Nucleotide sequence determination of Cadang-Cadang-like viroids in the Pacific area

Coconut palm is mainly a smallholder crop, with about 50 million people involved in its cultivation worldwide. In Asia, another 30 million people depend on coconut and its processing for a livelihood. Much of the value of the crop accrues to developing countries, where it has traditional essential uses as food, fibre and timber as well as being an important cash crop.

However, in the Philippines, coconut cadang-cadang viroid (CCCVd) causes a disease of coconut palm that costs the country approximately US$30-50 million annually in lost production. For this reason, major concern followed an ACIAR-sponsored survey of 29 other countries in the Pacific area in 1988-91, which showed that viroids similar to CCCVd are present at levels of 20-90% in populations of coconut, oil palm and other monocotyledons. However, at this stage, no disease symptoms were apparent on any of the coconut palms infected with viroids similar to CCCVd. It is possible that the newly discovered viroids may be responsible for as yet unrecognised losses in production or death of palms; and exchange of germplasm between countries may induce overt disease when the viroids are transferred to different environments.

Viroids - the smallest known pathogens - have been found only in plants. Unlike viruses, they do not have a protein coat, consisting only of small, circular, single-stranded, infectious RNA molecules. They replicate in the host cell and can be transmitted by man's cultural practices, by insects, seeds and pollen, and by other, unknown, means.

This project - urgently requested by the countries involved - follows on from the survey. Collaborating scientists from Australia, Vanuatu and the Philippines will collect material from a number of trees that were examined for viroids in the survey, isolate and clone viroid strains, and sequence their nucleotides. They will then compare nucleotide sequences of selected isolates, uncover relationships between the new viroids and between the new viroids and CCCVd, and assess the threat the organisms may represent.

Australia is in a unique position to lead the work. Researchers at the Waite Institute, in collaboration with colleagues in Asian and Pacific countries, established the aetiology of cadang-cadang disease (viroid) and coconut foliar decay disease (virus). The Waite researchers have also developed facilities for molecular characterisation of coconut viroids. No research is being done on viroids of monocotyledons elsewhere, and there is no international central research organisation for coconut. As a result, until now, sustained research on coconut diseases has been limited, and breeding programs have continued in the absence of information on intracellular pathogens such as viruses or viroids.

The collaborators from the partner institutions in Vanuatu and the Philippines will continue to maintain the source palms found to contain CCCVd viroids and assist in the collection of bulk material, either during visits of the Australian scientists or by harvesting leaf and shipping it to Australia. Both developing countries have received financial support during previous ACIAR projects and have trained staff available to carry out the tasks efficiently.

Laboratory work will be done in Adelaide employing biochemical analysis methods established during the previous ACIAR project and computer analysis of nucleotide sequences. As well as determining relationships among CCCVd-related viroids, the research will develop specific probes for use in diagnosis of viroid isolates, which will lead to rational design of quarantine guidelines; and develop methodology to provide viroid-free parents and progeny for coconut breeding and selection programs.

Project results will be used to design guidelines for the safe movement of coconut germplasm and to provide the necessary screening methods. Technology developed in Adelaide will be available to Philippine Coconut Authority staff who have received training there, and will provide all coconut-producing countries with the prerequisites to continue breeding research for improved lines. Viroid-tested in vitro germplasm will be made available to countries such as Papua New Guinea for the development of plantlets for breeding programs and, in the long term, for the distribution of improved hybrid seedlings to farmers. This will protect countries from potential outbreaks of new diseases similar to cadang-cadang. The chances of success of the project are high, and in the long term it is likely to benefit all coconut-growing countries.

Australia will benefit by the establishment of a research base for viroids of palms and other monocotyledons; by the establishment of appropriate international quarantine guidelines for the protection of palms and other monocotyledons; and by the development of good relationships and collaborative links with neighbouring countries. Further, when the viroid testing screening system is established, there may be an opportunity for partial cost recovery from users.