Apr 2, 2012

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Plant breeding and genetic engineering for improved nitrogen efficiency

Less fertilizer, higher yields

Plants take up nitrogen, a key component of many biological molecules, from the soil. For efficient crop cultivation, nitrogen has to be added to the soil at regular intervals. The large-scale use of artificial nitrogen fertilizers since the middle of the 20th century has led to considerable yield increases for farmers, but has also damaged the environment. Plant researchers are working on ways of improving the take-up and utilisation of nitrogen by crop plants. The most ambitious aim is to develop staple crops that can use nitrogen from the atmosphere.

Blue lupins from the legume family.

Root nodules on the hedge vetch, another legume. The nodules provide a home for bacteria that convert nitrogen from the air into a form that plants can use. It is a special characteristic of legumes that they are able to form this kind of symbiotic relationship with nitrogen-fixing bacteria.

Photo: Frank Vincentz, CC-BY-SA 3.0

Plants need nitrogen for their metabolic processes and for growth. It is a key component of amino acids, the building blocks of proteins, and of chlorophyll. Nitrogen is in fact a common element. It is primarily present as a gas and in this form accounts for 78 per cent of the Earth’s atmosphere. However, plants cannot use nitrogen in this form. They can only use it in the form of nitrate or ammonium ions. Since these compounds are present in only small quantities in the soil, farmers have to keep adding them when they want to cultivate crop plants.

Traditional methods of adding plant-available nitrogen compounds to arable land involve spreading slurry and manure on fields or including legumes like field peas, field beans, clover and lupins in the crop rotation. Legumes form a symbiosis with certain bacteria called rhizobia. Unlike plants, these bacteria can process elementary nitrogen from the air because they possess a certain enzyme called nitrogenase. If there are rhizobia in the soil, the legumes form nodules on their roots that provide a home for the bacteria. They convert the nitrogen into ammonia, which the plants can then use to produce amino acids. Until the middle of the 20th century it was common practice in this part of the world to plant legumes regularly as a catch crop. This changed when artificial nitrogen fertilizers became widely available and affordable. In organic farming, which bans the use of artificial fertilizers, legumes and animal manure are the only sources of nitrogen permitted.

The large-scale use of artificial fertilizers since the middle of the 20th century has helped produce considerable yield increases for farmers, but has also brought environmental problems. Not all the nitrogen applied to the fields is taken up and used by the crop plants. Some of it always stays in the soil, particularly if too much fertilizer has been used. The nitrogen is then washed out of the soil as nitrates and ends up in the groundwater and in lakes and rivers, where it can lead to over-fertilization and the subsequent death of a large number of organisms. In addition, when nitrogen compounds are converted by soil bacteria, laughing gas (nitrous oxide) is produced, which damages the ozone layer. All these problems also occur if large quantities of slurry and manure are spread on the fields in association with intensive animal farming. The production of artificial fertilizers also consumes a lot of energy.

Attempts are now being made to use nitrogen fertilizers more sparingly and efficiently. For some years now, people have been developing new fertilizers that release nitrogen slowly and over a longer period of time, so that it does not accumulate as much in the soil. Another approach is precision crop management, which is already widespread in the USA. Here, technological methods are used to calculate fertilizer requirements for each square metre of land and the fertilizer is applied accordingly.

Plant-breeding objective: Improved take-up and utilisation of nitrogen

Plant researchers are working on ways of improving the take-up and utilisation of nitrogen by crop plants. One important approach is to optimize the plant’s metabolism in terms of nitrogen utilisation. Researchers at the University of Tokyo have introduced a maize gene into the genome of rice that makes the plants produce more biomass. This forces them to take up and utilise nitrogen more efficiently. These plants thrive even when there is not much nitrogen available.

US company Arcadia Biosciences, in which BASF holds a stake, has genetically modified oilseed rape and rice so that they produce more amino acids when nitrogen is scarce. In field trials in low-nitrogen soil, the modified plants took up more nitrogen and produced higher yields than the reference plants. The release experiments have been running for several years. The plan is to carry out similar genetic modifications on other crop plants, including wheat. Arcadia Biosciences works in international partnerships with private companies, foundations and government agricultural research institutes, including Australia’s national science agency CSIRO and the African Agricultural Technology Foundation.

In 2012, release trials will begin in Sweden with transgenic barley that is designed to need less nitrogen fertilizer. For a few years now, scientists have been focusing more on the fact that as well as taking up ammonium and nitrate ions, plants can take up amino acids from rotting organic material in the soil. The transgenic barley contains two new genes that help the plant take up amino acids more efficiently from the surrounding soil. Release trials are planned from 2012 to 2016.

At the same time, researchers are trying to obtain other crop plants apart from legumes that can enter into a symbiosis with nitrogen-fixing bacteria. For a long time, they have been investigating which plant metabolic processes are involved in the symbiosis between legumes and rhizobia. They now know that they are largely similar to the metabolic processes involved in the formation of mycorrhiza, a symbiotic association with fungi, which most land plants are capable of forming. Researchers from the John Innes Centre in Norwich in the UK are taking these metabolic pathways, which exist in most plants, as a starting point and attempting to modify them so as to enable even non-leguminous plants like cereals to form a symbiosis with rhizobia.

Researchers at the University of Bremen have observed a similar symbiosis to the one that forms between legumes and rhizobia between a Pakistani grass that is closely related to rice and nitrogen-fixing bacteria of the genus Azoarcus. They are still investigating how this symbiosis occurs. Once scientists have discovered which genes in the grass plants are involved in developing the symbiosis, they can try to cross the relevant genome segments into rice.