"The rapid chain-reaction among the different pores allows the system to shut down extremely fast and can even memorize previous 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 organisms—cyanobacteria in the ocean, fungi that grow on grapes and make our wine, plants that provide our food—and even in our kidneys, which excrete nitrogen. We also suspect other different types of transporters will be discovered to work in this way."
The scientists 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: "Loqué, 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 ma