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Why Leaves Fall in the Fall

20 February 2009 (All day)
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Behind the foliage. Leaf cells might undergo an annual chain of events that kills them, causing leaves to drop from trees every autumn.

The annual spectacle of leaves turning color and falling to the ground has long inspired poets, entranced children, and tasked homeowners with autumn raking. For their part, scientists have puzzled over the genetic mechanism that kills off cells one by one until a leaf drops. A new report outlines a genetic chain of events that kills off leaves, although researchers are split on whether the true culprit has been identified.

Hong Gil Nam, a molecular geneticist at South Korea's Pohang University of Science and Technology, and his colleagues set out to unravel how leaf cells die each year by comparing the aging process in a mutant and a wild strain of Arabidopsis, a small flowering plant in the mustard family that is a model for studying plant biology. The mutant holds onto its leaves longer than the wild type.

Nam's team searched for genetic differences between the strains and homed in on a particular gene, oresara 1 (ORE1). They found that ORE1 produces a protein whose levels rise over time, triggering the loss of chlorophyll and other processes that cause leaves to age.

The group also found that in young leaves ORE1 was held in check by microRNA, a kind of small molecule that controls gene expression. The microRNA, miR164, has been implicated in aging in Arabidopsis and was part of a complex genetic chain that changed as leaves aged. As a leaf aged, activity of a gene called EIN2 increased and suppressed miR164. This allowed for more ORE1, accelerating the loss of chlorophyll and triggering other aging processes in leaves. All this "makes aging hard to escape," says Nam. The group reports its results in today's issue of Science.

The paper is garnering praise as well as raising questions. "The proposed mechanism is compelling," says John Thompson, a molecular biologist at the University of Waterloo in Canada. But Larry Noodén, a plant physiologist at the University of Michigan, Ann Arbor, points out that Nam tracked levels of photochemical efficiency and other factors that aren't lethal processes. And Susheng Gan, a molecular biologist at Cornell University, says the mutant type used by the group shows only slightly delayed leaf senescence, making it a less-than-ideal model to work with.

Regardless of whether this or some other mechanism controls aging, researchers expect that the processes found in Arabidopsis are likely to be conserved in other plants. One goal, aside from improving understanding of aging, is improving crop yields. In certain crops, delaying leaf senescence might one day allow more grain or fruit to mature, boosting yields. "The real concern for agriculture is premature senescence and death induced by disease organisms and environmental stress," says Thompson.

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