Droopy plants drop hints of enzyme's role.

Droppy plants drop hints of enzyme's role

Even at its death knell, after the last traces of chlorophyll and other vital chemicals have vanished, a withering plant keeps producing one compound--a red-pigmented enzyme called peroxidase. Since the turn of the century, scientists have studied this intriguing and easily detectable enzyme, which assists plants in wound healing, oxidation and cell elongation. But because a plant may contain up to 35 different types of peroxidase, each reaching with several plant compounds, researchers have been frustrated in their attempts to pin down the enzyme's fundamental role in plant development.

Now, with a bit of genetic sleight-of-hand, investigators have uncovered hints that the enzyme may play a part in plant aging and fertility.

In 1987, molecular biologist L. Mark Lagrimini of Ohio State University in Columbus cloned the gene that codes for one type of peroxidase found in tobacco plants. He and his colleagues at Ohio State and the University of Guelph in Ontario went on to combine a peroxidase gene with fragments of cauliflower mosaic virus, which stimulates overproduction of peroxidase. They inserted this gene-virus combination into two species of tobacco plants, which began producing up to 10 times the normal amount of peroxidase. Genetically altered plants containing at least twice the usual enzyme levels were outwardly identical to their normal relatives until flower buds appeared. Then the plants began to droop in sunlight, at first recovering during the night, but after several weeks wilting permanently.

Wilting and the stunted growth associated with it were more severe in plants with larger amounts of peroxidase, Lagrimini and his collaborators report in the January PLANT CELL. None of the altered plants died prematurely, and all the leaves remained green, unlike those that droop from drought. But the wilting, combined with peroxidase's well-known ability to thrive in dying plants, suggests a link between peroxidase and plant aging, Lagrimini says.

More recent observations, he told SCIENCE NEWS, indicate peroxidase action may concentrate in the roots. Genetically altered tobacco stems grafted onto normal roots no longer drooped, although normal stems grafted onto roots containing the altered gene still wilted.

But these observations fill in only one piece of the peroxidase puzzle. Other research by the same group suggests another role for the enzyme: regulating fertility. Tobacco plants altered to produce unusually low peroxidase levels grew normally but yielded about one hundredth as much seed as unaltered controls. Lagrimini, who is now extending his peroxidase pursuit to tomato plants and sweet gum trees, says the enzyme's primary function remains unclear.

"Even after [publication of] three to four thousand papers on plant peroxidase, we're still ignorant about what it does," says Fred B. Abeles, a plant physiologist at the USDA's Appalachian Fruit Research Station in Kearneysville, W. Va. "But [Lagrimini's] work is a new strategy" for narrowing the possibilities, he says.