Since ancient times in the Americas, maize, bean and squash have been grown together in a polyculture known as the ‘three sisters’. This polyculture and its maize/bean variant have greater yield than their respective monocultures. Zhang et al. grow mono- and polycultures in field plots with different nutrient availabilities and show that one cause of this yield advantage is that the crops have different, possibly complementary, root foraging strategies. Maize forages relatively shallower, common bean explores the vertical soil profile more equally, while the root placement of squash depends on P availability. Species differences in root foraging strategies increase total soil exploration, with consequent positive effects on the yield and resilience of these ancient polycultures.
Asymmetric cell divisions define plant development. High-throughput genomic and modelling approaches can elucidate their regulation, which in turn could enable the engineering of plant traits such as stomatal density, lateral root development and wood formation. Asymmetric divisions are formative divisions that generate daughter cells of distinct identity. These divisions are coordinated by either extrinsic (‘niche-controlled’) or intrinsic regulatory mechanisms and are fundamentally important in plant development.
A recent review in Annals of Botany describes how asymmetric cell divisions are regulated during development and in different cell types in both the root and the shoot of plants. It further highlights ways in which omics and modelling approaches have been used to elucidate these regulatory mechanisms. For example, the regulation of embryonic asymmetric divisions is described, including the first divisions of the zygote, formative vascular divisions and divisions that give rise to the root stem cell niche. Asymmetric divisions of the root cortex endodermis initial, pericycle cells that give rise to the lateral root primordium, procambium, cambium and stomatal cells are also discussed. The authors provide a perspective on the role of other hormones or regulatory molecules in asymmetric divisions, the presence of segregated determinants and the usefulness of modelling approaches in understanding network dynamics within these very special cells.
Kajala, Kaisa, Priya Ramakrishna, Adam Fisher, Dominique C. Bergmann, Ive De Smet, Rosangela Sozzani, Dolf Weijers, and Siobhan M. Brady. Omics and modelling approaches for understanding regulation of asymmetric cell divisions in arabidopsis and other angiosperm plants. (2014) Annals of Botany 113(7): 1083-1105.
Shining a laser onto biological material produces light speckles, and patterns of such biospeckle activity reflect changes in cell biochemistry, developmental processes and responses to the environment. Ribeiro et al. use a portable laser and a digital microscope to observe in situ biospeckle activity in roots of Zea mays, Jatropha curcas and Citrus limonia, and find that when a root encounters an obstacle the intensity of biospeckle activity decreases abruptly throughout the root system. The response becomes attenuated with repeated thigmostimuli. The data suggest that at least one component of root biospeckle activity results from a biological process, which is located in the zone of cell division and responds to thigmostimuli. The methodology presented is relatively inexpensive and portable, the analysis can be automated and the technique provides a rapid and sensitive functional assay.
Root cortical aerenchyma (RCA) provides an adaptation to low nutrient availability by reducing the metabolic cost of soil exploration. Hu et al. use radiolabelling to investigate uptake of phosphate, sulphate and calcium in roots of maize (Zeay mays) differing in their degree of RCA formation. They find that in each of the three genotypes studied the rate of phosphate exudation of high RCA genotypes is significantly less than that of low RCA genotypes, and that radial nutrient transport of phosphate and calcium is negatively correlated with the extent of RCA for some of the genotypes. The results support the hypothesis that RCA can reduce radial transport of some nutrients in some genotypes, which could be an important trade-off against the reduced cost of root production.
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.]
Agricultural practices determine world food supply and, to a great extent, the state of the global environment because of their influence on ecosystems and the inputs of nutrients including nitrogen (N) and phosphorus (P). Intensive crop production has resulted in environmental pollution and soil acidification due to excessive application of nitrogen fertilizers. So if we could get an increased understanding of the physiological and genetic regulation of plants response to low-nitrogen (LN) stress, we might be able to develop new crop varieties with increased nutrient-use efficiency through LN tolerance.
MicroRNAs (miRNAs) are small [approx. 21 nucleotides (nt)] endogenous RNAs that can play key regulatory roles in plants and animals by targeting mRNAs for cleavage or translational repression. miRNAs help plants sense and reduce nutrient stresses such as phosphate, sulfate and copper. Nitrate is a major form of inorganic N taken up by cereal crop roots. A recent paper in Annals of Botany identifies miRNAs and their targets in maize (Zea mays) subjected to low-nitrogen stress. Of 85 potentially new miRNAs, 25 show a more than two-fold relative change in response to low-nitrogen compared to optimal conditions. This increases our understanding of the physiological basis for low-nitrogen tolerance and adaptation in maize. Increasingly, systems biology approaches such as whole-scale miRNA analysis can accelerate our integrated knowledge of plant biology through such discoveries as this.
