Viral Diseases In Africas Food Crop Cassava Biology Essay

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Food security can be defined as a situation where by 'all people, at all times, have physical and economic access to sufficient, safe and nutritious food to meet their dietary needs and food preference for an active and healthy life' (FAO 1983). The main three factors affecting food security are the availability (usually due to changes in crop productivity), access (affordability and allocation) and utilization (production, distribution and exchange) of food (Gregory et al. 2005); conflict affecting any one of these factors may cause an increased susceptibility to food insecurity in a given area.

Currently 21 out of the 36 reported countries worldwide facing food insecurity are situated in sub-Saharan Africa (UN 2009). The current state of Africa's food security is exacerbated by a number of problems including shortage of food, political instability, social conflict, population increase and a decrease in productivity due to plant and human disease. Achieving food security is a particular problem for Africa because it was by-passed by the "Green Revolution" of the 1960s and 70s where many other food insecure countries benefited from increases in agricultural production and modern farming techniques. As a result, it is predicted that by 2020, Sub-Saharan Africa will account for 59 per cent of the total number of food insecure people in the world, exceeding 500 million (USDA 2010); the only continent predicted to become more food insecure in the future if the current situation is left to continue.

In this essay particular emphasis will be put on viral diseases of Africa's key food security crop cassava and how biotechnology is providing promising solutions in the form of resistant varieties; secondly I will highlight how global attitudes are having both positive and negative impacts on Africa's food security and the recent acquisition of land in Africa by foreign investors; and finally, whilst the last two factors have persisted for some time, the fairly recent challenge of combating climate change will also be briefly discussed.

Viral diseases of cassava.

Plant diseases in Africa have been estimated to cause enormous pre-harvest losses of 48.9 per cent (Oerke et al.1994); these include diseases caused by bacteria, protozoa, pests and weeds. A particular challenge to crop productivity are plant viruses which have been difficult to overcome as there is not yet an effective method of conferring 100 per cent resistance. Attempts by scientists at creating more virus resistant strains of key crops such as cassava and maize may be the only feasible solution to preventing large crop losses where conventional disease prevention techniques are failing.

In some cases non conventional approaches provided by engineering genetically modified (GM) crops overcome the drawbacks of conventional breeding. One conventional control method of plant viruses is the cross breeding and growing of resistant crop varieties that already have highly effective sources of resistance. However, where this "natural" resistance has not been identified for certain crops scientists must look for alternative solutions.

An example of a key food security crop that has no such "natural" resistance is Cassava (Manihot esculenta, Family Euphorbiaceae), which was originally brought over to Africa from Latin America. It is a major source of food in Africa due to its drought resistant traits and ability to grow in poor soil conditions and latest figures show that it is cultivated in a total area of 12.3 million hectares with a total production of 12.5 million tonnes, contributing to over half (53%) of the world's total production of Cassava (FAOSTAT 2009). The importance of cassava as a food crop in Africa is evident when comparing the worlds average annual cassava consumption of 16.46 kg/capita, with Africa's 74.12 kg/capita (FAOSTAT 2007).

Unfortunately cassava is vulnerable to one of the world's most damaging viral diseases, African Cassava Mosaic Disease (ACMD), which results in an estimated world economic loss of US$1.9-2.7 billion (£1.2-1.7 billion) per year (Patil & Fauquet. 2009). The problem is exacerbated by the viruses ability to affect not only cassava but other important African crops such as soybean (Mgbechi-Ezeri et al. 2008) which highlights the urgency in finding a general geminivirus solution capable of conferring broad spectrum resistance to ACMD.

ACMD is thought to be caused by six circular single stranded RNA viruses (Family: Geminiviridae; Genus: Begomovirus) (Fauquet et al. 2008) with the majority of work carried out on African cassava mosaic virus(ACMV) and east African cassava mosaic virus (EACMV). All six viruses have a bipartite genome consisting of a DNA-A component encoding six genes (AC1-AC4, AV1 and AV2) which are responsible for processes involved in replication of viral material. The second component DNA-B encodes two genes (BV1 and BC1) which are required for intra- and intercellular movement of viral DNA around the plant (Jeske, 2009). Both components are necessary for successful infection and spread of ACMD in a given host plant.

