This passage is excerpted from Yan Zhao, “Aspirin-Like Compound Primes Plant Defense Against Pathogens,” © 2014 by Yan Zhao.
Willow trees are well-known sources of salicylic acid, and
for thousands of years, humans have extracted the compound
from the tree’s bark to alleviate minor pain, fever, and
inflammation.
Now, salicylic acid may also offer relief to crop plants by
priming their defenses against a microbial menace known as
“potato purple top phytoplasma.” Outbreaks of the cell-wall-
less bacterium in the fertile Columbia Basin region of the
Pacific Northwest in 2002 and subsequent years inflicted
severe yield and quality losses on potato crops. The
Agricultural Research Service identified an insect accomplice
—the beet leafhopper, which transmits the phytoplasma to
plants while feeding.
Carefully timed insecticide applications can deter such
feeding. But once infected, a plant cannot be cured. Now, a
promising lead has emerged. An ARS-University of
Maryland team has found evidence that pretreating tomato
plants, a relative of potato, with salicylic acid can prevent
phytoplasma infections or at least diminish their severity.
Treating crops with salicylic acid to help them fend off
bacteria, fungi, and viruses isn’t new, but there are no
published studies demonstrating its potential in preventing
diseases caused by phytoplasmas.
Wei Wu, a visiting scientist, investigated salicylic acid’s
effects, together with molecular biologist Yan Zhao and
others at ARS’s Molecular Plant Pathology Laboratory in
Beltsville, Maryland. “This work reached new frontiers by
demonstrating that plants could be beneficially treated even
before they become infected and by quantifying gene activity
underlying salicylic acid’s preventive role,” according to
Robert E. Davis, the lab’s research leader.
For the study, published in the July 2012 Annals of
Applied Biology, the team applied two salicylic acid
treatments to potted tomato seedlings. The first application
was via a spray solution 4 weeks after the seedlings were
planted. The second was via a root drench 2 days before
phytoplasma-infected scions were grafted onto the plants’
stems to induce disease. A control group of plants was not
treated.
In addition to visually inspecting the plants for disease
symptoms, the team analyzed leaf samples for the
phytoplasma’s unique DNA fingerprint, which turned up in
94 percent of samples from untreated plants but in only 47
percent of treated ones. Moreover, symptoms in the treated
group were far milder than in untreated plants. In fact,
analysis of mildly infected treated plants revealed
phytoplasma levels 300 times below those of untreated
plants, meaning that the salicylic acid treatment must have
suppressed pathogen multiplication. Significantly, the
remaining 53 percent of treated plants were symptom-and
pathogen-free 40 days after exposure to the infected scions.
Researchers credit salicylic acid with triggering “systemic
acquired resistance,” a state of general readiness against
microbial or insect attack. Using quantitative polymerase
chain reaction procedures, the team also identified three
regulatory defense genes whose activity was higher in treated
plants than in untreated ones.
Why salicylic acid had this effect isn’t known. Other
questions remain as well, including how treated plants will
fare under field conditions. Nonetheless, such investigations
could set the stage for providing growers of potato, tomato,
and other susceptible crops some insurance against
phytoplasmas in outbreak-prone regions.
Willow trees are well-known sources of salicylic acid, and
for thousands of years, humans have extracted the compound
from the tree’s bark to alleviate minor pain, fever, and
inflammation.
Now, salicylic acid may also offer relief to crop plants by
priming their defenses against a microbial menace known as
“potato purple top phytoplasma.” Outbreaks of the cell-wall-
less bacterium in the fertile Columbia Basin region of the
Pacific Northwest in 2002 and subsequent years inflicted
severe yield and quality losses on potato crops. The
Agricultural Research Service identified an insect accomplice
—the beet leafhopper, which transmits the phytoplasma to
plants while feeding.
Carefully timed insecticide applications can deter such
feeding. But once infected, a plant cannot be cured. Now, a
promising lead has emerged. An ARS-University of
Maryland team has found evidence that pretreating tomato
plants, a relative of potato, with salicylic acid can prevent
phytoplasma infections or at least diminish their severity.
Treating crops with salicylic acid to help them fend off
bacteria, fungi, and viruses isn’t new, but there are no
published studies demonstrating its potential in preventing
diseases caused by phytoplasmas.
Wei Wu, a visiting scientist, investigated salicylic acid’s
effects, together with molecular biologist Yan Zhao and
others at ARS’s Molecular Plant Pathology Laboratory in
Beltsville, Maryland. “This work reached new frontiers by
demonstrating that plants could be beneficially treated even
before they become infected and by quantifying gene activity
underlying salicylic acid’s preventive role,” according to
Robert E. Davis, the lab’s research leader.
For the study, published in the July 2012 Annals of
Applied Biology, the team applied two salicylic acid
treatments to potted tomato seedlings. The first application
was via a spray solution 4 weeks after the seedlings were
planted. The second was via a root drench 2 days before
phytoplasma-infected scions were grafted onto the plants’
stems to induce disease. A control group of plants was not
treated.
In addition to visually inspecting the plants for disease
symptoms, the team analyzed leaf samples for the
phytoplasma’s unique DNA fingerprint, which turned up in
94 percent of samples from untreated plants but in only 47
percent of treated ones. Moreover, symptoms in the treated
group were far milder than in untreated plants. In fact,
analysis of mildly infected treated plants revealed
phytoplasma levels 300 times below those of untreated
plants, meaning that the salicylic acid treatment must have
suppressed pathogen multiplication. Significantly, the
remaining 53 percent of treated plants were symptom-and
pathogen-free 40 days after exposure to the infected scions.
Researchers credit salicylic acid with triggering “systemic
acquired resistance,” a state of general readiness against
microbial or insect attack. Using quantitative polymerase
chain reaction procedures, the team also identified three
regulatory defense genes whose activity was higher in treated
plants than in untreated ones.
Why salicylic acid had this effect isn’t known. Other
questions remain as well, including how treated plants will
fare under field conditions. Nonetheless, such investigations
could set the stage for providing growers of potato, tomato,
and other susceptible crops some insurance against
phytoplasmas in outbreak-prone regions.