Life's a Gas - if you're a plant
Plants making fertiliser from fresh air?
The fundamental importance of plants to life on Earth reflects their success in using gases; as raw materials for growth and to store energy, to control their development and reactions to their environment, and to defend themselves against attack by pests and pathogens. Even where plants are unable to directly use the gas nitrogen, an essential component of living organisms, they enlist the help of microbes to convert it into a chemical form they can use.
Nitrogen is an important element in plant nutrition and one of the main reasons for applying fertilisers and manures is to supply nitrogen to crops. Seventy-eight percent of the atmosphere is nitrogen gas (along with 21% oxygen and 0.033% carbon dioxide), but it must be ‘fixed’ – converted into ammonia or nitrate salts – before plants can use it. The only biological processes for fixing nitrogen are carried out by a few microbes; plants are unable to do it themselves.
Legumes (peas and beans) have evolved an intimate symbiotic relationship with bacteria (called rhizobia) found living in most soils. The roots of legume plants produce a chemical signal that stimulates any nearby rhizobia to secrete their own signals. These cause the plant’s root hairs to curl and a, so-called, infection thread develops that grows into the root hair and enters the root. Rhizobia enter the root through the infection thread and colonise specialised outgrowths of root tissues, called root nodules. The bacteria in the nodules undergo a dramatic change in their biology, which enables them to ‘fix’ nitrogen gas from the air and supply it, in a useable form, to the plant they are associated with. Pea and bean plants typically have up to 100 nodules on their roots while other legumes, such as peanuts, may have up to 1000. The bacteria do not supply the nitrogen for ‘free’ and some legumes divert up to 20% of their photosynthates to their root nodules when these are actively fixing nitrogen. However, many legumes can supply all their nitrogen needs via the rhizobia in their root nodules, in some cases fixing 45 – 90 kg of nitrogen/hectare/year.
Legumes are used in crop rotations because they require little or no application of nitrogen fertiliser and when the non-harvested parts of the plant are ploughed in they return nitrogen to the soil.
To be able to make any plant ‘fix’ its own nitrogen via an association with Rhizobia has been a dream of scientists for many years, but the plant-Rhizobia relationship is highly specialised and complex. However, recent research suggests that the dream may not be so difficult to achieve. Many plants form intimate relationships with ‘mycorrhizal’ fungi, these are soil-living fungi that grow over the plant roots and can push hyphae (fungal filaments) into the cells on the root surface, without damaging the plant or the plant resisting the fungal invasion. These specialised feeding hyphae allow the fungus to absorb sugars from the cells in the root. In return the fungus takes up nutrients from the soil and passes them on to the plant, increasing the efficiency of the plant’s nutrient uptake from the soil. We now know that some of the key genes controlling these mycorrhizal associations, which are commonly found in many plants, are also needed for the plant-rhizobia association. Studies have shown that Legume plants with mutations in specific genes are unable to establish a normal association with mycorrhizal fungi and also cannot make a normal association with rhizobia. Thus it seems some of the genetic instruction book, needed to build the complex and rare nitrogen-fixing relationship, may already be present in a wide range of plants.