Scientists discover genetics of nitrogen fixation in plants - potential implications for future agriculture

Some plants have the capacity to grow well in nutrient poor soils without additional fertilizers. This is the result of a very efficient symbiosis between either nitrogen fixing bacteria that interact with the plant's roots, or between these roots and mycorrhizal fungi. These symbioses allow plants to strongly improve their uptake of nitrogen, phosphorus and water. Now a team of French and German scientists has discovered [*.pdf French/Spanish] the common genetic mechanism at work that allows the elements of the symbiosis to interact.

Their findings might make it possible to transfer the nitrogen fixing capacity of legumes to a wide range of crops that do not have this ability, including maize and rice. Ultimately, this could lead to a massive reduction of inorganic fertilizer consumption. The discovery is reported in the early edition of the Proceedings of the National Academy of Sciences.

The team of researchers from the Institut de Recherche pour le Développement (IRD) and the University of Munich have been collaborating for years on the project. They found that one of the genetic elements of nitrogen fixing plants called SymRK (Symbiosis Receptor Kinase), used by leguminous plants (pea, alfalfa...) to join Rhizobia bacteria and mycorrhizal fungi, is also essential for the establishment of the symbiosis between the tropical tree Casuarina - an actinorhizal plant that thrives in poor sandy soils - and nitrogen fixing bacteria belonging to the genus Frankia. This new understanding unlocks the keys to the genetics of the nitrogen fixing capacity of plants, and could make it possible to apply the mechanism to the development of crops that massively cut back on fertilizers.

Inorganic fertilizers are an essential but expensive input for farmers. World wide consumption of nitrogen fertilizers was around 130 million tonnes in 2007. Phosphate demand stood at around 37 million ton. Prices are rising steadily because of high oil and gas prices. Some crops like maize require large applications that have to be repeated each growing seaon. Particular cropping systems - such as growing nitrogen-fixing crops after other crops - can limit the need for fertilizers marginally and temporarily.

But what if crops like maize and rice could be designed in such a way that they do not require any additional inorganic fertilizers? That would revolutionise agriculture on a global scale and would greatly limit the different types of pollution and ecosystem damage caused by artificial fertilizers. The discovery of the genetic basis for the efficient N2-fixing capacity in plants might make the development of such crops possible.

The association between mycorrhizal fungi and plants is estimated to be more than 400 million years old. It helped plants colonize the land. Today, the symbiosis can be found in more than 80% of the all plant species. More recently, approximately 60 million years ago, a new symbiosis developed between soil bacteria known as rhizobia, and leguminous plants, which granted them the unique capacity to nourish themselves by extracting nitrogen from the air to use it as a nutrient.

Rhizobia establish themselves inside the root nodules of legumes, where they transform nitrogen into ammonium that can be directly taken up by the plant. In return, the plant provides to the micro-organisms with nutrients in the form of complex glucides:
energy :: sustainability :: biomass :: bioenergy :: agriculture :: fertilizer :: nitrogen fixation :: mycorrhizae :: soil microbes :: symbiosis :: molecular biology :: genetics ::

Unlocking the mechanism
For several years, scientists have tried to unlock the genetic mechanisms responsible for these mutually beneficial relations between plants and bacteria on the one hand, and bacteria and fungi on the other.

Already in 2000, IRD researchers discovered a genetic signaling mechanism common to the way in which legumes interact with rhizobia and to the way in which mycorrhizae work. The symbioses use a common genetic element baptized SymRK. This gene intervenes in recognizing Nod factors - the signaling molecules that are crucial for the rhizobia to establish themselves in root nodules.

So-called actinorhizal plants have formed a second group of plants that have acquired the capacity to benefit from a symbiosis with another type of nitrogen-fixing bacteria called Frankia. The genetic mechanisms of these plants' relationship with their symbiont has not been studied in-depth so far.

The actinorhizal plants can be found in disturbed environments, such as volcanic soils or mining terrain and in soils starved of nitrogen, such as sandy moraines.

There are approximately 260 species of actinorhizal plants distributed over 24 genera and 8 families of flowering plants. To study the symbiosis, the French and German researchers were particularly interested in the tropical Casuarina tree, better known under the name filao. Casuarinas thrive at tropical beaches, in poor sandy soils.

Using techniques from molecular biology, the scientists looked for the sequence coding the SymRK gene within the Casuarina genome. Once they identified the gene, they wanted to find out whether it is again responsible for the establishment of the symbiosis between filao and the Frankia bacteria.