One way of increasing crop productivity is to increase the amount of grain or other harvestable product that is actually harvested from the plant. To that end scarecrows were invented by human beings, although their success in that regard is inconsistent at best (is there a scientific study on the effectiveness of scarecrows just waiting to be done..?). However, another variation on the scarecrow theme aims to tackle productivity more directly, and shows quirkily that clues to above-ground productivity can come from ‘down-below’. Investigating any similarities between the endodermis in roots [‘the central, innermost layer of cortex in some land plants… a… ring of endodermal cells that are impregnated with hydrophobic substances (Casparian Strip) to restrict apoplastic flow of water to the inside’] and the sheath of mesophyll cells that surround the vascular bundles in leaves of C4 photosynthetic plants (the so-called Kranz anatomy, which is the site of net CO2 fixation into photosynthesis in those plants) such as maize, Thomas Slewinski et al. have discovered that a transcription factor called ‘SCARECROW’ is involved in development of both. [A transcription factor is a protein that ‘binds to specific DNA sequences, thereby controlling the flow (or transcription) of genetic information from DNA to m(essenger)RNA’.] Scarecrow is more usually associated with various issues of cell identity and cell-patterning in subterranean roots [ a ‘wiki’ that incidentally has the serious scientific credibility of combining ‘collaborative and largely altruistic possibilities of wikis with explicit authorship’ – Robert Hoffman]. So, establishing its role in above-ground Kranz anatomy is both interesting and testament to a high degree of molecular economy in plant design principles. But the real hope is that this knowledge can now be exploited to convert C3 photosynthetic plants into Kranz-bearing C4 ones, which are photosynthetically more efficient than their C3 poor-relations. Set against a backdrop of global concerns about the ability of current crops to provide enough food for a growing world population [‘food security‘], this C3 to C4 conversion is one of the holy grails (e.g. Richard Leegood; Udo Gowik and Peter Westhoff; Rowan Sage and Xin-Guang Zhu), if not the Grand Challenge (Sarah Covshoff and Julian Hibberd), of plant physiology, and doubtless has many more years to run. However, rather than add extra cell layers, etc, into C3 plants, might it not be easier to engineer the rather neat trick of having both C4 and C3 photosynthesis in the same cell, as naturally exists in such plants as the hydrophyte Hydrilla verticillata (e.g. Srinath K. Rao et al.)? Sadly, I can take no credit for that suggestion(!), but see the experiences of Mitsue Miyao et al. and their attempts to effect this in C3 rice. However, if you want to dabble in such areas, you’ll probably want to keep such work under wraps – or in the confines of the lab – since Hydrilla has been hailed as ‘the perfect aquatic weed’ by Kenneth Langeland. Which gives me an idea: if it is allowed to escape and colonise the rest of the planet’s waterways with regrettable – but necessary! – elimination of native flora we’d have converted huge areas of the planet to more productive C4 photosynthesis at a stroke. If only we can eat the stuff, future food security will have been secured..? Isn’t science and a little bit of imagination great!? Who said scientists can’t be creative?
[Please do not attempt to test mischievous Mr P. Cuttings’ ‘Hydrilla hypothesis’ at home; and certainly not outdoors! – Ed.]
The contentious matter of plant GM (genetic manipulation – which always sounds more menacing and mankind-meddling-with-nature than GE, genetic engineering, or the other GM – genetic modification…) has been put in the spotlight recently with Gilles-Eric Séralini et al.’s paper. Entitled ‘Long term toxicity of a Roundup herbicide and a Roundup-tolerant genetically modified maize’, it ascribes health and ‘longevity-shortening’ (i.e. earlier deaths…) effects in rats not only to the endocrine-disrupting effects of Roundup (commercial name for glyphosate, a herbicide), but also the over-expression of the transgene for glyphosate tolerance in GM maize and its metabolic consequences. Strong stuff indeed, and which has caused not a little concern and understandable ‘interest’ amongst the media (e.g. prompting a press release by the European Food Safety Authority and an article from the Agricultural and Rural Convention).
Leaving aside considerations about what this episode might tell us about the process of peer-review of scientific research, given the long-standing and enduring interest/concerns about Roundup and GM crops, suggestion of an alternative to glyphosate will probably be welcomed. Encouraging news then that Sarah Barry et al. have elucidated a key step in production of thaxtomin. Thaxtomin, which exhibits herbicidal activity by inhibiting cellulose biosynthesis and thus interfering with formation of plant cell walls, is made naturally by Streptomyces species, actinomycetous bacteria that cause the disease known as potato scab. Although thaxtomin’s herbicidal nature has been known for some time, its commercialisation was not realistically possible without fuller understanding of its biosynthetic pathway. With Berry et al.‘s identification of the particular P450 cytochrome enzyme – TxtE – that catalyses an important step in thaxtomin synthesis, it is expected that the phytocide might now be made in amounts that could be commercially exploited. And, as a ‘natural product’, it is apparently able to be used in agricultural systems that have the cachet (to say nothing of any ‘sales-price-premium’) accorded by their ‘organic’ status/certification. (But probably best not to dwell on the fact that commercial amounts of this natural, organic herbicide may need to be produced by GM’d bacteria.)
