Researchers reveal the ‘dark side’ of beneficial soil bacteria

September 21, 2012 under CANR News

It’s a battleground down there — in the soil where plants and bacteria dwell.

Even though beneficial root bacteria come to the rescue when a plant is being attacked by pathogens, there’s a dark side to the relationship between the plant and its white knight.

According to research reported by a University of Delaware scientific team in the September online edition of Plant Physiology, the most highly cited plant journal, a power struggle ensues as the plant and the “good” bacteria vie over who will control the plant’s immune system.

“For the brief period when the beneficial soil bacterium Bacillus subtilis is associated with the plant, the bacterium hijacks the plant’s immune system,” says Harsh Bais, assistant professor of plant and soil sciences, whose laboratory group led the research at the Delaware Biotechnology Institute.

In studies of microbe-associated molecular patterns (MAMPs), a hot area of plant research, the UD team found that B. subtilis produces a small antimicrobial protein that suppresses the root defense response momentarily in the lab plant Arabidopsis.

“It’s the first time we’ve shown classically how suppression by a benign bacteria works,” Bais says. “There are shades of gray — the bacteria that we view as beneficial don’t always work toward helping plants.”

In the past, Bais’ lab has shown that plants under aerial attack send an SOS message, through secretions of the chemical compound malate, to recruit the beneficial B. subtilis to come help.

In more recent work, Bais and his collaborators showed that MAMP perception of pathogens at the leaf level could trigger a similar response in plants. Through an intraplant, long-distance signaling, from root to shoot, beneficial bacteria are recruited to forge a system-wide defense, boosting the plant’s immune system, the team demonstrated. In that study, the Bais team also questioned the overall tradeoffs involved in plants that are associated with so-called beneficial microbes.

In the latest work, involving the testing of more than 1,000 plants, the researchers shed more light on the relationship. They show that B. subtilis uses a secreted peptide to suppress the immune response in plants. It is known that plants synthesize several antimicrobial compounds to ward off bacteria, Bais says.

The team also shows that when plant leaves were treated with a foliar MAMP — flagellin, a structural protein in the flagellum, the tail-like appendage that bacteria use like a propeller — it triggered the recruitment of beneficial bacteria to the plant roots.

“The ability of beneficial bacteria to suppress plant immunity may facilitate efficient colonization of rhizobacteria on the roots,” Bais says. Rhizobacteria form an important symbiotic relationship with the plant, fostering its growth by converting nitrogen in the air into a nutrient form the plant can use.

“We don’t know how long beneficial bacteria could suppress the plant immune response, but we do know there is a very strong warfare under way underground,” Bais says, noting that his lab is continuing to explore these interesting questions. “We are just beginning to understand this interaction between plants and beneficial soil bacteria.”

The lead author of the research article was Venkatachalam Lakshmanan, a postdoctoral researcher in the Department of Plant and Soil Sciences; Sherry Kitto, professor of plant and soil sciences; Jeffrey Caplan, associate director of UD’s Bio-Imaging Center; Yu-Sung Wu, director of the Protein Production Facility; Daniel B. Kearns, associate professor in the Department of Biology at Indiana University; and Yi-Huang Hsueh , of the Graduate School of Biotechnology and Bioengineering at Yuan Ze University, Taiwan.

The research was supported by grants from the National Science Foundation.

Article by Tracey Bryant

Animation and images courtesy of Harsh Bais

This article can also be viewed on UDaily.

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UD researchers show how beneficial soil bacteria can boost plant immunity

August 29, 2012 under CANR News

With the help of beneficial bacteria, plants can slam the door when disease pathogens come knocking, University of Delaware researchers have discovered.

A scientific team under the leadership of Harsh Bais, assistant professor of plant and soil sciences in UD’s College of Agriculture and Natural Resources, found that when pathogens attempt to invade a plant through the tiny open pores in its leaves, a surprising ally comes to the rescue. Soil bacteria at the plant’s roots signal the leaf pores to close, thwarting infection.

The fascinating defense response is documented in video and micrographs of live plants taken by confocal and scanning electron microscopes at UD’s Bio-Imaging Center at the Delaware Biotechnology Institute.

The research, which explored the interaction between the soil bacterium Bacillus subtilis and the laboratory plant Arabidopsis thaliana, is published in the August issue of The Plant Journal. The findings underscore both the importance of root-based processes in plant defense and the potential for bolstering plant immunity naturally through the emerging field of probiotics.

Postdoctoral researcher Amutha Sampath Kumar is the lead author of the journal article. In addition to Bais, the co-authors include postdoctoral researcher Venkatachalam Lakshmanan, researchers Jeffrey L. Caplan, Deborah Powell and Kirk J. Czymmek of UD’s Bio-Imaging Center, and Delphis F. Levia, associate professor of geography. The National Science Foundation, University of Delaware Research Foundation and Delaware Experimental Program to Stimulate Competitive Research (EPSCoR) provided funding for the study.

