Dutch, British and American scientists have developed a method to more
quickly identify and isolate genes that can be used to make potatoes
resistant to Phytophthora infestans, the dreaded potato blight. With this
method, multiple resistance genes from different species of potatoes can be
isolated and possibly used simultaneously. This offers the prospect of
achieving sustainable resistance against the pathogen because it is less
capable of breaking the resistance of the potato when multiple genes are
involved.
According to researchers at Wageningen University in the Netherlands, the
Sainsbury Laboratory at the John Innes Centre in the UK and Ohio State
University in the USA, the best strategy to make potatoes resistant to the
stubborn fungal pathogen Phytophthora is to develop so-called broad spectrum
resistance. In their article, published on 6 August in the journal PLoS One,
they explained that the current methods to discover resistance genes are too
slow. Moreover, because they often concern only a single gene, these methods
do not lead to sustainable resistance because Phytophthora can break
single-gene resistance relatively quickly and easily.



Interaction

The newly developed method is based on the interaction of genes of the
pathogen and genes of the potato. The response of the potato involves
resistance genes in the plant, and the response of P. infestans involves
so-called avirulence genes. The avirulence gene produces proteins
(effectors) that are recognised by the resistance gene proteins of the
potato; an interaction then takes place. By using effectors (proteins that
are secreted by Phytophthora into the plant after infection takes place),
researchers can relatively quickly identify and isolate the genes that are
crucial to the interaction. Because the pathogen (Phytophthora) cannot
switch off these proteins, but produces them constantly, genes that can
recognise these proteins can potentially serve as resistance genes.



In the study, a set of 54 effectors (of an estimated 500 effectors in total)
were tested on a large set of wild potato species. In many cases, this led
to reactions from the wild potato species (the hypersensitivity response:
the location where the effector protein was applied begins die off) and in
one case to the actual identification of the effector protein ? known as
IPiO. This effector turned out to be directly correlated with the resistance
of three wild species, S. stoloniferum, S. papita and S. bulbocastanum. This
means that a positive response against the effector always occurred in
plants that had the resistance gene. In additional studies, the researchers
were able to show that the effector in this case was the avirulence gene of
the resistance gene. Because the researchers realised that the resistance
genes from the three species had to be very similar, they were quickly able
to isolate the resistance genes in S. papita and S. stoloniferum by using
their knowledge of the previously isolated resistance gene from S.
bulbocastanum.



Permanent threat

Since Phytophthora first ravaged the potato ? an event epitomised by the
notorious Irish Potato Famine in the 19th century ? this pathogen has been a
permanent threat, and has repeatedly led to disastrous crop damage and high
production costs. Until now, a very labour-intensive process of searching
for sustainable resistance has yielded few or no results, and the use of
fungicides has been essentially the only way to control the disease in
modern agriculture.

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