Arabidopsis 2010: The role of nutrient sensing and signaling for ammonium nutrition in plants

Carnegie Institution Of Washington, Washington DC

Investigators

Abstract

The uptake of essential nutrients in the form of ions is critical for plant growth. Approximately 70% of all ions acquired by plants contain nitrogen. Thus quantitatively, nitrogen is the most important nutrient. In agriculture, nitrogen derives from mainly fertilizer application, (100-250 kg ha-1 a-1). Fertilizer production is costly, energy-consuming, and causes significant run-off with dramatic negative environmental and health impact. A better understanding of nitrogen acquisition in plants may help to engineer plants with improved nitrogen efficiency or optimize fertilization practice. Nutrient acquisition requires a fine balance between maximal nitrogen uptake and minimization of toxicity. The two dominant nitrogen forms are ammonium and nitrate. Ammonium can cause toxicity, thus requiring tight control over uptake and conversion efficacy inside cells. The molecular nature of ammonium transporters remained unknown for many decades. The PIs lab, together with B. André, Brussels, identified the founding members of the ammonium transporter family (AMT/Mep/Rh). Plant AMTs helped identifying the long sought-for bacterial and human counterparts (Rhesus factors). Bacterial and fungal AMTs have function as transceptors, dual function proteins mediating transport and sensing. AMTs integrate information on ammonium levels, cellular energy supply, and availability of precursors for assimilation of ammonium. The PIs lab found that AMTs form trimeric complexes and are subject to a novel allosteric regulatory mechanism involving the cytosolic C-terminus as a trans-regulatory domain. The unique trans-regulation in a complex of three similar/identical proteins is regulated by extracellular ammonium (and potentially other factors), and thus may be key to protecting against ammonium toxicity by fine-tuning ammonium accumulation. Given the role of the bacterial AMT counterparts in signaling, it is conceivable that plant AMTs are involved in sensing and signal integration as well. The objective of this project will be to unravel the regulatory mechanisms that control ammonium uptake, its integration with carbon- and energy status, and identify mechanisms that help protecting against ammonium toxicity. The proposal has five specific aims: Aim 1 uses Next Generation Sequencing and phosphoproteomics to identify plant responses to ammonium exposure. Aim 2 will study the role of the regulatory AMT1 C-terminus in sensing and regulation. Under Aim 3, yeast will be used to test for regulatory systems controlling ammonium transport. Aim 4 will attempt to develop genetically encoded Förster resonance energy transfer sensors for ammonium and alpha-ketoglutarate (a technology pioneered by the PIs lab), and to deploy such sensors to monitor ammonium in vivo in wild type and transporter mutants. Finally in collaboration with crystallographers, Aim 5, will attempt to generate structures of bacterial and plant AMTs. The project aims at identifying allosteric control mechanisms and feed-back loops as key elements of integration of signaling in plant nutrition. The project applies state of the art technology (Nextgen sequencing and biophysical tools, i.e. genetically encoded FRET sensors). The long-term goal will be to learn how plants integrate information on the availability of inorganic and organic nitrogen forms and the nitrogen status of the plant. It will provide a basis for learning how plants render decisions on prioritization of nitrogen forms, soil exploration and root architecture, key processes for optimal growth in a spatially and temporally complex soil system. Broader impacts: The insights gained are expected to be relevant to improvement of use of fertilizer and prevention of damage by nutrients in agricultural and forest ecosystems. The project will provide training for high school and undergraduate students and postdocs with an emphasis on minorities at the interface between plant nutrition, cell biology and biophysics.