Phytoplasma diseases of coconuts: Understanding their transmission and the sustainable breeding of resistant and tolerant varieties

Coconut palm is a vital crop in coastal tropical regions of much of East and West Africa (eg Ghana, Nigeria, Cote D'Ivoire, Tanzania, Kenya, Mozambique). It is referred to worldwide as the 'Tree of Life' and is the main source of livelihood for several disadvantaged groups (eg women and landless poor) providing food, fuel-wood, drink, edible oil, fibre, animal feed, building materials and a source of income with minimum capital outlay. Coconut oil is high in saturated fats including lauric acid and is regarded as one of the healthiest dietary oils on earth, helping to prevent heart disease and maintain the body's immune system, and coconuts grow in poor soils that are unsuitable for other crops and help to sustain the environment and beautify coastal landscapes. However, Lethal-yellowing like diseases have devastated plantations in much of Africa resulting in 100% losses in many areas and severe economic hardships in these coconut producing communities. These diseases are caused by a group of organisms called phytoplasmas, which inhabit the phloem and kill the crowns of palms to result in bare trunks. There is no effective remedial action that can be taken to save a palm from death once it has become infected, and the most appropriate way of controlling disease is to replant with resistant or tolerant palm varieties. Coconut breeders have identified some palm varieties from different parts of the world with high resistance/tolerance to the diseases in Africa, and these need to be exploited in breeding programmes to provide improved varieties and hybrids for replanting, to ensure a secure and sustainable industry. Phytoplasmas themselves are poorly characterised because they can not be grown in culture, but new diagnostic techniques and DNA sequencing projects have started to provide us with new tools for studying these diseases. Generally, they are transmitted between plants by insect vectors, but recent evidence has suggested that they may also be transmitted through seed, which is a major concern to Plant Protection, Quarantine and Regulatory Services who have imposed severe restrictions on the movement of germplasm as a result. In this research, the University of Nottingham will provide technical expertise and strategic resources in collaboration with the coconut breeding programme of the Oilpalm Research Institute in Ghana to achieve a greater understanding of the transmission of these diseases and about the nature of disease resistance. Specifically, we will investigate whether seed transmission occurs, by culturing and planting embryos derived from diseased palms and examining them for the presence of phytoplasmas using improved diagnostic and microscopic techniques developed at Nottingham. This will result in conclusive scientific evidence about the risks of phytoplasma transmission through movement of coconut germplasm, which will be communicated to regulatory authorities to help them make informed decisions. We will also use techniques that quantify the levels of phytoplasmas in plants to confirm the distinction between resistance (immunity to phytoplasmas) and tolerance (the presence of phytoplasmas without symptoms) in different coconut cultivars, and establish which lines should be used in breeding programmes and which should be distributed to growers for replanting to minimise the risks of future phytoplasma infection. We will also develop a long term strategy to discover more about the genes that control resistance and tolerance in coconut, so that breeding programmes can be maintained and improved in the future to produce more resistant varieties. In summary, this project will result in the distribution of improved varieties for replanting, and will provide the stakeholders in developing countries with the short and long-term support that they are requesting to sustain and expand their indigenous industries.

Coconut is a vital crop in much of Africa as a source of food and income, but many palms are succumbing to Lethal Yellowing like (LYD) diseases caused by phytoplasmas. There is no effective remedial treatment for infected palms, so replanting with resistant or tolerant varieties is the best way to manage the disease. However, distribution of germplasm is being restricted because evidence suggests that phytoplasmas might be transmitted through seed and embryo cultures. This project will produce evidence on the risk of seed and embryo transmission, establish which palm varieties possess resistance / tolerance and whether environmental factors influence this, and develop molecular markers to track resistance / tolerance in breeding programmes. To investigate transmission we will produce embryo cultures from infected seed and use molecular diagnostics to determine whether LYD is present, and will plant seeds from infected palms to determine whether LYD is transmitted into progeny plants. Antibodies raised against the LYD SecA protein will be used to develop immunofluorescence microscopy techniques to localise phytoplasmas within infected embryos and inflorescences to show how the organisms might pass into seeds. To examine tolerance and resistance, qPCR or T-RFLP will be used to quantify the levels of LYD in a range of coconut cultivars in Ghana over a three year period. By monitoring specific palms and taking samples during dry and wet seasons, we will show which varieties are immune to disease and which contain phytoplasmas without showing symptoms (tolerance). Established SSR and AFLP markers, and markers that we develop based on resistance gene analogues, will be mapped onto the palms that we screen for LYD to identify markers that are useful for identifying resistant palms to distribute for replanting. At the same time an F2 mapping population will be established as part of a long term programme to map resistance / tolerance traits onto the coconut genetic map.