Genetically Engineered Plants and the Arbitrary Line in the Sand

Jul 16, 2012

External inspectors make annual visit to CIMMYT's Seed Health Laboratory (SHL)

Photo courtesy of CIMMYT

By: Dr. John McMurdy, International Research and Biotechnology Advisor, Bureau for Food Security, USAID. The views expressed here are his own, and do not necessarily reflect the views of USAID.

While public opinion has generally been supportive of the use of science to improve agricultural productivity, it is obvious that opinions drastically differ when it comes to the process of genetic engineering (GE). This difference in opinion may be explained by the general public perception that a plant is somehow more fundamentally changed by the process of genetic engineering than by other means. On this point, it is constructive to explore the different ways in which science has quite successfully contributed to the improvement of crop performance through genetic “modification”, both in a controlled and random fashion.  

In order to utilize existing genetic diversity (or biodiversity), traditional plant breeding has been applied in some fashion as long as humans have practiced agriculture. Sexually compatible plants are cross-pollinated (or crossed) to produce offspring, each of which contains a blend of traits from the parent plants. Offspring displaying a trait of interest, say insect resistance, are selected based on the plants physical characteristics, or phenotype (e.g. does it resist insects?). This process is then repeated over successive generations with further selection based on other characteristics (yield, grain quality, etc.) until the breeders/farmers are satisfied with the overall phenotype and the variety is released.  

While plant breeding has served society well for centuries, there are two fundamental limiting factors. The accessible biodiversity is limited by sexually compatible species and the selection of offspring based on phenotype is imprecise and time consuming. Modern technology has allowed plant breeders to overcome both these limiting factors.

Laboratory or “tissue culture” processes such as embryo rescue and protoplast fusion allow viable offspring to be created from species not sexually compatible in nature, greatly increasing the accessible genetic base. Marker Assisted Selection (MAS) drastically speeds up the breeding process by utilizing gene sequencing technology. In this approach, breeders leverage the existing knowledge of linkages between gene sequences and plant characteristics to identify offspring likely exhibiting beneficial traits, which is significantly more efficient than waiting for plants to first grow, and then assessing the phenotype.

Doubled Haploid Breeding approaches use a “doubling” of chromosomes to hasten plant breeding. In order to generate offspring with predictable genetic traits, it is advantageous that the parental plants have identical genes on each paired chromosome (in the case of “diploid” organisms with two copies of each chromosome), termed “homozygous”. Creating homozygous plants can be time consuming for plant breeders, usually taking at least 6 generations. In the doubled haploid process, single chromosomes are “copied” during the reproductive process using both tissue culture techniques and crosses with specific varieties. As a result of this copying, wholly homozygous plants can be created in a single generation.

Although these tools have tremendously improved the use and utilization of existing biodiversity, important farmer desired traits still may not exist in the wider accessible genetic pool. Accordingly, additional tools have been developed to create new sources of biodiversity including chemical and radiation induced mutation breeding and genetic engineering.

Mutation Breeding uses mutagenic chemicals or irradiation to create artificial biodiversity by inducing random genetic mutations. These mutations can be unstable and lethal to plants; however, they may also fortuitously create novel and beneficial genetic elements not available in other sources of accessible biodiversity. The plants are mutated and cultivated, and viable plants are selected on the basis of characteristics of interest. Mutagenic breeding has produced thousands of varieties (cereals, fruits, vegetable, cotton) released for breeding programs and direct use, and includes such products as ruby red grapefruit and premium barley for scotch whiskey. 

Somaclonal Variation capitalizes on the random genetic variation that occurs when plants are regenerated through a tissue culture process. While this variation can be problematic when tissue culture approaches are used to create uniform planting materials, it can also be an asset in creating new genetic diversity and potentially beneficial characteristics through the rearrangement of chromosomes and mutations. 

Genetic Engineering (GE) utilizes recombinant DNA (rDNA) technology to insert genetic elements conferring beneficial traits (i.e. insect resistance, water use efficiency, herbicide tolerance). By using rDNA, the pool of biodiversity that can be utilized is widened beyond what is possible using other means, and the targeted nature of the genetic element insertion prevents the introduction of superfluous and undesirable additional genetic information, as occurs in other breeding approaches.

While all these preceding techniques fall under the umbrella of “biotechnology”, aside from GE approaches, they are generally accepted without controversy or public concern while only the GE methods are highly regulated and scrutinized for potential unintended consequences. As novel and “unnatural” genetic elements can be introduced by tissue culture methods, mutation, or rDNA, the argument that genetic enhancement through GE presents a wholly unique set of challenges regarding food safety or environmental impact is inaccurate. Additionally, unlike traditional approaches to crop improvement, varieties developed using GE are evaluated on both phenotype and genotypic characteristics (i.e. was the gene inserted? where was it inserted? how many copies were inserted? how does the gene work?). On this point, one can straightforwardly argue that the additional knowledge of a gene’s function and how it has been inserted makes plants developed using GE less likely to exhibit any unintended characteristics than crop improvement using methods that select only based on the plant’s phenotype (i.e. does it look good?).

Furthermore, as technologies to manipulate genetic elements continue to mature, categorizing varieties as either GE or non-GE will become more challenging. For example, approaches such as RNA interference (RNAi) and intraspecies shifting of gene location (cisgenics) do not express any new proteins, while new technologies such as zinc finger nucleases allow targeted cleaving and inactivation of genes. Even more transformational, as synthetic biology continues to progress, an entire plant genome may someday be synthesized from scratch, possibly using only genes from the species of interest, but having unparalleled control over gene expression and interaction. What will be regulated? What will be “acceptable”?

As many have stated, it is critical that all tools be utilized to improve global food security and develop more sustainable agricultural production systems, including the use of genetic engineering. Let’s not let the misperception that GE is somehow more “unnatural” than other breeding techniques define an arbitrary line in the sand and limit our arsenal.