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Fiji disease virus is a propagative, persistently transmitted virus that multiplies in species of the delphacid planthopper genus Perkinsiella, and in sugarcane, the feeding host of the insect. Efforts to improve and modify the disease rating system for Fiji disease have largely focussed on the planthopper as individual vectors of the virus, rather than as a population of the principal, or at least an alternative, host of the virus. This perspective has resulted in key parameters of disease incidence resulting from plant infection by propagative, persistently transmitted viruses being largely overlooked or misunderstood during efforts to improve the rating system. These parameters include the relatively long acquisition, latency, and transmission times, the percentage of the population containing virus, or viruliferous, in the above periods, and the effects of population density and number of plants visited on disease incidence. Suggestions to modify trial design to improve virus transmission to the plant, based on the disease incidence parameters of the propagative, persistent transmission class, are presented and the practical difficulties of implementing these proposals are discussed. In the context of fully understanding the underlying biology of this virus–insect–plant system, the hypothesis that Fiji disease virus, as a plant-infecting member of the Reoviridae, is primarily an insect virus with a secondary plant host, and may have diverged from an insect-infecting virus relatively recently is proposed and compared with other members of the family Reoviridae.

A transgenic line of the sugarcane cultivar Q124 with significantly enhanced resistance to Fiji disease was produced by microprojectile-mediated transformation with a transgene encoding a translatable version of Fiji disease virus (FDV) segment 9 ORF 1 under the control of the maize polyubiquitin promoter. Sixty-four transgenic lines were tested in glasshouse trials by caging the plants with viruliferous Perkinsiella saccharicida planthoppers. After 2 weeks, the planthoppers were removed and the plants monitored for symptoms. One transgenic line showed significantly enhanced resistance to Fiji disease compared with the Q124 parent and other lines showed varying levels of resistance. The molecular phenotypes of the transgenic plants at both the DNA and RNA levels were not entirely consistent with a resistance mechanism based on post-transcriptional gene silencing but were consistent with reports from other sugarcane-virus resistance systems. This is the first report of transgenic sugarcane containing an FDV-derived synthetic resistance gene showing resistance to FDV, although the mechanism of resistance has not yet been elucidated.

The fungus Pachymetra chaunorhiza Croft & Dickcauses a root rot in sugarcane (Saccharum interspecifichybrid). Suspension-cultured sugarcane cells prepared from cultivars Q114(P. chaunorhiza resistant) and Q90(P. chaunorhiza susceptible) were inoculated with aheat-derived elicitor preparation from P. chaunorhizaand the cellular responses monitored by measuring phenylalanine ammonia-lyase(PAL) and peroxidase (POD) activity and the production of additional phenoliccompounds.Introduction of the P. chaunorhiza elicitor inducedmarked changes in the biochemistry of both sugarcane cell lines. Both celllines produced additional phenolic compounds not present in untreated cellsand different compounds were produced by each cell line. Induced enzymeactivities also differed between the cell lines with Q90 (susceptible) showinga large and transitory increase in PAL activity that was far greater than thatobserved for Q114 (resistant). POD activity increased more in Q114 than inQ90, although the differences between the resistant and susceptible cell lineswere not as great as for PAL.

Sugarcane plants expressing the firefly luciferase gene were regenerated from microprojectile-bombarded meristematic tissue. The concentration and type of cytokinin added to the basal media, both prior to bombardment and during regeneration, significantly affected the number of plantlets produced and the percentage of plantlets expressing luciferase. Maximum shoot initiation from meristematic tissue of sugarcane cultivar Q137 was achieved on media supplemented with N6-benzylaminopurine (BAP). There were differences in the numbers of shoots initiated from different sugarcane cultivars, and some optimisation for each cultivar was required. No luciferase activity was observed in plantlets regenerated in the absence of cytokinin. Sugarcane meristematic tissue preconditioned by exposure to cytokinin appears to be a suitable target tissue for the introduction of genes of agronomic importance into sugarcane. This approach should be readily adaptable to most commercial varieties of sugarcane with minor modification to the cytokinin treatment necessary for optimal regeneration.