Increasing Crop Resistance To Droughts Through The Genetic Modification
Globally we face escalating demand on food production due to an increasing population, a greater consumption of animal products as affluence grows and increasing biofuel consumption. By 2050 global food production needs to increase by approximately 60%. When combined with the ongoing effects of climate change, our ability to meet this demand comes under question as crop growing conditions deteriorate. We face, therefore, the reality of global food insecurity becoming worse and potentially unmanageable over time unless new food production methods are found which are both resilient to the changing climate and able to feed the growing population while reducing environmental impact. Whilst the causes of food insecurity are complex, this review will focus on increasing crop productivity through the genetic modification (GM) of major crops. In particular, it will look at the ability to develop GM crops resistant to drought stress, an especially dangerous threat to our food supply. The review will first analyse levels of food security today, before explaining how climate change is predicted to worsen the situation in future years. It will then frame GM as an important tool in sustainable intensification. To illustrate this recent successful research will then be highlighted. The paper will end with a consideration of whether this potential solution is realisable given the socio-political and economic issues surrounding GM.
Food security has long been a global issue. In 1996 at the World Food Summit food security was defined as “when all people, at all times, have physical and economic access to sufficient, safe and nutritious food to meet their dietary needs and food preferences for an active and healthy life''. Achieving food security and promoting sustainable agriculture by 2030 was outlined as a sustainable development goal. Sadly, the FAO’s (2018) recent report stated that after a long decline since 2014 food insecurity has been rising, with their currently being an estimated one in nine people undernourished worldwide. For example, nearly 21% of Africa’s population is undernourished. Whilst there are a multitude of reasons for this, rising temperatures and extreme weather are undoubtedly a leading cause (Ibid).
Climate change is, arguably, the biggest crisis of the current generation and is now an irreversible consequence of the Anthropocene. Despite ongoing attempts to reduce CO2 emissions and minimise climate change, the past four years have been the warmest on record. Climate change is thus not a future problem but a worsening crisis, and as many reports have shown that continued increases in temperature and aridity can be expected; ways must now be found to adapt to these changes. For every degree celsius increase, there will be a decrease in the yield of wheat, rice and maize by 6.0%, 3.2% and 4.4% respectively unless significant changes are made. It is not just an increasing temperature that impacts crop yields, however, but increasing occurrences of extreme weather such as hurricanes, flooding and drought. There is a direct correlation between climate variability and yield variability as globally one third of the observed variation in yield is explained by climate variability and in some areas over 60% of yield variability. Crucially it is the country’s most vulnerable to food insecurity that are most affected by climate extremes - there are more than double the number of undernourished people in countries with high exposure to these climate extremes than those without (WMO, 2018). It is now imperative that we find ways to adapt and prepare our agriculture for this changing climate, to minimise the devastation it could have on food security.
One of the most damaging aspects of climate change on agriculture is drought. Climate change affects many abiotic stresses including salt, light, and cold but primarily impacts crops via heat and water stress. As can be seen in the figure above, drought stress is the leading cause of agricultural damage; this review will thus focus specifically on the case of drought. Indeed, of the overall damage and loss caused by drought, 83% is on agriculture. Agricultural drought is expected to increase as precipitation not only becomes less frequent but more intense, actually increasing flash floods, run off and soil erosions and diminishing soil moisture (Dai, 2010; Reynolds et al., 2016). Some research has argued that the effects of drought will be ameliorated by an increase in CO2, increasing photosynthesis and reducing stomatal conductance, improving yield and decreasing water use. Contrary to this however, Gray et al. (2016) in an eight year field study on soybean showed that intensifying drought will eliminate any expected benefits of elevated CO2 and also highlighted the negative impacts of climate change on major crops. It is therefore imperative that the threat of increasing droughts is considered a serious one, for which appropriate and sustainable solutions are sought.
As a potential solution, increasing yields could be achieved by expansion but this would further reduce biodiversity and contribute to carbon emissions - only worsening the original problem. If current global trends of such extensification continue by 2050 an estimated one billion ha of land will be cleared at great environmental cost, releasing vast amounts of CO2 into the atmosphere. Instead the focus has moved to sustainable intensification, which has the purpose of making our existing crops more efficient through better agronomic practices, but also through the use of more resilient crops. Improving soil quality through practices such as conservation tillage, using drip irrigation where appropriate, using cover crops and making suitable choices about what crops to grow are just a few ways in which agronomic practice can make crops more resilient to drought. These practices alone however, are unlikely to be sufficient in the face of climate change and will need supplementing with a toolbox of other approaches to drought tolerance. One such tool is GM plants to achieve greater yield or resilience.
