Plant Cuttings

This image showing flowers of Theobroma cacao by Vinayaraj is made available under the Creative Commons Attribution-Share Alike 4.0 International license.

In this modern age, ‘going viral’ is supposedly a good thing. But, it’s not so good when it concerns infection of a plant with a virus, a “microscopic infectious agent which attacks living organisms in order to use the machinery of the cells in the organism to replicate itself (the cell that they use to multiply is the host cell)” (Freeha Anjum)*. Although they are non-living entities, they cause much harm to the living things with which they interact. Viruses infect all known life-forms on Earth, e.g. archaea (Eugene Koonin et al., Biol Direct 1, 29 (2006); https://doi.org/10.1186/1745-6150-1-29), bacteria (Laura Kasman & La Donna Porter), fungi (Bianca Hough et al., Viruses 2023, 15(5), 1202; https://doi.org/10.3390/v15051202; Paul Rowley), protists (Matthias Fischer), and animals (ME Reichmann). Amongst the Plant Kingdom, more than 700 viruses (Richard Strange & Peter Scott, Annual Review of Phytopathology 43: 83-116, 2005; https://doi.org/10.1146/annurev.phyto.43.113004.133839) cause diseases in a range of plants, particularly those of economic value such as tomato, tobacco, and potato (Karen-Beth Scholthof et al., Molecular Plant Pathology 12: 938-954, 2011; https://doi.org/10.1111/j.1364-3703.2011.00752.x), and cereals (e.g. rice, wheat, and maize (Edward Rybicki, Arch Virol 160: 17–20, 2015; https://doi.org/10.1007/s00705-014-2295-9)).

This blog item primarily concerns cacao swollen shoot virus disease (CSSVD) (Ebenezer Antwi Gyamera et al., Plant Disease 107: 1261-1278, 2023; https://doi.org/10.1094/PDIS-10-22-2412-FE; George Ameyaw et al., Viruses 2024, 16(1), 43; https://doi.org/10.3390/v16010043). Although this virus causes disease in several plants, the cacao tree (Theobroma cacao) is considered to be the main one affected by CSSV, hence the virus’ name. Famously, the cacao tree is the botanical source of cocoa beans from which chocolate is made (Gemma Whitaker; Kate Castleden & Emma Williams; Naomi Blumberg et al.). CSSVD is a major threat to cacao plantations in Ghana (Ebenezer Antwi Gyamera et al., Plant Disease 107: 1261-1278, 2023; https://doi.org/10.1094/PDIS-10-22-2412-FE), and neighbouring Côte d’Ivoire (Gilberte Koffi) which together produce approx. 50% of the world’s chocolate (Oliver Morrison).

Cacao trees become infected with CSSVD when virus-carrying mealybugs (Harold Box, Nature 155: 608–609, 1945; https://doi.org/10.1038/155608b0; Ebenezer Antwi Gyamera et al., Plant Disease 107: 1261-1278, 2023; https://doi.org/10.1094/PDIS-10-22-2412-FE) feed upon their ‘sap’ [the contents of the sieve tubes in the phloem (AE Douglas, Journal of Experimental Botany 57: 747–754, 2006; https://doi.org/10.1093/jxb/erj067)%5D. Once introduced into the plant’s vascular system (WJ Lucas et al., J Integr Plant Biol. 55(4): 294-388, 2013; doi: 10.1111/jipb.12041), the virus can spread throughout the plant, and far from the original puncture site of its introduction. When infected, symptoms – which are seen throughout the plant, in leaves, stems, roots, and developing fruits – include: discoloration of foliage; reddening of young leaf veins; chlorosis; small distorted pods; swelling of stems; and dieback. One of the consequences of such a suite of symptoms is that the productivity of the cacao tree is severely reduced, “within one year of infection yields decrease by 25%, and within two years by 50%. The trees are usually killed within 3 to 4 years”. Disease of the cacao tree is therefore of concern to all who eat chocolate, and/or who earn a living from cultivation of the tree or any aspect of the chocolate production pathway.

