To many people, the scent of jasmine flowers suggests a romantic interlude in an exotic locale. But jasmonate, the main component of the lush scent, carries far different meanings for plants. It is a hormone they use to regulate reproductive development, immunity to pathogens, defense against insect herbivores and other critical aspects of their biology.
Despite jasmonate’s importance in plant development and function, the chemical steps that convert the hormonal signal into genetic and cellular action have remained elusive. Now researchers at Washington State University and Michigan State University have identified the family of proteins that allow a plant to perceive and respond to the hormone. They have also proposed a model for how the proteins, dubbed JAZ proteins, work.
“Jasmonate is the last major hormone for which the central signaling components have not been described,” said co-author John Browse of Washington State University. He said understanding how the jasmonate system works will shed light on all the processes that jasmonate is involved in, notably plant reproduction and defense.
It’s been known for some time that jasmonate causes dozens of specific genes to be turned on, and that a protein complex called SCF-COI1 is somehow involved. Browse and his colleagues set out to find the other proteins that convert the hormonal signal into cellular action.
Working with Arabidopsis, a small mustard species commonly used in plant-biology research, they first treated seedlings with jasmonate and looked for proteins that the plant produced in response. They determined that eight of those proteins had features in common, including a short stretch known as a Zim domain. That gave the proteins their name: JAsmonate-Zim, or JAZ.
The newly-identified JAZ proteins were localized in cell nuclei, suggesting they might be involved in gene regulation. They did not, however, possess a DNA-binding site. That suggested that rather than directly turning genes on or off, the JAZ proteins might be interacting with other proteins that do. They also turned out to interact with the previously-identified SCF-COI1.
According to the model the researchers developed, proteins called transcription factors (T) are bound to jasmonate-responsive genes whether jasmonate is present or not. In the absence of jasmonate, JAZ proteins bind to the transcription factors and prevent them from transcribing (or turning on) the genes. When jasmonate is present, SCF-COI1 causes the JAZ proteins to be removed and degraded. With the repressive JAZ proteins gone, the transcription factors are able to turn on the genes.
Browse said the jasmonate signaling system has been hard to figure out in part because mutations in the signaling proteins have been hard to identify. A general approach in such studies is to look for individuals with defects in function, and then find the specific mutant genes responsible for these defects. That’s been difficult to do with the jasmonate system because there are at least a dozen JAZ proteins, and at least some of them have what Browse calls “overlapping functions”—if one of the proteins is rendered nonfunctional by a mutation, the other proteins substitute for it so that there is no loss of function.
The authors of the current study produced and tested seven different mutant forms of a JAZ protein before finding one that lost its response to jasmonate. The mutation, which involved deletion of a short segment of the protein, appears to prevent JAZ from being recognized by COI1 and degraded. The mutant protein will therefore remain attached to the transcription factor even when jasmonate is present, so the genes that would normally be turned on by jasmonate remain turned off.
The researchers found that plants with the mutant JAZ protein lack normal responses to jasmonate with respect to root growth, male reproductive development, resistance to a fungus and other activities.
Another notable finding was that among the genes that JAZ proteins repress are the genes for JAZ proteins themselves. In the presence of jasmonate, those genes are turned on, producing more JAZ proteins that will shut down further response to the jasmonate.
Browse said that enables the plant to keep its response to jasmonate brief. If the jasmonate-responsive genes stayed turned on too long, the plant could suffer damage, rather like a person having an inflammatory response that won’t quit.
“That is absolutely, classically, the way almost every signaling system that’s been studied in all organisms works,” said Browse. “There are very careful mechanisms put into place, on the one hand, to bring about passage of the signal, and then on the other hand, to feed back control and keep it shut down all the time when the response is not required. You want to regulate the signal very carefully to bring about its effect and then shut it down again.”