Garden Soaps - intimacy, deceit and betrayal in the plant world.
Silencing the Opposition.
Plant viruses are extremely simple but sophisticated pathogens. They consist of an outer protective coat of protein and an innter component of genetic material (either DNA or RNA). The latter carries the few genes that viruses need to be effective disease agents. Viruses are transferred from plant to plant by stealth, commonly by a pest feeding on an uninfected plant shortly after it has fed on an infected plant. Typically viruses are transported by feeding insects, although some viruses are transmitted by nematode worms or fungi.
Once inside a plant cell, the virus particle loses its protein coat to expose the genetic material, or genome. This genome exists as ‘foreign’ genetic material within the host cell. The virus then uses the cell’s resources to produce thousands of copies of itself, which can then be transferred to new plants.
Plants have an in-built mechanism that checks for the presence of ‘foreign’ genetic material, which they destroy by cutting into small pieces. The recognition process is very specific, so only particular nucleic acid sequences are targeted for destruction, thus a virus genome can be eliminated without damaging the plant’s genetic material.
Chopping up the virus genome into pieces prevents the virus from establishing an infection. By this mechanism (called ‘gene silencing’), the virus genes are rendered inactive, or silenced. Scientists are studying how some viruses are able to deceive the plant’s recognition system to avoid being silenced, so that they can establish successful infections.
Gene silencing is not the only mechanism of defence that a plant has against viruses. In resistant plants a wider cascade of processes can be activated (see the Raising the Alarm’ handout). Among the chemical, physical and genetic defences used by resistant plants to defend themselves, one of the commonest is cell suicide. Plant cells are genetically programmed to die in response to particular triggers, one of which is pathogen infection. This programmed cell death (PCD) in cells around sites of infection is called the hypersensitive response (HR). The strategy cuts off the supply of nutrients to invading microbes, while the dying cells often produce chemicals that are toxic to the invader. PCD is not unique to pathogen defence. Plants use PCD extensively in their normal growth and development, for example the formation of xylem vessels in vascular tissues and during leaf senescence. How the different forms of PCD are triggered and controlled is of importance for our understanding of plant development in general and specifically for disease resistance studies.
The greater our understanding of how gene silencing and HR are triggered
and controlled, and how some pathogens are able to defeat the plant’s
many layered defences, the better equipped scientists are to develop plants
that are naturally better able to protect themselves against attack.