Cocoa, the food of the gods, and plant tissue culture

Cocoa, the food of the gods, and plant tissue culture

Did you know that the Mayans used cocoa beans as a currency? A bar of chocolate in Mayan times may have been worth more than its weight in gold.

Chocolate’s first recorded occurrence was in 1500 B.C. However, it was not a sweet confectionary as we know it today, but rather a bitter drink. Moreover, the accounts of Spanish colonials suggest that cocoa beans were used to pay workers in the 16th century. Humans have always held cocoa in high regard.

What is cocoa?

Cocoa (cocoa beans) comes from the seeds of Theobroma cacao pods (fruit). Drying and fermentation of the seeds create cocoa, the key ingredient in chocolate. Additionally, the solids from the cocoa fruit create cocoa butter. Cocoa products occupy a wide market range including both gastronomical and pharmaceutical.

These magical beans exhibit many health benefits.  Eating cocoa (not sugary chocolate!) decreases inflammation, improves blood sugar control, and helps support healthy skin, to mention a few. Additionally, research has found that chocolate positively affects brain chemistry. This is probably why when we are sad we tend to eat an entire box of chocolates.

Cocoa agriculture

The cocoa tree is an Amazonian native that was domesticated over 5000 years ago. Subsequently, the tree was introduced to Central America, Western Africa, West Indies, Indonesia, and the Philippines. In 2019 the greatest cocoa bean producer came from a West African country, the Republic of Côte d'Ivoire (Ivory Coast).

There are three main varieties of cocoa: Forastero, Criollo, and Trinitario. The Forastero variety is responsible for between 80 - 90% of the world’s cocoa production. The Criollo variety is a delicacy that is often plagued by diseases. Lastly, the Trinitario variety is a cross of the two former varieties.

Challenges for cocoa producers

There are three main challenges to overcome in cocoa production. The first challenge is over-aged stock, the second is finding suitable environments as well as the impact of climate change, and the third is pest infestations.

The cocoa tree takes five to six years to mature to fruiting age. Thereafter, they live until around 25 years old. However, in Africa, some trees live well into their 40s. Older trees make fewer cocoa pods, but they do become more pest resistant. The result is a greater number of older trees responsible for much of the cocoa bean yield.

These trees are notoriously fussy. Being Amazonian natives, they enjoy growing with other trees, but not really with other cocoas as they compete for the same resources. However, breeding has resulted in cocoa genotypes that can grow in orchards. But there is still the need for favorable tropical conditions and nutrient-rich soil for cocoa trees to flourish. Therefore, all these factors leave few geographical areas suitable for cocoa production.

Climate change is not making things any easier for cocoa farmers. Cocoa trees exhibit extreme sensitivity to drought. This is really bad news as climate predictions for West African cocoa producers are not favorable. Predictions show a decrease in rainfall and an increase in temperature. Within the next decade, the ‘wet months’ are predicted to decrease from four months to three months. The temperature will see a 1.2 °C increase by 2030. Therefore, the forests in which cocoa thrive will likely be compromised.

Pesky parasites! Some estimates state that 30—40% of global cocoa production is lost to pests and diseases. Most cocoa farmers are not equipped to handle this problem. Many of these farmers come from impoverished areas and lack the education and/or resources to combat parasites and diseases. The main threats to cocoa farmers are:

Diseases, such as:

  1. Witches’ Broom;
  2. Frosty pod rot;
  3. Phytophthora pod rot; and
  4. Vascular-streak dieback.

Insects, such as:

  1. Mirids (capsid bugs); and
  2. Cocoa pod borer (moth).

Some viruses can cross from the maternal plant into the seeds. Consequently, farmers incur devastating losses. Such diseases can affect generations of cocoa crops thereby affecting generations of farmers. However, plant tissue culture techniques can combat these nasty ailments.

History of plant tissue culture and cocoa

Cocoa trees show a great deal of genetic diversity. This genetic diversity also relates to diversity in cocoa production. For example, in Costa Rican cocoa hybrids, 60% of production was the result of just 3% of the trees. Therefore, it is particularly important to create clones of high-yielding, pest-resistant trees. However, techniques such as grafting on rootstocks have been ineffective in propagating many healthy cocoa trees. Especially when several cuttings serve as disease carriers. Therefore, tissue culture techniques are much more beneficial to produce disease-free starting material for cocoa producers.

Several studies have indicated that the major limiting factor in cocoa agriculture is high seedling mortality. Most of the seedlings planted do not make it to maturity. This is due to the high amount of genetic diversity. Many of the groups do not handle stress well. This results in major losses at the seedling establishment phase.  However, some cocoa genotypes do very well at the seedling establishment phase and can be used to increase establishment. Plant tissue culture methods can help isolate these genotypes and facilitate large-scale production.