To find out, they developed transgenic plants in which the expression of the SymRK gene was strongly reduced. They then compared the capacity of these plants to form symbiotic nodules on their roots with that of wild plants. According to these analyses, the plants whose SymRK gene's potency was reduced, produced half the number of root nodules compared with the control plants. The formation of mycorrhizae also strongly decreased compared with the wild Casuarina trees.

These results indicate that the reduction of the expression of the SymRK gene, in filao, causes a major reduction in its capacity to fix atmospheric nitrogen as well as a reduction of its aptitude to form mycorrhizae. More generally, these conclusions highlight the fact that there is a common genetic element at work in nitrogen fixing plants that seems essential for the installation of the three types of symbiotic associations utilizing bacteria (Rhizobium and Frankia) or mycorrhizal fungi.

Implications
A better comprehension of these genetic mechanisms could contribute, in the years to come, to the development of techniques to transfer the genetic material necessary for the nitrogen fixing capacity to crops that are unable to perform this task, such as cereals like maize and rice.

Whereas rice does establish a symbiotic relation with a mycorrhizal fungus, it is indeed inapt to develop nitrogen fixing nodules. However, by modifying its genome in such a way that rice plants too are capable of feeding off the atmospheric nutrient, it would become possible to significantly limit the nitrogen fertilizer needs in rice cultivation. This would have major economic and environmental effects: reduced production costs for farmers world wide and less pollution from nitrogen runoff.

If the N2-fixing capacity is transferred to all the major grain crops currently produced, world agriculture would be transformed forever.

Translated for Biopact by Laurens Rademakers.

Image: Frankia is genus of nitrogen-fixing bacteria that live in the soil and have a symbiotic relationship with many plants. By focusing on the genome of Frankia, French and German scientists discovered a genetic mechanism responsible for root-fungal and root-bacterial symbioses. Credit: MicrobeWiki.

References:
Hassen Gherbi, Katharina Markmann, Sergio Svistoonoff, Joan Estevan, Daphné Autran, Gabor Giczey, Florence Auguy, Benjamin Péret, Laurent Laplaze, Claudine Franche, Martin Parniske, and Didier Bogusz, "SymRK defines a common genetic basis for plant root endosymbioses with arbuscular mycorrhiza fungi, rhizobia, and Frankia bacteria", Published online on March 3, 2008, Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.0710618105,

IRD: Un mécanisme génétique universel découvert chez les plantes fixatrices d’azote [*.pdf] - Fiche n°288 - Février 2008

AlphaGalileo: Un mecanismo genético universal descubierto en las plantas fijadoras de nitrógeno - March 7, 2008.

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Renergie receives $1.5 million grant for sweet sorghum ethanol production

Renergie, Inc. is one of the eight recipients, selected from 139 grant applicants, to share $12.5 million from the Florida Department of Environmental Protection’s Renewable Energy Technologies Grants Program (previous post). It received $1,5 million (partial funding) in grant money to design and build Florida’s first sweet sorghum juice mechanical harvesting system and ethanol plant capable of producing fuel-grade ethanol solely from sweet sorghum juice. The fuel is competitively priced at 15 percent less per gallon than regular gasoline, in part because of the crop's high yield. Renergie focuses on a decentralised production approach, firmly rooted in local farming communities.

Renergie was formed in March 2006 by Meaghan M. Donovan for the purpose of raising capital to develop, construct, own and operate ethanol plants in the parishes of the State of Louisiana which were devastated by hurricanes Katrina and Rita. Each ethanol plant in Louisiana has a production capacity of 5 million gallons per year of fuel-grade ethanol. Upon completion of the initial network of ten ethanol plants, Renergie will have an annual production capacity of fifty 50 million gallons (189 million liters). Renergie intends to replicate its Louisiana decentralized network of ethanol plants in Florida.

Sweet sorghum advantages
Renergie produces ethanol solely from sweet sorghum juice. This crop has received growing interest from the bioenergy community because it outperforms most alternative, firts generation energy crops. According to Renergie, the main advantages of producing ethanol from sweet soghum juice are:

High Yield – Sweet sorghum yields between 500 to 800 gallons of ethanol per acre (4700 to 7500 liters per hectare);

Water Efficient Crop – Sweet sorghum requires one-half of the water required to grow corn and one third of the water required to grow sugarcane;

Ability to Grow in Marginal Soil – Sweet sorghum can grow in marginal soils, ranging from heavy clay to light sand. Sweet sorghum has been called a “camel among crops,” owing to its wide adaptability, its marked resistance to drought and saline-alkaline soils, and tolerance to high temperature and waterlogging;

Not Harmful to the Environment – Sweet sorghum requires the use of only 40 to 60 pounds of nitrogen per acre whereas corn growers use more than 150 pounds per acre, according to the U.S. Environmental Protection Agency. Less fertilizer reduces the risk of water contamination. Producing ethanol from sweet sorghum, rather than increasing corn-to-ethanol production, reduces the risk of the continued formation of dead zones in the Gulf of Mexico;

Rapid Growth – Sweet sorghum takes only 4 months to reach maturity, which is short enough to allow harvesting twice a year. Sugarcane requires 14 months to reach maturity; and

Energy Efficient – The energy requirement for converting sweet sorghum juice into ethanol is less than half of that required to convert corn into ethanol. This is due to the fact that the sugars in sweet sorghum juice are fermented directly. There is no need to excessively heat the juice to breakdown starch into sugars as required for corn.

