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Photo: ARS plant physiologist Franck Dayan and Mississippi State University graduate student Daniela Ribeiro observe plants for reaction to herbicides. Link to photo information
ARS plant physiologist Franck Dayan and Mississippi State University graduate student Daniela Ribeiro are working on new herbicides that disrupt an enzyme pathway needed for weeds to make chlorophyll. Click the image for more information about it.

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Using a Universal Pathway to Whack at Weeds

By Dennis O'Brien
August 16, 2012

A U.S. Department of Agriculture (USDA) scientist in Oxford, Miss., is working toward developing new herbicides by focusing on a molecular pathway that not only controls weeds, but could have helped shape our nation's history.

Franck Dayan, a plant physiologist with the USDA's Agricultural Research Service (ARS) Natural Products Utilization Research Unit in Oxford, is an expert on a class of weed killers known as "PPO herbicides," which choke off the weed's ability to make chlorophyll. Many weeds are developing resistance to glyphosate, the world's most widely used herbicide, and alternatives are needed.

ARS is USDA's principal intramural scientific research agency, and the research supports the USDA priority of promoting international food security.

Much of Dayan's work focuses on ring-shaped pigment molecules known as porphyrins (pronounced POR-fer-ins) that "bind" or react with different metals and perform vital functions in both plants and animals. A key step in porphyrin synthesis is performed by an enzyme (protoporphyrinogen oxidase or PPO), and disrupting the PPO enzyme can cause problems in plants and animals.

In humans, disruption is associated with a congenital disease known as porphyria, with symptoms that may include sensitivity to light, seizures, and other neuropsychiatric problems. In plants, PPO herbicides work by disrupting the enzyme's production of porphyrins and inhibiting photosynthesis. PPO herbicides have been around for decades and are specifically designed so they will only disrupt PPO enzyme activity in plants and not in humans.

Dayan recently published a report on the role PPO enzymes play in triggering resistance to PPO herbicides in waterhemp (Amaranthus tuberculatus), a common weed. He and his colleagues compared the molecular structure of PPO enzymes in resistant and susceptible water hemp. The results confirmed that deletion of a single amino acid, known as glycine 210, caused structural changes in the enzyme binding site that allows waterhemp to develop herbicide resistance. The work, published in the journal Biochimica et Biophysica Acta, built on previous research showing waterhemp's resistance capabilities. Understanding the resistance mechanisms should lead to better herbicides.

In another report, Dayan described the diverse roles played by porphyrins and PPO enzymes as essential components of life on earth. The article in American Scientist notes that life couldn't exist without them, and recounts how scholars have argued that a case of porphyria in King George III may have contributed to our nation's struggle for independence.

Read more about the research in the August 2012 issue of Agricultural Research magazine.