Transgenic crops were first commercialized in 1996. Today, genetic modification has largely been used for herbicide and pesticide tolerance, as this is where there is the most commercial benefit. Utilizing GM to create more resilient and better yielding crops alongside improving management practices will be crucial in mitigating the effects of climate change on crops (International Service for the Acquisition of Agri-biotech Applications (ISAAA), 2017). If we are to tackle the effects of climate change using GM technology then it must be developed now, as it can take between 15 and 30 years for agricultural techniques to get maximum returns.
GM Case Studies
Drought stress impacts plants at every level from the biochemical to the plant’s entirety (Farooq et al., 2012). Mechanisms of plant tolerance are highly complex and in recent years much work has been done to understand them. For example, Lamaoui et al. (2018) provide an overview of the physiological, biochemical and genetic aspects of heat and drought stress. In addition, Liu et al. (2018) provide a comprehensive study of ABA dependent and independent pathways in response to drought stress. Greatest improvements may be found by pyramiding or stacking multiple genes into a single genotype. This review does not attempt to provide an overview of plant responses to drought but rather highlights three recent promising areas of research while also illustrating that there are many different routes to achieving drought tolerance.
Cold Shock Proteins
Bacterial cold shock proteins (CSPs) are produced in response to a sudden drop in temperature. However, CSPs have been found to do far more than aid acclimatization to cold, having been shown to have a much broader role in stress tolerance including osmotic, oxidative starvation, PH and ethanol stress. Castiglioni et al. (2008) observed transgenic CSP plants had improved water-deficit and heat stress tolerance in rice, and water-deficit tolerance in Maize when compared to controls showing the potential of CSPs in developing drought-tolerant crops in the future. The first and only commercially available GM drought resistance crop is Monsanto’s DroughtGard, a CSPB transgenic maize, first planted in 2013. This trait reduces leaf growth, therefore increasing water availability throughout the crop’s crucial flowering stage and allowing for increased ear growth, improving productivity especially when under drought stress (Nemali et al., 2014). This trait also decreases water loss, lessening the need for irrigation as it can reduce transpiration by 175% under stress. Each year in America an increasing amount of drought tolerant maize is planted; in 2017 the figure stood at 1.4 million hectares. Recently modified CSPA transgenic wheat was shown to have drought tolerance in the field. They found a grain yield improvement of the control plants by over 24% compared to the control. This confirms the potential of CSPs in improving drought tolerance in several major crops and the need for ongoing trials.
Ethylene
The phytohormone ethylene has been shown to modulate various aspects of growth and development, particularly when under abiotic stress tolerance such as water deficit or high temperature. Habben et al. (2014) found that down regulating the ethylene pathway via an ACC synthase enzyme reduced ethylene emission levels by 50% and demonstrated that this improved grain yield of maize under drought stress with no difference to the control in normal conditions. Shi et al. (2015) identified that ARGOS genes are negative regulators of ethylene and modulate ethylene signal transduction. In particular, they found that overexpression of some of these genes in maize reduced ethylene sensitivity and improved grain yield under drought and normal conditions. Shi et al. (2016) have continued their work by showing that where conventional breeding was unable to produce lines with expression levels close to that of the original ARGOS8 transgenic events, genome editing using CRISPR-Cas is invaluable. Another avenue is using ethylene response factors (ERFs) to regulate ethylene and attain improved yield under stress. There have been some successes in using ERFs to improve yield in rice, particularly when using regulated promoters to avoid undesirable effects by controlling ERF expression. However, there can be unwanted side effects; improved stress tolerance comes at the cost of yield as shown in rice, demonstrating the need to fully decipher the complex networks around ERFs (Dubois et al., 2017). This understanding will allow future progress to be made in utilising the ethylene pathway for drought tolerance.
Galactinol
The raffinose family of oligosaccharides (RFOs) are crucial in transport and storage including in seeds as a desiccation protectant, in phloem sap as a transport sugar and as storage sugars in plants (Sengupta et al., 2015). A higher concentration of RFOs is suspected to confer stress tolerance. Galactinol (Gol) and therefore galactinol synthase enzymes (GolS) are essential to the biosynthesis of RFOs. Particularly the Arabidopsis thaliana galactinol synthase 2 gene (AtGolS2) has improved drought tolerance in Arabidopsis. Selvaraj et al. (2017) tested AtGolS2 transgenic rice in an extensive field study and found improved grain yield and biomass under drought stress. This drought tolerance was thought to be due to leaves having a higher relative water content, increased photosynthesis, less impact on plant growth and better recovery (Ibid).
