Hemoglobin may be common in plants.

Hemoglobin may be common in plants

Researchers in Australia reported this week the discovery of hemoglobin in the roots of a plant in the elm family. The finding represents the first time hemoglobin has been found in a plant lacking specially adapted "root nodules," and leads the researchers to suggest that hemoglobin genes might be present in all plants.

Scientists have for years been puzzled by the presence of hemoglobin, the oxygen-carrying component in blood, in some plants. It exists in single-unit monomers in plants, while in humans it combines into four-unit tetramers. Mysteriously, it has been found solely in the root nodules of a specialized class of plants that associate with nitrogen-fixing bacteria -- microorganisms capable of capturing atmospheric nitrogen and making it available to plants. It is not obvious, however, why hemoglobin would appear only in such plants. In light of this narrow plant-host range, some researchers have proposed that the gene for hemoglobin might have been transferred from an animal, such as an insect, early in the evolution of those particular plants.

W. James Peacock and his colleagues at the Commonwealth Scientific and Industrial Research Organization in Canberra used DNA probes to identify the hemoglobin gene in the non-nodulating plant, and confirmed by the presence of messenger RNA and protein that the hemoglobin gene was indeed active. Their research appears in the Jan. 14 Nature.

"All of this is building up to a concept that the hemoglobin gene is a regular part of the plant genome," Peacock says. If the gene can be found in a variety of other plants, such as cereals, ferns and pines, he says, "it would indicate that very likely all plants have it and that probably the organism that gave rise to both the animal kingdom and the plant kingdom had this hemoglobin gene." Such a theory would eliminate the need for an animal vector.

It's not clear what role hemoglobin might play in plants. "It seems unlikely to us that it has a function in facilitating oxygen diffusion, because there just isn't enough of it there," Peacock says. But it's possible, he theorizes, that hemoglobin may be involved in detecting the amount of oxygen available to the roots. Oxygen levels can get very low after heavy rain or flooding, he says, triggering a plant's roots to undergo a major change of metabolism.

"Hemoglobin might be involved in the cascade of signals that say, 'Hey, there's low oxygen here, so switch off oxidative metabolism and switch on the anaerobicresponse gene.'"