Should banana streak virus-infected germplasm be distributed?

The International Transit Centre, which holds the largest in vitro collection of Musa cultivars and species, only distributes plant material that has been certified free of pathogens and diseases. Andrew Geering, a plant pathologist at the Department of Primary Industries and Fisheries in Australia, argues that it should make an exception for material infected with banana streak viruses, as long as approval is granted by the relevant plant protection authority in the importing country. Pierre-Yves Teycheney, a virologist at the French Agricultural Research Centre for International Development CIRAD argues that there is not enough information to make an informed decision.

January 2008

Background

Bioversity’s International Transit Centre (ITC) is the only source of clean Musa germplasm available for distribution internationally. Rigorous culture procedures ensure that the plantlets are free of fungi and bacteria but viruses present a special challenge. Each accession is inspected for virus symptoms and undergoes extensive testing for Banana bunchy top virus (BBTV), Cucumber mosaic virus (CMV), Banana bract mosaic virus (BBrMV), Banana mild mosaic virus (BanMMV) and banana streak viruses (BSVs), of which Banana streak Obino l’Ewai virus (BSOLV) and Banana streak Goldfinger virus (BSGfV) are probably the main species concerned (J. N. Parry and A. D. W. Geering, unpublished results). Of the more than 1100 Musa accessions held in the ITC, about one-third are infected with one or two viruses, generally a banana streak virus and/or BanMMV, and are therefore not available for distribution until they receive a clean bill of health.

Therapies exist to remove viral particles from infected plantlets (Thomas et al. 1995, Helliot et al. 2001, Helliot et al. 2002, Helliot et al. 2003), so eventually the ITC could distribute all its material, with one notable exception: plantlets infected with BSVs that also contain the B genome. The reason is that the complete genome of various BSV species is integrated in the B genome derived from Musa balbisiana. Under certain circumstances, these sequences can excise themselves and produce infectious particles. Therapies exist to clean plantlets of episomal viral particles but not of integrated sequences. Cleaning B-genome plantlets that have tested positive for a banana streak virus is thus pointless since viral particles can be produced again after the plantlet has been cleaned.

Currently, about 20% of ITC’s accessions are infected with a banana streak virus in single or mixed infections. The ones that contain only the A genome derived from Musa acuminata should eventually become available for distribution after therapy. Under the current guidelines, however, the vast majority of the infected accessions will not since they belong to the AAB, ABB or AAAB genome groups, with plantains – an important staple food in many countries – being the most affected of them all.

CIRAD, the French Agricultural Research Centre for International Development, has gone one step further than the ITC. Not only does it not distribute the B-genome accessions in its collection, regardless of whether they have tested positive or not for the presence of BSVs, it has also stopped using them in breeding.