Each year, 20 to 30 percent of the global production of cocoa pods, which are used to make chocolate, is destroyed by plant diseases.
Plant scientists from Penn State University believe they may have found a solution to the destruction the diseases cause.
Use of the powerful gene-editing tool CRISPR-Cas9 could help to breed cacao trees that exhibit desirable traits such as enhanced resistance to diseases.
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A gene-editing tool CRISPR-Cas9 could help breed cacao trees that exhibit desirable traits such as enhanced resistance to diseases, a Penn State study finds. Pictured: lead author Andrew Fister in the Ivory Coast
The cacoa tree grows in tropical regions produces the cocoa beans that are the raw material of chocolate.
Reliable productivity from cacao plants is essential to the multibillion-dollar chocolate industry, the economies of producing countries and the livelihoods of millions of smallholder cacao farmers.
‘In West Africa, severe outbreaks of fungal diseases can destroy all cacao fruit on a single farm,’ said lead author Andrew Fister, postdoctoral scholar in plant science, College of Agricultural Sciences, Penn State.
‘Because diseases are a persistent problem for cacao, improving disease resistance has been a priority for researchers. But development of disease-resistant varieties has been slowed by the need for sources of genetic resistance and the long generation time of cacao trees.’
The researchers reported the study results in Frontiers in Plant Science, which were thought to be the first to demonstrate the feasibility of using CRISPR technology to improve Theobroma cacao.
CRISPR stands for ‘clustered regularly interspaced short palindromic repeats’. It is a way to modify an organism’s genome by precisely delivering a DNA-cutting enzyme, Cas9, to a targeted region of DNA.
The resulting change can delete or replace specific DNA pieces, thereby promoting or disabling certain traits.
Previous work in cacao identified a gene, known as TcNPR3, that suppresses the plant’s disease response.
The researchers hypothesized that using the technique to knock out this gene would result in enhanced disease resistance.
They tested it by adding CRISPR-Cas9 components into detached cacao leaves. They then infected the leaves with a naturally occurring plant pathogen, and the leaves showed graded resistance to the disease.
Previous work in cacao identified a gene, known as TcNPR3, that suppresses the plant’s disease response. The researchers hypothesized that using the technique to knock out this gene would result in enhanced disease resistance
The results suggested that the mutation of only a small amount of the copies of the targeted gene may be sufficient to trigger downstream processes, resulting in systemic disease resistance in the plant.
‘Our lab has developed several tools for the improvement of cacao, and CRISPR is just one more tool,’ said senior author Mark Guiltinan, professor of plant molecular biology and leader of Penn State’s endowed cocoa research program
‘But compared to conventional breeding and other techniques, CRISPR speeds up the process and is much more precise. It’s amazingly efficient in targeting the DNA you want, and so far, we haven’t detected any off-target effects.’
There may be thousands of genes involved in disease resistance, Fister added. ‘We want to evaluate as many as we can,’ he said.
WHAT IS CRISPR-CAS9?
CRISPR-Cas9 is a tool for making precise edits in DNA, discovered in bacteria.
The acronym stands for ‘Clustered Regularly Inter-Spaced Palindromic Repeats’.
The technique involves a DNA cutting enzyme and a small tag which tells the enzyme where to cut.
The CRISPR/Cas9 technique uses tags which identify the location of the mutation, and an enzyme, which acts as tiny scissors, to cut DNA in a precise place, allowing small portions of a gene to be removed
By editing this tag, scientists are able to target the enzyme to specific regions of DNA and make precise cuts, wherever they like.
It has been used to ‘silence’ genes – effectively switching them off.
When cellular machinery repairs the DNA break, it removes a small snip of DNA.
In this way, researchers can precisely turn off specific genes in the genome.
The approach has been used previously to edit the HBB gene responsible for a condition called β-thalassaemia.
The ultimate goals of Penn State cacao research are to help raise the standard of living for smallholder growers and stabilize a threatened cocoa supply by developing plants that can withstand diseases, climate change and other challenges, according to co-author Siela Maximova, senior scientist and professor of horticulture.
‘Any production increases in the last 20 years have been mostly due to putting more land into production,’ said Maximova, who co-directs the cacao research program. ‘But land, water, fertilizer and other inputs are limited.
To enhance sustainability, we need plants that are more vigorous and disease resistant and that produce more and better-quality beans.
‘This study provides a ‘proof of concept’ that CRISPR-Cas9 technology can be a valuable tool in the effort to achieve these goals,’ she said.