The Martin laboratory studies the molecular basis of bacterial pathogenesis, plant disease susceptibility, and plant immunity. Most of our research focuses on bacterial speck disease which is caused by the infection of tomato leaves with the bacterial pathogen Pseudomonas syringae pv. tomato. This is an economically important disease that can decrease both the yield and quality of tomato fruits. It also serves as an excellent model system for understanding plant-pathogen biology because much is known about the molecular biology of this pathosystem and many genomics resources are available for both tomato and P. s. pv. tomato.
Symptoms of bacterial speck disease of tomato caused by the bacterial pathogen Pseudomonas syringae pv. tomato
In the tomato-Pseudomonas interaction, the virulence proteins AvrPto and AvrPtoB are delivered into
the plant cell by the bacterial type III secretion system. Both proteins then act to suppress host basal
defenses and thereby promote plant disease susceptibility. Some tomato genotypes express the Pto gene
which encodes a protein kinase that detects the presence of AvrPto and AvrPtoB and confers resistance
to bacterial speck disease. This resistance is activated by the physical interaction of the Pto kinase
with AvrPto or AvrPtoB and also by the interaction of Pto with Prf, a protein containing a
nucleotide-binding site and a region of leucine-rich repeats
(i.e., an NB-LRR protein).
This early recognition event activates a complex series of signaling events that leads ultimately to host defense responses, including transcriptional reprogramming and localized host cell death, that restrict growth of the pathogen. We have found recently that a C-terminal domain of AvrPtoB encodes an E3 ubiquitin ligase that, in certain tomato genotypes, can interfere with activation of this host resistance response. Thus, some bacterial virulence proteins appear to have evolved to suppress both basal and resistance-gene mediated host defenses and plants have, in turn, evolved to interfere with both of these activities.
To further understand the molecular basis of bacterial virulence, plant immunity, and susceptibility in this pathosystem we are using various experimental approaches including: genomics, biochemistry, cell biology, molecular biology, forward and reverse genetics, and structural biology. Our long term goal is to use the knowledge we gain about plant-pathogen interactions to engineer plants for increased resistance to diseases and thereby lessen the need for synthetic chemical inputs.
There’s an arms race underway in the plant world in which plants and disease-causing bacteria are continually evolving ways to outsmart each other. Plants have developed a defense system that enables them to resist disease, but some pathogens have evolved survival methods that undermine this system. Understanding the details of this race for dominance could lead to crop plants with more effective, natural resistance to disease.
Gregory Martin’s laboratory studies a bacterium called Pseudomonas syringae, which causes bacterial speck disease of tomatoes. When P. syringae invades a tomato plant, it injects a disease-promoting protein called AvrPtoB into the plant cells. However, the plant is ready and waiting with the protein Fen,which was recently discovered by Martin’s team. Fen recognizes AvrPtoB and, in doing so, activates the plant’s defense system.
Fighting back, P. syringae has cleverly engineeredAvrPtoB to act as a tomato E3 ligase, a protein that tags other proteins to be destroyed. When AvrPtoB binds the Fen protein, Fen is tagged and the plant’s own system takes Fen to the cell’s “garbage bin” before Fen can activate the plant’s defenses. This eliminates the plant’s ability to resist speck disease and ensures the survival of the bacterium.
In further studies,Martin’s laboratory found that the Fen gene is present in many wild species of tomatoes suggesting it is an ancient plant defense strategy. But if the bacterial protein AvrPtoB is so effective at destroying the Fen protein,why would the Fen gene be so prevalent? Martin answers that there are some strains of P. syringae that produce a version of AvrPtoB that cannot destroy Fen, and, therefore, cannot turn off the plant’s defense system. Consequently, he reasons that the bacteria have only recently evolved the version of AvrPtoB that can sabotage the plant’s defenses.
Martin’s work helps to explain how the plant/pathogen arms race works at the molecular level and sheds new light on how disease-resistant plants can suddenly become susceptible again. Understanding the strategies pathogens use to overcome plant defenses against disease may lead to crops that have more effective, longer lasting resistance – an advance that could lead to more productive varieties and less dependence on pesticides.
Millions of years of evolution have equipped viruses, bacteria, and other attackers with clever ways to slip past a plant’s defenses. One strategy Greg Martin’s lab has studied recently is molecular mimicry: evolving proteins that mimic plant proteins and can alter the plant’s physiology for the benefit of a pathogen.
Martin’s lab recently found two instances of this mimicry. In the first case, graduate student Jeff Anderson was looking for ways in which infection by the bacteria Pseudomonas syringae affects tomato cells. Specifically, he wanted to find whether the Pseudomonas protein AvrPto, which makes the bacteria more virulent, influences phosphorylation of host proteins, since attaching or taking away a phosphate group is a common way to change protein function.
He discovered instead that AvrPto itself gets phosphorylated by a host protein, and not by chance: a phosphate group is added at a specific amino acid. Mutating AvrPto so that it cannot be phosphorylated reduces the ability of AvrPto to promote plant disease. This observation suggests that a host enzyme specifically recognizes AvrPto and adds the phosphate. For that to happen, AvrPto likely mimics a host protein the enzyme normally acts on.
In a second case of molecular mimicry, graduate student Rob Abramovitch was looking for tomato proteins that interact with AvrPtoB, another Pseudomonas virulence protein. He came up with ubiquitin, a small protein plant cells commonly use to mark unneeded proteins for degradation.
Why would AvrPtoB bind to ubiquitin? Part of the answer came when Abramovitch found that AvrPtoB acts as a ubiquitin ligase, a host enzyme that attaches ubiquitin to host proteins. A collaborator on the project showed that the three-dimensional crystal structure of AvrPtoB looks like a typical host ubiquitin ligase. Martin and Abramovitch speculate that AvrPtoB makes plants more susceptible to disease by attaching ubiquitin to plant proteins involved in defense responses, thus tagging them for demolition.
