Gene-Edited Crops and the New Green Revolution
There has been a long-drawn out debate concerning the ethics, regulation and – principally – safety of introducing genetic modifications to organisms intended to be consumed by humans. Researchers have largely failed to convince regulators that their products are safe whilst anti-GMO campaigners have persuaded much of the public into believing that modified organisms are inherently dangerous and their developers cannot be trusted. The contentious nature of the issue has largely stalled the progress of such products to market.
One of the key regulatory issues concerning techniques such as CRISPR-Cas9 is one of definitions. In the EU, regulations talk of “genetically modified organisms” as ones that contain exogenous DNA. However, genome editing techniques alter plants in a way that has an end-result identical to that of natural or induced mutations, albeit more targeted. From a scientific perspective, CRISPR-edited crops are not GMOs. Consequently, CRISPR-Cas9 and associated technologies are currently defined as a “New Breeding Technique” and, along with other genome editing technologies, is under review by the European Commission. The gap in the current regulation means that any organisms edited by CRISPR are regulated as GMOs, but how that will change after the review is complete remains to be seen.
The discrepancy between the United States and the EU is once again demonstrated in their approach to genetically edited crops when, in 2016, the US Department for Agriculture (USDA) ruled that regulating crops edited using CRISPR-Cas9 was outside its remit as it does not contain any exogenous DNA. As a result, a genetically modified, anti-browning mushroom will become the first organism edited using the CRISPR system to be given the go-ahead by the US government (Waltz, 2016). The anti-browning effect was created by deleting a polyphenol oxidase (PPO) gene, which encodes an enzyme that leads to browning in a wide variety of fruits and vegetables (Kuijpers et al., 2014).
To many, the USDA’s ruling underscored inadequacies in their own regulations and, like the EU, the US has found it necessary to begin reshaping policy around genome editing.
Regardless of the current regulatory environment, CRISPR-Cas9 offers the opportunity to quickly introduce new traits into crops and livestock whilst circumventing traditional selective breeding methods. These now-outdated methods suffer from low efficacy and are limited by the alleles present in the population (Fu, 2015). Improved techniques, such as marker-assisted selection, are based on more recent understanding of genetics and cellular biology (ISAAA, 2010) though still lack the accuracy and precision that can be achieved using CRISPR-Cas9 and other NBTs.
Such NBTs are defined by their ability to introduce highly targeted changes into an organisms’ DNA. There are seven principle NBTs including CRISPR-Cas9, which falls under the classification of “site-directed nucleases” alongside Zinc-Finger Nucleases (ZFNs) and Transcription Activator-Like Effector Nucleases (TALENs). Cisgenesis, another NBT, involes the introduction of genes from a closely related species. Oligonucleotide-directed mutagenesis, where a few bases of DNA can be changed using hybridisation. Increased understanding of epigenetics means that RNAs that alter DNA methylation can be introduced to a cell and selectively silence or activate genes. In agro-infiltration, where parts of the plant are dipped in a suspension of Agrobacterium that will then transfect the desired gene. Other NBTs include reverse breeding and grafting a plant onto a GM root (Lusser and Davies, 2013). As it stands, these techniques don’t result in products currently classed as GMOs, though they are generally considered to be in a legal limbo.
Whilst regulators determine how to class and oversee the commercial use of NBTs, researchers continue to use them in laboratories both to develop potential products and investigate more fundamental research questions.
One major threat to global food security comes in the form of agricultural pests and pathogens (Strange and Scott, 2005). Projections indicate that, if left uncontrolled, the economic impact of weeds on soybean and corn crops in North America alone could reach $43 billion per annum (Kansas State University, 2016). However, the potential range of herbicides is limited, as they must selectively kill the undesired plant (the weed) without significantly damaging the desirable plant (the crop).
One way to tackle this issue is by using CRISPR-Cas9 technology to edit resistance gene anaologs (Sekwhal et al., 2015). These genes have very high sequence similarity to resistance genes in related species but do not themselves confer resistance. One such case is the plant Acetyolactase Synthase (ALS) gene (Svitashev et al, 2015). In Arabadopsis thalania and soybean (Haughn et al., 1998 and Walter et al., 2014 respectively) it was shown that a single amino acid change in the ALS protein conferred resistance to chlorosuforon, a sulfonurea (SU) herbicide. Such SU herbicides were first commercialised in 1982 and their low toxicity to animals lead to their widespread use across eighty countries (Walter et al., 2014). As with many herbicides, some SU pesticides are non-selective, killing any plant with which they come into contact. However, certain alleles of ALS allow the plant to detoxify SUs and thus lead to resistance. Editing non-resistant alleles such that they now confer resistance is of obvious benefit.
Other possible applications include nutritionally enhanced crops. Golden Rice, the paradigm example of “biofortification”, is a strain of rice that has been modified to increase its vitamin A content. Dietary deficiencies in vitamin A cause hundreds of thousands of incidences of blindness annually, which children being particularly susceptible. Golden Rice was developed to help address the problem, which is particularly widespread in the southern hemisphere. Since its conception, it has faced many regulatory hurdles and is not yet available to farmers anywhere in the world. Proposed plans to introduce it to the Philippines have been met with fierce opposition, with critics claiming that it will lead to the extinction of traditional “heirloom” rice grown in the region (Stone and Glover, 2017).
Golden Rice wasn’t created using CRISPR-Cas9, though it is an exemplar of what could be achieved. Alongside rice, wheat is one of the most intensively studied crops due to its high agricultural and economic importance. Genome editing of wheat comes with some unique challenges: its hexaploid genome means that it is very difficult to target all alleles of a gene, whilst the highly repetitive nature of the genome lends to a high rate of off-target cleavages (Wang, 2014). Nevertheless, it has been suggested that if an efficient method of engineering wheat could be developed, it could be used to address other aspects of malnutrition, such as iron or zinc deficiencies (Borrill et al., 2014).
However it must come about, there is little debate that we will need a new Green Revolution to tackle impending climate change and predicted food shortages. With adequate regulation that strikes the balance between ensuring safety and encouraging innovation, it is highly likely that the NBTs – particularly CRISPR-Cas9 – will be instrumental in solving many of these crises and we may at last see the end of widespread famine and malnutrition.
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