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Life scientists, colleagues differentiate microbial good and evil

TGF-FruitImageJan. 9, 2014 — To safely use bacteria in agriculture to help fertilize crops, it is vital to understand the difference between harmful and healthy strains. The bacterial genus Burkholderia, for example, includes dangerous disease-causing pathogens — one species has even been listed as a potential bioterrorist agent — but also many species that are safe and important for plant development.

Can the microbial good and evil be told apart? Yes, UCLA life scientists and an international team of researchers report Jan. 8 in the online journal PLOS ONE.

“We have shown that a certain group of Burkholderia, which have just been discovered in the last 12 years as plant-growth promoting bacteria, are not pathogenic,” said the study’s senior author, Ann Hirsch, a professor of molecular, cell, and developmental biology in the UCLA College of Letters and Science. “This opens up the possibility of using these particular species for promoting plant growth through the process of nitrogen fixation, particularly in areas of climate change. This will have a major impact, especially on people in the developing world in producing protein-rich crops.”

Nitrogen fixation is a process by which helpful bacteria that have entered the roots of plants convert nitrogen in the atmosphere into ammonia, which helps the plants thrive. The findings of Hirsch and her colleagues indicate that several recently discovered Burkholderia species, including Burkholderia tuberum, could be used — cautiously — in nitrogen fixing. These species, the scientists discovered, lack those genes that make other Burkholderia species harmful agents of infection.

“Bacteria that fix atmospheric nitrogen into ammonia, such as Burkholderia,are critical for plant growth,” said Hirsch, whose laboratory studies many aspects of the complex symbiosis between plants and bacteria. “We’re especially interested in these recently described Burkholderia species because they are found primarily in the dry and acidic soils of the Southern Hemisphere, making them potentially important for agriculture in less productive areas.”

For their study, the UCLA life scientists performed a bioinformatics analysis of four symbiotic Burkholderia species, all of which fix nitrogen and one, B. tuberum, which “nodulates legumes.” They found a strong distinction between genes in these beneficial strains and in pathogenic strains. They searched for genes typically involved in infection — for attaching to and invading cells or for secreting toxins. Unlike their dangerous cousins, the four symbiotic Burkholderia species did not have genes associated with the virulence systems found in the pathogenic species.

Burkholderia were first discovered as plant pathogens in 1949 by Walter Burkholder, who identified them as the agent causing onion-skin rot. Later, Burkholderia species were identified as the causative agent of the disease melioidosis, a public health threat, especially in tropical countries like Thailand and in parts of Australia. B. pseudomallei, which causes melioidosis, is classified by the Centers for Disease Control and Prevention as a potential bioterrorist agent.

Other Burkholderia belong to the Burkholderia cepacia complex, a group of related bacteria that are not true pathogens but can cause “opportunistic” or hospital-acquired infections in people with weakened immune systems or with cystic fibrosis. Although some members of the Burkholderia cepacia complex have been used to protect plants from dangerous fungal infections, their potential to cause infection has resulted in severe limits on their use in agriculture.

It wasn’t until many decades after Burkholder’s discovery that closely related Burkholderia species were found to enter plant roots not as pathogens but as helpful symbionts — generating root nodules in which the bacteria provide nitrogen fertilizer to the plant. Bacteria that cause the formation of these nodules in legumes, such as soybeans, alfalfa and peanuts, are crucial to sustainable agricultural systems, Hirsch said.

Although the nodulating, symbiotic species of Burkholderia are related to the more dangerous species, a detailed analysis of their evolutionary relationships published earlier this year by Hirsch and her colleagues showed that the two groups have a distinct evolutionary lineage.

The harmful Burkholderia species are more resistant to antibiotics than the symbiotic and agricultural strains. In addition to the bioinformatics analysis in the current study, the team analyzed resistance to a panel of common antibiotics, and tested the potential of different Burkholderia species to cause infection in laboratory conditions.

Experiments testing the potential of the four symbiotic species to cause infection in the small nematode worm known as Caenorhabditis elegans and in human cells grown in culture verified the bioinformatics analysis, showing that the bacteria were not harmful.

“We used a variety of detailed experiments to make sure that the symbiotic species are safe to put into farmers’ fields and home gardens, just like currently used nitrogen-fixing bacteria,” Hirsch said. “Our goal is to have these newly discovered nitrogen-fixing bacteria be used for a more sustainable approach to agriculture in the future.”

Co-authors of the PLOS ONE research included Annette Angus and Christina Agapakis, UCLA postdoctoral scholars in Hirsch’s laboratory; Stephanie Fong, Paul Yang, Nannie Song and Stephanie Kano, former UCLA undergraduate researchers in Hirsch’s laboratory; Shailaja Yerrapragada of the Baylor College of Medicine in Houston; Paulina Estrada-de los Santos of the department of microbiology at Mexico’s Instituto Politécnico Nacional, Prolongación de Carpio y Plan de Ayala; Jésus Caballero-Mellado (now deceased) of the Genomic Sciences Center at the National Autonomous University of Mexico; Sergio de Faria of Brazil’s Embrapa Agrobiologia; Felix Dakora of the chemistry department of Tshwane University of Technology in South Africa; and George

ScienceDaily: Agriculture and Food News

AU: Asparagus imports a necessary evil

AU: Asparagus imports a necessary evil

An asparagus grower in Western Australia’s Southern growing region says imports are an unfortunate necessity, with greater market competition coming from within Australia.

Coles supermarket was this week fined $ 61,200 for displaying Australian Grown signs above imported asparagus and other fruit and vegetables.

Grower Dirk Mostert says that while he was disappointed to hear of those incidences, the nature of the seasonal asparagus production cycle makes imports unavoidable.

“Because it’s only a short growing season, it’s a Spring vegetable, it’s just a reality of life that once that stops, then supermarkets import it,” says Mr Mostert.

Mostert says that when the two do overlap in the market, he has no concern about pairing South West WA asparagus against imported product.

“I’ll back my asparagus against any imported asparagus,” says Mr Mostert.

“Our wholesaler sells imported asparagus up in Perth, and he always likes a bit of pre-warning when our stuff comes up because the buyers in Perth, they just walk straight past the imported stuff. As soon as they see a box of local asparagus, they are prepared to wait a week or two before they go back for imported asparagus.”

Instead, the threat of competition to WA markets comes from production in eastern states.

“When Victoria starts to fill their market, then they tend to load their trucks up and send it over to WA,” says Mr Mostert.

“That really affects our market. You see the prices really come back then. That’s our major competitor.”

In spite of the challenges, Mostert says the South West region equally affords growers opportunities, with the mining boom prompting an unexpected growth in markets.

“With the mining last year, there was a request to send our asparagus up to the chefs in the mining camps. They were really looking for it. Our wholesaler in Perth was getting quite a few orders for it. He was pleased to be able to send it on to them and they were quite rapt with it.”
Mostert is not overly concerned about how a flagging resources sector might affect those newly developed markets.

“It’ll be interesting to see how it goes. People are still up there and they still have to eat. Once you’ve got a taste for nice asparagus, it’s a bit hard to lose it.”

He is confident that with an established regional banner to grow under and favourable seasonal conditions, South West growers are in a healthy position leading into harvest over the next two months.

“Over the years, we’ve really pushed the South West asparagus. We have a name for it, we have a reputation for it and our quality is first of anybody in Australia. I’ll back it, no worries at all.”


Publication date: 7/3/2013