Leaf bacteria fertilize trees, researchers claim

Posted on May 27, 2015

The fastest growing trees outside the tropics are poplars. Tall and slender, they can reach 30 meters in less than a decade despite the seemingly inhospitable ground they favor—burned areas and sandy riverbanks, for example. Sharon Doty says the credit goes to microbes in their leaves and other tissues. While the poplar's leaf cells are busy converting sunlight to energy, she says, bacteria between those cells are transforming nitrogen from the air into a form the tree needs to sustain this rapid growth.

That's a radical notion, because nitrogen fixation is generally thought to happen primarily in bacteria-rich nodules on the roots of legumes and a few other plants, and not in the treetops. “We are completely fighting dogma,” says Doty, a plant microbiologist at the University of Washington, Seattle.

Earlier this month at the Fifth Annual Yosemite Symbiosis Workshop here, Doty bolstered her case. She reported the first direct evidence that poplars do get nitrogen from certain microbes, and she got support from Carolin Frank, an environmental microbiologist at the University of California (UC), Merced, who studies a different tree that thrives on poor soil. Frank reported that nitrogen fixation may also occur in the needles of limber pines, which grow on stony, high-elevation slopes in western North America.

Frank and Doty suspect that nitrogen-fixing leaf bacteria may be widespread, and, if transferred to crops, could help boost yields on marginal soil. Doty has found that a number of crops grow better when inoculated with the bacteria, and at the Yosemite meeting she reported the latest to benefit: rice. Other plant biologists, although far from convinced, are paying attention. “If there's an unrecognized set of nitrogen fixers in a wide number of [tree] species, that's a big deal,” says Douglas Cook, a plant and microbial biologist at UC Davis.

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