By Roxanna Pignolet, Biochemistry and Molecular Biology 20’

Author’s Note: Since I started working on plant metabolites as an undergraduate researcher in the Shih Lab, I’ve developed a great appreciation for the power of plant genetic engineering to address a wide variety of problems. A uniquely global and increasingly relevant concern is how to continue to feed the world’s growing population in the face of climate change. I decided to write this paper to provide a snapshot of the current research being done to innovate crop species that will survive in the face of climate change. As part of this review. I also wanted to address ongoing concerns about the safety and impact of GMOs on consumers and the environment, and whether these genetic engineering strategies have the potential to make a positive impact on food security.

Introduction

As the world population continues to rise, climate change is also having an increasingly large impact on agriculture in the form of rising temperatures and intensified weather variations. Population growth is challenging researchers and farmers to find new ways to increase crop yields without access to more land or freshwater. Population is expected to increase from the current 7.7 billion to 9 billion by 2050 (1,2). However, it was found in 2000 that about 70% of the available freshwater was already in use. Meanwhile, climate change is introducing new challenges to crop productivity and stability. By 2050, the global crop demand may increase as much as 110%, which emphasizes the need for new, powerful strategies for crop improvement.

Genetically engineered crops have been used in agriculture since the mid-1990s, and have been instrumental in overcoming serious agricultural challenges such as disease outbreaks and overuse of toxic insecticides (3). In contrast to traditional breeding, genetic engineering allows for a direct transfer of one or more genes of interest from either closely or distantly related organisms. In some cases, a plant is modified solely by turning on or off one of its own genes (4). These methods allow for fast and precise changes that target a specific trait. Since their introduction, numerous studies have measured their potential for health and environmental risks, as well as their benefits. This review will discuss the impacts of genetically engineered crops from an environmental and health perspective. Additionally, I will look at how genetically engineered crops are currently being applied to address food security concerns in the face of climate change.

What is the Impact of Genetically Engineered Crops?

Environment

As genetically engineered crops have now been used in the field for many years, the environmental impacts can be assessed. The most abundant type of genetically engineered crops are insect resistant crops, specifically Bacillus thuringiensis (Bt) resistant corn and cotton. Bt is a soil bacterium which produces proteins that are toxic to certain insects (5). Bt crops have been modified to produce Bt genes as protection against specific pests (3). These crops have been grown commercially since 1996 (2), which has allowed long term environmental studies to be conducted. In a two-year field trial on the impact of transgenic maize on soil fauna, Fan et al. found that there was no impact on biodiversity, abundance or composition of the soil fauna. They compared samples taken in varying conditions from either transgenic maize or non-transgenic maize controls. The researchers found that the insecticide transgene did not affect the soil ecosystem, while factors such as time of year, pH, sampling time, and root-biomass all had significant effects (6). In a 2003 review on Bt crops, Mendelsohn et al. also found that there were no negative impacts observed on species of endangered insects, earthworms, or non-target insects. However, one negative that applies to all insecticides is that pests will eventually gain resistance. Engineering crop varieties to have several different resistance genes has been shown to slow this process (2).

Another class of genetically modified crops that are currently in use are herbicide-tolerant crops. Herbicide-tolerant crops are designed to be tolerant to broad-spectrum herbicides that can be used to control surrounding weeds. Use of herbicide-tolerant corn and soybeans has been shown to decrease the use of highly toxic herbicide sprays in favor of an amino-acid derived, non-toxic alternative (Roundup), and has also encouraged low-till farming practices which have been correlated to significant reductions in greenhouse gasses (2). Weed resistance is a concern with herbicide-resistant crops, especially when a single herbicide gene is overused. In some cases, high selection pressures caused by overuse of a single broad-spectrum herbicide have led to resistant weeds. If unchecked, these resistant weeds can spread across farms and negatively impact crop growth (7). New varieties of crops resistant to multiple types of herbicides should help mitigate this problem by allowing farmers to rotate several types of herbicides. A widespread adaptation of these new varieties and consistent practice of sustainable herbicide application will be important to avoiding negative outcomes of herbicide-tolerant crop use.

Implementing these genetically engineered crops has contributed to overall decreases in the amount of toxic insecticide and herbicide sprayed. Just as with chemical pesticide and herbicide sprays, proper steps must be taken with insect-resistant or herbicide-resistant crops to delay resistance in the affected insect or weed. These steps include rotating planting of herbicide-resistant crops and using weed control tactics with different modes of action to avoid putting high selection pressure on one type of resistance.

Health

The consensus from long term studies carried out to address biosafety concerns of genetically modified crops, is that they are just as safe as their natural counterparts. Genetically engineered crops are subjected to a variety of tests on a case by case basis before they are implemented, and now long term data shows that there have been no side effects from possible unintended chemical compositions of crops, making them just as safe as those derived from traditional breeding. There are, however, concerns about next generation genetic engineering, which targets regulator genes instead of a single functional gene. Targeting regulator genes could allow scientists to target plant stress response pathways, and engineer plants to have multiple desirable traits (8). Additional research must be conducted to assess the plant-wide changes caused by affecting a player in a signaling cascade.

