Spotlight on macronutrients (Part 2)
This month we conclude our look at essential plant macronutrients that started in the December issue, 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[1], which are the building blocks of proteins[2] – and hence cell membranes[3], enzymes[4] and nutritionally important storage or reserve proteins[5,6]; and it is an important constituent of nucleotides, which are major components of nucleic acids[7] such as RNA (ribonucleic acid[8]) and DNA (deoxyribonucleic acid[9]), and of the ‘energy molecule’ ATP (adenosine triphosphate[10]). 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 is approx. 78 % of this gaseous element[11,12] in the form of dinitrogen, N2[13]. Sadly, in that state plants cannot use it; it must be converted into 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[14] – have teamed up with bacteria that can undertake that chemical reaction in the process known as nitrogen fixation[15]. 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[16]. 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[17,18]. In agricultural settings this deficiency is usually remedied by the addition of chemical fertilisers[19], 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[20]. Not only is that damaging to the environment, it is costly – ‘Nitrogen fertilizer costs US farmers approximately $8 billion each year …’[21]. So, 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.[22] 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[23] that are released by the would-be symbiotic rhizobial bacteria[24], 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 15 essential plant nutrients[25], cobalt (Co)[26] is additionally required by the bacteria of the N-fixing nodules[27], so indirectly Co is a 16th essential nutrient in those cases – Ed.]
Image: Wikimedia Commons.
[1] http://en.wikipedia.org/wiki/Amino_acid; [2] http://en.wikipedia.org/wiki/Protein; [3] http://en.wikipedia.org/wiki/Cell_membrane; [4] http://en.wikipedia.org/wiki/Enzyme; [5] http://en.wikipedia.org/wiki/Storage_protein; [6] http://bit.ly/1jq9jot; [7] http://en.wikipedia.org/wiki/Nucleic_acid; [8] http://en.wikipedia.org/wiki/RNA; [9] http://en.wikipedia.org/wiki/DNA; [10] http://en.wikipedia.org/wiki/Adenosine_triphosphate; [11] https://www.webelements.com/nitrogen/; [12] http://en.wikipedia.org/wiki/Nitrogen; [13] http://www.thefreedictionary.com/Dinitrogen; [14] http://en.wikipedia.org/wiki/Legume; [15] http://en.wikipedia.org/wiki/Nitrogen_fixation; [16] http://en.wikipedia.org/wiki/Root_nodule; [17] http://bit.ly/1eNEINX; [18] http://bit.ly/1eJCd0O; [19] http://en.wikipedia.org/wiki/Fertilizer; [20] http://en.wikipedia.org/wiki/Eutrophication; [21] http://bit.ly/1lWa1vt; [22] Science 341: 1384–1387, 2013; [23] http://en.wikipedia.org/wiki/Nod_factor; [24] http://en.wikipedia.org/wiki/Rhizobia; [25] http://en.wikipedia.org/wiki/Plant_nutrition; [26] http://en.wikipedia.org/wiki/Cobalt; [27] http://bit.ly/1bjjp8f.
Stressed-out sulphur …
Amongst its many roles in plants, sulphur (S) is found in two of the 20 standard amino acids that form proteins[1,2], namely cysteine and methionine, and is therefore important in crucial cell components such as membranes[3] and enzymes[4]. Sulphur is also present in the organic compounds that give plants such as onion, garlic and mustard their characteristic odours[5]. Sulphur is generally taken up from the environment by plants as the sulphate ion (SO42–), which is frequently produced by bacterial activity in the soil[6]. Well, as much as plants need sufficient amounts of S to maintain growth, development and ‘health’, some forms of S in the environment can be damaging. Take for example H2S – hydrogen sulphide, a gas with the ‘characteristic foul odor of rotten eggs’[7] – which is found naturally in oxygen-poor areas as bacteria metabolise SO42–. Sediment-derived H2S can impact deleteriously on the growth and health of seagrasses[8] – flowering plants that live a submerged existence and that provide important marine habitats[9] often covering large areas (up to 600 000 km2 of the oceans[10]), which, because of their similarity to terrestrial meadows, are termed seagrass meadows[11]. In view of the inter-relatedness of marine ecosystems, damage to seagrass stands can have knock-on effects upon such iconic habitats as coral reefs[12]. Monitoring seagrass health is therefore important. And an important diagnostic technique to assess seagrasses' well-being has been developed by Kieryn Kilminster et al.[13], and has a S dimension. Outwardly, seagrass that is ‘compromised’ may look healthy, so an internal diagnostic test is needed to indicate its state of health. Such a test was provided when the Dano-Australian team discovered that elemental S accumulated in tissues of the seagrass Halophila ovalis[14] when their environment was stressful. The incorporated sulphur resulted from the plant's uptake of H2S from the sediment, whose microbial production was in turn an indication that the sediment had become anaerobic, which is a stressful state of affairs for the aerobic seagrasses … Another marine-sulphur–stress dimension has been revealed by Melissa Garren et al.