description
- Legume plants have the special ability to interact with soil bacteria called rhizobia. Rhizobia colonise plant roots and are taken up inside the cells of special outgrowths called nodules. Inside the nodule, the rhizobia use sugars provided by the plant as energy to convert nitrogen gas from the air into ammonia which is a form of nitrogen the plants can use as fertilizer (fixed nitrogen). This process of bacterial colonization, nodule formation and nitrogen fixation is called nodulation. Since plants cannot fix nitrogen themselves, and the availability of fixed nitrogen is a major limiting factor for plant growth, nodulation provides a big advantage for legumes in nitrogen poor soils. In agricultural settings this means that legumes like pea, soybean or lucerne require less artificial fertilizer. Another advantage is that legumes leave more nitrogen in the soil than other crops, a quality which has led to legumes having a special role in agriculture as they can be grown in rotation with non-legume plants like wheat or vegetable crops in order to provide these crops with extra nitrogen. In fact, legumes sometimes are grown solely for the purpose of increasing the levels of fixed nitrogen in the soil for the next crop as a type of 'green manure'. Aside from the benefits to soil fertility, legume seed crops such as soybean, field bean, and navy bean are valuable as a protein source and are grown throughout the world. Either as a rotation crop or as a green manure legumes require less industrial fertilizer inputs which are expensive and harmful to the environment, which makes legumes a key component of sustainable agriculture. Nitrogen fixation of legumes in agricultural systems is sometimes very low. The main reason for this is that high levels of soil nitrogen inhibits nodulation. Legumes have evolved to utilize fixed nitrogen available from the soil (such as ammonia or nitrate) rather than fix nitrogen by nodulation that would require using its own sugars. In order for the legume plant to choose between using fixed nitrogen from the soil or to fix nitrogen by nodulating it must be able to sense soil nitrogen. The aim of this study is to identify the legume genes involved in the sensing of external soil nitrogen. We hypothesize that if we create legume plants with a reduced ability to sense fixed nitrogen they will continue to nodulate and fix nitrogen even when soil nitrogen is abundant. A gene called NRT1.1 was identified as being important for nitrogen in the non-legume plant Arabidopsis. Interestingly, legumes appear to have multiple copies of NRT1.1. It is possible that since legumes nodulate and produce their own fixed nitrogen they may have developed a more sophisticated nitrogen sensing system. To investigate this possibility, we will test each of the legume NRT1.1 genes to see if they are important for sensing fixed nitrogen. To do this we will identify and characterize legume mutants that have non-functional NRT1.1 genes. In particular, we will test these mutants to see if they are still able to nodulate and fix nitrogen even when there are high levels of fixed nitrogen available. In addition, we will biochemically characterize these mutants and the NRT1.1 proteins. Finally, we will use a similar strategy in an important UK crop, peas. Peas that can nodulate and fix nitrogen in nitrogen rich soils could be grown in rotation with crops such as wheat to lessen the need for industrial nitrogen inputs thereby lowering costs and decreasing environmental damage.