New mechanism for nutrient uptake discovered

Stanford, CABiologists at the Carnegie Institution's Department of Plant Biology have discovered a new way that plant cells govern nutrient regulationneighboring pore-like structures at the cell's surface physically interact to control the uptake of a vital nutrient, nitrogen. It is the first time researchers have observed that the interaction of neighboring molecules is essential to this regulation. Since plants, animals, bacteria, and fungi all share similar genes for this activity, the researchers think that the same feature could occur across species. The discovery, reported in the February 11th on-line edition of Nature, has widespread potentialfrom understanding human diseases, such as kidney function, to engineering better crops.

"Every cell in every organism has a system for bringing in nutrition and expelling waste," explained lead author Dominique Loqu. "Some are through pore-like protein structures called transporters, which reside at the surface of the cell's outer membrane. Each pore is capable of transporting nutrients individually, so we were really surprised to find that the pores simply can't act without stimulation from their neighbors".

In earlier research the Carnegie scientists, with colleagues, identified the genes responsible for initiating nitrogen uptake in plants. That identification has helped other scientists find the relatives of these genes in a variety of species from bacteria to humans. In this study, the researchers wanted to identify how ammonium transport is regulated.

Plants import nitrogen in the form of ammonium from the soil. The scientists observed that the end portion, or so-called C-terminus, of the protein Arabidopsis ammonium transporter AtAMT1;1, located at the surface of the cell membrane, acts as a switch.

"The terminus is an arm-like feature that physically grabs a neighboring short-chain molecule, binds with it, and changes the shape of itself and its neighbor thereby activating all the pores in the complex," continued Loqu. "The pores can't function without this physical stimulation".

"The rapid chain-reaction among the different pores allows the system to shut down extremely fast and can even memorize prior exposures," noted co-author Wolf Frommer. "Imagine a large animal marking its territory. A sudden flow of ammonia could be toxic to the plant. If it weren't for a rapid-fire shutdown plants could die. The conservation of this feature in the related transporters in bacteria, fungi, plants, and animals suggests that an ancient organism, which was a precursor to all known organisms on Earth, had developed this feature because there was much more ammonia on the early Earth. The ubiquitous presence of this structure in all of the known ammonium transporters suggests that the regulation is still necessary today for all of these organismscyanobacteria in the ocean, fungi that grow on grapes and make our wine, plants that provide our foodand even in our kidneys, which excrete nitrogen. We also suspect other different types of transporters will be discovered to work in this way".

The researchers don't yet know what triggers the rapid shut-off. They think it might be a very common regulatory event called phosphorylation, where a phosphate molecule is introduced to another molecule, changing the latter, and preparing it for a chemical reaction. They have found a site for phosphorylation and are looking at this possibility further.

A leading expert in transporters, Professor Dale Sanders, head of the biology department at the University of York in the U.K. commenting on the work said: "Loqu, Frommer and co-workers have demonstrated very beautifully how plant ammonium transporters are controlled. A switch domain in the protein facilitates rapid and sensitive control of ammonium transport to preclude over-accumulation of an ion that is beneficial at low concentrations, but potentially toxic at high concentrations. This is a major advance in the field of plant mineral nutrition."


Posted by: Erica    Source