The main form of transmission of this virus from plant to plant is by a whitefly vector, Bemisia tabaci, feeding on infected plants. Planting of infected stem cuttings by farmers can also assist the spread of ACMD between countries. Once the plant is infected chlorosis of the leaves and stunted growth occur with the severity of symptoms varying with viral strain and environmental conditions. ACMV transmission and symptom development is currently being controlled by a number of methods including conventional cross breeding and vector management. Although these measures are slowing the spread, biotechnological approach should complement conventional techniques by finding desirable genes to provide longer lasting crop protection, improve crop production and thus increase food security (Borlaug 2000).

So far, varying levels of resistance have been reported regardless of the biotechnological techniques used. Scientists have mainly focussed on the AC1 gene as it is involved heavily in replication and interacts with several viral and host proteins. It is also highly conserved among bipartite geminiviruses so scientists may be able to confer multiple strain resistance to ACMD where plants have been infected by more than one viral strain.

More recently, RNA interference (RNAi) has achieved partial resistance to ACMV in the laboratory (Vanderschuren et al. 2009; Vanderschuren et al. 2007). This technique involves targeting viral double stranded RNA for destruction. It works by inserting RNA that contains sequences homologous to invading viral RNA and so is recognised by the host plant defence mechanism, which then leads to the breakdown and destruction of viral RNA. This prevents translation from occurring thus silencing the expression of the gene, vital for virus replication, from which the mRNA was transcribed.

Results using RNAi have showed a significant decrease in infection rate with the majority of transgenic lines transformed with the double stranded AC1 gene insert showing no mosaic symptoms and some plants showing increased resistance when compared to the wild type(plants that had been inserted with the AC1 gene) (Vanderschuren et al. 2009). However, when testing the resistant plant further with increased amounts of virus they found that the initial 100 per cent resistance from the first experiment was not enough to resist the increased viral load. In this case, wild type and transgenic lines showed similar symptom severity. Thus, the level of resistance depended on the amount of inserted double stranded RNA; those with a high accumulation had higher resistance and less severe symptoms. Not only does there need to be sufficient amounts to ensure complete and efficient silencing of viral mRNAs the efficiency of the process is dependent on the high level of sequence homology between inserted RNA and the targeted viral RNA.

In cases where a plant has been infected with two strains of the virus or the recombination of the two the symptoms of both viruses act synergistically to produce a more severe form (Adjata et al. 2009). ACMV and most plant viruses have a high recombination rate allowing them to overcome host plant defences and environmental challenges such as drought. There is evidence for a co-evolutionary arms race between the host plant and geminiviruses with the latter having recently evolved mechanism to produce suppressor proteins that counteract the hosts defence response of gene silencing, suggesting that gene silencing is a potent anti-viral defence (Raja et al. 2010).

Scientists are hopeful that by injecting multiple double stranded RNA they may be able to confer resistance to multiple strains of geminiviruses that plants are exposed to in the field; which could be helpful where incidences of severe mixed infections of ACMV and EACMV strains are occurring (Harrison et al 1999; Pita et al. 2001). Mixed infections are likely to become more common as different strains spread and overlap geographically with one another and there are recent reports that ACMV-EACMV form has spread across central Africa to the Republic of the Congo, Kenya and Tanzania (Ntawuruhunga et al. 2007). To prevent further spread molecular markers are being used to tag genes for CMD resistance genes, in particular the CMD1 and CMD2 genes, which may ease screening and selection process of resistant varieties to be grown by farmers (Bi et al. 2010).

An emerging threat to cassava in coastal lowlands of East Africa and more recently to highlands around Lake Victoria (Mbanzibwa et al. 2009a) is Cassava brown streak disease (CBSD). CBSD is thought to be caused by at least two diverse Ipomovirus species that belong to the family Potyviridae (Monger et al. 2001a), Cassava brown streak virus (CBSV) and Cassava brown streak Uganda virus (CBSUV), which are both monopartite, positive-sense, single stranded RNA viruses (Winter et al. 2010). Unlike ACMD, CBSD has received little research attention until recently.

