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UD researchers show how beneficial soil bacteria can boost plant immunity
http://www.udel.edu/udaily/2013/aug/beneficial-soil-bacteria-082712.html

http://www.udel.edu/udaily/2013/aug/beneficial-soil-bacteria-082712.html

Behind closed doors

UD researchers show how beneficial soil bacteria can boost plant immunity

"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 pathogen Pseudomonas 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 subtilis bacteria 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