Natural Plant DefensesFight or
Built-in response protects against invading bacteria.
When bacteria invade a resistant plant, an active oxygen burst
may signal plant cells to produce lignin that slows the bacterias passage
through cell walls. Or it may signal the cells to collapse around and trap the
Lost, you hurry across the dark, empty street but stop short as a
threatening figure emerges from the shadows of a nearby alley. A sudden rush of
adrenaline shifts your heart rate, breathing, and sugar metabolism into
overdrive, readying your muscles for flightor to fight, if necessary.
An imperiled plant is somewhat like a person in this regard. It, too, relies
on built-in safety mechanisms to ensure its well-being. While it can't exactly
flee an imminent threatsuch as from disease-causing bacteria or a hungry
insectit can fight back. First, however, the plant must know what it is
up against, says Jacyn Baker, a plant pathologist at the ARS Molecular Plant
Pathology Laboratory in Beltsville, Maryland.
He is studying chemical cues or signals that help plants recognize harmful
microorganisms such as Pseudomonas solanacearum, a soilborne bacterium
that causes brown rot disease in potato crops. From such basic research, Baker
hopes to eventually speed or improve the plant's ability to mobilize natural
defenses against microintruders like Psuedomonas and thus arrest the
diseases they cause.
"Plants do have all the means to defend themselves. The key is whether
they can trigger these mechanisms or turn them on in time," notes Baker.
"Our strategy has been to find the first plant responses that signal
recognition of a pathogen and resistance to it."
Plants that hold their own against such threats, he says, would need less of
the chemical pesticides growers now use to protect them. In one sense, Baker's
plant-signaling work might be likened to a technician adjusting the sensitivity
of a home alarm system to deter would-be burglars.
His colleagues on the project are University of Maryland plant pathologist
Elizabeth Orlandi and biological lab technician Norton Mock of the Molecular
Plant Pathology Laboratory. Their chief focus is a signaling event called the
active oxygen (AO) burst. Using a liquid medium of plant callus cells and other
lab techniques, they have studied AO bursts in alfalfa, soybeans, tobacco, and
most recently, potatoes.
Their work showed that AO is produced in two phases outside of the membranes
of plant cells. The second phase, their main interest, is apparently triggered
by microorganisms dubbed "incompatible"essentially those to
which the plant is resistant.
One defensive response the AO burst may mediate in plants is the production
of lignin, a substance that can impede a pathogen's progress through cell
walls. Baker is also investigating whether the burst may induce a plant
response called apoptosisa kind of suicidal cell death in which the cell
collapses around the pathogen, trapping it.
When bacteria invade a resistant plant, an active oxygen burst may signal
plant cells to produce lignin that slows the bacteria's passage through cell
walls. Or it may signal the cells to collapse around and trap the invaders.
"One of our goals is to find out whatif anyvalue this
active oxygen burst might have in the whole scheme of things," says Baker.
Toward that end, his lab is investigating new techniques to accurately
measure the duration, intensity, and locale of the signaling event in
individual plant cells and tissues.
So far, a promising strategy is to use human oxidase genes as probes to
locate genes that regulate AO production in the plants.
Baker's lab is doing the work with Beltsville colleague Frank Turano, who is
in the ARS Climate Stress Laboratory. Immunology researchers at the National
Institutes of Heath in Bethesda, Maryland, furnish the human oxidase genes for
Pinpointing AO genes in plants may eventually open the door for
biotechnology research. If tobacco plants are found to harbor AO-regulating
genes that help them recognize a certain pathogen more quickly than soybeans,
scientists may then be able to bolster the soybean plant's defensive response
by inserting AO-regulating genes borrowed from tobacco, Baker says. -- By
Jan Suszkiw, ARS.