Methylammonium-resistant mutants of Nicotiana plumbaginifolia are affected in nitrate transport

Methylamine treatment changes the allocation of carbohydrate to roots in rice plants

Methylamine (MA), an ammonia analogue, has been used to investigate ammonia uptake. This compound competes with ammonia to be taken up and also inhibits the nitrate assimilation pathway. However, the effect of MA on plant growth is unknown. In this paper, we describe the responses of the rice plant to MA. The growth of MA-treated plants was inhibited in the aerial parts and stimulated in the roots. MA-treatment also induced a decrease of starch and hexose in shoots whereas hexose, sucrose and starch contents are increased in MA-treated roots. These results indicate that MA can change the mass allocation of biomass to the roots. The properties of MA suggest that a plant alters its growth via a change in the distribution of carbohydrate in resposes to the nitrogen status.

Evidence that fungal MEP proteins mediate diffusion of the uncharged species NH(3) across the cytoplasmic membrane

Methylammonium and ammonium (MEP) permeases of Saccharomyces cerevisiae belong to a ubiquitous family of cytoplasmic membrane proteins that transport only ammonium (NH(4)(+) + NH(3)). Transport and accumulation of the ammonium analog [(14)C]methylammonium, a weak base, led to the proposal that members of this family were capable of energy-dependent concentration of the ammonium ion, NH(4)(+). In bacteria, however, ATP-dependent conversion of methylammonium to gamma-N-methylglutamine by glutamine synthetase precludes its use in assessing concentrative transport across the cytoplasmic membrane. We have confirmed that methylammonium is not metabolized in the yeast S. cerevisiae and have shown that it is little metabolized in the filamentous fungus Neurospora crassa. However, its accumulation depends on the energy-dependent acidification of vacuoles. A Deltavph1 mutant of S. cerevisiae and a Deltavma1 mutant, which lack vacuolar H(+)-ATPase activity, had large (fivefold or greater) defects in the accumulation of methylammonium, with little accompanying defect in the initial rate of transport. A vma-1 mutant of N. crassa largely metabolized methylammonium to methylglutamine. Thus, in fungi as in bacteria, subsequent energy-dependent utilization of methylammonium precludes its use in assessing active transport across the cytoplasmic membrane. The requirement for a proton gradient to sequester the charged species CH(3)NH(3)(+) in acidic vacuoles provides evidence that the substrate for MEP proteins is the uncharged species CH(3)NH(2). By inference, their natural substrate is NH(3), a gas. We postulate that MEP proteins facilitate diffusion of NH(3) across the cytoplasmic membrane and speculate that human Rhesus proteins, which lie in the same domain family as MEP proteins, facilitate diffusion of CO(2).

Nitrate transport: a key step in nitrate assimilation

The nitrate assimilation pathway has been the matter of intensive research during the past decade. Many genes involved in low and high affinity nitrate uptake have been identified in fungi, algae and, more recently, in plants. The plant genes so far isolated are transcriptionally regulated; their inducibility by nitrate seems to be a common feature, shared by their homologs in fungi and algae. A number of questions remain to be elucidated regarding the physiological roles of these transporters and the regulation of their expression.