Millions of stomata, consisting of microscopic pores surrounded by guard cells, cover the above-ground parts of plants, from the stems to the flower petals. The pores resemble tiny mouths, or doors, which the guard cells open and close to allow carbon dioxide, oxygen, water and minerals in and out of the plant.

Pathogens also can slip through these stomata and begin infecting the plant. However, as Bais’s team confirmed, this invasion is halted when the beneficial bacterium Bacillus subtilis is present in the soil where the plant is rooted. The finding was based on tests of approximately 3,000 Arabidopsis plants inoculated with the foliar pathogenPseudomonas syringae pathovar tomato DC3000 (PstDC3000) during a year-long period.

When a foliar pathogen attacks, as shown in previous research by Bais and his group, the plant recruits Bacillus subtilis to help and facilitates its multiplication. The Bacillus subtilisbacteria bind to the plant’s roots and invoke abscisic acid and salicylic acid signaling pathways to close the stomata.

Abscisic acid and salicylic acid are both important hormones involved in plant defense. When a plant encounters adverse environmental conditions, such as drought, for example, abscisic acid triggers the stomata to shut tightly to prevent the plant from dehydrating.

In addition to ramping up plant disease resistance, the use of this rhizobacteria to promote drought tolerance in plants could be a very promising avenue, Bais notes.

“Many bacterial pathogens invade plants primarily through stomata on the leaf surface,” Bais says. “But how do plants fight off infection? In our studies of the whole plant, we see this active enlistment by Bacillus subtilis, from root to shoot.”

Strikingly, the research team’s data revealed that of different root-associated soil bacteria tested, only Bacillus species were effective in closing the stomata and for a prolonged period.

“We know only 1 to 5 percent of what this bug Bacillus subtilis can do, but the potential is exciting,” Bais notes, pointing out that there is increasing commercial interest in inoculating crop seeds with beneficial bacteria to reduce pathogen infection. “Just as you can boost your immune system, plants also could be supercharged for immunity.”

Article by Tracey Bryant

Photo by Ambre Alexander

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Prestigious travel grants allow UD students to attend plant biology conference

March 30, 2012 under CANR News

Harsh Bais, assistant professor in the University of Delaware Department of Plant and Soil Sciences, has chosen his research team well. Two members of his group, postdoctoral researcher Venkatachalam Lakshmanan and graduate student Emily Alff, have received travel grants from the American Society of Plant Biologists (ASPB), which will enable them to attend the society’s annual meeting this summer in Austin, Texas.

According to Bais, the number of ASPB travel grants is limited to 20 for postdocs and 30 for graduate students worldwide.

Alff received the ASPB travel grant for her project that explores the role of rhizobacteria in rice growth promotion and defense against the fungus Magnaporthe oryzae, commonly known as rice blast.

Her research examines the natural relationships between rice plants and the microbial communities that inhabit the rhizosphere, the area surrounding their root systems. Secretions from the root system are rich in nutrients, which sustain microbial communities that can be detrimental or beneficial to the plant.

Rice blast can cause devastating crop losses, but Alff’s research has demonstrated that certain bacteria can significantly decrease the effects of rice blast and improve plant growth. The goal of the project is to provide a basis for inoculating seeds with beneficial microbes, which is cost-effective for farmers and more environmentally sound than fungicides.

Lakshmanan’s research was also selected for oral presentation in a “mini-symposium” on plant-microbe interactions as part of the conference. He studies microbe-associated molecular patterns, or MAMPs, which are responsible for triggering a plant’s immune response if it is attacked by a pathogen. This signaling process is well understood in response to foliar pathogens; however, the role of MAMPs in response to the belowground microbial community is largely unknown.

Lakshmanan’s project indicates that certain beneficial rhizobacteria are able to block MAMPs signaling and subdue an immune response from the plant, allowing them to colonize the plant’s root system. The bacteria are beneficial because they subsequently activate the plant’s immune response if it is attacked by another pathogen. Lakshmanan’s research is expected to expand the current understanding of intra-plant signaling and its relationship with microbial communities.

Awards for current research in the field, which affects many of today’s top issues, will be presented at the Plant Biology conference. Alff is eager to see how it will play out.

“It is extremely important to me to see the impact that plant biology research is making towards the vital issues of food security and safety, climate change, bioenergy, and medicine,” she said.

Lakshmanan sees the plant biology symposium as “a unique opportunity to network and receive feedback from peers.” At the conference, Alff and Lakshmanan will present and discuss their research with plant biology faculty, postdocs and students from around the country.

Alff says, “This meeting will help in my transition from a graduate student to a professional scientist. Receiving feedback from the plant biology community will help in preparation for my thesis defense and eventual job interviews.”

The research conducted by Alff and Lakshmanan in Bais’ lab is supported by grants from the National Science Foundation and the Delaware EPSCoR program.

Article by Jacob Crum

Photo of Emily Alff by Kathy F. Atkinson

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