By Krishna Ramanujan
An arms race is under way in the plant world. It is an evolutionary
battle in which plants are trying to beef up their defenses against the
innovative strategies of pathogens.
The latest example of this war is a bacterium (Pseudomonas syringae)
that infects tomatoes by injecting a special protein into the plant's cells
to undermine the plant's defense system.

"Plant breeders often find that five or six years after their release,
resistant plant varieties become susceptible because pathogens can evolve
very quickly to overcome plant defenses," said Gregory Martin, Cornell
professor of plant pathology, a scientist at the Boyce Thompson Institute
for Plant Research (BTI) on the Cornell campus and the senior author of the
research paper, published in the July 19 issue of the journal Nature.
"However, every now and then, breeders develop a plant variety that stays
resistant for 20 years or more."

Understanding why some varieties have more durable disease resistance
is important to the development of more sustainable agricultural practices,
he said.

The study by Cornell and BTI scientists describes how a single
bacterial protein, AvrPtoB, which is injected by P. syringae into plant
cells through a kind of molecular syringe, can overcome the plant's
resistance. Normally, the plant's defense system looks out for such
pathogens and, if detected, mounts an immune response to stave off disease.
As part of this surveillance system, tomatoes carry a protein in their cells
called Fen that helps detect P. syringae and trigger an immune response.

But some strains of P. syringae have evolved the AvrPtoB protein that
mimics a tomato enzyme known as an E3 ubiquitin ligase, which tags proteins
to be destroyed. Once injected, AvrPtoB binds the Fen protein, and the
plant's own system eliminates it, allowing the bacteria to avoid detection
and cause disease.

"This paper explains how a pathogen can evolve to escape detection,"
said lead author Tracy Rosebrock, a graduate student in Cornell's Department
of Plant Pathology and BTI. "The bacterium has one specific protein that it
uses to turn off the plant's immunity."

The researchers found that the Fen gene is present in both cultivated
tomatoes and many wild tomato species, leading them to believe that the gene
is likely ancient in origin and that many members of the tomato family have
used it to resist P. syringae infections over the years. Since the Fen
protein still detects AvrPtoB-like proteins from some strains of P.
syringae, prompting an effective immune response, the researchers believe
new P. syringae strains have only recently evolved a version of AvrPtoB that
includes an E3 ubiquitin ligase enzyme that interferes with the plant's
surveillance.

"This paper provides molecular data that supports the evolutionary
'arms race' theory" that as pathogens develop new ways to spread and attack
organisms, the organisms in turn create novel defenses, each in a continuous
battle to outdo the other, said Rosebrock.

The research was funded by the National Institutes of Health, the
National Science Foundation and the Triad Foundation, a private charitable
trust.


[www.cornell.edu]