Crop parasite may spread with global warming
Scientists from the John Innes Centre on Norwich Research Park have shed new light on a plant pathogen that causes yield losses in crops worldwide.
Global warming could increase the threat from leaf hoppers – tiny sap-sucking, highly mobile and opportunistic agricultural pests.
Certain species can acquire and transmit plant pathogens including viruses and phytoplasmas, which are small bacteria. They cause severe yield losses to sugar beet, oil-seed crops, fruit trees, vegetables, maize and grapevine among other crops.
Dr Saskia Hogenhout and her team focused on a phytoplasma strain called Aster Yellows Witches’ Broom, isolated from infected lettuce fields in North America. It causes deformity in plants and prevents flower formation so the plant does not produce seed, becoming sterile.
“It is timely to better understand phytoplasmas as they are sensitive to cold and could spread to new areas as temperatures rise through climate change,” said Dr Hogenhout.
Aster yellows phytoplasmas and other phytoplasmas have become an increasing problem in northern Europe, because the insect vectors are able migrate northwards with rising temperatures.
Infected plants grow clusters of multiple stems which can look like a witches’ broom or in trees like a bird’s nest. This makes them more attractive to insects for egg-laying, increasing the number of insects that can spread the parasite further.
As the phytoplasma needs both the leafhopper and the plant host to stay alive it cannot be maintained independently, making it difficult to study. Fortunately the John Innes Centre is home to its own insectary for research on crop pests.
Leafhoppers are exotic plant disease vectors so were kept under quarantine. They are also very active, so carbon dioxide was used to temporarily put them to sleep to move them onto the plants used in the experiment.
It was already known that pathogens can alter their hosts, for example malaria parasites can make humans more attractive to mosquitoes, but how they do it has remained a mystery. This study is the first time that scientists have identified a specific molecule from a parasite that manipulates plant development to the advantage of the insect host.
“Our findings show how this pathogen molecule can reach beyond its host to alter a third organism,” said Dr Hogenhout.
The scientists sequenced and examined the genome of the witches broom phytoplasma and identified 56 candidate molecules, called effector proteins, which could be key to this complex biological interaction.
They found that a protein effector SAP11 reduces the production of a defence hormone in the plant that is used against the leafhopper. As a consequence, leafhoppers reared on plants infected with witches broom laid more eggs and produced more offspring.
The higher reproductive rate is probably matched by a similar increased rate in transmission of the witches broom phytoplasma by leafhoppers to other plants.
“Phytoplasmas that can enhance egg-laying and offspring numbers in leafhoppers are likely to have a competitive advantage,” said Dr Hogenhout.
“If we can interfere with these processes, then plants could become more resistant.”
The research was funded by the Biotechnology and Biological Sciences Research Council (BBSRC) and The Gatsby Charitable Foundation.
For many phytoplasmas, the insect vectors have not been identified so it is not understood how they spread. Dr Hogenhout is a taking part in and EU-funded project to control phytoplasma epidemics in Europe.
She and her EU research colleagues will sequence the genomes of multiple phytoplasmas in collaboration with The Genome Analysis Centre on Norwich Research Park. This information will help with the detection and identification of phytoplasmas in insect vectors and plants.