By Tracey Bryant

Researchers at the University of Delaware have discovered that when the leaf
of a plant is under attack by a pathogen, it can send out an S.O.S. to the
roots for help, and the roots will respond by secreting an acid that brings
beneficial bacteria to the rescue.
The finding quashes the misperception that plants are ?sitting ducks?- at
the mercy of passing pathogens - and sheds new light on a sophisticated
signaling system inside plants that rivals the nervous system in humans and
animals.

The research was led by Harsh Bais, assistant professor of plant and soil
sciences at UD, former postdoctoral researcher Thimmaraju Rudrappa, who is
now a research scientist at the DuPont Co., Kirk Czymmek, associate
professor of biological sciences and director of UD's Bio-Imaging Center,
and Paul Paré, a biochemist at Texas Tech University.

The study is reported in the November issue of Plant Physiology and also is
featured on the journal's cover. Rudrappa is the lead author of the research
paper.

?Plants are a lot smarter than we give them credit for,? says Bais from his
laboratory at the Delaware Biotechnology Institute.

?People think that plants, rooted in the ground, are just sitting ducks when
it comes to attack by harmful fungi or bacteria, but we've found that plants
have ways of seeking external help,? he notes.

In a series of laboratory experiments, the scientists infected the leaves of
the small flowering plant Arabidopsis thaliana with a pathogenic bacterium,
Pseudomonas syringae. Within a few days, the leaves of the infected plants
began yellowing and showing other symptoms of disease.

However, the infected plants whose roots had been inoculated with the
beneficial microbe Bacillus subtilis were perfectly healthy.

Farmers often add B. subtilis to the soil to boost plant immunity. It forms
a protective biofilm around plant roots and also has antimicrobial
properties, according to Bais.

Using molecular biological tools, the scientists detected the transmission
of a long-distance signal, a ?call for help,? from the leaves to the roots
in the plants that had Bacillus in the soil. The roots responded by
secreting a carbon-rich chemical - malic acid.

All plants biosynthesize malic acid, Bais explains, but only under specific
conditions and for a specific purpose - in this case, the chemical was
actively secreted to attract Bacillus. Magnified images of the roots and
leaves showed the ratcheted-up defense response provided by the beneficial
microorganisms.

Czymmek captured the definitive proof using a state-of-the-art LSM 510 DUO
laser scanning confocal microscope in UD's Bio-Imaging Center. UD is among
only a few universities that own one of these million-dollar instruments.

?A plant is a challenge to image because at least half of it is below ground
in the form of roots,? Czymmek notes. ?Here at UD, we use modern
technologies including hydroponic growth systems with see-through chambers
and sophisticated optical techniques that will enhance the image clarity
when visualizing plants and the pathogens attacking them.?

Bais and his colleagues are now working to determine what the aerial signal
is from the infected leaf to the root using different pathogen-associated
molecular markers (PAMPs).

The research not only sheds light on the remarkable signaling system in
plants, but also is important to understand how invasive plants conquer new
territory with the aid of plant microbes.

?Plants can't move from where they are, so the only way they can accrue good
neighbors is through chemistry,? Bais notes.

The research was funded by the National Science Foundation's Division of
Integrative Organismal Systems, the University of Delaware Research
Foundation and the Delaware Experimental Program to Stimulate Competitive
Research (EPSCoR).
www.chechbiotech.org