.html

David G. Robinson
http://www.zmbh.uni-heidelberg.de/kolleg/kolleg-zellbio/Robinson.html