DURHAM, N.C. -- Biologists have discovered that the air pollutant nitric oxide acts as a plant hormone to delay flowering in plants. The scientists discovered that while plants produce their own internal nitric oxide to regulate flowering, they are also influenced by external concentrations of the chemical.
The scientists said that although their findings are basic in nature, they suggest that the massive amounts of nitric oxide emitted as air pollutants from burning fossil fuels could affect the critical process of plant flowering. Since the decision to flower is so critical to reproduction, a delay in flowering could have important impacts on ecosystems, both plant and animal, they said.
The researchers, led by Duke University biologist Zhen-Ming Pei, published their findings in the Sept. 24, 2004, issue of the journal Science. Co-lead authors were Yikun He and Ru-Hang Tang from Duke. Other co-authors were Yi Hao, Robert Stevens, Charles Cook, Sun Ahn, Liufang Jing, Zhongguang Yang, Longen Chen, Fabio Fiorani and Robert Jackson, all of Duke; and Fangqing Guo and Nigel Crawford from the University of California at San Diego. The research was funded by the National Science Foundation and Duke University.
"The floral decision signaling pathway in plants has been studied for many, many years because the decision to flower is so critical to reproduction," Pei said. "And it was known that some of these pathways integrate external environmental signals, such as daily and seasonal changes in light, while others are autonomous pathways that act independently of external cues.
"However, nobody knew that nitric oxide was involved in these pathways. And nobody knew that plants would be affected by external concentrations of nitric oxide, as might be caused by air pollution." Such biochemical pathways are networks of protein enzymes that make up the signaling machinery that controls the flowering process.
"While our work is very much at a very detailed molecular level, I would bet that these findings have large-scale ecological implications regarding the effects of air pollution on flowering. It is entirely possible that global pollution by oxides of nitrogen could delay plant flowering worldwide," Pei said.
Pei and his colleagues began their studies by exposing the seedlings of the mustard plant, scientific name Arabidopsis, to a chemical that produced nitric oxide. The researchers found that such exposure enhanced the vegetative growth of the plants and significantly delayed flowering in a dose-dependent manner.
Arabidopsis is a widely used model plant in plant biology research, and its genetics and biology have been thoroughly studied.
"This finding that externally applied nitric oxide delayed flowering was a big surprise," Pei said. "It had been known that internal nitric oxide regulated growth in plants. And, it was known that stimuli, such as drought, salt stress and pathogen infection, induced internal nitric oxide production. But it was not expected that external nitric oxide affected the flowering pathway."
To explore how the molecular machinery of the plant was affected by nitric oxide, the researchers identified mutant plants that produced high levels of nitric oxide and were hypersensitive to the chemical. They found that these plants were also late-flowering. Conversely, mutant plants that produced lower nitric oxide levels flowered early.
Genetic studies of such mutants revealed that nitric oxide appeared to affect genes that control both the environmentally sensitive pathways and the autonomous pathways that lead to flowering, Pei said. Thus, he said, nitric oxide may "integrate" both external and internal cues into the decision to flower.
Further studies, Pei said, will concentrate on understanding the nitric-oxide-controlled regulatory machinery in the flowering decision. Also, he said, the researchers will seek the receptor that nitric oxide plugs into in plant cells -- an activation process like a key fitting a lock.
"This study will validate the concept that nitric oxide is an important plant hormone," Pei said. "It provides the hard evidence that is needed to demonstrate nitric oxide's role, since flowering is such a fundamental, well-studied process."
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