On infection, plants show external symptoms of chlorosis along the veins and brown streaks on the stem. Internal symptoms vary greatly but mainly yellow or brown necrosis of the storage tissue render the crop unsuitable for human consumption. Consequently, this disease has serious implications on food security, as the extent of loss caused by the virus does not become apparent until the crop is harvested; wasting valuable resources and time put into growing it.

The vector for CBSD was originally thought to be the same as ACMD but there is now evidence that two white fly species, B. tabaci and Aleurodicus disperses, are responsible for CBSV infection of Cassava in coastal Kenya (Mware et al. 2010). Although these two whitefly have been linked to the CBSD the transmission rate in the laboratory is lower than when compared to those found in field conditions so there is difficulty in finding an efficient method of transmitting the virus readily into a susceptible laboratory host plant (Ogwok et al. 2010). Recently, biolistic inoculation, a technique to inject genetic material into a plant cells, has been used for ACMD with some success (Ayeh & Ramsell 2008) and it is thought that this technique will yield similar results for CBSD too.

Unlike ACMV, CBSV has some "natural" resistance which makes cross breeding a possibility for control. However, this would be very laborious and bio-technological approaches similar to those described for ACMV have already yielded high success in the lab, particularly by targeting the coat protein (CP) gene through RNAi. Results have shown that plants with the resistant CP gene inserted were immune against all six diverse isolates (from east Africa) of CBSV and CBSUV that were tested (Patil et al. 2011). As with all research into plant resistance plants are tested in specific environmental conditions in the laboratory and as virus severity varies greatly with environmental factors successful results in the laboratory may not be replicated if approved for field trials.

CBSV can be detected in plants whose symptoms are shown on their leaves using reverse transcriptase polymerase chain reaction (Monger et al. 2001b) but this form of screening has been criticised for being time consuming and expensive for African companies to test plants in bulk (Abarshi et al. 2010). Also as viruses evolve and recombine it is necessary to be able to detect presence of CBSV in plants infected with more than one strain; this has recently been developed (Mbanzibwa et al. 2010) and will be useful to restrict the spread of more severe forms. As with ACMV, sequencing genomes of CBSV isolates, of which there are several complete genome sequenced so far, may help distinguish genes that are able to confer resistance such as the P1 gene , which confers viral gene suppression (Mbanzibwa et al., 2009a D.R. Mbanzibwa, Y.P. Tian, S.B. Mukasa and J.P.T. Valkonen, Cassava Brown Streak Virus (Potyviridae) encodes a putative Maf/HAM1 pyrophosphatase implicated in reduction of mutations and a P1 proteinase that suppresses RNA silencing but contains no HC-Pro, J. Virol. 83 (2009), pp. 6934-6940. Full Text via CrossRef | View Record in Scopus | Cited By in Scopus (8)Mbanzibwa et al. 2009b).

In order for research into cassava to continue funding is very important. The Great Lakes Cassava Initiative project started in 2008 and funded by the Belinda and Bill Gates foundation is a 48 month long project that has allowed an estimated 1.15 million African farmers in six countries to benefit from research into improving varieties of cassava. Their goal is to improve food security by providing resistance to ACMD and CBSD by 2011. Results are yet to be published but if this project has proved successful similar initiatives targeting other key crops such as legumes and bananas will be implemented in the future.

Hypothetically speaking, if within the next decade scientists are able to increase cassava productivity through genetic engineering what will the effect on the Africa's food security be? Increasing the amount of crop harvested per hectare may allow greater access to food if distributed fairly but it will also put greater strains on resources needed to grow such crops. For example, a harvest of 25 tonnes/ha of cassava removes about 60 kg/ha of nitrogen, 40 kg/ha of phosphate and 136 kg/ha of potassium oxide (NARP Roots and Tubers Research Programme, 1996) from the soil. So when biotechnology increases the amount of crop that can be grown in a given area it needs to be met with an increase in resources such as fertiliser and water which are already scarce in Africa.

