Plant Cuttings

For as long as the Casparian strip (CS) – an impervious component of the endodermal cell wall that blocks the passive flow of water and solutes into the vascular tissues of a plant (http://en.wikipedia.org/wiki/Casparian_strip) – has been known (discovery credited to Robert Caspary in the mid 1860s; http://www.biologie.uni-hamburg.de/b-online/e06/06h.htm#caspari), it has been considered to contain lignin (http://en.wikipedia.org/wiki/Lignin) and/or suberin (http://en.wikipedia.org/wiki/Suberin). And, since both of those materials have waterproofing properties, does it really matter whether one or other or both were found there? Well, it certainly mattered to Sadaf Naseer et al., whose investigations in arabidopsis allow the unequivocal conclusion that the CS is made of lignin ( … or a ‘closely related lignin-like polymer’; PNAS109: 10101–10106, 2012). Furthermore, Naseer et al. unambiguously demonstrated that ‘completely abrogating suberin accumulation’ (i.e. using suberin-deficient plants) still allows the establishment of an efficient barrier to penetration of PI (propidium iodide, used as an apoplast tracer in the study; http://en.wikipedia.org/wiki/Propidium_iodide). This elegant structure–function investigation presents precise developmental staging of the appearance of various histochemical stains for suberin and lignin using whole-mount staining procedures, combined with functional assays, reporter gene expression analysis and various pharmacological and novel genetic manipulations of lignin and suberin production. But for those who like a news items with no loose ends, this study is not quite as definitive as one might like, because the CS does contain suberin … And with almost precision timing this internal water-proofing revelation follows the announcement that Trevor Yeats et al. have determined the enzymatic basis of the polymerisation of cutin (Nature Chemical Biology8: 609–611, 2012), the polyester polymer of plant cuticles (which famously waterproof the exterior of aerial plant parts; http://en.wikipedia.org/wiki/Plant_cuticle). Experimenting with cutin deficient 1 (cd1), a cutin-deficient mutant(!) of tomato, the team propose that not only is it an acyltransferase – CD1 – the principal catalyst of cutin polymerisation, but also that the polymerisation process is extracellular and occurs at the site of cuticle deposition. Image: Wikimedia Commons. This year's transit of Venus (http://en.wikipedia.org/wiki/Transit_of_Venus,_2012) reminds me – albeit belatedly – to applaud the hard-working botanists of the Universities of Nottingham (UK), Ghent (Belgium), Leeds (UK) and Lyon (France) who have been exploring a more down-to-Earth – and ultimately more useful? – kind of Venus. Like all good phytologists they are interested in understanding the role of auxin in plant development (http://en.wikipedia.org/wiki/Auxin). In order to get a better understanding of auxin biology it is important to identify where it actually works inside plant cells; a tall order, locating a tiny molecule in a small cell. Hitherto, a so-called DR5-based protocol was employed to monitor auxin distribution in planta. However, this reporter activity does not directly relate to endogenous auxin abundance. Recognising this limitation, Géraldine Brunoud et al. have developed ‘A novel sensor to map auxin response and distribution at high spatio-temporal resolution’ (Nature482: 103–106, 2012). The group have engineered a new synthetic auxin signalling sensor in the model plant Arabidopsis thaliana using VENUS (a fast-maturing form of yellow fluorescent protein; http://en.wikipedia.org/wiki/Yellow_fluorescent_protein) fused to the Aux/IAA auxin-interaction domain (termed domain II; DII), hence its name DII-VENUS. Amongst DII-VENUS' attributes are that its abundance is dependent on auxin, and it provides a map of relative auxin distribution at cellular resolution in different tissues. As proof of its usefulness, its rapid degradation in response to auxin has been exploited to visualise dynamic changes in cellular auxin distribution during two developmental responses, root gravitropism and lateral organ production at the shoot apex. A most pleasing symmetry, celestial transience mirrored in a more terrestrial context. Fiat lux! Image: ‘Crabtree watching the Transit of Venus A.D. 1639’ by Ford Madox Brown; a mural at the Town Hall of Manchester, UK. 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’ (http://www.gatesfoundation.org/Grants-2012/Pages/John-Innes-Centre-OPP1028264.aspx) 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 (http://en.wikipedia.org/wiki/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 (http://en.wikipedia.org/wiki/Mycorrhiza) 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’; Lobell et al., Annual Review of Environment and Resources34: 179–204, 2009)? Because such fertilisers are not only too expensive 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 (http://news.jic.ac.uk/2012/07/cereals-self-fertilise/). 