These three examples illustrate convincing GM drought tolerance because they have undergone rigorous field testing to corroborate lab-based successes. However, for many other studies one of the key issues in genetically engineering stress tolerance is avoiding a yield penalty under normal conditions. It is crucial that drought tolerant crops are tested in field trials. Drought is also almost always concurrent with other stresses such as salinity that all interact; the controlled environment of a laboratory or greenhouse cannot be representative of these complex and interacting factors in the field (Gaudin et al., 2012). Another issue in developing a drought tolerant crop is ensuring that it maintains its resilience under varied frequency, severity and duration, since a crop may perform well under one scenario and not others. Tardieu (2012) explains this nuance and identifies that it is not whether a trait confers tolerance that matters but more realistically whether in a specific area an improved yield is shown over a considerable number of years and scenarios. The successful studies discussed previously are excellent examples, having carried out extensive field testing, which come to the conclusion that these are promising mechanisms of drought tolerance. However, it is critical that new studies also adopt the same field testing.
In theory, drought tolerance could be developed using conventional breeding technologies; conventional breeding (simply crossing varieties with a desired trait, selecting and repeating until a stable variant is created) has been used to enhance crop productivity throughout history with great success. However, as discussed previously, yields of major crops are starting to stagnate. Drought tolerance is very difficult to achieve through conventional breeding due to its multigenic nature at quantitative trait loci, although there has been some limited success. The problem with conventional breeding techniques is that undesirable traits may be introduced through linkage drag and so several rounds of crossing and selection are needed, making the process time consuming and inefficient. It is worth noting that new technologies such as next generation sequencing technology, genomic selection and speed breeding are drastically improving the cost and speed of conventional breeding. Ultimately, conventional breeding is still limited to the genetic variation available in major crops and lacks the scope of GM, so the need for additional technology such as GM still remains in tackling food security.
GM overcomes the limits of conventional breeding by creating new avenues for stress tolerance, particularly drought. Since being commercialised transgenic crops have provided significant benefits to farmers and consumers, but despite this, have faced major hurdles in regulatory requirements and the public perception of their use in practice. These hurdles originate from a concern for the safety of these crops with respect to the environment and public health due to foreign DNA, despite risk assessments in over thirty countries finding no greater risk stemming from GM crops than from conventional breeding. Whilst the United States, Canada and Australia have adopted GM crops to differing extents, it continues to be predominantly developing countries who are taking on the technology. Programmes such as Water Efficient Maize for Africa (WEMA) aim to increase GM accessibility in developing countries and the provision of sufficient support to these will be crucial in determining whether those that need this technology the most have access to it.
Part of the explanation for this might be that the EU is particularly opposed to GM crops, imposing a ‘stringent regulatory framework’ which causes a minimum five year delay in getting new varieties approved compared to other countries. Strict regulation aimed at commercialising GM crops discourages research funding in this area, particularly in the public sector, because current technologies are held predominantly by wealthy private sector organisations who have not published their material. Additionally, even if GM drought tolerant crops were to be successfully produced, the public may not be willing to consume them due to the stigma perpetuated by non-governmental environmental groups. As food security deteriorates further and the severity of climate change increases, it is critical that the scientific community communicates to policy makers and the public the role GM can play in this. By removing red tape and improving the public perception of GM, its ability to create drought tolerant crops can be accessed and further maintained through sustained public demand.
New gene editing technologies such as CRISPR-Cas have invigorated the quest for stress tolerant crops, allowing precise, efficient and cost-effective genome editing. Significantly, since CRISPR-Cas promises results without the use of foreign DNA many of the safety concerns of transgenic GM crops can be overcome. Therefore, as GM technology progresses its ability to offer a potential solution to food insecurity only strengthens because a safer image can be marketed to both policymakers and the public. Despite this, the European Union voted in 2019 for gene editing to come under the same regulations as GMOs, only delaying the development of more efficient and marketable GM crops by continuing to burden smaller companies with additional costs and longer production timelines. As gradual yield improvements can be expected through conventional breeding techniques it should be considered carefully whether the time it takes to bring a GM crop to market negates its yield benefits, given the barriers of regulation and public perception. In time however, such problems may solve themselves as the pressures of climate change and food insecurity become more apparent, changing government and public priorities. Hence whilst using GM today may seem to confer little advantage over conventional breeding due to the current barriers in place, investing in GM for the future may prove a superior strategy in meeting the challenge of food insecurity.
Conclusion
Developing a sustainable and productive agriculture is a priority as climate change increasingly threatens food security. Crops that can withstand abiotic stresses such as drought are thus vital and this review has endeavoured to highlight how GM - particularly recent gene editing technologies - can be invaluable in creating these crops. Using these technologies, a future where a crop is engineered to be tolerant of multiple stresses and diseases specific to a location is foreseeable. However, this future can only be realised if policy makers fully support the use of GM by removing limiting regulations and creating an environment where research can be shared and built upon. It is also crucial that the public perception of GM improves if this research is going to lead to high yielding, resilient crops. Whilst improved conventional breeding practices and sustainable intensification will also play a key role in meeting the challenge of food security, collaboration between policymakers, scientists, businesses and the public is what is necessary to render GM one the most promising tools in the box.