Critics argue that, for health reasons, cassava should not be heavily relied upon to improve Africa's food security (Wobeto et al. 2007) due to the high presence of cyanogens that interfere with digestion and uptake of nutrients as well as poisoning the consumer in higher doses(Nhassico et al. 2008). Although, cases of poisoning are rare (Teles FFF 2002) this could pose a problem for those relying on a single crop, mainly subsistence farmers, who are unlikely to be able to afford or possess the time to process cassava adequately (Montagnac et al. 2009).

Overall, plant disease can have devastating effects on crop production in Africa and although biotechnology will not solve Africa's food insecurity it will certainly have a positive impact on those that have the opportunity to benefit from it once techniques are approved for field trials. Emphasis should be put on improvement of local crops which do not posses "natural resistance" so that small holder farmers can benefit directly from improved resistant cultivars that they are familiar with growing.

The challenges to increase crop production are huge and resources to achieve this are scarce. Whilst improved varieties will improve productivity, it is soil fertility and irrigation practices that are likely to be limiting factors in boosting cassava growing in Africa. Rapid development over the last decade in cassava research is promising and is boosting food security in places that rely heavily on cassava. One of the main issues is that much of the research is carried out by the private sector so GM crops may not become widely available to small holder farmers who are unlikely to be able to afford the new varieties.

The effects of just two types of disease on a single crop are highlighted above but the reality is multiple diseases of multiple crops. The challenge of the future will be to confer broad spectrum resistance for all major crops in Africa whilst still keeping important traits that farmers and consumers desire. It is therefore vital that funding by foreign bodies for such projects is increased in the future.

Global attitudes and the "Land Grab".

Africa relies heavily on food aid, particularly from the U.S, and therefore much of Africa's food policy is shaped by input and attitudes from other countries. World attention is being drawn to agricultural development in Africa as President Obama's administration recently announced plans to make food security central to its Africa policy (Reuters 2010). This has been coupled with an acknowledgement by policy makers that there needs to be a significant increase in public investment in African agriculture (Financial Times, 2010). In light of this increased awareness will Africa finally get the level of help it needs to improve its food security or will global interference exacerbate the problem further in the form of "land grabs"?

A major conflicting issue is the use of GM food in African food aid and the adoption of EU's negative attitudes towards GM food by African policy makers. The extreme effects of European attitudes concerning GM food on African people is demonstrated by Zambia's rejection of food aid from America during the drought of 2002 over fears of losing its commercial export markets in Europe because the risk of african crops becoming contaminated with G.M seed (Science 2002). Consequently, George.W.Bush accused Europe of impeding America's effort at reducing hunger in Africa by voicing fears over GM crops sparking the GM debate further.

When considering GM food aid African governments do not seem to take into account that the needs of Africans are much different from those living in the EU where the rejection of GM food does not lead to mass starvation. Furthermore, despite sceptics accusing the U.S of using Africa as a testing ground (BBC News 2008), there is no significant evidence that it has had negative health effects on Americans who have been growing and consuming it for many years. However, the lack of evidence does not mean that there is no risk involved.

This is a stark example of how the decisions countries have about what they decide to grow has life and death impacts elsewhere. So much so that European views stunt the development of GM food in places like Africa where it is most needed and where full potential could be realised. Of course relying on food aid will not improve Africa's food security in the long term but it many need to start thinking about following in America's footsteps in the near future if it wants to be able to feed its growing population and become more self sufficient.

As one of the consequence of the food price crisis of 2007-08 there has been an increased interest in acquisition of farmland in Africa by foreign investors in order to secure their own future food security sparking a controversial debate over land. The major investors tend to be oil rich and food insecure countries like Saudi Arabia or countries with large and rapidly growing populations such as China (Gorgen et al. 2009). The main contentious issue is whether the procurement of Africa's land could be seen as a "land grab", as commonly reported by the media, or a development opportunity, a chance for countries to exchange land and labour for the modern technology and investment that is vital for developing their own food security (FAO 2009b).

