Wageningen, THe Netherlands
June 15, 2009
 Source:
Wageningen UR
Potato blight, false mildew,
sudden oak death and a disease in salmon are all caused by a
group of miniscule, yet destructive, organisms called Oomycetes.
Because of their changeability and huge numbers, they are able
to overwhelm the defence mechanisms of both plants and animals.
The use of chemicals is usually the only remedy but this is also
undesirable. Will future research offer a sustainable means of
combating these blights and preventing failed harvests? Francine
Govers, a professor at Wageningen University, can see sporadic
openings left by the pathogens and they provide a strategic
starting point for combating the infections.
There are between 1000 and 2000 species in the group of
micro-organisms, the Oomycetes (‘egg moulds’). They are not
fungi. In fact, fungi (including toadstools) are more closely
related to man than to these one-celled egg moulds. However, the
effect of the latter on crops and animals is disastrous as was
demonstrated by the potato blight that entered Europe via
Belgium in 1845, advancing very rapidly, and causing the Great
Irish Famine.
Prof. Francine Govers listed the limited number of strategies
available for keeping the pathogen, Phytophthora infestans ('the
destroyer of plants ') under control. The approach also provides
an opportunity for reducing the quantity of pesticides used per
hectare in the Netherlands, the highest levels of which happen
to be in potatoes.
Combat takes place on a microscopic scale, the pathogen trying
to work its way through the biological defences of the host, the
potato plant. Phytophthora attacks the plant using a special
group of proteins, the RXLR effectors. It has a huge and diverse
arsenal available with some 560 RXLR effectors, so the odds of
it finding a suitable weapon to break through the plant's
defence mechanism is very high. If the attack succeeds, the
effectors breach the plant's defences by suppressing its
resistance. The spores of P. infestans can then make the most of
the foodstuffs available and reproduce themselves, causing the
death of the plant. Wild potato plants which grow in South
America are reasonably resistant to such attacks because
potato-resistant proteins recognize the invaders and block their
advance.
Research efforts, including those at Wageningen, have meanwhile
identified more than 10 resistant genes which make resistant
proteins. Seven of those are known to contain the RXLR effector.
If recognition is not 100% as, for example, when the RXLR
effector looks slightly different, the invader escapes attack
and can reproduce after all. This is how the pathogen, after a
certain number of years, becomes able to break through the
resistance in potatoes crossed with wild strains.
Predictions
Phytopathologists are trying to understand the interaction
better. The challenge is to predict whether a pathogen strain is
going to infect a field where resistant potato cultivars are
growing. By taking samples, a DNA chip can be used to determine
which strains are present in the field and what sort of RXRL
arsenal they have. It is then possible to establish which potato
cultivars will not be affected by Phytophthora and which will.
Only in the latter case is it necessary to spray. How long it
will take to put this method into practice depends, according to
Prof. Govers, on how quickly new combinations of resistant gene
and RXLR effectors with all the variants can be identified.
Researchers are not putting all their money on one horse; they
are also looking at the weak links in the life cycle of
Phytophthora and at the genetic properties that are unique to
the Oomycetes. Thus, it has been shown that a certain enzyme,
phospholipase D, takes on novel forms in Oomycetes. Precisely
these forms are ideal for applying control because the specific
inhibition technique has no direct effect on other useful
organisms, including the crop itself.
To finish, Prof. Govers pointed to a biological control
technique. Soil bacteria belonging to the genus Pseudomonas
attack spores of Phytophthora, making use of a small, special
protein. It is not yet understood how this protein destroys the
spores. By analysing 15,000 genes in Phytophthora, researchers
have found candidates which may provide new and specific points
of inquiry. |
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