[permaculture] Horticultural hijacking: The dark side of beneficial soil bacteria

Lawrence London lfljvenaura at gmail.com
Fri Sep 21 20:09:43 EDT 2012

Horticultural hijacking: The dark side of beneficial soil bacteria
Horticultural Hijacking: The Dark Side of Beneficial Soil Bacteria

ScienceDaily (Sep. 21, 2012) — 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

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,

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

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