Unfortunately, land deals are often elusive due to the lack of transparency and reluctance of investors and governments to publish the size and nature of deals. As a consequence, there are limited scientific reports on this matter and most of what is published comes from the media of which most are contradictory and of limited reliability. Thus, conclusions drawn must be treated with caution.

The media tend to take the view that the acquisition of land in Africa will have negative impacts on food security (FAO 2009; Independent 2009). These views are not unfounded as reports of resistance to the settlement of thousands of Chinese workers in Mozambique and the overthrowing of government in Madagascar over the lease of 1.3 million hectares for maize and palm oil highlight just some of the conflict land acquisitions have caused in recent years (von Braun & Meinzen-Dick 2009).

Ethiopia is a key example of the enormous scale in terms of size and number of these land acquisitions. It has approved 815 foreign-financed agricultural projects since 2007 and has offered 3 million hectares of fertile land to investors (Guardian 2010a); as a consequence reports of local people losing their land and resources on which they depended on for their own food security (Cotula et al. 2009a; Smaller and Mann 2009; UN 2010) implying that many deals were likely to be in the form of "land grabs" rather than development opportunities for Ethiopia.

The likelihood of conflict could be lessened if emphasis was placed on well-structured land deals which promote transparency. Where governments are not doing enough to protect local land owners international law should put in blanket provisions that all parties should abide by. However, enforceability of such laws may be a problematic as locals may not be aware or informed of their rights and are therefore more likely to be exploited by their government.

Another problem is that the African governments are creating the perception that there is lots of unused land when in reality most of the land is in use by locals. This leads to investors viewing Africa as the last place where there are large places of good agricultural land that is not occupied. Unfortunately there is not the legal framework in place to suggest otherwise (Cotula & Vermeulen 2009b).

The negative reporting of foreign input and attitudes by the media overshadows the positive effect global input can have on Africa's food security with the majority of the current agricultural development grants aimed at helping African countries. An example of a recent investment is from the Group of Eight who recently pledged US$20 billion over three years to help small-scale farmers in Africa to improve productivity (Financial times 2009). An increase in projects like this in recent years is an indication that there is an increasing global awareness that Africa has the land to become self sufficient and therefore could rely less on food aid in the future with financial support.

It could be argued that lack of funding is not the key issue it is making sure that money is channelled into well managed programmes and research. By investing in farmers rather than farmland (Cotula 2010) for example, through contract farming where a private company provides input in the form of fertilisers, seed etc, in exchange for fixed market price for the farmers produce. Investment like this that targets not only agricultural practices that will increase productivity but also improve market access by making trade easier for farmers in isolate locations to trade their produce (Anim 2010).

In addition, most of Africa's labour forces are unskilled when it comes to modern agricultural techniques so it is vital that investment includes skills training for local farmers to create a more sustainable solution to food insecurity. . Without adequate training there is a risk of migration of foreign skilled labour which displaces local workers, would lead to further vulnerability to food insecurity. Large scale acquisitions can be beneficial if in the form of contract farming but the recent World Bank report (World Bank Report 2010) suggest deals like these are not carried out and investors are targeting countries that do not have the infrastructure to protect its people.

Whilst global attitudes and input can negatively impact Africa through exploitation of farmland and labour they also provide the biggest support in terms of funding. If investment is channelled into well-managed programmes and leads to high productivity this may lead to greater food availability which will in turn push down food prices, while increased income for poor are likely to mean greater access to food (Wiggins 2009). In poor area's the problem of human disease affecting work force may be more of a problem than adequate food supply so by providing adequate sanitation and clean water there may be an indirect improvement in productivity.

The extent to which countries will invest in Africa in the future will be dramatically affected by the emerging threat of climate change which may lead to large amounts of land being converted for growing bio fuels, placing further pressure on food prices and firing the debate about land use in Africa further.

The emerging threat: climate change.

Climate change, although a fairly recent phenomenon is becoming the most reported threat to Africa's food security due to its ability to affect not only crop production but also access and utilization of food supply (Schmidhuber & Tubiello 2007). Research has mainly investigated changes in crop productivity caused by extreme weather conditions such as drought (Gregory et al. 2005). However, because climate change affects all aspects of food security this approach provides only partial assessment.