[15] for hard corals – those mutualistic symbiotic organisms that comprise an animal coral polyp and an internalised microalga, a zooxanthella[16]. When the coral Pocillopora damicornis was heat-stressed (to 31 oC), concentrations of DMSP (dimethylsulphoniopropionate) in its mucus increased 5-fold and the chemotactic response of the pathogenic bacterium Vibrio coralliilyticus was enhanced. The bacterium appears to be using the DMSP as an ‘infochemical’ to home in on stressed coral hosts, which it subsequently attacks. Vibrio coralliilyticus is associated with many coral diseases and infects them at temperatures above 27 oC[17]. And what is the relevance of all of this? Think heat-stress, think global warming[18]. Interestingly, DMSP – which is produced by a wide range of marine algae when variously ‘damaged’, not just heat-stressed corals – is the precursor for DMS (dimethylsulphide[19]), which ultimately acts as a nucleating agent for cloud formation in the atmosphere. The clouds can act as reflectors of some of the incoming solar radiation, which would otherwise serve to increase the temperature of the Earth (global warming[20]). Thus, DMS might actually contribute to global cooling (and features in the CLAW Hypothesis[21]), and which DMS may have been formed from DMSP produced by corals as a response to global warming … Nature: she's complicated(!). For more information on the range of S-compounds in plant biology, see Katharina Gläser et al.'s paper that explores the so-called ‘sulphur metabolome’ of arabidopsis[22]. [By way of fuelling the debate, Mr Cuttings says that he will continue to spell sulphur with a ‘ph’. He knows the ‘f-spelling’ is the standard form of spelling for this element in ‘chemistry and other technical uses’[23,24], but he prefers consistency of spelling, so SULPHUR (NOT sulfur …), please. And anyway, our cousins across the Atlantic pond have won the war and got us all to use ‘program’ for those computer programme things …, so let's make a principled stand; there's no ‘f’ in sulphur! – Ed.]
Image: Wikimedia Commons.
[1] http://bit.ly/1iGwdVD; [2] http://en.wikipedia.org/wiki/Amino_acid; [3] http://en.wikipedia.org/wiki/Cell_membrane; [4] http://en.wikipedia.org/wiki/Enzyme; [5] http://bit.ly/1npPScP; [6] http://bit.ly/1npPv1L; [7] http://en.wikipedia.org/wiki/Hydrogen_sulfide; [8] http://bit.ly/1jqfMj7; [9] http://www.project-seagrass.co.uk/; [10] http://www.seagrass.org.uk/; [11] http://en.wikipedia.org/wiki/Seagrass; [12] http://www.seagrassnet.org/about-seagrassnet; [13] Ecological Indicators 36: 280–289, 2014; [14] http://en.wikipedia.org/wiki/Halophila_ovalis; [15] The ISME Journal in press; doi:10.1038/ismej.2013.210; [16] http://en.wikipedia.org/wiki/Coral; [17] The ISME Journal 6: 835–846; [18] http://en.wikipedia.org/wiki/Global_warming; [19] http://bit.ly/1dCfLR5; [20] http://en.wikipedia.org/wiki/Global_warming; [21] http://en.wikipedia.org/wiki/CLAW_hypothesis; [22] The Plant Journal 77: 31–45; [23] http://bit.ly/1dCfNsb; [24] https://www.webelements.com/sulfur/.
Touchy-feely calcium …
Most essential plant nutrients exert their roles when integrated into organic compounds and macromolecular structures – e.g. nitrogen and sulphur (see other items in this month's collection). Others – such as magnesium (Mg) – may also act in their ionic form as ‘enzyme activators’[1]. But calcium (Ca)[2] is almost in a class of its own as it acts – amongst other things![3] –as a so-called ‘second messenger’[4], and participates in many processes of plant growth and development[5,6]. As a second messenger, levels of Ca2+ in the cytoplasm vary dramatically in response to many environmental and developmental stimuli, which subsequently trigger different physiological responses[7]. Such a role for Ca is also relevant to interactions between plants and other organisms, as demonstrated by Lehcen Benikhlef et al. in the case of microbial attack[8]. However, their work goes even further than that because they showed that light ‘mechanical sweeping’ of leaves of arabidopsis led to development of a strong resistance to Botrytis cinerea (a nectrotophic fungus that attacks plants and causes ‘grey mould’[9]). This was preceded by a rapid change in Ca concentration and a release of ROS (reactive oxygen species[10]), and was accompanied by ‘changes in cuticle permeability, induction of the expression of genes typically associated with mechanical stress and release of biologically active diffusates from the surface’. OK, so, it's a bit more than just Ca, but what a fascinating chain of events. Maybe we should all be handling our plants more often to encourage them to develop pathogen resistance. After all, they do talk of ‘healing hands’[11] …
Image: Wikimedia Commons.