In 2007, China and India produced 1.3 billion gallons of ethanol from sweet sorghum juice. The Renergie project will be the first time that ethanol will be produced solely from sweet sorghum juice in the U.S.

Decentralisation
The company focuses on creating a decentralized network of smaller ethanol plants with a commitment to local rural economic development. The distributed nature of a smaller ethanol production plant network reduces Renergie’s feedstock supply risk, does not burden local water supplies and provides broad-based economic development:
energy :: sustainability :: biomass :: bioenergy :: biofuels :: ethanol :: sweet sorghum :: efficiency :: Florida ::

In Louisiana, Renergie is headquartered in the small city of Kaplan (population of less than 5,000). Renergie has agreed to donate two cents of every gallon of ethanol it sells to the City of Kaplan. Renergie firmly believes that the success of the ethanol industry requires a long-term commitment to rural economic development.

Market focus
The Renergie philosophy is to produce ethanol locally and market ethanol locally. There is not an oversupply of ethanol. The major obstacle to widespread ethanol usage continues to be the lack of fueling infrastructure. Only 1,347, of the nearly 180,000 retail gasoline stations in the United States, offer E85. Moreover, ethanol is slowly moving from being just a blending component in gasoline to a truer fuel alternative in the form of E85. If it were up to the company, the day of building 100 million gallon per year corn-to-ethanol plants in the Midwest corn belt, for the sale of E-10 to consumers on the U.S. East Coast and West Coast, is over.

Renergie is focusing its efforts on growing ethanol demand beyond the 10% blend market. Initially, Renergie will directly market E85, a blend of 85 percent ethanol and 15 percent gasoline for use in FFVs, to fuel retailers under the brand Renergie E85. Renergie’s unique strategy is to blend fuel-grade ethanol with gasoline at the gas station pump. Currently, ethanol providers blend E10 and E85 at their blending terminal and transport the already blended product to retail gas stations.

Once state approval is received, Renergie’s variable blending pumps will be able to offer the consumer a choice of E10, E20, E30 and E85. Via use of the Blender’s Tax Credit, Renergie will be able to ensure that gas station owners are adequately compensated for each gallon of fuel-grade ethanol that is sold via Renergie’s variable blending pumps at their gas stations.

Cost of Feedstock
Renergie will not fall victim to rising feedstock costs. Farmers in Louisiana and Florida will share in the profits realized from the sale of the ethanol made from their crops. Renergie enters into long-term feedstock supply contracts with area farmers. Currently, the profits from corn-to-ethanol projects go primarily to the wealthiest farmers, major corporations, e.g., Archer Daniels Midland, and out-of-state investors.

Renergie ensures that there is a link between the compensation paid to its feedstock producers and ethanol market conditions. Farmers will receive a lease payment for their acreage and a royalty payment based on a percentage of Renergie’s gross sales of ethanol. The Renergie ethanol project will mark the first time that Louisiana and Florida farmers will share in the profits realized from the sale of value-added products made from their crops.

Research
Sorghums are receiving a great deal of interest from the bioenergy community. Recently, scientists from the U.S. Agricultural Research Service released new low-lignin sorghums that are ideal for biofuel and feed (previous post). Several projects are underway to develop drought-tolerant varieties, high sugar varieties and high biomass varieties (earlier post). Some sorghums promise great opportunities for use in developing countries, where they can be grown with low inputs to yield both fuel, food, fiber and fodder (more here and here).

Late last year, a major breakthrough was achieved when researchers succeeded in engineering a sorghum that can grown in soils plagued by aluminum toxicity. Such acidic soils limit crop production in as much as half the world's arable land (previous post).

Thanks to Renergie's Brian J. Donovan.

References:
Biopact: Florida awards $12.5 million grants for renewable energy: includes sweet sorghum ethanol, biodiesel distribution, multi-feedstock biofuels - March 07, 2008

Biopact: Researchers use IV to monitor flow of sugar in sweet sorghum, analysis aimed at maximizing biofuel potential - September 13, 2007

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