A team of researchers at the Pennsylvania State University has made a discovery that may help accelerate breeding efforts aimed at improving the disease-ridden Theobroma cacao, a cash crop and the sole source of cacao beans from which the primary ingredients in chocolate products — cocoa powder and butter — are derived.
A cacao tree (Theobroma cacao) with fruit pods in various stages of ripening. Image credit: Marco Vasquez.
Native to tropical Mesoamerica, Theobroma cacao is an understory tree principally grown in rainforest areas within 20° latitude of the equator around the world.
Its unique and critical role in the chocolate manufacturing industry makes it an important export for developing countries in Africa, Central and South Americas and in South Asia, where cacao is predominantly cultivated.
Cultivation of cacao is limited by many factors including several fungal, oomycete and viral diseases that cause global losses of 20-30%.
Massive pathogenic losses make research and breeding for improved disease resistance crucial for the future sustainability of the crop and to improve farmer livelihoods.
In addition to improved disease resistance traits, cacao breeders actively pursue avenues for the improvement of cocoa quality traits such as flavor, health beneficial metabolites, climate resiliency and improved yield.
However, progress in breeding programs is severely limited by cacao’s juvenile longevity and high costs of breeding typical of tree crop systems and thus the control of flowering time is of scientific and practical interest.
“Characterizing the Flowering Locus T gene in cacao, responsible for the production of florigen — a protein that triggers flowering in most plants — is important,” said senior author Professor Mark Guiltinan, a researcher in the Department of Plant Sciences and the Huck Institutes of the Life Sciences at the Pennsylvania State University.
“We expect this advancement to enable scientists to develop disease-resistant trees faster, which is critical because 20% to 30% of the world’s cacao crop is lost to disease annually.”
To find the flowering gene in cacao, Professor Guiltinan and colleagues first looked at genes known to be responsible for flowering in the Arabidopsis plant.
Before finding the cacao flowering gene, they tested an Arabidopsis flowering gene in cacao to see how the plant developed.
Testing their theory, they overexpressed that gene to trigger very early flowering in cacao ‘plantlets’ in the lab and showed that those tiny flowers produced grains of pollen that were viable.
“To find the flowering gene in cacao, we used a bioinformatics approach, taking the sequence of the gene from Arabidopsis and looking for similar genes in the cacao genome,” said first author Sarah Prewitt, a doctoral candidate in the Department of Plant Sciences at the Pennsylvania State University.
“We found the cacao gene that promotes flowering because the sequences look very similar.”
“Geneticists consider the function of the florigen flowering gene to be highly conserved. That means the gene is extremely consistent — it does what it does in every plant genome that you look in.”
“The florigen flowering gene certainly has been looked at in a lot of plants, and it’s very reliable. It controls the timing of flowering.”
“While intriguing on a scientific level, the discovery of the cacao Flowering Locus T gene could have a potentially significant, real-world impact by helping to improve the lives of millions of cacao farmers in developing countries sooner than previously thought possible,” Professor Guiltinan said.
“The breeding of cacao varieties with high yields, disease resistance, resilience to climate change and desirable quality traits is an important component of a broader goal to develop sustainable farming systems for cacao.”
“Better cacao varieties can increase the income, and thus the well-being, of cacao farmers who live in some of the most impoverished regions of the world, such as West Africa.”
“In turn, this will benefit the economies of these countries and the environment and will provide a sustainable source of the main raw ingredient for the chocolate industry.”
The findings were published in the journal BMC Plant Biology.
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S.F. Prewitt et al. 2021. Inter-species functional compatibility of the Theobroma cacao and Arabidopsis FT orthologs: 90 million years of functional conservation of meristem identity genes. BMC Plant Biol 21, 218; doi: 10.1186/s12870-021-02982-y
