Wheat decision: to respond or not respond to available nitrogen

Crop varieties and agricultural practises developed as part of the Green Revolution have contributed to feeding the population growth of the 20th century. However, some of these practises are not sustainable because they are detrimental to the environment. In particular, high yield cereal crop such as wheat have benefited from the application of nitrogen fertiliser generated through the Haber-Bosh process, which produce large amount of greenhouse gases. In addition, the application and uptake of nitrogen in the field are inefficient processes, which lead to escape of nitrogen in aquatic ecosystems causing eutrophication or in terrestrial ecosystems leading to greenhouse gases volatilisation. In order to continue producing cereal crop sustainably, it is important to grow crop varieties that can produce high yield with lower need for nitrogen application. Increasing the availability of nitrogen in the soil has a strong effect on yield, this is driven by an increase in the number of spikes/heads on each plants. However, under increased nitrogen supply, the amount of yield produced per unit of nitrogen added decreases. The plant response to increasing nitrogen availability, how much yield is produced, decreases under increasing nitrogen levels. Within this project we aim to gain a better understanding of how plants respond to nitrogen available in the soil and coordinate this response with how much they have already acquire and what is their need. We will work with wheat because it is relevant to the food security of 4.5 billion people throughout the world and because tools to study this species have become available. We will focus on strigolactone as these are small molecules produced mostly in the roots when plants are growing under low phosphorus and low nitrogen conditions, that can be transported to the shoot. These can modulate the number of spikes produced on each wheat plant and we aim to test whether their production and their perception is necessary for plant to respond to nitrogen. For this, we will use wheat plants that are lacking specific genes which render them insensitive to strigolactone. These experiments will allow us to generate new knowledge on the role of specific genes known to be involved in the perception and signalling of strigolactone, in nitrogen responsiveness. We are also interested in understanding how strigolactone production is regulated under low nitrogen conditions, and whether it is linked to the local external availability of nitrogen or the nitrogen status of the plants i.e. whether the plants have been grown under sufficient nitrogen. The dampening effect of nitrogen application under increased nitrogen availability also affect grain protein content. This trait is particularly important for determining the use and price of the grains, high grain protein content is necessary for breadmaking, and is linked to the capacity of the plant to take up nitrogen from the soil when the grains are filling. We have shown that this capacity is reduced when plants are growing under increased nitrogen availability. Within this project, we will study how high nitrogen status can also lead to lower post-anthesis nitrogen uptake. We aim to generate new information that is relevant to the fundamental understanding of plant and that is useful to develop new crop varieties which can be grown under more sustainable agricultural conditions.

Nitrogen (N) application is necessary to achieve the production of grain yield of high quality. The positive effect of increasing N availability both on grain yield and grain N content (an indicator of quality) declines with increasing N supply. A group of phytohormones called strigolactone (SL) have been shown to mediate the plant response to low nutrient availability in particular phosphate and N, though their role in wheat N response remain to be tested. These also play a key role in plant development notably regulating shoot branching or tillering, and root architecture in crop species. Within this proposal, we will pinpoint the role of SL in modulating the N response in wheat. Specifically, we will measure SL production and expression levels of genes involved in the SL synthesis pathway under increasing N availability. We will use a series of wheat Targeting Induced Local Lesions IN Genomes (TILLING) lines insensitive to SL because they lack elements of the perception pathway to establish whether SL perception is necessary for the regulation of N responsiveness. Using split-root experiments and transgenic lines over-expressing a nitrate transporter, we will establish the contribution of N status on SL regulation of N responsiveness. To further explore the mechanism of N homeostasis regulation of N responsiveness, we will investigate the role of strigolactones in the down-regulation of post-anthesis N uptake (a trait underpinning grain N content). Understanding the molecular basis for the regulation of N response under increasing N availability will provide fundamental information that can be exploited to develop crop varieties with lower N requirements, better suited for more sustainable production systems, thus limiting the negative environmental effect of fertiliser application.