The research also provides insight into a previously little-understood area of plant biology.
William R. Woodson, professor of horticulture at Purdue, and graduate students Hanan Itzhaki and Julie M. Maxson have identified the genes that causes flowers to wilt, and they have provided a detailed description of how this process works. The research is reported in the September issue of The Proceedings of the National Academy of Science, U.S.A.
Before Woodson's research, it was commonly thought, even among scientists, that flowers wilted because of an unregulated death of the tissues. Woodson is the first to prove that flowers undergo "senescence," (si-NES-n(t)s) as scientists call it, because of a specific genetic response.
In other words, the flowers don't die of old age. The plants intentionally kill the blooms.
This makes biological sense, Woodson says. The flower of a plant exists to attract insects, or sometimes birds or animals, so that it can be pollinated. A flower can be pollinated only once, so if it did not die after pollination, it would compete with adjacent unpollinated blossoms with no benefit to the plant. Also, because the flower petals don't contain chlorophyll, which produces nutrients from sunlight, they are a drag on the plant's nutrients.
"It's not a matter of life span for the flower," Woodson says. "Take, for example, the orchid, which is just a beautiful flower. Some orchids will keep a flower for two, three, even four months.
"The reason for the long life of the orchid blossom is that orchids have very elaborate pollination requirements. Only certain species of birds or insects can pollinate them. It makes sense from an evolutionary point of view for them to have long-lasting flowers. But, if the plant is pollinated, the flower will die within 12 hours.
"Contrast that to a plant that is self-pollinated, such as cotton. Its flowers last only one day, because it doesn't need to attract insects for reproduction.
"Our research is another spoke in the wheel of understanding how plants work. We know very little about the physiology of plants compared to what we know about the physiological systems of animals.
"For example, how does a plant know it's been pollinated? It doesn't have a nervous system to pass the message along. How do its tissues know to respond to pollination? Plants don't have circulatory systems to carry hormones through the plant."
Previous research by Woodson has shown that plants send messages through their system by a process known as hormone signaling, and that hormones are sent to outlying plant tissues via auto-catalytic waves.
When an insect pollinates a flower, or when a flower is cut from the plants, the flower begins producing the plant hormone ethylene. Plants produce and react to ethylene in levels that are measured in parts per billion of air molecules.
Ethylene production in plant cells is auto-catalytic, which means that its presence causes a plant cell to begin to produce it. As cells in the plant's ovary produce ethylene, the adjacent cells begin producing it too. This results in an auto-catalytic ethylene wave that moves through the plant flower, eventually reaching the petals. Genes in the petals react to the ethylene, causing the petals to die.
Genes in the petal tissue respond differently to the hormone than genes in other tissues. Although the petals die, the plant's ovary doesn't, for example. In fact, the ovary grows. "So this is a very specific response," Woodson says. "It shows how plant hormones are sometimes used as a signal mechanism in the plant."
"This is important scientifically because it's the first detailed description of the regulation of genes by the plant hormone ethylene," Woodson says.
By better understanding the biology of flowers, Woodson expects to possibly increase crop yields and to create flower varieties that will last much longer than flowers now available.
Cut flowers begin producing ethylene immediately after being cut. If they don't receive the physiological message to kill the bloom, the flowers can last several times longer than normal. For example, carnations typically die within 3 to 5 days after being cut, but Woodson has been able to keep cut flowers from genetically engineered carnations alive in his laboratory for as long as three weeks.
"You might think that longer-lasting blooms would result in fewer sales of flowers, but the opposite is true," Woodson says. "All of the marketing studies done by the floral industry have indicated that more people would buy flowers if the blooms lasted longer."
Longer-lasting blooms also would mean that florists could stock some types of flowers that are not now sold at the retail level because they take too long to ship, or that new varieties of bedding flowers might begin appearing in nurseries.
Although the floral industry is not generally recognized as a major part of agriculture, in 1993 it generated $7 billion of retail sales in the United States. Indiana took home $170 million of that, divided between the state's 700 flower shops and 200 greenhouses.
"It's a real small-business industry with a lot of mom-and-pop type operations," says Bruno Moser, head of Purdue's Department of Horticulture. "There are a lot of classic American success stories in that industry."
Crop plants also may benefit from Woodson's research. "Absolutely all crops grow flowers," Woodson says. "Conveniently, the product of the crop is the product of reproduction. So if we find ways to increase the pollination, we could increase yields. And, in the case of fruits, ethylene is involved in abscission, or dropping of the fruit. So if we can find ways to control the release of ethylene, we can increase the fruit yields."
Woodson's research in plant pollination is funded by the U.S. Department of Agriculture's Agricultural Research Service.
HOMETOWN NOTE: WIlliam Woodson's hometown is Fordyce, Arkansas. The hometown for Julie Maxson is Dayton, Ohio, and the hometown for Hanan Itzhaki is Bet Dagan, Israel.
Sources: William R. Woodson, (765) 494-1337; Internet, firstname.lastname@example.org
Bruno Moser, (765) 494-1306; Internet, Bruno_Moser@hort.purdue.edu
Writer: Steve Tally, (765) 494-9809; Internet, email@example.com
Purdue News Service: (765) 494-2096; e-mail, firstname.lastname@example.org
The increased production of ethylene during carnation petal senescence regulates the transcription of the GST1 gene encoding a subunit of glutathione-S-transferase. We have investigated the molecular basis for this ethylene-responsive transcription by examining the cis elements and trans-acting factors involved in the expression of the GST1 gene. Transient expression assays following delivery of GST1 5' flanking DNA fused to a beta-glucuronidase reporter gene were used to functionally define sequences responsible for ethylene-responsive expression. Deletion analysis of the 5-prime flanking sequences of GST1 identified a single positive regulatory element of 197 bp between 667 and 470 necessary for ethylene-responsive expression. The sequences within this ethylene-responsive region were further localized to 126 bp between 596 and 470. The ethylene-responsive element (ERE) within this region conferred ethylene-regulated expression upon a minimal cauliflower mosaic virus035S TATA-box promoter in an orientation-independent manner. Gel electrophoesis mobility-shift assays and DNase I footprinting were used to identify proteins that bind to sequences within the ERE. Nuclear proteins from carnation petals were shown to specifically interact with the 126-bp ERE and the presence and binding of these proteins were independent of ethylene or petal senescence. DNase I footprinting defined DNA sequences between 510 and 488 within the ERE specifically protected by bound protein. An 8-bp sequence (ATTTCAAA) within the protected region shares significant homology with promoter sequences required for ethylene-responsiveness from the tomato fruit-ripening E4 gene.
NOTE TO JOURNALISTS: A color photograph of the researcher is available. Contact Steve Tally, (765) 494-2096.
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