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Research team examines molecular basis of plant immune responseBasic research on the biological signaling systems of plants could lead to "souped-up" plant immune systems, said Ping He, Ph.D., a Texas AgriLife Research scientist. "Differential innate immune signalling via Ca2+ sensor protein kinases" was published Jan. 17 in "Nature," a prominent scientific journal. The article describes a mechanism discovered by a collaborative team that includes Ping He, with the Texas A&M University department of biochemistry and biophysics, and Libo Shan, Ph.D., Texas A&M department of plant pathology and microbiology. Other members of the team included Marie Boudsocq, Ph.D., Institut des Sciences du Végétal, France; Matthew R. Willmann, Ph.D., department of biology, University of Pennsylvania; and Matthew McCormack, Ph.D.; Horim Lee, Ph.D.; Jenifer Bush, Ph.D.; and Jen Sheen, Ph.D., all of the Harvard Medical School department of genetics. It's long been known that both plants and animals react to microbials by first recognizing them as foreign, then launching cascades of immune responses to fend off the attack, He explained. Both kingdoms recognize the microbes by their molecular patterns, then use a very complex system of enzymes and intracellular processes to boost the natural immune response. It's also known, He said, that calcium ions are used as a kind of messenger in this signaling process to activate cellular proteins. But exactly how calcium ions were sensed and relayed in the process was a mystery. To try to solve this mystery, the research team used Arabidopsis, a small flowering plant in the same family as cabbage and mustard. It is often used in such studies because it was one of the first plants to have its entire genome mapped, He said. The team found four calcium-dependent enzymes were critical to plants' immune system response. All four enzymes were involved in the plant cells synthesizing defense mechanisms such as peptides and other metabolites to fight microbial threats. To understand how the calcium-dependent process worked, team members developed a new technology called "protoplast transient assay." Protoplasts are naked plant cells without cell walls, a characteristic that makes it easier to introduce foreign genes into plant cells for study, He said. "The results clearly suggested that specific calcium dependent processes are central regulators in integrating multiple signaling pathways," he said. "The identified calcium kinases (enzymes) have great potential to improve plant resistance to multiple pathogens, including bacteria, fungi and phytophthora." There were some surprises along the way. For example, neither a common toxin associated with gram-negative bacteria (LPS) or a component of the bacteria cell wall (PGN), elicited what the researchers considered a "stereotypical" immune response, He said. Instead, it would seem that a relatively few target genes were the same. "This was presumably due to (the toxins) eliciting distinct Ca2+ (calcium ion) dependent signatures," He said. Different toxins or microbial signals stimulate slightly different defense responses. Some are quicker, others are stronger, and still others are longer-lasting. This was thought due to the distinct signatures elicited by different toxins, He said. "It has long been known that calcium-dependent kinases function upstream of another group of evolutionarily conserved proteins called MAPK in animals," He said. "However, the new study suggests that calcium kinases (enzymes) and MAPK (mitogen-activated protein kinases) can function in parallel in a cell." "Evolutionarily conserved" means enzymes such as MAPK have a common evolutionary origin among both the plant and animals, He explained. He said this discovery suggests there are more genetic tools to manipulate plants and animals for beneficial characteristics that previously suspected. "I feel this was an exciting surprise," He said.
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