tag:theconversation.com,2011:/africa/topics/plant-health-5309/articlesplant health – The Conversation2015-12-22T11:22:06Ztag:theconversation.com,2011:article/472352015-12-22T11:22:06Z2015-12-22T11:22:06ZPlant viruses: from crop pathogens to key players in bio-nanotechnology<figure><img src="https://images.theconversation.com/files/106468/original/image-20151217-8081-2ykemk.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Orchid infected with the Tobacco mosaic virus. </span> <span class="attribution"><span class="source">Department of Plant Pathology Archive North Carolina State University - USDA Forest Service</span></span></figcaption></figure><p>Plant viruses are sub-microscopic parasites that have been studied primarily because they cause devastating diseases in crop plants. But in recent years, scientists have discovered they’re not just bad news; they also form symbiotic relationships with plants and the microbes they host. </p>
<p>Moreover, plant virus genomes have been engineered for use in <a href="http://www.hindawi.com/journals/bmri/2015/932161/">gene delivery</a> and are also being increasingly used as <a href="http://www.ncbi.nlm.nih.gov/pubmed/22038411">nanoparticles in bio-nanotechnology</a>. </p>
<h2>A range of symptoms</h2>
<p>More than 1000 virus species have been described that infect cultivated plants worldwide. They have the capacity to cause major epidemics and total crop loss, which is particularly devastating in developing countries that depend on a few staple crops for food security and income. For example, the <a href="http://www.cabi.org/isc/datasheet/2747">Cassava Mosaic Disease</a> epidemic in many African countries is caused by several species of <a href="http://viralzone.expasy.org/all_by_species/111.html">Begomoviruses</a>, including African Cassava Mosaic Virus. These are transferred from plant to plant by whiteflies.</p>
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<img alt="" src="https://images.theconversation.com/files/106490/original/image-20151217-8093-b3n0oy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/106490/original/image-20151217-8093-b3n0oy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=305&fit=crop&dpr=1 600w, https://images.theconversation.com/files/106490/original/image-20151217-8093-b3n0oy.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=305&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/106490/original/image-20151217-8093-b3n0oy.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=305&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/106490/original/image-20151217-8093-b3n0oy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=383&fit=crop&dpr=1 754w, https://images.theconversation.com/files/106490/original/image-20151217-8093-b3n0oy.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=383&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/106490/original/image-20151217-8093-b3n0oy.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=383&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">Plum pox in apricot.</span>
<span class="attribution"><a class="source" href="https://en.wikipedia.org/wiki/Plum_pox#/media/File:Plum_pox_in_apricot.jpg">wikimedia</a></span>
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<p>Often viruses do not completely destroy crops but decrease their yield and quality. In fruit and vegetable crops, <a href="http://www.cabi.org/isc/datasheet/42203">Plum pox</a> virus affects stone fruit trees, while <a href="http://www.apsnet.org/edcenter/intropp/lessons/viruses/Pages/TomatoSpottedWilt.aspx">Tomato spotted wilt </a> and <a href="http://westernfarmpress.com/controlling-tomato-yellow-leaf-curl-virus">Tomato yellow leaf curl</a> viruses cause severe losses in tomatoes. <a href="http://www.potatovirus.com/index.cfm/page/pvyinfo.htm">Potato virus Y</a> affects potatoes and many vegetables.</p>
<p>Even more benign viruses can build up in planting material of crops propagated through cuttings, roots or tubers. This leads to degeneration and yield losses in subsequent growing seasons. In potato production systems, virus incidence can be high with tubers infected with five or six different viruses. Such degeneration depresses both yield and quality. <a href="http://link.springer.com/article/10.1007%2Fs11540-011-9190-5">Studies in sub-Saharan Africa</a> have shown that yield increases of 30-50% are possible when farmers replenish their stock with healthy planting material. </p>
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<img alt="" src="https://images.theconversation.com/files/106502/original/image-20151217-8109-148y7ns.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/106502/original/image-20151217-8109-148y7ns.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=928&fit=crop&dpr=1 600w, https://images.theconversation.com/files/106502/original/image-20151217-8109-148y7ns.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=928&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/106502/original/image-20151217-8109-148y7ns.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=928&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/106502/original/image-20151217-8109-148y7ns.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1167&fit=crop&dpr=1 754w, https://images.theconversation.com/files/106502/original/image-20151217-8109-148y7ns.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1167&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/106502/original/image-20151217-8109-148y7ns.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1167&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">17th century watercolor.</span>
<span class="attribution"><a class="source" href="https://en.wikipedia.org/wiki/Tulip_mania#/media/File:Semper_Augustus_Tulip_17th_century.jpg">wikimedia</a></span>
