Beans and their bugs - a beautiful relationship

The relationship between vegetables such as peas and beans and nitrogen-fixing bacteria is even closer than previously thought, with bacteria acting similar to an intrinsic part of the plant, according to research undertaken in England.

Scientists from the University of Reading and the John Innes Centre, Norwich have found that nitrogen-fixing bacteria provide more than just a supply of useable nitrogen to the plants.

They discovered that amino-acid cycling between the plant and the bacteria controls the fixation process, with the bacteria acting as an 'organelle' (part of the cell) on which the plant is totallydependent.

"This evens things up in terms of the symbiotic relationship between plant and bacteria and also provides some reason for how the symbiotic relationship might have evolved," explained Dr Philip Poole from the University of Reading, near London, and who headed the research team.

"We used to think that the bacteria fixed nitrogen in return for a supply of carbon and energy but now we can see that the two are mutually dependent. Their complete dependency on each other helps explain how selective evolution might have driven the development of such an advantageous relationship," he added.

Plants cannot use nitrogen directly from the atmosphere but rely on it being converted to a usable form, such as ammonium, in order to use it to make proteins and other essential molecules, including DNA.

A type of bacteria, known as rhizobia, can 'fix' nitrogen directly from the atmosphere. Rhizobia infect nodules on the roots of legumes, producing ammonia from atmospheric nitrogen that the plant can use.

This symbiotic relationship between legumes and rhizobia accounts for about 65% of the global nitrogen that can be used by other plants (when the legume dies) or animals (when the legume is eaten), said the researchers.

They found that the cycling of amino-acids between the legume and rhizobia was the key driver of the nitrogen fixation process. When the cycle was blocked, nitrogen fixation stopped. Similarly in the absence of amino-acid cycling, rhizobia use carbon very poorly and legumes are poor at assimilating nitrogen released from the bacteria.