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UNT Resource magazine >> Flower Power

Plants don't have nervous systems or immune systems.
      But they do have their own type of cellular communication systems through lipid signals, says Kent Chapman.
      Chapman, University of North Texas associate professor of biological sciences, and Swati Tripathy, postdoctoral research associate, are the first researchers to show that some lipid compounds act as signaling molecules in plants that encounter pathogens or other elements that cause stress to their cells and lead to cell death. These lipids appear to block enzymes that contribute to cellular collapse and tissue death.
      This finding in basic cellular research could have many applications — from delaying the ripening of fresh fruits to preserving the freshness of cut flowers.
      Chapman compares these signaling lipids, which are derived from N-acylphosphatianolamine (NAPE), a cellular membrane component in all multicellular organisms, to the signaling lipids in human cells.
      "Unlike humans, plants don't have circulating antibodies, and they can't go inside to avoid stress from hot and cold weather. They can't put on sunscreen to avoid stress from UV rays," he says. "They really have to be much more sophisticated in responding to their environments than we do."

A stress defense for plants
Chapman has studied plant lipid biochemistry since he became a UNT faculty member in 1993. He was attracted to the unusual chemical structure of NAPE — three fatty acids instead of the usual two found in typical membrane lipids.
      In addition, Chapman's UNT research team had found the lipid at higher concentrations in seeds than in any other parts of a plant.
      "Seeds are only 10 percent water, and there isn't any cellular stress that is worse than dehydration," he says. "It's not proven, but we may see an accumulation of these lipids in seeds as a result of dehydration stress."
      In 1995, Chapman and Tripathy began to experiment with NAPE as a defense signal in tobacco plants. They introduced pure proteins from pathogens into the plants' leaves, believing the proteins would activate the same defense response that bacteria or other pathogens would trigger.
      "We chose to use protein because if you use bacteria, you have to deal with the bacteria's own lipids," Chapman says. "The proteins did activate the metabolism of NAPE."
      In additional experiments, release of the lipid activated the plants' own defense systems.

Prolonging the life of cut flowers
Chapman and researcher Shea Austin-Brown next searched for other biological functions for this obscure class of lipids. They found that these lipids inhibit a type of membrane-degrading enzymes.
      These enzymes are normally activated in plants during tissue collapse, which occurs in flowers when they are cut from their stems. Chapman wondered whether lipid supplements could delay the degradation and tissue death in cut flowers and other plant systems. He and Austin-Brown soaked the cut flower stems in a solution of synthetic lipids and compared them to flowers soaked in water or solutions commonly used by florists.
      Degrading cells in cut flowers lead to droopy blossoms and stems after a few days. However, these plant lipids seem to inhibit the enzymes that participate in that process, resulting in full and fresh blossoms and rigid stems for a longer period of time, Chapman says.
      "We measured the diameters of the blossoms before soaking in the solution, then measured them again days later to note cell degradation," he says. "The blossom measurements varied depending on the quality of the flower and whether the correct dosage was soaked up. But generally, the flowers in our solution were far superior to others. This process is almost like putting a protectant into the flower."
      Chapman experimented with carnations, roses, daisies and wildflowers, all with the same results.
      "We also tested the concentration range. Higher concentrations of our solution extend the life of flowers for a longer time while they are in a vase," he says.
      UNT has applied for a patent for the method of using these solutions to preserve cut flowers. Chapman says the method could revolutionize the cut flower industry — a $7 billion industry in the United States alone.
      "If only a small percentage of those handling flowers treated them in the solution, it would lead to a significant savings in terms of shipping, inventory costs and customer satisfaction," he says. "When you buy cut flowers, you want them to last for a while. But right now, some flowers last only 24 to 48 hours before wilting." Flowers in Chapman's experiments were still fresh after 17 days.
      He adds that the UNT-developed solution could replace a silver compound solution florists have used to preserve flowers. That solution, he says, is heavily regulated by the Environmental Protection Agency because of concerns about heavy metal pollution in wastewater.

Other applications
In addition to cut flowers, the freshness of fruits and vegetables might be preserved with variations of the lipid formulas, Chapman says.
      "Tomatoes, for example, taste the best when they ripen on the vine," he says. "But if you let them ripen on the vine and then cut them for shipping to grocery stores, the cellular degradation process begins and the tomatoes start getting too ripe and soft. Dosing them could be a good way to temporarily delay further ripening after they're cut."
      Chapman and his research team plan to continue to investigate whether lipid signaling is involved in different types of environmental stress.
      "NAPE metabolism may be the central mechanism activated in response to temperature stress," he says. "If we can manipulate the levels of NAPE metabolites in response to different environmental stress, we could successfully grow plants in climates that are warmer or cooler than normal."
      Part of Chapman's research on NAPE will be published in a special issue of the journal Chemistry and Physics of Lipids later this year. He notes that scientists had long believed that NAPE had no real function in any organism.
      "We found out not only is that false, but the lipid participates in a vast array of biological functions," he says. "Science is meant to be skeptical, but there's really nothing in an organism that is there by accident."