Researchers in the United Kingdom say the bacteria that cause bacterial speck in tomatoes use a unique approach to infect a variety of important crops.
Scientists at the Department of Life Sciences at Imperial College in London have discovered that a bacteria known as Pseudomonas syringae (PS) has two genes that work together to launch an attack on plants’ cells.
Bacterial speck causes disease and significantly reduces crop quality and yield.
It was first recognized in the United States in the 1930s but tomato farmers did not suffer from serious losses until the mid-1970s when Florida’s winter tomato crop was affected in 1977-78.
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The infection spread to northern states when some infected transplant tomatoes were inadvertently shipped north.
Pseudomonas syringae is found worldwide and is responsible for major disease outbreaks in a wide range of plants including cauliflower, celery, corn, Japanese apricot, kidney beans, kiwi, wheat, barley, oats, rice, kiwi and zucchini.
The pathogen strains are found throughout North America, Europe, Asia and parts of Africa. They are a problem in wild plants and one strain of the bacteria has recently infected half of all chestnut trees in the U.K.
“These bacteria have quite a sophisticated system for infecting plants,” said Jorg Schumacher, senior researcher and senior author of the study published inNature Communications.
“They use remarkable needle-like structures called pili to penetrate and inject a range of proteins into a plant’s cells, which then work to suppress its immune response and kill infected cells.
“It has been suggested that three factors induce the pili’s infection. (These include) the plant’s tissue environment, the plant’s specific molecules, and contact with the plant’s cell wall.”
By understanding the molecular basis for how the pathogen attacks plant cells, scientists will be able to recommend better strategies for disease management and provide information for the development of more effective pesticides.
“The motivation for this study was to find out how having a duplicated gene could provide Pseudomonas syringae with the ‘edge’ in terms of evolutionary advantage,” said Schumacher.
“We have studied related systems in other bacteria in great detail (but) what we have found here is that the two-gene system in PS is an evolutionary innovation that had not been described in bacteria.”
According to the Ontario Ministry of Agriculture, bacterial speck caused by PS is common in tomato fields.
It needs moisture to multiply. Wet conditions in the plant canopy due to rain, fog, dew, high humidity or irrigation provide a suitable growth environment.
The degree of infestation varies according to weather conditions each year but in optimum environments, yield losses can be as high as 60 percent.
Lesions can occur anywhere on the foliage, stem or fruit and they appear as tiny black specks often surrounded by a yellow halo.
They can cause distortion of the leaf and multiple lesions may come together causing the leaf to die. Severely infected plants are stunted in growth.
Schumacher said when the PS pathogen infects tomatoes, its effector proteins counteract the plant’s hypersensitive response by silencing the plant’s warning signals.
Some scientists refer to PS infections as being accomplished by stealth.
Schumacher described the effector proteins as proteins that are injected into the host cell and operate behind enemy lines, eventually deactivating the plant’s “burglar alarm” system.
What distinguishes PS from other related pathogens that also use pili to infect plants is that, during its evolution, PS duplicated a gene involved in the production of the pili.
This duplicated gene is present in all the strains of PS that the researchers have studied.
The duplicate genes suggest that the duplication provides some selective advantage in the infection process. The presence of two genes may allow for more subtlety, or stealth, during the pathogen’s attack on the plant.
Schumacher said PS pathogen is one of the most hazardous bacteria to infect agricultural crops.
A better understanding of the bacteria could lead to drug development.
“PS is a persistent and slow plant killer and targeted drugs are not available,” Schumacher said.
“If these could be developed, agricultural food production could be increased.”
