Some species still have some immunity, and therefore have "sustained resistance." Until recently, this kind of immunity was considered to be founded on a totally different mechanism than the non-sustained form of resistance.

Species with this type of resistance carry resistance genes (called the R genes) which bring about a hypersensitive reaction in potato plants. The reaction causes plant cells to die at the spot where the pathogen enters the plant, resulting in the pathogen itself dying off.

The genes responsible for this are not active against all forms of the pathogen, which makes the resistance non-sustainable.

Resistance against fungi is generally based on a number of characteristic mechanisms. A few years ago, however, DNA research showed that Phytophthora infestans is a oomycete rather than a fungus. This type of microorganism is more closely related to plants than to fungi.

The potato and other related varieties of the Solanum species contain scores or perhaps even hundreds of genes that can give the plant a degree of resistance to potato blight, which is caused by Phytophthora infestans.

This is one of the conclusions that came to light in Vivianne Vleeshouwers' thesis, with which she recently earned her doctoral degree at Wageningen University in The Netherlands. Vleeshouwers conducted her studies at Plant Research International and Wageningen University.

According to the thesis, research scientists and breeders should be made aware of the fact that Phytophthora infestans is no fungus but an oomycete. It is possible, therefore, that the interaction between plants and Phytophthora infestans is entirely dissimilar to the well-researched interaction between plants and fungi. This could have an important impact on the study and the development of resistant species of potato.

To her amazement, Vivianne Vleeshouwers discovered that all plants are, to a certain extent, resistant to Phytophthora infestans. And all plants that come into contact with the pathogen show a more rapid rate of cell death.

The speed and effectiveness of the hypersensitive reaction, however, is different for each plant. The hypersensitive mechanism is therefore not only responsible for non-sustained resistance, but plays an essential role in all forms of resistance, including sustained resistance.

Microscopic studies of plants with partial resistance showed Vleeshouwers that the pathogen sometimes escapes from the dying cells in one corner. When that happens, the pathogen can still grow and multiply, meaning these plants are not fully resistant.

Vleeshouwers makes the assumption that potatoes and related species are "overflowing" with genes that could contribute to the development of varieties with a sustained resistance to Phytophthora.

Plants showing a hypersensitive reaction must therefore certainly not be rejected in the course of the cultivating process. Vleeshouwers' new insights will likely have an effect on potato research and cultivation worldwide.

Her theories and their application should open up new perspectives for the development of varieties with sustained resistance.

[Contact: Erik Toussaint]

05-Feb-2001