A common genetic mechanism discovered in nitrogen-fixing plants

March 10, 2008

Some soil microorganisms are capable of forging associations with plant roots in the form of symbioses. Certain of these relationships play a highly important ecological and agronomic role. Arbuscular mycorrhizal symbiosis (which links a plant to a fungus) thus gives plants a mechanism for improving their supply of water and mineral nutrition.

This association has been in existence for 400 million years and appears to have accompanied plants in their colonization of the terrestrial environment. At present it involves about 80% of plant species. In a more recent era, about 60 million years B. P., the symbiosis which became established between soil bacteria, Rhizobium species, and leguminous plants doted them with the ability, unique among mass-produced crop plants, to capture nutrient nitrogen from the air.

Rhizobium forms specialized organs, nodules, on the plant roots. These are capable of transforming atmospheric nitrogen into ammonium that can be directly assimilated by the plant. In return, the plant supplies the microorganisms with nutrients in the form of complex carbohydrates.

Scientists have for many years been seeking to unravel the genetic mechanisms that govern such mutually beneficial relationships, on the one hand between plants and bacteria, on the other between plants and fungi. Investigations by a French team in 2000 had shown that some genetic signalling mechanisms operating in the symbiosis between leguminous plants and Rhizobium type bacteria and such plants and mycorrhizal fungi involved a common genetic element named SymRK. This type of gene was already known to operate in the recognition of Nod factors, signalling substances emitted by the Rhizobium type bacteria which are essential for root nodule formation.

The actinorhizal plants make up another category of plants which have acquired the ability to live symbiotically with a nitrogen fixing bacterium, in this case Frankia. These pioneer plant species, whose host-symbiont mechanisms remain little studied, generally colonize disturbed environments, such as volcanic soils or mining-affected ground, and nitrogen-poor terrains such as moraines or sandy soils. About 260 species of actinorhizal plants exist, spread among 24 genera and classified into eight families of angiosperms, flowering plants.

An IRD team, jointly with a laboratory of the University of Munich, turned particular attention to the tropical tree Casuarina, or Australian pine. The first step employed molecular methods to find the sequence coding for the SymRK gene in the Casuarina genome.

Once isolated, the question was whether or not Casuarina needed this gene to establish its symbiosis with the bacterium Frankia. The team therefore developed transgenic plants in which SymRK gene expression was strongly reduced. Subsequent comparison of these plants’ ability to form symbiotic root nodules with that of control plants showed that the plants with lowered SymRK gene expression produced only half as many root nodules as the controls.

The same modified individuals also showed strongly reduced mycorrhization compared with the unaltered Australian pine. The results therefore demonstrated that the weakened SymRK gene expression produced a considerable loss of Casuarina’s nitrogen-fixing ability and also a reduction in its aptitude to form mycorrhiza. More generally, these conclusions bring out the fact that, in nitrogen fixing plants, a common genetic factor seems essential for setting-up the three types of symbiotic association involving bacteria (Rhizobium or Frankia) or a mycorrhizal fungus.

Improved understanding of these genetic mechanisms could in the coming years contribute to the development of procedures for performing the transfer of the genetic material necessary for atmospheric nitrogen fixation to plants like cereals, which do not possess this faculty. Although rice, for example, establishes a symbiotic relation with a mycorrhizal fungus, it is incapable of developing nitrogen fixing nodules. Modification of its genome to equip it with this ability could then open the way to considerable reduction of input of nitrogen fertilizers on this crop and thus cut down the resulting soil pollution.

Source: Institut de Recherche Pour le Développement

Explore further: Human activity affecting microbes in soil

Related Stories

Human activity affecting microbes in soil

September 24, 2015

New research from an Iowa State University ecologist shows that agricultural inputs such as nitrogen and phosphorous alter soil microbial communities, which may have unintended environmental consequences.

New disease resistant pea developed

September 24, 2015

Commercial pea growers stand to benefit from the release of Hampton, a new edible dry pea variety that resists some of the legume crop's most costly scourges, including pea enation mosaic virus (PEMV) and bean leaf roll virus ...

The world's nitrogen fixation, explained

September 23, 2015

Yale University scientists may have cracked a part of the chemical code for one of the most basic, yet mysterious, processes in the natural world—nature's ability to transform nitrogen from the air into usable nitrogen ...

Recommended for you

Chimpanzees shed light on origins of human walking

October 6, 2015

A research team led by Stony Brook University investigating human and chimpanzee locomotion have uncovered unexpected similarities in the way the two species use their upper body during two-legged walking. The results, reported ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.