Scientists in France, examining plant reactions to nitrogen, have made a breakthrough they claim could lead to improving crops' response to nitrate availability.

Maintaining crop yields while reducing nitrate inputs into fields is a challenge that requires a clearer understanding of how plants respond to nitrogen supplementation.  As part of work being undertaken across the world to improve understanding of how plants react to nitrogen, researchers at INRA in Versailles-Grignon and their colleagues from ENS-Paris focused on the role of a transcription factor (the first stage in the process of change from DNA to a protein, or in other words from a gene to its product) NLP7, in the signalling pathway for nitrates - the principal source of nitrogen – in the plant Arabidopsis thaliana (thale cress).

Thale cress was chosen for study as it is a model organism, meaning it has characteristics which lend themselves to examination, such as a quick growth rate or a make-up that provides results which can be inferred to other species.

The scientists discovered that, depending on the availability of nitrogen, the NLP7 transcription they were examining was not always present in the same area of the plant cell, and that this had an influence on its activity.

They said that, "When nitrate was available in sufficient quantities for the plant, NLP7 was present in cell nuclei, but it was exported into the cytoplasm in the case of nitrate deficiency."

Nitrogen plays an important role in plant metabolism and growth, though most plants are incapable of utilising the nitrogen present in air (those that can are known as nitrogen fixers). For most plants, the essential nitrogen must therefore be absorbed, principally in nitrate form, from the soil via the roots.  

Inorganic nitrogen absorbed through the roots is assimilated to produce organic nitrogen (present in carbon molecules) which will be used to produce proteins and nucleic acids the plant needs to grow and carry out other functions.  Thus the growth and yields of crops are often restricted by soil nitrate availability.  The use of nitrate fertilisers has enabled a considerable increase in crop yields, even though this can sometimes have harmful effects on the environment.

The French scientists said that, based on their observations of NLP7, it may be responsible for triggering plants' conversion of nitrate from soil into the organic nitrogen (amino acids, etc.) needed for growth and development. They said their observation led to a novel finding; that by retaining NLP7 in the cell nucleus plants can benefit from an "early and rapid response… to nitrates." 

The scientists added that "This mechanism ensures the rapid adaptation of plants to nitrate availability and among other things permits its rapid absorption from the soil."

Although this to-and-fro mechanism between two cell compartments has already been observed in plants in other signalling pathways, including those associated with response to pathogens and reaction to light, the INRA study is the first to describe the process occurring in the case of a signalling pathway for an essential nutrient.

The INRA authors concluded that, "Although the maintenance of biomass production and grain quality while limiting nitrate fertilisation currently constitutes the principal challenge for modern agriculture, this discovery opens the way to interesting opportunities to improve control of the response of plants to nitrate availability."

More information is available from INRA here