Scientists have developed ‘biosensors’, handheld detectors, which they say can be used to tackle the spread of cacao swollen shoot virus disease (CSSVD).
Cocoa Swollen Shoot Virus Disease (CSSVD) can infect cocoa plants at any stage of development. The disease causes a wide range of symptoms depending on the strain of the virus, the stage of infection and the susceptibility of the cocoa variety.
Symptoms include, discoloration of leaves, stem and root swellings and abnormally shaped pods.
CSSVD is transmitted by infectious mealybugs and infected bud wood. The risk of spreading the disease is reduced if mealybugs and live plant material are not transported deliberately or accidentally from one location to another.
The disease is a serious constraint to the production of cocoa in West Africa, particularly in Ghana where the disease was first recognized in 1936. Several different strains of the virus exist and can cause defoliation, dieback of the plant and severe yield losses.
The Institute of Bio-Sensing Technology, and the Faculty of Applied Science at the University of the West of England (UWE) aims to develop highly sensitive and specific bio-sensing platforms to target a range of different diseases affecting humans, animals and even plants.
One of the most recent projects funded by Innovate UK, is to develop a biosensor to detect CSSVD in cocoa plants in West Africa.
The research group at UWE is collaborating with Mars Wrigley on this project, as they work towards a shared goal of tackling the prevalence of CSSVD and enhancing cocoa production in the region.
Côte d’Ivoire and Ghana supply around 70% of the world’s cocoa production, but the cocoa decline has led to CSSVD eradication programs to fight the infection head on by destroying countless infected cocoa trees.
So far these efforts have failed to prevent the spread of CSSVD, resulting in the death of many more cocoa plants and threatening the livelihoods of cocoa farmers.
Dr. Jackie Barnett, Senior Research Fellow, Institute of Bio-Sensing Technology explains,
Our aim is to be able to monitor the epidemiology of the disease, as well as test new planting material.
As a result of developing highly sensitive and specific tests to detect CSSV in pre-symptomatic trees, we can make sure that new plants are not sent out that would grow into infected trees.
Early detection of disease enables a chance to prevent it from spreading further and also gives farmers a peace of mind knowing that their crops are less vulnerable and that new planting material is free of infection.
The team at UWE is developing a biosensor to detect different strains of the virus.
Dr Barnett further explains,
Many CSSV papers are outdated and many have developed immunoassays using polyclonal antibodies
Using polyclonal antibodies generated against the purified virus via animal immunization typically results in a high level of unwanted background in the assay.
Despite attempts by the scientists to take this unwanted background into account, it is often too significant, which impacts the sensitivity. To combat this, Dr Barnett and Dr Allainguillaume, Associate Professor in Conservation Science Faculty of Applied Science at the University of the West of England (UWE) chose to collaborate with Bio-Rad, using its custom antibody reagents to develop sensitive immunoassays for the detection of pre-symptomatic CSSV infections.
In addition to the antibody tests, they use quantitative PCR assays at UWE for comparison. These tools enable them to analyze the CSSVD infection status of cocoa plants grown in the Envirotron at UWE.
As well as working in the lab, the team travelled to Ghana in an early project to pursue their research efforts further at the Cocoa Research Institute of Ghana (CRIG). The group has tested a range of infected samples in Ghana, and gained some promising preliminary results.
They believe antibodies form the key to early disease detection.
To develop the most sensitive and specific immunosensors for detection of distinct antibody-antigen interactions, high-quality antibodies are required for the bioreceptor components.
The EU directive 210/63/EU described in Gray et al, 2020 recommends that animals are no longer used to produce unique antibodies. Instead, Dr Barnett explains, “By collaborating with Bio-Rad, we were able to ensure that no animals were used in production of the monoclonal antibodies, also avoiding the drawbacks of polyclonal antibodies.”
“I found that the custom antibodies we commissioned from Bio-Rad, based on the antigen target information we gave them, were the most cost effective and most reliable way to go, whilst maintaining the highest level of sensitivity we required. As a result, the antibodies Bio-Rad produced for us were ideal for our research.”
The biosensor will work by detecting a specific biological interaction and converting the chemical information obtained into a quantifiable output signal.
Dr Barnett adds,
We now know our antibodies work well, and our goal is to make sure the sensor is reliable and robust.
Looking ahead, the team intends its sensor to function as an optical sensor. Currently, they are developing a bespoke system to help them achieve exceptional sensitivity in the presence of plant material and are also working to define concentrations and learn how to improve the plant tissue samples.
They will have cassettes made to their design specification and a reader. These will be used to perform field trials in Côte d’Ivoire and in Ghana.
The researchers will be working with the World Agroforestry Institute (ICRAF) to explore a range of cocoa samples from sites across Côte d’Ivoire and Ghana.
Dr Allainguillaume concludes,
Realistically, we cannot remove CSSVD as there are far too many CSSVD infected trees, however, we can create areas where you can develop plant material and control a particular area of growth.