New Approaches to Crop Improvement

While the current genetically engineered crops have been found to have a positive effect on crop yields, the increases are not enough to keep up with projected population growth. Additionally, climate change is predicted to cause stressors to crops such as drought, rising temperatures, and weather variations among other things (2). Therefore, scientists are looking for new and creative genetic engineering techniques to create robust and high-yielding crops for our future.

One of the main targets for genetically engineered crops is adaptions to grow and produce quality yields under higher temperatures. In a study investigating the genes responsible for creating lower quality, chalky rice grains under high temperature conditions, Nakata et al. looked at the role of a starch metabolizing enzyme, known as amylase, in the packing of starch into rice grains. Their team used transgenic rice modified with a reporter gene attached to each isotype of the amylase gene. By comparing the activity of plants overexpressing each variety, they were able to identify specific amylase genes as targets for genetic modification. Rice variants with these modifications would remain higher quality, with tightly packed starch, even if grown under non-optimal higher temperatures (9). Another study tested the responses of a previously created transgenic rice line called HOSUT under high amounts of carbon dioxide (CO2), a heat wave, and nitrogen enriched conditions. They found that the transgenic line, which has enhanced sucrose transport, has a superior yield than the control line (Certo), and that increased CO2 conditions resulted in higher yields in Certo with only minimal increases for HOSUT. They concluded that the minimal response of HOSUT to the increased CO2 was indicative of HOSUT already being saturated due to its optimized transport capabilities. The HOSUT line is already optimized for translocation of carbon, which they were able to show by increases in starch in the grains in HOSUT only. HOSUT also produced more yield in response to increased nitrogen, making it a good option for producing high rice yields under variable climate change conditions (10) The HOSUT line is a great example of how genetic engineering can be used to fortify and optimize crops to both survive under atypical conditions and produce enough yield to keep up with demand.

Another problem that researchers are addressing through genetic engineering, is drought. Selvaraj et al., developed and field tested two drought tolerant rice lines, created by introducing an Arabidopsis stress response gene (galactinol synthase) with a maize promoter. Galactinol synthase produces galactinol, a sugar that functions as an osmoprotectant, keeping water from leaving the cells. These galactinol synthase genes were introduced into two commercially available rice lines and tested in the field under drought and well-watered conditions. Under drought conditions, the collection of galactinol resulted in higher grain yields, while under well-watered conditions no significant yield increase was observed. Galactinol is a sugar that functions as an osmoprotectant, keeping water from leaving the cells. The results of these field trials show that these rice lines are ready to be integrated into ongoing breeding programs (11). Wang et al. also tackled the problem of drought stress caused by global warming on fruit such as apple trees. They transgenically expressed an aquaporin gene found in Fuji apples that has increased expression during fruit growth in tomato. The transgenic plants did have an increased drought tolerance, observed as an increased sensitivity of their stomata to water loss, and a larger fruit size when compared to wild type. This research will be continued in apples next with the goal of producing plants with larger fruits when well-watered, which will also be more tolerant to drought due to increased water transport efficiency (12).

A third target for genetic engineering solutions is circadian rhythms. Understanding and controlling circadian rhythms in crop plants has the potential to adapt plants to radically different environments. One group at the Guru Jambheshwar University of Science and Technology is tackling this challenge in rice. This group expressed an Arabidopsis transcription factor known as Circadian Clock Associated1 (CCA1) under the Timing Of Cab Expression 1 (TOC1) promoter, which are both part of the circadian clock machinery in Arabidopsis. They found that overexpression of the CCA1 in rice had negative results, while repressing it caused positive changes to plant morphology. The researchers used RNAi, which is a biological process where small fragments of RNA are used by the cell to target complementary mRNA for destruction, thus silencing expression of the encoded protein. By comparing RNAi constructs based off of three different parts of the CCA1 gene for silencing the gene expression, they found that the RNAi derived from the 3’-terminal end of the CCA1 gene had the best impact on plant morphology (13). This study is an important first step towards unlocking the power of using circadian clock genes to breed plants better adapted to a changing environment.

One new strategy being considered is a CRISPR/Cas9 genome editing method that could be used to quickly develop improved crop varieties without transgenes. CRISPR/Cas9 can introduce specific changes into a plant genome without being limited by existing variation. Applying this method, scientists will be able to stack multiple edits into a plant within a single generation, resulting in transgene-free progeny. One benefit of this method is that it may allow for more complex changes to polygenetic traits or signaling pathways. For example, this could be helpful for targeting complex plant stress response pathways. This technology is currently limited by the availability of annotated reference genome sequences for plants other than Arabidopsis. Scheben et al. suggest that taking a genomics-based approach would allow for a comparison of species-wide genome diversity, making differences in copy-number visible and thus available for editing. While the authors suggest that this method creates plants that are indistinguishable from those created through natural breeding and random mutations, bans against genetically modified crops may target methodologies rather than the final result (14).

Conclusion

Currently implemented genetically engineered crops, have been shown, through years of testing and trials to be at least as safe, both towards the environment and in terms of human health, as naturally bred varieties. While new transgenic lines must be screened and tested on a case-by-case basis, the overall benefits of this technology make it an important tool that may be necessary to confront upcoming challenges to agriculture. Climate change and population growth are putting steep demands on crops to survive in more hostile environments while also producing higher yields. Current efforts are focusing on vital crops, such as rice, corn, wheat, and fruits, to create drought-tolerant, heat-tolerant, and yield-optimized plants.

References

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