The molecular genetics of plant-nutrient interactions. Of the many functions performed by plant roots, the most important is the extraction of water and nutrients from the soil. To achieve this efficiently in the face of competition from other organisms and physical processes such as leaching, roots have acquired a sophisticated suite of adaptations. Understanding the molecular basis of these adaptations will be important for improving crucial agronomic traits such as nutrient-use efficiency. We are particularly interested in the molecular processes that underlie patterns of ?foraging? behaviour in plant roots. The efficiency with which roots explore the soil is greatly enhanced by their ability to preferentially colonise nutrient-rich soil patches. Using Arabidopsis as a model, we previously identified a MADS-box gene (ANR1) which is a key component of a signalling pathway that enables root growth to respond positively to the external presence of nitrate. By engineering transgenic lines in which ANR1 over-expression is dexamethasone-inducible, we have shown that up-regulation of ANR1 in one part of the root system is sufficient to initiate a ?foraging? response, even in the absence of nitrate (unpublished results). Recent progress, in collaboration with Alain Gojon?s group at INRA-Montpellier, suggests that the identity of the long sought-after ?nitrate sensor? that lies upstream of ANR1 may now be known. Current evidence suggests it takes the form of a protein previously known for its role as a dual-affinity nitrate transporter, NRT1.1. Other work is looking at the ability of root tips to respond to external signals from the amino acid L-glutamate (L-Glu). It now appears that the external presence of L-Glu (but not other amino acids) is able to induce major changes in Arabidopsis root architecture that are quite distinct from those elicited by nitrate. Using a combination of approaches, including QTL mapping and forward and reverse genetics, we are trying to determine the molecular basis of the root?s ability to sense external L-Glu. A long-term goal is to use the knowledge gained in these studies to develop plant varieties that are more efficient at using fertilisers, so reducing costs and minimising the environmental impact of arable farming.