Plant respiration not just an evolutionary leftover, study shows

07/21/04

A biological process in plants, thought to be useless and even wasteful, has significant benefits and should not be engineered out -- particularly in the face of looming climate change, says a team of UC Davis researchers.

The researchers have found that the process, photorespiration, is necessary for healthy plant growth and if impaired could inhibit plant growth, particularly as atmospheric carbon dioxide rises as it is globally. Their findings are published this week in the Proceedings of the National Academy of Sciences.

Over the past two hundred years, scientists have come to understand that plants are amazing biochemical factories that harness energy from sunlight to convert water and carbon dioxide into sugars that fuel the plant, while giving off oxygen.

Though elegantly simple in concept, this process, known as photosynthesis, is remarkably complex in detail. And for years, researchers have been puzzled by another process, photorespiration, which seems to have annoyingly associated with photosynthesis down the evolutionary pathway.

Photorespiration has appeared to be downright wasteful because it virtually undoes much of the work of photosynthesis by converting sugars in the plant back into carbon dioxide, water and energy.

Believing that photorespiration is a consequence of the higher levels of atmospheric carbon dioxide in long past ages, many scientists concluded that photorespiration is no longer necessary. Some have even set about to genetically engineer crop plants so that the activity of the enzyme that initiates both the light-independent reactions of photosynthesis and photorespiration would favor photosynthesis to a greater extent and minimize photorespiration.

The result, they have thought, would be more productive crop plants that make more efficient use of available resources.

But the new UC Davis study suggests that there is more to photorespiration than meets the eye and any attempts to minimize its activity in crop plants would be ill advised.

"Photorespiration is a mysterious process that under present condition dissipates about 25 percent of the energy that a plant captures during photosynthesis," said Arnold Bloom, a professor in UC Davis' vegetable crops department and lead researcher on the study. "But our research has shown that photorespiration enables the plant to take inorganic nitrogen in the form of nitrate and convert it into a form that is useful for plant growth."

The UC Davis team used two different methods to demonstrate in both wheat and Arabidopsis, a common research plant, that when plants are exposed to elevated levels of atmospheric carbon dioxide or low levels of oxygen -- both conditions that inhibit photorespiration -- nitrate assimilation in the plant's shoot slows down. Eventually, a shortage of nitrogen will curtail the plant's growth.

"This explains why many plants are unable to sustain rapid growth when there is a significant increase in atmospheric carbon dioxide," said Bloom. "And, as we anticipate a doubling of atmospheric carbon dioxide associated with global climate change by the end of this century, our results suggest that it would not be wise to decrease photorespiration in crop plants."

The UC Davis study was supported by the National Science Foundation, the U.S. Department of Agriculture and an Israel Binational Agricultural Research and Development Fund fellowship.

Source: Eurekalert & others

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