What is a ‘GMO’? There is much uncertainty among citizens and regulators as to where the bright line distinguishing varieties of plant breeding one from the other should be drawn (Johnson 2015). The genomics revolution in biology enabled new molecular plant breeding techniques to complement or supersede conventional plant breeding. Marker Assisted Selection (MAS) allows plant breeders to identify improved traits in plants more rapidly than is possible in conventional breeding (Barabaschi et al., 2016). Agricultural biotechnology can also include – in contrast to previous plant-breeding practices — manipulation of recombinant DNA to generate new or improved traits in plants. ‘Transgenic plants’ – containing DNA from sexually incompatible species – form the core of both regulatory scrutiny and popular opposition to GMOs. These plants may have unique nutritional or agronomic traits resulting from recombinant DNA (rDNA) techniques (Kamthan et al., 2016), but are restricted in much of the world.
Misgivings about biotechnology often target the ‘unnatural’ alteration of a crop’s genome by rDNA. What most consumers do not realize is that many varieties of crops available today have had their genomes altered by a technology that existed long before the advent of recombinant DNA. Derived from mutation research that originated in the 1930’s, ‘mutagenesis breeding’ involves the introduction of random mutations to plant cuttings using chemical or irradiation mutagenesis. Plant tissues expressing novel traits are then propagated from these mutation events into new varieties of crops (Barabaschi et al., 2016). Over 3,000 varieties of crops have been developed using mutagenesis breeding– including the popular ruby red grapefruit — according to the Mutant Variety Database (https://mvd.iaea.org). Mutagenized plants face neither stigmatization as GMOs nor special regulation. Indeed foods that sell at premium prices for being labeled ‘organic’ may be produced with mutagenized plants, in practice if not in purist theory (Nuijten, Messmer, and Lammerts van Bueren, 2017).
Genetic engineering in a broad sense enhances the potential for introducing novel traits into crops through the manipulation of their genetic material, either by adding new genes or making small changes to pre-existing genes that are already part of the crop genome. New genetic material can be incorporated into the plant genome through several delivery methods: chiefly Agrobacterium-mediated transformation and particle bombardment (gene gun). In the United States, genetically modified (GM), or ‘transgenic’ crops have been commercially available since 1996 (ISAAA, 2014). One of the most well known examples of a transgenic crop is Golden Rice, which expresses β-carotene and was created philanthropically with the intent of alleviating vitamin A deficiency (VAD) in developing countries. Golden rice contains genes derived from different species, such as maize, which together contribute to a synthetic β-carotene pathway (Al-Babili et al., 2005, Beyer, 2010). Golden rice can easily be distinguished from its conventional counterparts by its yellow hue, unlike many transgenic plants that defy easy detection, monitoring or regulation. Yet golden rice has yet to make it to farmers’ fields for a number of reasons: political, regulatory and agronomic.
Transgenic crops have been engineered to address many of the world’s most significant agricultural challenges, including insect resistance and herbicide tolerance (Ricroch and Henard-Daman, 2016). Today, nearly 90% of all transgenic crops cultivated across the world are herbicide tolerant (ISAAA, 2014). Herbicides can be sprayed on these crops without causing damage to the crop itself while the growth of neighboring weeds is retarded. Insect resistance is the second most commonly used trait generated in transgenic crops. Bt (an insecticidal protein from Bacillus thuringiensis) is used globally to prevent insect infestation. Insects that ingest the transgenic plant which expresses the precursor Bt protein are killed, while non-target insects that may reside near the crop but are not pests remain unharmed (Kumar et al., 2008).
Cisgenic crops are those that do not contain a transgene from another species, but rather a gene from a sexually compatible variety of the same plant – e.g. a blight-resistant Chinese chestnut with a blight-vulnerable American chestnut. Cisgenesis creates plants that express genes from closely-related plants and are also being designed to regain useful genes that have been lost over years of conventional crop breeding. For example, the Wheat Stem Rust Initiative works toward designing cisgenic versions of wheat containing multiple resistance genes to the fungal pathogen Ugg99 from wheat relatives (Singh et al., 2015).
‘Gene silencing’ (RNA interference technology — or RNAi) also could be considered a form of genetic engineering that is proving increasingly useful for agriculture. Plants are engineered to express the antisense RNA version of a specific gene that may be part of the plant genome or part of an invading pathogen’s genome, such as a virus. Expression of the targeted gene is then blocked by a phenomenon known as gene silencing. Genetically modified papaya that has been generated using this technology is resistant to papaya ringspot virus by expressing an antisense RNA to the viral genome. This technology is responsible for having saved the papaya industry in Hawaii (Gonsalves, 1998). China’s small papaya sector is almost entirely based on this technology. Though the RSVR papaya has failed to gain wide market presence in many countries because of political resistance, farmers elsewhere have spread the technology informally and found it effective in fighting the fatal virus (Evanega and Lynas 2015).
Despite wide adoption, and evident usefulness to many farmers in many countries, the technologies described above have shown limitations that have disappointed some early expectations. Long delays from multi-year field trials and legal challenges have limited progress. Moreover, plant breeding, even with improved technologies, is invariably complex. Golden rice technology, for example, has experienced numerous challenges in breeding into land races and has yet to have the long-awaited impact on Vitamin A deficiency. To date, successful crops have mainly been those protecting harvest yield from biotic stress – weeds and pests. Multi-gene traits such as yield have proved more elusive. The frontier looks different with the advent of genome editing.