Pleasingly, there are a number of ways in which the disease can be controlled, e.g. use of CSSV-resistant cultivars of cacao, and removal – ‘cutting-out’, or ‘roguing’ (Ebenezer Antwi Gyamera et al., Plant Disease 107: 1261-1278, 2023; https://doi.org/10.1094/PDIS-10-22-2412-FE) – of infected trees (and those in contact with them) to save non-infected ones . Additionally, chemical interventions can be used to control the virus-transmitting mealybug, and even organic treatments employing neem oil, tea-tree oil, mineral oil and detergents/soaps designed especially for agricultural usage. Unfortunately, all of these ‘interventions’ have their associated costs (which are an added burden to the cacao farmers who are often already on narrow profit margins and many are experiencing poverty), and benefits, and consequences (e.g. use of chemicals may harm the environment, the human operator, and organisms other than the target mealybugs, plus, the insects are highly-resistant to pesticides (Ben Cost)).

Cross-protection – the plant equivalent of immunization or vaccination – is another disease-control option. In this approach, milder strains of a virus are used to offer protection to trees against subsequent infection with a virulent variant of the virus (Ebenezer Antwi Gyamera et al., Plant Disease 107: 1261-1278, 2023; https://doi.org/10.1094/PDIS-10-22-2412-FE). But, this technique is expensive, and immunized trees are less productive than virus-free trees (Kathryn Egan Bennett). However, if that’s the intervention that’s appropriate and chosen it needs to be as cost-effective as possible. And one way of reducing the overall cost is to avoid the need to ‘vaccinate’ every tree in a plantation. But, what is the best planting distance to achieve this, i.e. how far apart can ‘vaccinated’ trees be from unvaccinated trees in order to prevent the spread of the virus?

Folashade Agusto et al. ((2024) PLoS ONE 19(3): e0294579; https://doi.org/10.1371/journal.pone.0294579)** used a mathematical approach to tackle this question. Their solution was to propose creation of a protective layer of vaccinated trees surrounding unvaccinated ones – “like elephants forming a circle to protect their young” (Ben Cost) –  which disrupted the mealybugs’ routes of travel from tree-to-tree (Anjali Thakur). “While still experimental, these models are exciting because they would help farmers protect their crops while helping them achieve a better harvest … This is good for the farmers’ bottom line, as well as our global addiction to chocolate” (Benito Chen-Charpentier [last-named author of the study], quoted here). Chocolate-lovers everywhere will no doubt be somewhat heartened by Agusto et al’s work***.

But, just as CSSVD is bad news for cacao plants (and chocoholics everywhere…), another plant virus may be good news for mammals suffering from cancer****. Working with mice (as a model species to understand human beings (Kristina Rydell-Törmänen & Jill Johnson (2019); https://doi.org/10.1007/978-1-4939-9086-3_1), Young Hun Chung et al. (Advanced Science 11(18) May 15, 2024 2308237; https://doi.org/10.1002/advs.202308237) show that “Systemic administration of cowpea mosaic virus demonstrates broad protection against metastatic cancers”. Unfortunately, we have no space left to say more about that good news story – other than to point interested readers to the scientific paper, and the interpretation of the work by Russell McLendon.

* For more on viruses that infect living organisms, see here, here, here, and here. For more on computer viruses, see Jonathan Tanner, here, and here.

** For more scicomm articles about the CSSVD story, see Katherine Egan Bennett, Ashley Palya, Ben Cost, Kendra Stacy, Anjali Thakur, Oliver Morrison, here, and here.

*** In a world where CSSVD causes uncertainty about affordable and continued supply of chocolate, some encouraging news is provided by Kim Mishra et al. (Nature Food (2024), doi: 10.1038/s43016-024-00967-2) in their study entitled “Valorization of cocoa pod side streams improves nutritional and sustainability aspects of chocolate”. Interpreting this work for a non-specialist audience, Deborah Kyburz tells us that “Researchers at ETH Zurich have joined forces with the chocolate industry to investigate the potential for making maximum use of the cocoa fruit, which would increase the profitability of cocoa cultivation while making chocolate a healthier indulgence”. To find out more, Mr P Cuttings encourages readers to look at Kyburz’s article – in conjunction with Mishra et al. (2024)’s scientific paper. For another scicomm article on this news see Mike McRae’s piece here.

**** Mindful that I don’t want to stray too far away from plant territory, I just want to mention this interesting article by Nala Rogers whose subject is insects that “have tapped wild viruses in a seriously frightening manner”. Mr P Cuttings is happy to help his more zoologically-inclined readers.