Frédéric Hénin
Part 3
IN AFRICA, GENOME EDITING MAKES AGRICULTURE MORE EFFICIENT AND SUSTAINABLE
Genome editing enables the acceleration of plant breeding by modifying the genetic information of plants to promote their growth. The newly obtained varieties of sorghum and cassava are more productive as they become more resistant to their bio-aggressors.
Striga weeds cause havoc in sorghum crops in Sub-Saharan Africa. These parasitic plants, known as “witchweed,” attach themselves to the roots of host plants to divert and absorb the water and mineral salts intended to nourish the sorghum leaves and ears.
Researchers have succeeded in creating a sorghum variety resistant to infestation by this parasitic plant. The procedure is as follows: the new host plant prevents the Striga roots from connecting to its own roots by releasing a substance that inhibits the germination of Striga seeds. As a result, the parasitic plant can no longer thrive at the expense of sorghum by extracting the water and mineral salts necessary for its growth through its roots.
Striga hermonthica Plants (@Aneth David (SLU))
“The new sorghum variety was obtained through genome editing,” reports Georges Freyssinet, president of the French Association of Plant Biotechnology. “It is a set of technologies that allows for the targeted modification of the genetic information of plants (see box). This modification is achieved through the addition, subtraction, or exchange of nucleotides, the ‘building blocks’ that make up the DNA of plants.”
An increasing number of African countries see opportunities in genome editing (or targeted mutagenesis) to quickly increase yields of local crops by creating more productive plant varieties without relying on massive amounts of inputs (such as pesticides, fertilizers, water, etc.). It will make their agriculture both more efficient and more environmentally friendly.
“Genome editing represents a limited additional cost compared to conventional breeding if in vitro culture is mastered,” asserts George Freyssinet. “Countries that have adopted it have already adjusted their regulations to make the newly obtained plant varieties through genome editing commercially viable.”
In Nigeria, edited plants that do not contain recombinant DNA are not considered genetically modified plants (GMOs).
In Kenya, plants in which gene expression has been inhibited through genome editing are also not considered GMOs. The same applies to plants with a modified genetic heritage through the introduction of genes from plants of the same species.
Sorghum Harvest in Burkina Faso (Rik Schuiling / TropCrop-TCS)
Adapted Regulation
Other African countries are motivated to develop varietal research using genome editing and will adapt their regulations to make the newly obtained plants commercially viable. However, edited varieties must be validated in each country by a competent authority.
However, South Africa has chosen a different path. Its government has already stated that it will continue to classify edited plants as GMOs.
Genetic editing does not alter the reproductive mode of selected plants. Therefore, farmers who cultivate more productive edited plant varieties will be able to produce and share their own seeds.
“Over the years, the range of edited plants has expanded,” observes Georges Freyssinet. “The goal in each territory is to meet the needs of farmers by enabling them to produce better for better food security. But for now, only two edited plants are commercially available: high oleic acid soybeans in the USA and GABA-enriched tomatoes in Japan.”
Edited maize varieties are now resistant to lethal necrosis or drought.
Finally, edited cassava plants are resistant to mosaic disease, and wheat plants have been selected for their ability to be cultivated in highly alkaline soils. Tests on these plants are underway.
Compared to the parent plants they originate from, these newly edited varieties can be selected to have nutritional and food qualities that reduce the occurrence of deficiencies when consumed. For example, Ethiopian mustard is enriched with essential fatty acids.
Genome Editing or Targeted Mutagenesis or NGT
Genome editing has revolutionized the possibilities of increasing genetic variability. “It allows for the targeted modification of plant genetic information through the addition, subtraction, or exchange of nucleotides,” explains Georges Freyssinet, president of the French Association of Plant Biotechnology.
“These obtained mutations are targeted because the genes to be modified must be known and located before being modified,” he adds. “Therefore, genome editing or targeted mutagenesis does not involve the introduction of genes from plants of other species.”
Genome editing technologies were developed at the end of the 20th century. They involve exploiting the ability to cut both strands of DNA, which then repair themselves automatically.
Through genome editing, it will be possible to confer a multitude of new resistance traits to plants, both biotic and abiotic stresses, that other plants of the same species, whether cultivated or not, possess in their natural state.
Edited plants will be able to better absorb nitrogen from the soil, reducing the need for fertilizer inputs in cultivation. The aim is to make emerging countries less dependent on chemical fertilizer imports.
“In fact, a wide range of traits can be modified through genome editing,” reports the president of the French Association of Plant Biotechnology. “It is possible to act on plant yield or their aboveground or belowground architecture.”
The most well-known targeted mutagenesis technique is the CRISPR technique, known as “molecular scissors.” It was invented in 2012 by Emmanuelle Charpentier and Jennifer Doudna. It achieved immediate success, and eight years later, they were awarded the Nobel Prize in Chemistry.
The CRISPR technique exploits the natural ability of a bacterium to defend against viral invasions by cleaving their DNA or RNA, preventing their replication. Its genetic heritage gives it the ability to synthesize a nuclease dedicated to this function.
To edit plants using CRISPR, this bacterial ability to cleave a DNA strand is transferred to them via transgenesis, by associating the nuclease with an RNA sequence that recognizes the gene segment where the cut will be made. The desired gene modification is then carried out. Finally, the DNA is repaired, and the transgene is removed.
The other, more random, method of mutagenesis aims to introduce the nuclease responsible for cutting the host plant’s DNA, for example, into protoplasts. Plants with the modified gene are then selected through PCR and evaluated.