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<p>However, in the past not all virus symptoms were thought to be bad. The symptoms of <a href="https://www.rhs.org.uk/advice/profile?PID=254">Tulip breaking virus</a> – broken colour on the petals – can be seen in the 17th century paintings of Dutch masters, for example. At that time, bulbs giving rise to such flowers were highly prized. It was only in the 20th century that these bulbs were known to be infected with a virus. </p>
<p>Plant viruses can be detected by fast and sensitive laboratory methods. In addition, virus spread can be controlled with pesticides which kill the natural vectors such as insects or nematodes that transmit viruses from plant to plant. Many types of insects transmit viruses, including aphids, thrips and whiteflies – and their populations are increasing in size and geographic range, an effect expected to increase with global warming. For that reason host resistance is the most sustainable and effective way to control viruses in crop plants.</p>
<h2>Good viruses</h2>
<p>Recent studies have shown that many plants in natural populations are infected with viruses – many of which cause <a href="http://www.nature.com/nrmicro/journal/v9/n2/full/nrmicro2491.html">no obvious symptoms</a> but persist and are transmitted by seed to subsequent generations. The reason for these cryptic virus-host associations is not yet clear. However, as occurs in other host-virus interactions in mammals or bacteria, some viruses can exist in symbiotic association and have a beneficial effect. A good example is <a href="http://www.sciencemag.org/content/315/5811/513.abstract">a study</a> of <a href="http://www.tropicalforages.info/key/Forages/Media/Html/Panicum_maximum.htm">tropical panic grass</a> growing in Yellowstone National Park where the soil temperature can exceed 50°C. The grass can only tolerate these conditions through a symbiotic association with a fungus infected with a virus. </p>
<p>Several disease-causing plant viruses can also protect their host plants from abiotic stresses such as cold or drought conditions. Plants infected with <a href="https://www.rhs.org.uk/advice/profile?pid=143">Cucumber mosaic</a>, <a href="http://viralzone.expasy.org/all_by_protein/134.html">Brome mosaic</a> and <a href="http://www.extension.umn.edu/garden/yard-garden/vegetables/tomato-tmv-disease/">Tobacco mosaic viruses</a> tolerate drought conditions better than uninfected plants. Tobacco mosaic virus was first recognised as an infectious agent in 1898 and research on it continues to push the boundaries of knowledge, including in bio-nanotechnology. </p>
<p>Plant viruses are natural nanoparticles, and viruses such as the tubular rod-shaped Tobacco mosaic virus and icosahedral <a href="http://www.ncbi.nlm.nih.gov/pubmed/20455698">Cowpea mosaic virus</a> have been used in a number of applications. By genetically manipulating the virus genomes and inserting gene coding for other molecules, scientists have managed to make host plants infected with the virus produce additional molecules. The molecules can be expressed fused to the surface of the virus particles or expressed as free protein. We can then harvest the leaves of the plants and purify the molecules for vaccines or other <a href="http://www.pharmaceutical-technology.com/features/feature122248/">valuable pharmaceuticals</a>.</p>
<p>An emerging area is to use virus particles as “nanoscaffolds”. Here, the particle surfaces are genetically or chemically altered and certain compounds, peptides and enzymes are added. For example, nano structures based on the Tobacco mosaic virus can stabilise magnetic liquids in transistor components. In fact, the future use of plant viruses as components in materials-, plant- and biomedical sciences is only limited by our imagination.</p><img src="https://counter.theconversation.com/content/47235/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Lesley Torrance receives funding from the BBSRC and the Scottish Government's Rural and Environment Science and Analytical Services Division (RESAS). </span></em></p>They’re good, they’re bad and they’re useful: we are still discovering what we can do with plant viruses.Lesley Torrance, Professor of Plant Biology, University of St AndrewsLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/398902015-04-10T15:03:51Z2015-04-10T15:03:51ZThe famous olive trees of Puglia are ravaged by disease – here’s how we can save them<figure><img src="https://images.theconversation.com/files/77534/original/image-20150409-15244-1r08rsu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The whole olive-growing region of Puglia has been devastated.</span> <span class="attribution"><span class="source">olive grove by risteski goce/www.shutterstock.com</span></span></figcaption></figure><p>A common, humble field bug is spreading a disease that has already infected millions of olive trees in Italy. Olive and citrus fruit crops throughout the Mediterranean are threatened, yet there has been a collective failure to recognise the danger and take decisive action.</p>
<p>In Puglia in southeastern Italy, <a href="http://www.euractiv.com/sections/agriculture-food/italy-resists-massive-destruction-olive-trees-because-bacteria-313357">241,000 hectares of olive groves</a> have been restricted, with at least 10% of the 11m olive trees in the worst-affected region, Lecce, infected by the <em>Xylella fastidiosa</em> bacterium. The disease has almost entirely wiped out the region’s olive trees in the two years <a href="http://www.oliveoiltimes.com/olive-oil-making-and-milling/europe-puzzled-by-olive-tree-epidemic/37219">since it was first detected</a>, and threatens the olive crop elsewhere in the country.</p>
