Genetic Variation and Unintended Risk in the Context of Old and New Breeding Techniques

Abstract

For thousands of years, humans have been improving crops to better suit their needs. These enhancements are driven by changes in the genetic makeup of the plant. While this was initially unintentional, there has been a steady push to increase the pace and precision of crop breeding, something that has occurred alongside a growing understanding of genetics and an escalating capacity to thoroughly assess genomes at the molecular level. With the advent and rapid uptake of molecular breeding techniques, such as transgenics and genome editing over the past few decades, there has been much trepidation regarding the possibility of off-target effects derived from unanticipated mutations at loci other than those intended for alteration, and the unintended risks that this might confer. These concerns persist regardless of the fact that a growing number of studies indicate that the occurrence of off-target mutations derived from newer biotechnological breeding techniques are negligible compared to what is observed with many conventional breeding approaches, and even spontaneously from one generation to the next. Given the impending food security crisis that we are facing in the short-term, there is a critical need to implement a wide range of breeding tools as a means of meeting growing demand, withstanding climate change-related pressures, increasing nutrition, and providing environmental benefits. While food safety is clearly of the utmost importance, now is certainly not the time to prevent the use of particular breeding technologies based on unfounded doubts. Therefore, in this review, we attempt to shed light on these apprehensions by putting purported "risks" into the context of plant breeding as a whole by comparing frequencies of spontaneous mutations with those (both anticipated and unanticipated) that occur through various conventional and biotechnological breeding approaches, including transgenics and genome editing. We then consider how these changes may, or may not, translate into unanticipated risk, and discuss the current global regulatory asynchrony surrounding genome edited crops.

The CRISPR/Cas9-Mediated Modulation of SQUAMOSA PROMOTER-BINDING PROTEIN-LIKE 8 in Alfalfa Leads to Distinct Phenotypic Outcomes

“…Unfortunately, despite the potential of such genotypes for economic benefit, their implementation will almost certainly be hindered by their transgenic nature. Since crops developed using RNAi technology are considered to be “genetically modified” (“GM”), they would raise public concern and be subject to prohibitively costly and lengthy regulatory processes ( Singer et al, 2021a ).…”

“…Due to the unlinked nature of the resulting edit and the introduced transgene, this technology allows for the rapid production of non-transgenic germplasm bearing mutations that are identical in nature to those achieved spontaneously or through conventional breeding approaches such as chemical mutagenesis ( Subedi et al, 2020a , b ). While the regulatory status of crop varieties derived from genome editing is still uncertain in some countries, many others, including the United States, have concluded that in the absence of foreign DNA they are not “GM,” and will therefore not be subjected to costly and burdensome regulatory processes ( Schmidt et al, 2020 ; Singer et al, 2021a ).…”

In vivo and in vitro random mutagenesis techniques in plants

“…Ionising radiations include X‐ray, γ‐rays, neutrons and high‐energy ion beams. Ionising radiations are the most commonly used physical mutagens (Mba, 2013 ; Navjot et al., 2018 ; Singer et al., 2021 ). Radiations are capable of dislodging electrons from the orbits of the atoms that they hit, transforming those atoms into ions.…”

“…O4 alkylthymine, a lesion generated by ENU, can mispair with guanine and lead to A:T – G:C transitions. MMS, another important alkylating agent, induces the formation of N3 alkylguanine and N3 alkyladenine, leading to transversions or transitions (Leitao, 2012 ; Singer et al., 2021 ).…”

Frequent spontaneous structural rearrangements promote rapid genome diversification in a Brassica napus F1 generation

“…Mutations of this kind occur in plants at different rates depending on the genome size and ploidy. For example, in Arabidopsis thaliana, between 1 and 5 de novo intra-varietal mutations have been shown to occur per generation, whereas higher rates are found in rice and other plants with larger genomes (Singer et al, 2021).…”