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Feature: Genomics provides clues to sustainable potato protection

Summer 2009

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Plant scientists are shedding light on the strategies used by the potato blight pathogen to infect cells. Their work is helping to accelerate breeding of more durable disease resistant potato varieties.

In the 1840s, the first outbreaks of potato blight in northern Europe led to widespread crop failures. These were felt most dramatically in Ireland where failed potato harvests, combined with political factors, led to the starvation of over 1 million people and the emigration of millions more. Today, Phytophthora infestans is still the most significant pathogen of potatoes, responsible for large yield losses. Costs associated with crop losses and chemical control of the disease exceed £3Bn globally per year.

Attempts to control potato blight by developing potato varieties with genetic resistance to P. infestans, have been readily overcome by variation in pathogen populations.

"What we have seen is an evolutionary arms race between a pathogen and its host and, so far, the pathogen has been winning," explained Professor Paul Birch of the University of Dundee at the Scottish Crop Research Institute (SCRI).

Understanding the enemy

Professor Birch is collaborating with colleagues at SCRI, Warwick HRI and the University of Aberdeen in BBSRC-funded projects to address the problems faced by existing control measures, which currently rely on aggressive prophylactic chemical treatments. They are looking to understand how P. infestans causes disease and to identify essential pathogen virulence genes that may be durable targets for host disease resistance proteins.

Their work has focussed on so-called ‘effector’ proteins, which are secreted by the pathogen and go on to manipulate the plant cell structure, defences and metabolism to establish disease. Recently, a short genetic ‘motif’ called RXLR was discovered, which is present in many P. infestans effectors. Importantly, RXLR is required by effectors to enter plant cells.

"We were really excited by the discovery of RXLR. This has provided a signature to search for proteins that are delivered inside host cells, where they may be exposed to plant defence surveillance systems," reported Prof. Birch.

Using modern genomics techniques, the entire complement of RXLR-containing proteins has been identified in the P. infestans genome. The discovery of more than 500 genes encoding these effectors, along with recent advances in technology to study protein-protein interactions, provides an unparalleled opportunity to investigate how plant defences are suppressed by invading microbes.

Targets for crop breeding

The team’s discoveries could have implications, not just for potato blight but, for many other plant diseases. "We can imagine that many of the host plant proteins targeted by Phytophthora effectors will also represent key ‘pressure points’ that are manipulated by other pests and pathogens to establish disease. These disease agents are a considerable obstacle to global food production, so we hope this research will have wide implications for food security," said Prof. Birch. "We hope that understanding how these effectors interact with their targets in the host will lead to novel strategies to control or prevent crop losses and environmental damage".

In related work, the team will search through a wild potato biodiversity collection at SCRI – The Commonwealth Potato Collection – for plants containing surveillance proteins that recognise key effectors from P. infestans.

These resistances are likely to be highly durable and will be prioritised for introduction into cultivated potatoes in commercially supported breeding programmes at SCRI.

Evolutionary insights into P. infestans protein trafficking

Phytophthora infestans belongs to a group of organisms called oomycetes, which look and behave very much like fungi but are actually more closely related to algae.

Phytophthora penetrates plant cell walls and forms structures called haustoria that live in intimate contact with host cells. The pathogen secretes effector proteins from these haustoria, which are translocated across the plant cell membrane to interact with defence proteins and manipulate their functions.

Effector translocation is dependent on a short protein motif, RXLR. This motif is similar to the host targeting signal, ‘RXLXE/D/Q’, which is required for translocation of effectors from the malaria parasite, Plasmodium falciparum, inside human blood cells. This suggests that distantly related pathogens of plants and animals have evolved a common means of effector delivery into their hosts.

Discovering the mechanisms underlying these translocation processes may present scientists with new ways of combating diseases of both plants and animals. 

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