Successful propagation via somatic embryogenesis has already occurred for 100 different cocoa genotypes. And the clones were disease-free! This method allows farmers to have disease-free clones of high-yielding genotypes on their farms. This results in higher cocoa harvests. One study also discovered that it is possible to produce 4000 somatic embryos from a single cocoa flower in a year, using somatic embryogenesis ('SE').

If this excites you, there is more to read in our article on "Somatic embryogenesis for plant tissue culture".

Tissue culture improvements in cocoa

Scientists have found methods to decrease the cost per somatic embryo to grow a cocoa tree. Joining plant tissue culture and classical propagation techniques often results in cheaper production of cocoa trees. One such method, that is showing great promise is joining secondary SE with classical greenhouse cutting propagation.

Secondary SE creates somatic embryos using the cells of other somatic embryos. Essentially, the process follows two steps. First, using SE we create somatic embryos. Then using cells from the first step we can create more somatic embryos. Somatic embryos making more somatic embryos: how efficient!

The classical greenhouse propagation techniques use cuttings and stress them in the soil to yield positive results. In a similar way, micro cuttings from juvenile cocoas of secondary SE are harvested and subjected to root induction. This improves meristematic growth in the lower regions of the plant, creating healthy taproots. These strong taproots make for hardier cocoas that can survive adverse growing conditions. Thus, supplying disease-free and genetically superior plants to farmers.

Researchers have also found the best carbon source for the culture medium. Findings show that glucose is the best carbon source for the three main cocoa varieties. Plantlets grown on glucose develop shoots with normal cocoa leaves. While other carbon sources such as sucrose, fructose, and maltose sometimes produce cotyledon-like leaves (Cotyledons are the first leaves produced by plants and are not considered true leaves as they are part of the embryo).

Some of the new improvements in cocoa tissue culture include:

  • Better identification of genetics (better selection for cloning favorable genetics);
  • Better disease identification (infected tissues can be excluded from cloning);
  • Favorable carbon sources found;
  • There is efficient use of antioxidants;
  • Better selection of PTC techniques and induction of secondary SE; and
  • Improvements in mass clone production including post tissue culture plant propagation.

Recent developments in cocoa tissue culture are creating exciting possibilities for increasing cocoa production for growers.

Chocolate wasted

Wasting chocolate should never happen! Some might even consider it a sin. Modernization and incorporation of micropropagation in cocoa farming is the way forward. Allowing us to get chocolate-wasted.

We hope this article will be useful for many of you, especially cocoa growers for understanding the scope of increasing production using tissue culture techniques. For more articles like this, keep checking our space!

By Christos Tripodis | 15th November 2021

References

  • Guiltinan, M.J., Maximova, S. (2000). Recent advances in the tissue culture of Cocoa from somatic embryos to Bentwood Gardens. Proceeding of the International Workshop of New Technologies and Cocoa Breeding 163—170.
  • Learn, J. (2018). The Maya civilization used chocolate as money. Retrieved 19 October 2021, from https://www.science.org/content/article/maya-civilization-used-chocolate-money
  • Modeste, K.K., Eliane, M.T., Daouda, K., Brahima, S.A., Tchoa, K., Kouablan, K.E., Mongomake, K. (2017). Effect of antioxidants on the callus induction and the development of somatic embryogenesis of cocoa [Theobroma cacoa (L.)]. Australian Journal of Crop Science 11(1):  25—31. DOI: 10.21475/1jcs.2017.11.01.pne174
  • Padi, F.K., Adu-Gyamfi, P., Akpertey, A., Arthur, A., Ofori,A. (2013). Differential response of cocoa (Theobroma cacao) families to field establishment stress. Plant Breeding 132(2), 229—236. DOI: 10.1111/pbr.12039
  • The International Cocoa Organization. (2020). Pests & Diseases. Retrieved 19 October 2021, from https://www.icco.org/pests-diseases/
  • Traore, A., Guiltinan, M.J. (2006). Effects of carbon source and explant type on somatic embryogenesis of four cacao genotypes. HortScience 41(3): 753—758.
  • Quainoo, A.K. (2006). Germplasm conservation of cocoa (Theobroma cacoa L.) and virus elimination through tissue culture. Thesis for Doctor of Philosophy, The University of Reading.
  • Läderach, P., Martinez-Valle, A., Schroth, G. Castro, N. (2013). Predicting the future climatic suitability for cocoa farming of the world’s leading producer countries, Ghana and Côte d’Ivoire. Climatic Change 119, 841—854. DOI: 10.1007/s10584-013-0774-8