WSU Scientists Discover Key Proteins Involved In Plant Reproduction And Defense

By Amelia Bachleda

The pleasant steam from a nearby mug of jasmine tea may remind the café-goer of a far off place and time.

For a plant, however, the molecule responsible for that light scent and its derivative, jasmonate, mean something quite different. The presence of jasmonate in a plant system acts as a signal, initiating a cascade of molecular responses involved in plant reproduction and defense.

All terrestrial land plants are believed to be receptive to jasmonate. And while the effects of the jasmonate hormone are crucial to the ultimate survival of the organism, how the signaling pathway works has been a mystery. That is, until recently, when scientists at Washington State University discovered a key family of molecules, the Jasmonte-Zim (JAZ) proteins, which play a crucial role in the jasmonate pathway.

"It is the last major hormone for which the central signaling components have not been described,” explains Regents Professor and co-author of the study, John Browse. Published in the August 9th issue of Nature, the research paper was among the top ten most frequently downloaded from the Nature website that month.

The discovery will make it easier for researchers to study and understand the mechanisms plants use to defend themselves against insects and diseases, invaluable information for agricultural and horticultural fields.

"It's a big breakthrough. Before this discovery, researchers in the field were somewhat blocked when it came to asking questions about the mechanism of Jasmonate perception and how jasmonates control gene expression. A large piece of the puzzle was missing,” says Edward Farmer, professor of plant molecular biology at the University of Lausanne in Switzerland. The knowledge of the jasmonate signaling pathway is now on par with other, better-understood hormone signal pathways, Farmer says. "We are now in a better position to see how these pathways interact.”

The story of that puzzle piece, the newly discovered JAZ proteins, is the story of a former graduate student in Browse's lab, co-author, Bryan Thines. At the time, Thines was studying the role of jasmonate in proper sexual maturation of the common laboratory plant, Arabidopsis. By adding jasmonate to a plant that is unable to synthesize this hormone of its own accord, he could measure the changes in gene activity (expression) directly related to its application.

Thines examined the changes in expression at different times after the initial application, and he saw something interesting. "He came into my office and said, ‘you know, in the shortest time (one-half hour after jasmonate treatment), only thirty-one genes increased expression'” Browse recalls, "'and eight encode proteins of unknown function.'” The proteins encoded by these eight genes were all similar. Collectively, they were the missing puzzle piece: the genes that encode the JAZ proteins.

Further research identified the JAZ proteins as repressor proteins, working to suppress the action of the jasmonate responsive genes. Jasmonate promotes the destruction of the JAZ repressor proteins, enabling the activation of genes involved in defense and development. Only when jasmonate is present is the inhibition of these genes released.

Surprisingly, the genes encoding JAZ proteins are also activated by the jasmonate signal. These were the genes of unknown function originally identified by Thines and Browse. The expression of the JAZ proteins in response to jasmonate is a safety feature for the plant. The presence of JAZ ensures that the jasmonate signaling will eventually be shut down again by initiating the formation of the proteins that it originally marked for destruction.

In biology, this type of system is referred to as a negative feedback loop; a signal sets off a series of reactions that ultimately results in the cessation of the original signal. This built-in safety break is "extremely common,” Browse says, "Almost every system studied has these checks and balances.” Nevertheless, He was extremely pleased to discover the presence of a feedback loop, as it fits into this paradigm. "It helped confirm that JAZ proteins were a central player.” For Browse, everything seemed to fit into a system that one would expect to find.

Amelia Bachleda is an undergraduate student in the University of Washington's Department of Neurobiology.

Image: Washington Sate University regents professor and co-author of the research, John Browse. Photo: Shelly Hanks, WSU