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 (http://en.wikipedia.org/wiki/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 (http://www.n2africa.org/), 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 (Current Opinion in Plant Biology13: 556–564, 2010), ‘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 (PNAS109: 12302–12308, 2012) second Green Revolution (GR 2.0; http://en.wikipedia.org/wiki/Green_Revolution), 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 (Ezezika et al., Nature Biotechnology 30: 38–40, 2012); regardless of how environmentally sympathetic the science may be, hearts and minds will need to be won over too. Image: Scene from a mural in the tomb of Sennedjem, Egypt. In this global-climate-change- obsessed world we frequently hear the term carbon sink, which is a ‘natural or artificial reservoir that accumulates and stores some carbon-containing chemical compound for an indefinite period’ (http://en.wikipedia.org/wiki/Carbon_sink). The emphasis on carbon is understandable since carbon dioxide is one of the most important greenhouse gases (http://en.wikipedia.org/wiki/Greenhouse_gas) whose accumulation in the atmosphere contributes to an increase in global temperatures – the greenhouse effect (http://en.wikipedia.org/wiki/Greenhouse_effect). Intuitively, one way to reduce that aerial CO2 burden is to reduce the amount added to the air, and/or increase the rate at which it is removed from the atmosphere. And the botanically inclined amongst you will surely suggest that maintaining, or even increasing, areas of vegetation – which by dint of that marvellous photobiochemical process known as photosynthesis (http://en.wikipedia.org/wiki/Photosynthesis) consume CO2 – will be a desirable thing. Which is why traditionally we have assumed large forest areas to be such important carbon sinks – not only do they remove CO2 in photosynthesis, much of the carbon is fixed within the trunks and stored for as long as the tree continues to grow. And hence major concerns over the rate at which forests are being removed in such areas as the Amazon (http://en.wikipedia.org/wiki/Amazon_deforestation). Well, it seems that in concentrating on the big things we have ignored very small things that are also major players that help to make a lot of this sink construction possible. It is easy to forget that building such massive structures as trees not only needs carbon, but also nitrogen and all of the other essential plant nutrients (http://www.soils.wisc.edu/~barak/soilscience326/listofel.htm). So, regardless of how much carbon is available, the value of trees as a carbon sink will be limited by the factors that limit plant growth. That is why you should not ignore the contribution of the photoautotrophic cryptogams (http://en.wikipedia.org/wiki/Cryptogam) – cyanobacteria, algae, fungi, lichens and bryophytes. Being so small they are easily overlooked, yet can cover large areas of such surfaces as soil, rocks and even other plants, and according to Wolfgang Elbert et al. (Nature Geoscience5: 459–462, 2012), globally they may account for almost 50% of terrestrial biological nitrogen fixation. Which, in turn, is necessary to help make atmospheric nitrogen biologically available to other organisms, such as trees, whose role as carbon sinks is so important in ameliorating atmospheric carbon levels … And we shouldn't underestimate the importance of these land- and oceanic- (see next item) based carbon sinks, whose global carbon uptake has almost doubled in the past 50 years according to Ashley Ballantyne et al. (Nature488: 70–72, 2012). Image: Jialiang Gao, Wikimedia Commons. Whilst forests – aided and abetted by cryptogams (see previous item) – have a major role as biotic carbon sinks on land, in the oceans that role is largely down to the activity of cryptogamous phytoplankton, which ‘draw-down’ vast amounts of CO2 during photosynthesis. However – and unlike trees – much of that aquatic primary productivity is consumed by herbivores, which in turn are preyed upon by various levels of carnivores. Ultimately, a lot of the CO2 that is fixed is released quite soon thereafter in respiration. Which is why attempts to consign such fixed carbon retained in the bodies of the algae (before it can be consumed and respired by hungry herbi-/carnivores) to the ocean depths – and thereby place it out of reach of the atmosphere where it could contribute to global warming – are quite attractive. Hence the notion of iron fertilisation (http://en.wikipedia.org/wiki/Iron_fertilisation), which aims to promote phytoplankton growth by addition of that essential