[1] http://bit.ly/1h4jBKP; [2] http://en.wikipedia.org/wiki/Calcium; [3] http://bit.ly/1jqjOIn; [4] http://bit.ly/1fZhhl9; [5] The Plant Cell 17: 2142–2155, 2005; [6] Plant Signaling & Behavior 2: 79–85, 2007; [7] In: Abiotic stress adaptation in plants, eds Pareek et al., pp. 75–90, Springer 2010; [8] BMC Plant Biology 13: 133, 2013; [9] http://en.wikipedia.org/wiki/Botrytis_cinerea; [10] http://bit.ly/LQ4i9L; [11] http://dailym.ai/1cpcx3G.
Magnesium and a food fight …
Whilst incorporation of essential elements into the body of the plant is undoubtedly important for and to the wellbeing of the plant, their presence in those green organisms constitutes a major source of elements that are also essential for the human organism (and other animals that ingest plant matter …). Consequently, plants provide an important source of elemental nutrition for us, whether by their direct consumption or via our feasting on the animals that ultimately feed on the plants. And different plants differ in their ability to provide those all-important nutrients. Take for example, quinoa – Chenopodium quinoa – a so-called ‘pseudocereal’ that originated in the Andean region of South America[1]. A 185-g serving of cooked quinoa provides 29·6 % of your RDA (recommended dietary allowance, now largely replaced by RDI – reference daily intake, ‘the daily intake level of a nutrient that is considered to be sufficient to meet the requirements of 97–98 % of healthy individuals in every demographic in the United States’[2]) of Mg[3]. [Aha, the Mg connection – eventually …! – Ed.] Although this is not as high as, say, seeds of pumpkin[4], where a serving a third of that of quinoa provides 47·7 % of Mg's RDI[5], or spinach[6], which provides about the same RDI for Mg in an equivalent serving[7] (although with about a seventh of the calories) and is in the same family as quinoa (the Amaranthaceae[8]), quinoa is still pretty good. Plus, that same serving of quinoa can also provide high levels of other essential nutrients – 58·5 % manganese (Mn), 40·1 % phosphorus (P), 40 % copper (Cu) and 18·3 % zinc (Zn). Given these fairly fascinating food facts it is perhaps no surprise that quinoa – despite its 4000 years history of cultivation and consumption in places today known as Ecuador, Bolivia, Columbia and Peru – has been widely touted as a ‘newly discovered, up-and-coming’ food. So much so that – apparently (well, it somehow passed me by …) – 2013 was the United Nations' International Year of Quinoa[9]. But this ‘must-have’ food status is not without its problems, and there are concerns that increased demand for quinoa has pushed up prices to the detriment of those people who traditionally used the crop as a staple of their diet in places like Bolivia[10,11]. When will this little planet of ours be free of battles over food? [For a more in-depth nutrient analysis of quinoa, visit the George Mateljan Foundation's website[12]. But you might want to wait, because – allegedly – Ethiopian tef[13] is set to overtake quinoa as the next ‘super grain’[14]. Despite quinoa not really being a grain[15], and tef producing probably the smallest grain in the world – you need approximately 150 of them to match the weight of a single grain of wheat[16] (and the apparent irony of Ethiopia feeding the rest of the world has not gone unnoticed). But flour produced from tef, unlike wheat, is gluten-free and suitable for those who suffer from coeliac disease[17], a digestive condition where a person has an adverse reaction to gluten, which symptoms include diarrhoea, abdominal pain, weight loss and feeling tired all the time[18] – Ed.]
Image: Wikimedia Commons.
[1] http://en.wikipedia.org/wiki/Quinoa; [2] http://bit.ly/1iROOxx; [3] http://bit.ly/1f9WgjV; [4] http://en.wikipedia.org/wiki/Pumpkin; [5] http://bit.ly/1iRP67C; [6] http://en.wikipedia.org/wiki/Spinacia_oleracea; [7] http://bit.ly/1bOdvts; [8] http://en.wikipedia.org/wiki/Amaranthaceae; [9] http://www.fao.org/quinoa-2013/en/; [10] http://nyu.academia.edu/GlenelysJimenez; [11] http://nyti.ms/1bjCven; [12] http://bit.ly/1iRPvXO; [13] http://en.wikipedia.org/wiki/Eragrostis_tef; [14] http://bit.ly/1bOdLbW; [15] http://bit.ly/1gmN7GS; [16] http://bit.ly/1lWUdbP; [17] New England Journal of Medicine 353: 1748–1749, 2005; [18] http://bit.ly/1jqskar.