In this and my next few posts we conclude our look at essential plant macronutrients that started in some previous articles, and this time concentrate on the last four of the nine elements – C, H, O, P, K, N, S, Ca and Mg – in that category (and try to bring a Cuttings-esque twist to that quartet).
Nitrogen, in a bit of a fix…
Nitrogen (N) is a major component of many compounds in plants, e.g. it is present in all amino acids, which are the building blocks of proteins – and hence cell membranes, enzymes and nutritionally important storage or reserve proteins; and it is an important constituent of nucleotides, which are major components of nucleic acids, such as RNA (ribonucleic acid) and DNA (deoxyribonucleic acid), and of the ‘energy molecule’ ATP (adenosine triphosphate). As a major component of plants, N is needed in relatively large amounts – which is why it is termed a macronutrient. Fortunate then, you might think, that plants are virtually surrounded by an unlimited amount of nitrogen in the atmosphere, which consists of approx. 78% of this gaseous element in the form of dinitrogen, N2. Sadly, in that state plants cannot use it; it must be converted to forms that they can use, such as the ammonium (NH4+, from ammonia – NH3) and nitrate (NO3–) ions.
Whilst plants cannot themselves convert N2 into NH3, many groups of plants – e.g. famously, the legumes – have teamed up with bacteria that can undertake that chemical reaction in the process known as nitrogen fixation. Some of that fixed nitrogen is used by the plant that hosts the mutualistic microbe, as a sort of rent for the home that the plant provides for the bacteria within root-sited nodules.
Unfortunately, many more plants are not blessed with this in-built nitrogen-fixing partnership and are reliant on appropriate forms of fixed nitrogen from the environment, e.g. NO3–. Since N is frequently in short supply in the soil, it is often referred to as a limiting nutrient – an essential nutrient whose amount limits overall plant growth and development. In agricultural settings this deficiency is usually remedied by the addition of chemical fertilisers, often containing phosphorus (P) and potassium (K) in addition to the N. Whilst desired increases in crop growth/yield are obtained by this human intervention, not all of that added nitrogen – and frequently phosphorus, too – is taken up by the crop; substantial amounts of N and P end up in freshwater systems where they can cause highly undesirable problems such as eutrophication. Not only is that damaging to the environment, it is costly – ‘Nitrogen fertilizer costs US farmers approximately US$8 billion each year…’.
Wouldn’t it be great if non-legumes could be persuaded to develop N-fixing bacterial partnerships? Yes, and work by Yan Liang et al. (Science 341: 1384–1387, 2013) encourages that view. The team from The Plant Molecular Biology and Biotechnology Research Center (South Korea) and University of Missouri (USA) have demonstrated that non-legumes – in this instance good old Arabidopsis thaliana, Zea mays (‘corn’) and Solanum lycopersicum (tomato) – do have the ability to respond to the rhizobial lipo-chitin Nod factors that are released by the would-be symbiotic rhizobial bacteria, and which are signal molecules that trigger nodulation in legumes. Although we are still some time away from nodulating N-fixing non-legume crops such as maize and tomato, this discovery does at least show that the rhizobia are recognized as ‘friendly bacteria’ – the plants just have to be trained to let them accept invasion of their tissues by the microbe, and build the nodule, etc, etc…
[Although there are generally recognised to be 17 essential plant nutrients, cobalt (Co) is additionally required by the bacteria of the N-fixing nodules, so indirectly Co is an 18th essential nutrient in those cases – Ed.]