plant nutrient, which is in short supply in large parts of the oceans. Whilst attempts at this manipulation to date have succeeded in promoting algal growth, none have unambiguously demonstrated the necessary mass deep-sinking events of the fixed carbon that would lead to it being appropriately sequestered at depths that preclude its rapid return to the atmosphere. However, Victor Smetacek et al., analysing the European Iron Fertilization Experiment (EIFEX) – carried out early in 2004 in ‘the closed core of a vertically coherent, mesoscale eddy of the Antarctic Circumpolar Current’ – conclude that at least half the diatom bloom biomass sank far below a depth of 1000 m and that a substantial portion is likely to have reached the sea floor (Nature487: 313–319, 2012) as a ‘fluff layer’. And – encouragingly – iron-fertilised diatom blooms ‘may sequester carbon for timescales of centuries in ocean bottom water and for longer in the sediments’. But I've yet to find out why these promising results were seemingly sequestered from public gaze for 8 years … And if this biological sequestration isn't up to the job, we'll just have to hope that the oceans themselves continue to soak up the excess CO2 (Jean-Baptiste Sallée et al., Nature Geoscience5: 579–584, 2012). Image: Hannes Grobe, Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany/Wikimedia Commons. Animals are well known for recognising a good thing when they see it. So, too it seems are carnivorous plants (http://en.wikipedia.org/wiki/Carnivorous_plants) – those erstwhile gentle botanics that are not averse to digesting the odd fly or two to supplement their nitrogen intake. Well, that seems to be the case for Swedish Drosera rotundifolia (common or round-leaved sundew; http://en.wikipedia.org/wiki/Drosera_rotundifolia) at least. Using nitrogen isotope measurements, Jon Millett et al. (New Phytologist195: 182–188, 2012) demonstrated this carnivore's remarkable opportunistic nutritional plasticity – plants in areas that received the greatest levels of N deposition (from the atmosphere) obtained a smaller proportion of N from prey (via their modified leaves) than did those with lower or intermediate depositional N levels (and who were less reliant upon root-sourced N). This may also be an example of ‘every cloud has a silver lining’ (http://www.goenglish.com/EveryCloudHasASilverLining.asp) since the N that is aerially sourced onto the studied ombrotrophic mires is derived from what is otherwise known as acid rain, which elsewhere has caused serious environmental damage to many Scandinavian lakes (http://nadp.sws.uiuc.edu/educ/acidrain.aspx). Canny critters, carnivores! However, the study also found that plants that gained more N from prey had an enhanced nutritional status (higher tissue %N). Which maybe also supports the notion that if you have to ‘forage’ for your food, you are fitter than those ‘couch potatoes’ (http://en.wikipedia.org/wiki/Couch_potato) who just sit around to be waited upon … ? Image: Petr Dlouhý, Wikimedia Commons. As one of the most obvious indications of growth, you might have thought that there was little new to discover about increase in length of plant stems. Wrong! Studying growth in Iriartea deltoidea (Arecaceae; http://en.wikipedia.org/wiki/Palmaceae), Heidi Rinninger and Nathan Phillips have uncovered an ‘efficient and novel method for height growth’ (American Journal of Botany99: 607–613, 2012). Typically, one is taught that increase in height of trees – primary growth (https://utextension.tennessee.edu/publications/Documents/W227.pdf) – is the result of apical meristem activity (formation of new cells near the end of the stem that increase in length as they develop and differentiate comparatively close to the meristem; http://en.wikipedia.org/wiki/Meristem). But in this palm the duo found that stems were lengthening after their tissues were differentiated and far below the apical meristem. Although such a phenomenon might be presumed to damage the non-extensible mature conducting cells of the vascular tissues, the vascular bundles are laid down in a spiral which apparently ‘unwinds’, resulting in the observed length increase. The pair consider this so-called ‘secondary lengthening’ not only to be the most efficient method for height growth in terms of carbon investment, but also a mechanism that permits plasticity in height growth rates for more rapid growth when short-lived canopy gaps are present than they would have with apical growth alone. As they tantalisingly conclude, ‘This finding also highlights the fact that in addition to biodiversity, tropical rainforests also contain a high degree of “physiological” diversity, the depth of which is just beginning to be discovered’. Slinky (http://en.wikipedia.org/wiki/Slinky)! Image: Roger McLassus, Wikimedia Commons.