<p>Olive trees often hundreds of years old are completely destroyed by the wasting disease, which leaves them looking dehydrated as if burnt. The disease has spread at epidemic levels – quickly, and with devastating effect. Besides the great economic damage, olive trees are a tremendous cultural symbol of the region, and their loss is a huge blow to the region’s identity. There’s also a convincing argument that the epidemic will spread to neighbouring regions in Italy, and on to other parts of the Mediterranean. So interested countries such as France and Greece are demanding Italy take rapid steps to control the disease, before it gets worse.</p>
<h2>Slash and burn</h2>
<p>Yet so far the only real action taken has been to apply the European Union protocol that requires the destruction of any olive tree and vegetation for a mile around the border of the affected zone, in this case the Salentinian Peninsula. Will this stop the diffusion of <em>Xylella</em>? The bacterium spreads easily as it cleverly uses a partner, the burrowing <a href="http://www.britannica.com/EBchecked/topic/371494/meadow-spittlebug">meadow spittlebug</a> (<em>Philaenus spumarius</em>), one of the most abundant field insects, in order to enter the tree.</p>
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<img alt="" src="https://images.theconversation.com/files/77533/original/image-20150409-15216-11c5qcx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/77533/original/image-20150409-15216-11c5qcx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/77533/original/image-20150409-15216-11c5qcx.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/77533/original/image-20150409-15216-11c5qcx.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/77533/original/image-20150409-15216-11c5qcx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/77533/original/image-20150409-15216-11c5qcx.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/77533/original/image-20150409-15216-11c5qcx.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">Olive trees are often hundreds of years old.</span>
<span class="attribution"><span class="source">fotometin</span></span>
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<p>The border management strategy so far reported by the Italian and EU authorities offers only tentative steps to limit the spread of the pathogen. It’s very hard to see that this will have any significant effect: the spittlebugs are not great flyers but can be carried many kilometres in a single day by the wind or on humans or animals. Officials have also discussed an export ban on the plants, but total control is quite impossible in a region where olive trees are so important. </p>
<p>I was consulted immediately after the disease was discovered, but various proposals to tackle the infection were rejected in favour of a policy that only followed the EU protocol, which we know to be ineffective. Since then I and many other researchers have worked in the lab to test other solutions.</p>
<h2>More than just pesticide</h2>
<p>Pesticides have performed an invaluable service around the world, preventing crop losses to various insect pests, and controlling the spread of insect-borne diseases such as malaria, dengue, or <a href="http://www.who.int/mediacentre/factsheets/fs100/en/">yellow fever</a>. But there are several chemicals that are no longer effective due to growing resistance – from microorganisms developing resistance to antibiotics, to insects becoming resistant to insecticides. It’s possible that in the next 20 to 30 years all the currently used pesticides will have lost their effectiveness. </p>
<p>It’s possible to attack the bacterium by treating the olive trees with antibiotics, but this is extremely expensive and too complicated to administer to so many trees. The spittlebugs can be controlled by insecticide — in fact the bug’s larval stage is the best route, as it’s possible to apply insecticide before they have grown wings to fly. But current chemicals are not useful, so we are working on a new line of insecticides based on natural products to tackle the <em>Xylella</em> problem.</p>
<h2>Three-point plan needed</h2>
<p>The olive tree’s disease has three main players, the bacterium, the insect, and probably a symbiotic fungus of which we understand very little, in addition to the olive plant itself. They work together, acting like a “superorganism”. It is a very complex system, but in some way also very efficient. </p>
<p>The only way to face the <em>Xylella</em> challenge is integrated pest management, tackling all elements at once. A soil treatment to sustain the plant, a larvicidal insecticide to control the spittlebug, an antifungal treatment, and a natural, low-cost and eco-friendly antibiotic to treat the plant to help prevent infection from becoming established. It’s vital that we act strongly and soon, as the disease has already spread far.</p>
<p>Finally, perhaps the biggest challenge will be to gain the active cooperation of the local population and farmers, who so far have only <a href="http://www.telegraph.co.uk/news/worldnews/europe/italy/11440639/Italy-warns-deadly-olive-tree-bacteria-could-spread-across-Europe.html">suffered damage from the loss of their trees</a>, and for whom the idea of slashing and burning millions of century-old trees is positively sacrilegious – even if it may be the best course of action in the long term.</p><img src="https://counter.theconversation.com/content/39890/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Marcello Nicoletti does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Thousands of miles of olive groves across southeastern Italy are threatened by the Xylella fastidiosa bacterium.Marcello Nicoletti, Professor of Pharmaceutical Biology, Sapienza University of RomeLicensed as Creative Commons – attribution, no derivatives.