Antarctic fungi found to be effective against citrus canker

June 14, 2018 by Peter Moon, FAPESP
Antarctic fungi found to be effective against citrus canker
Twenty of the isolated fungi with action against X. citri belonged to the genus Pseudogymnoascus and were extracted from terrestrial and marine samples. Next came Penicillium (five), followed by Cadophora (two), Paraconiothyrium (one) and Toxicocladosporium (one), all extracted from marine sediments. Credit: Daiane Cristina Sass (IB-UNESP)

A research team at the São Paulo State University's Bioscience Institute (IB-UNESP) in Rio Claro, Brazil, has identified 29 fungi with proven action against Xanthomonas citri, a bacterium responsible for citrus canker, an endemic disease in all citrus-producing countries. The origin of the fungi is surprising. They were isolated from samples of soil and marine sediment collected in Antarctica.

"These fungi live in isolated conditions and proliferate under inhospitable conditions including low temperatures and high levels of ultraviolet radiation," says Daiane Cristina Sass, a professor at UNESP who heads a project engaged in a search for microorganisms that produce with antibacterial action for use in agriculture.

"How have they adapted to survive in an environment so hostile to life? We wanted to see if they produced molecules with unique structures that protected them from infections and might therefore be capable of antibacterial action." Sass wrote an article published in Letters in Applied Microbiology - jointly with IB-UNESP colleagues Lara Durães Sette and Henrique Ferreira, among others.

Although the bacterium can be combated in several ways, none is sufficient to eradicate the disease. Therefore, new chemical or biological methods of protecting citrus groves have to be pursued.

The disease is controlled directly by growers. The recommended measures include spraying trees with copper-based products and replacing infected trees with healthy new plantings derived from more resistant varieties. Control of the citrus leaf miner (Phyllocnistis citrella) is also advisable. The wounds made by larvae of this moth in feeding on the plant exacerbate by serving as an entry point for X. citri.

"The main method for combating citrus canker consists of spraying trees with copper compounds. The downside is that when even small amounts are used for a long period, copper accumulates in the fruit, soil and water, eventually contaminating the entire environment. For this reason, we're looking for new compounds that are less aggressive to the environment and also less harmful to humans," Sass explained.

Collection and isolation of the Antarctic fungi

On the extent of the Sass-headed project and its research on biotechnology, the team came up with the idea of investigating the collection of fungi curated by Professor Sette, which resulted from Antarctic summer expeditions to the South Shetland Islands in 2013 and 2015 as part of Project Microsfera, conducted under the aegis of the Brazilian Antarctic Program (PROANTAR) and sponsored by the National Council for Scientific & Technological Development (CNPq).

Sette isolated 33 filamentous fungi from samples collected in soil under rotten wood on Deception Island and 53 from marine sediments at a depth of 20 meters in Admiralty Bay, King George Island. All fungal strains are kept at UNESP's Microbial Resource Center (CRM). The researchers found that 29 of the 86 Antarctic fungi they isolated (19 of marine origin and ten terrestrial) contained compounds with proven action against X. citri.

Isolating the compounds produced by the fungi and verifying their involved several stages. The process began with isolation of the fungi, which were then grown for several days in culture dishes with nutrients. The fungi were cultured in liquid medium and shaken for 20 days at 15 °C. The solid biomass was separated from the liquid portion, and both parts were submitted to processing with solvents to obtain intracellular and extracellular extracts.

The researchers obtained 158 extracts. Each extract was diluted at several concentrations (2.10 mg/ml-0.02 mg/ml) and tested against X. citri. In the case of the soil fungi, most of the extracts with antibacterial action were intracellular in origin, while for the marine fungi, only the extracellular extracts hindered the bacterium's growth. "We wanted to determine the lowest concentration of each extract that inhibited growth in 90% of cases," Sass said.

Some (12) of the extracts affected bacterial growth at lower concentrations than the highest tested, and ten of these inhibited growth in more than 90% of cases at concentrations of 1.5 mg/ml-1.0 mg/ml. "At maximum concentration, one extract inhibited growth by up to 98%, and another inhibited it by about 80% at 0.52 mg/ml," Sass said. "It's important to note that we're talking about extracts [which contain varying amounts of molecules]. If an extract contains only one compound that's responsible for this bioactivity, the compound may display good antibacterial activity at much lower concentrations."

Twenty of the isolated with action against X. citri belonged to the genus Pseudogymnoascus and were extracted from terrestrial and marine samples. Next came Penicillium (five), followed by Cadophora (two), Paraconiothyrium (one) and Toxicocladosporium (one), all extracted from marine sediments. Having identified the extracts with action against X. citri, the researchers are now working to find out which chemical compounds give them this antibacterial capability.

"We expect to identify and purify some of these bioactive compounds, as well as to complete toxicology testing on them, within 18 months or less," Sass said.

The researchers plan to patent the compounds they identify. They also hope to persuade pesticide manufacturers to develop commercial products for combating citrus canker based on these compounds.

Explore further: Researchers discover mechanism behind citrus canker bacteria's defense system for predators

More information: G. Vieira et al, Terrestrial and marine Antarctic fungi extracts active against Xanthomonas citri subsp. citri, Letters in Applied Microbiology (2018). DOI: 10.1111/lam.12890

Related Stories

Fungi respire millennium-old carbon from Antarctic soil

May 30, 2018

Fungi in Antarctic soils release carbon, as carbon dioxide, that is more than a thousand years old, a team led by scientists at the British Antarctic Survey (BAS) has found. This discovery sheds light on how carbon is released ...

Plant relationships break down when they meet new fungi

May 1, 2018

Gijsbert Werner, Postdoctoral Fellow and Stuart West, Professor of Evolutionary Biology, both in the Department of Zoology, explain the process of plant cooperation, in relation to their new study published in PNAS, which ...

Researchers find genetic cause for citrus canker

January 31, 2014

Researchers from the Institute of Food and Agricultural Sciences at the University of Florida are closer to finding a possible cure for citrus canker after identifying a gene that makes citrus trees susceptible to the bacterial ...

Marine fungi reveal new branches on tree of life

November 17, 2015

Researchers from the University of Exeter have discovered several new species of marine fungi inhabiting previously undescribed branches of the tree of life. Little is known about the fungi flourishing in the world's oceans ...

Recommended for you

The powerful meteor that no one saw (except satellites)

March 19, 2019

At precisely 11:48 am on December 18, 2018, a large space rock heading straight for Earth at a speed of 19 miles per second exploded into a vast ball of fire as it entered the atmosphere, 15.9 miles above the Bering Sea.

Revealing the rules behind virus scaffold construction

March 19, 2019

A team of researchers including Northwestern Engineering faculty has expanded the understanding of how virus shells self-assemble, an important step toward developing techniques that use viruses as vehicles to deliver targeted ...

Nanoscale Lamb wave-driven motors in nonliquid environments

March 19, 2019

Light driven movement is challenging in nonliquid environments as micro-sized objects can experience strong dry adhesion to contact surfaces and resist movement. In a recent study, Jinsheng Lu and co-workers at the College ...

Levitating objects with light

March 19, 2019

Researchers at Caltech have designed a way to levitate and propel objects using only light, by creating specific nanoscale patterning on the objects' surfaces.


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.