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.
Not content with just being grateful for all of the marvellous things that plants do and provide, we humans always seem to want them to do even more. Well, in that vein there has been a veritable avalanche of stories that exploit the impressive chemical synthetic abilities of plants. Moran Farhi et al. have managed to persuade tobacco to manufacture artemisinin. Why? Artemisinin – and its derivatives – are a group of drugs that possess the most rapid action of all current drugs against Plasmodium falciparum malaria, a mosquito-borne infectious disease estimated to kill 2.23% of the world’s population according to the World Health Organization’s (WHO) 2011 World Malaria Report. Yes, but why in tobacco? Although artemisinin is isolated from its namesake Artemisia annua, low-cost artemisinin-based drugs are lacking because of the high cost of obtaining natural or even chemically synthesized artemisinin. Elsewhere, Xing Xu and colleagues have created recombinant human collagen in transgenic maize. But not only that, they importantly demonstrate that such a ‘system’ has the ‘potential to produce adequately modified exogenous proteins with mammalian-like post-translational modifications that may be required for their use as pharmaceutical and industrial products’. And, exploiting another major crop for human protein ends, Yang He et al. have designed rice to make human serum albumin (HAS), at levels >10% of the total soluble protein of the rice grain. Proper human-derived HSA is in short supply because of limited availability of donated blood, but is widely used in production of drugs and vaccines, and in treatment for severe burns, liver cirrhosis, and haemorrhagic shock. As the authors conclude, ‘Our results suggest that a rice seed bioreactor produces cost-effective recombinant HSA that is safe and can help to satisfy an increasing worldwide demand for human serum albumin’. Finally, news that an anti-HIV (Human Immunodeficiency Virus, which causes AIDS, Acquired ImmunoDeficiency Syndrome) antibody produced in GM tobacco underwent clinical trials in the UK in 2011. Testing was intended to establish how safely and effectively the vaginally applied product stops HIV transmission and was carried out under the watchful eyes of the UK’s Medicines and Healthcare products Agency (MHRA) at the University of Surrey Clinical Research Centre. Apparently the UK was chosen for the honour of this pharmaceutical ‘first’ because the Pharma-Plant Consortium – which is leading the trial – were put off by the level of fees required by the EMA (European Medicine Agency). So, UK vs. The Rest of Europe (c’est la vie, again…). Supporters of this whole approach to human exploitation of plants – so-called molecular farming – argue that: protein drugs could be made more efficiently and cheaply inside GM crops, since plants are extremely cost-effective protein producers; mass producing medicines in GM plants uses lower-cost tech than those of biopharmaceuticals made in huge stainless steel fermentation vats containing bacteria or mammalian cells; production costs could be 10 to 100 times lower than using conventional bioreactors; and the relatively simple manufacturing process could be transferred to developing countries. Now, surely, those fantastic plant pharma feats have got to be more impressive than getting a silkworm to spin spider silk, as reported by Florence Teulé et al.! But, if I’ve (unintentionally!) awakened in you a desire to discover more arachnoid antics, try Martin Humenik and colleague’s review of recombinant spider silks.
This is a video of a flour mill with two stones in a small shop outside the gates of Punjab Agricultural University, Ludhiana, India. The film shows the grain being poured into the hoppers above the stones, and then going between the millstones. From one stone set, the brown/wholemeal flour goes into sacks; in the other, it goes straight onto a shaking sieve, with the bran and germ going to the back and the white flour coming out of the front of the sieve. It is then collected and bagged, here in 10kg sacks, for use without further grinding or bleaching. The mill also makes maize meal and gram (lentil/chickpea) flour. The wheat flour will be mostly used for making chapati (chappati) or roti breads, thin dough cooked on a heated stone. The flour has high gluten content to give the elasticity to the dough so it can be stretched to be thin.
The YouTube video of the mill grinding wheat flour and its sieves is at http://youtu.be/1tPWqpj4680
In the west, most flour is milled using multi-stage roller mills that first remove the bran and germ, and then grind the flour/starch/endosperm, usually slightly finer that this mill. In this shop, the separation of the two stones can be adjusted to change fineness while they are running, a process not shown in the video. In the Punjab and Ludhiana, there were several similar roadside shops for flour mills (some with only one stone pair), and other shops which crushed oilseeds for oil and meal.