Plant strategies for optimising nitrate intake

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The less nitrogen there is in the soil, the better plants are at using it. Researchers from INRA, CNRS and CIRAD, in cooperation with Czech colleagues, have recently shed light on the crucial role of a protein that enables plants to not only assess their environment but also activate the proper adaptive response based on the conditions. This research, published in the 2 March 2015 issue of Nature Plants, opens a number of possible directions, namely to the identification of plants that are well-suited to low quantities of fertilizer.

Modern agriculture is extremely dependent on nitrogen fertilizers. While such fertilizers boost crop health in the short term, they are not a long-term solution: these fertilizers have high energy costs and are harmful to the environment (nitrates pollute continental and coastal waters; nitrogen oxides pollute the atmosphere). The development of sustainable agriculture requires making crops less dependent on fertilizer yet still able to produce high yields even when soil has less nitrogen.

One research strategy has unfolded from the observation that plants are able to adapt to limited nitrogen intake. The mechanisms that make this adaptation possible are not well understood and their biodiversity is completely underexploited. Several years ago, researchers identified NRT1.1, a protein in root cell membranes responsible for recognising nitrate (the main source of nitrogen in soil) and transporting it through the roots. This protein enables plants to trigger adaptive responses to insufficient nitrogen. The plant will modify its root system architecture, the regulation of other proteins that transport nitrogen, the expression of various genes and more.

Researchers at the Joint Research Unit for Biochemistry and Plant Molecular Physiology (INRA/CNRS/Montpellier SupAgro/Université de Montpellier), in cooperation with the Joint Research Unit for the Genetic Improvement and Adaptation of Mediterranean and Tropical Plants (INRA/CNRS/Montpellier SupAgro) and Czech colleagues, have just revealed the crucial role of NTR1.1 as a hub that triggers these responses only under necessary conditions. This protein sets off not just one but several nitrate signalisation mechanisms that selectively activate different responses. Their results also show that NRT1.1 has various forms (phosphorylated or not) that handle specific signalisation actions. Depending on the conditions, the plant modifies NRT1.1 so that it can trigger the required mechanism to activate the correct adaptive response.

This work has opened the door to numerous research possibilities. They include basic principles such as the molecular characterisation of signalling pathways required upstream of NRT1.1. In the longer term, more finalised aspects can be studied, such as examining whether a genetic variability exists for adaptive responses by NRT1.1 and whether this variability could be exploited in plant breeding to develop new genotypes that are better suited to lower fertilizer quantities.

Histochemical localisation of the NRT1.1 gene expression in the roots of an Arabidopsis thaliana seedling. The line studied expresses the GUS reporter gene (which encodes beta-glucuronidase) under the control of the NRT1.1 promoter.

GUS activity colours the tissues expressing this reporter gene in blue. This shows that NRT1.1 is expressed in all root apices, i.e., the root tips used by plants to explore the soil. This is where NRT1.1 recognises nitrate and enables the plant to determine if the soil where the roots will grow is rich or lacking in nitrogen.

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