Congratulations are in order to the John Innes Centre (Norwich, UK) for its recent award of nearly US$10m ‘to test the feasibility of developing cereal crops capable of fixing nitrogen as an environmentally-sustainable approach for small farmers in sub-Saharan Africa to increase maize yields’. The funding – curiously, for 5 years and 1 month – from the Bill & Melinda Gates Foundation (BMGF) should allow Giles Oldroyd and his team to further their attempts to encourage cereals to develop a mutually beneficial symbiosis with nitrogen-fixing bacteria, as found within root nodules of legumes. And this notion is not as science-fiction fanciful as you might think because the pathway that facilitates development of mycorrhiza between flowering plants and fungi is similar to that involved in nodule development. Whilst cereals presently form mycorrhiza they don’t yet have N-fixing nodules, but a little molecular magic may be all the encouragement that’s needed to kick-start that ancient ‘dormant’ ability. But why go to such trouble when you could just add artificial fertiliser to reduce the yield gap (‘the gap between average and potential yields‘)? Because such fertilisers are not only too costly for farmers in that region (and elsewhere!), they are also environmentally expensive – apparently, making and applying nitrogen fertilisers contributes half the carbon footprint of agriculture and causes environmental pollution. Although Team Oldroyd will focus upon maize – the most important staple crop for small-scale farmers in sub-Saharan Africa – to speed the work along they will also exploit Setaria viridis, which has a smaller genome and shorter life cycle. And as an added bonus, results of this work should also be applicable to other major cereals such as wheat, barley and rice. This work will take place in tandem with another BMGF-co-funded initiative, N2Africa, a large-scale, science research project ‘focused on putting N-fixation to work for smallholder farmers growing legume crops in Africa’. So, it looks like we can now answer the question posed by Myriam Charpentier and Giles Oldroyd, ‘How close are we to nitrogen-fixing cereals?’ – US$9,872,613 closer! Let us hope that the investment pays off as we follow the path towards Prabhu Pingali’s second Green Revolution (GR2.0), and trust that needful nations can afford the solution that is reached. However, one is mindful that to date adoption of ‘agbiotech’ solutions in sub-Saharan Africa has been low; regardless of how environmentally sympathetic the science may be, hearts and minds will need to be won over too.
Unlike ‘Audrey 2’ – the plant which ate members of the cast from ‘The Little Shop of Horrors’ (botanically suspect but with some good songs) – the maize seed grows on the cob by extracting goodies from the mother plant.
YouTube has a great video of a production of Little Shop of Horrors: Feed me Seymour – embedding not possible so you need to jump to the link.
Now researchers at the Universities of Warwick and Oxford have discovered a key gene in this feeding process – prosaically named Meg1*. It seems that Meg1 converts the tissues surrounding the developing embryo into a placenta-like structure. The big surprise is that Meg1 is expressed only from the maternally-inherited copy, with the male copy remaining silenced. Some evolutionary biologists believe that this parent-of-origin ‘gene imprinting’, which also occurs in animals, is a result of a battle of the sexes in which the male sperm’s desire to make the ‘biggest and best’ seed is pitted against the female’s need to keep control over her resources so she has enough left to fill a number of seeds.
Whatever – Meg1 is almost certainly responsible for generating what you had for breakfast this morning and as such is a really, really important gene. Excitingly, the Warwick/Oxford researchers were also able to show that the output of Meg1 – like most animal imprinted genes – is strictly dosage-dependent – suggesting that it may be possible to improve seed yield by breeding plants with more copies of Meg1.
The Meg1 work was led by Jose Gutierrez-Marcos from Warwick’s School of Life Science, and Liliana Costa and Hugh Dickinson from Oxford’s Department of Plant Sciences. As Jose says “these findings have significant implications for global agriculture and food security, as scientists now have the molecular know-how to manipulate this gene by traditional plant breeding or through other methods in order to improve seed traits, such as increased seed biomass yield. To meet the demands of the world’s growing population in years to come, scientists and breeders must work together to safeguard and increase agricultural production”.
* Liliana M. Costa, Jing Yuan, Jacques Rouster, Wyatt Paul, Hugh Dickinson and Jose F. Gutierrez-Marcos, (2012) Maternal Control of Nutrient Allocation in Plant Seeds by Genomic Imprinting Current Biology.. 22, 160–165 doi:10.1016/j.cub.2011.11.059.