Many climate predicting models produce dramatically different results when assessing the impact climate change will have food security. However, one thing that is agreed is that warming in sub-Saharan Africa will be greater than the global average (IPCC 2007). As the affects of climate change will vary greatly across Africa, at different times and spatial scales, the poor are expected to suffer the most because they are less able to adapt to changes in their environment as they are more likely to be poverty stricken and heavily reliant on subsistence farming (UNICEF 2009). Like with the other two challenges to African food security it is important that coping strategies should be in the form of specific local policies rather than a general region wide approach because the affects vary greatly between regions and countries.

As a result of climate change higher food prices and a decline in cereal production are predicted with the greatest affect on crops such as wheat and sweet potato that require more water than hardy crops such as sorghum and millet. Africa relies heavily on rain with 89% of cereals rain fed in sub Saharan Africa (Igbadun et al, 2005) so by using biotechnological techniques such as molecular markers to find key genes in plant functioning to improve plant traits such as drought tolerance and water uptake this could improve productivity by creating higher yields of crops .

Another way to lessen the impact of climate change on Africa could be by improving food distribution and access to food aid during crisis by building more and better roads. For example, Sudan is the size of France but only has 40km of paved road. In areas like this people will suffer not because of the lack of food but because of the lack of investment in effective distribution of aid. The development of early warning systems (Huber 2010) so that food aid can be distributed to hard to access areas before the onset of a crisis would also increase access to food thus increasing food security.

Improving economic access to food by providing incentives to producers (to make more food) and consumers (to increase access to food) (Cooper 2004) could also improve regional food production. A consequence of this would be that more jobs are available, easy access to food, better incomes and increased food security.

In sum, there are multiple challenges and many conflicting views on how food security in sub-Saharan Africa should be achieved and not yet a coherent solution. Solutions proposed so far have been vague focussing on few aspects of food security, mainly looking at availability through increasing productivity, rather than looking at improving access and utilization and how these three factors interact.

Although Africa's food insecurity has caused much "doom-and-gloom" in the media it has spurred a rapid change in biotechnology and policies which will improve Africa's current food insecurity. One of the main problems is that achieving food security can be tautological in the sense that we aim to increase production of food to meet rising demands but this in turn will increase population rate further through higher reproduction and survival rates allowing competition to become fiercer for resources that are already scarce.

Plant disease is one of the main challenges to overcome in order for sub-Saharan Africa to reach its potential. Plant biotechnology is capable of increasing food supply to meet Africa's growing population but whether everyone will have equal access to it is questionable as the distribution and access to this new technology is still unbalanced across Africa. Funding research into Cassava is showing promising results in the form of resistant varieties but it is important that plans to increase production take into consideration the effects on future resources, particularly water security, as any benefits gained in the short term will not be sustainable.

Land deals are very much a global phenomenon, the effects of which are most widely felt in Africa where the legal framework is not in place to protect local farmers. It is because of this that land rights for small holder farmers should be central to agricultural development schemes because the market access, science and technology provided would be useless if farmers lost the land to utilize it. Pressure is mounting to feed rapidly expanding populations and governments are not always going to work in people's interests but it is in the interest of the investor and government to negotiate well-managed land deals as the outcome will affect quality and quantity of land available to use in the future.

Climate change will have a significant but uncertain affect on food security in the future; affecting plant disease epidemics (of which more research needs to carried out) and the amount of available fertile land (increased drought could render fertile land unusable) (Cecilie & Reenberg 2010). Looking at small or local solutions rather than bigger wide scale solutions proposed by international companies and governments to impending food insecurity may help us track progress easier and tailor make programmes to suit the specific food security issues a given area is facing.

Food insecurity is an urgent problem that can only be helped with a diversity of approaches tailored to specific crops, countries, and regions needs. It is no surprise that without an agreement on solutions in the near future the U.S department of defence predicts that the shortage of food will be the next cause of global war and conflict.