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The Secrets of Dark Green Islands

"Dark green islands" in plants are clusters of apparently healthy cells surrounded by paler, yellow-green or white, virus-infected cells found in leaves of plants infected with mosaic viruses. The cells appear to be structurally and biochemically normal, but they are immune to the virus infection, as are plantlets generated from cells taken from within the islands.

Dark green islands have intrigued researchers since the 1920s. Despite extensive studies, very little is known about what causes them or how they function.

Whereas cells in the yellow-green or white tissue contain high concentrations of virus, cells in the dark green islands contain very low levels or no infectious virus, and no detectable viral proteins or RNAs (viral DNA). Dark green islands can exist as single cells, as clusters of cells in a single tissue layer, or as clusters of cells across all the cell layers in a leaf. In the latter type, the junction between the dark green island and virus-containing cells is often a well-defined boundary running almost vertically between the layers.

Dark green islands form both in leaves that were very small at the time of virus invasion, and in leaves that develop subsequently. In leaves that are partially developed at the time of system invasion, all cell layers in the leaf are already established. In these leaves, islands that extend vertically through all the cell layers must arise by cell-to-cell spread of an agent during leaf development. A recognition system based on nucleic acids has been suggested to explain their formation.

There are a number of similarities between dark green island formation and a phenomenon called "gene silencing". This phenomenon was first recognised when plant biologists attempted to get genes to produce large quantities of their products by introducing genes from another source into the plant genome. Instead of the expected increases in "expression", decreased expression of both the introduced gene and host genes was often encountered. Extensive studies worldwide have not elucidated the mechanisms controlling gene silencing.

A team from HortResearch and the University of Auckland, led by Dr Richard Forster, is to investigate the mechanisms involved in forming dark green islands -- are they formed by specific inhibition of virus transport and replication between cells, or is the viral DNA being specifically targeted and degraded? They will also be trying to understand why these dark green islands arise: are they a generalised resistance response by the host plant or do they result from a viral adaptation to limit damage to the plant?

The team will also investigate the relationship between dark green islands and gene silencing, and, if silencing is involved, whether silencing events can occur in the cytoplasm in the absence of the viral sequences in the nuclear chromosomes. Using tissues from dark green islands for studying the mechanism of gene silencing should allow researchers to shed new light on a fundamental plant gene control system that involves an unknown form of intercellular communication. A better understanding of these processes will have a major impact on future research in plant genetics.

Sue Muggleston, HortResearch, Auckland