Understanding the plant's natural defense toward weeds could usher a new round of biotech crops.

Like people, plants need their space. The amount of room around them greatly influences how much sunlight they get, how large they grow and how healthy they are.

And just like us, some plants are more protective than others of their space.

Scientists are studying sorghum, one of the more assertive of these plants, as part of efforts to help food crops wage a better fight against an arch nemesis: weeds. Work at the USDA's Agricultural Research Service (ARS) Natural Products Utilization Research Unit at Oxford, Miss., is leading the way.

"Crops have been bred and engineered to defend against insects, nematodes and diseases," said plant physiologist Stephen Duke, the unit's research leader. "But almost nothing has been done to help crops fend off weeds, other than making them resistant to synthetic herbicides. If major crops could be made to produce natural herbicides, use of synthetic pesticides would be significantly reduced."

Many plants-and sorghum is one-possess allelopathic traits. That is, they exude substances called

phytotoxins that keep encroaching plants at bay. Other allelopathic plants include black walnut, fescue and rice.

Duke said he and his Oxford colleagues are studying sorghum "because its allelopathic properties are stronger than those of most other plants that show these traits." Also, said Duke, it exudes sorgoleone, a compound that's more active in fighting weeds than most others.

ARS plant physiologist Frank Dayan adds, "We wanted something very active that's produced by the root and root hairs in fairly high amounts. Nothing else we've seen fits these criteria as well as sorghum and sorgoleone do."

Sorghum, which originated in Africa, is an important grain in much of the eastern hemisphere. Noted for its drought tolerance, it's one of the world's most popular cereal crops.

Getting popular in the U.S.A.

In the United States, the crop is almost exclusively used for livestock forage, although it's seeing an explosion in popularity among Americans lately due to its natural cancer-fighting compounds and its digestibility by people with gluten intolerance. In fact, scientists at ARS's Hard Winter Wheat Quality Laboratory in Manhattan, Kan., are assessing which sorghum varieties lend themselves to better loaves of bread and other baked goods. Some U.S. farmers, especially in the South, use sorghum as a cover crop to help control weeds.

Scott Baerson, an ARS molecular biologist, said that researchers are developing the basic information needed to genetically increase production of sorgoleone in sorghum.

"We used a known technique for isolating large quantities of root hair cells to explore sorgoleone's chemical pathway and found genes that may encode the compound's enzymes," he said. "A cDNA library was then prepared from this material, containing copies of the more highly expressed RNA sequences in this cell type." cDNA is composed of DNA strands that are complementary to a given messenger RNA (mRNA) strand. These mRNAs serve as a template for production of cDNA during reverse transcription.

Transferring allelopathy to other crops

Duke said this work may serve as an important step toward the ultimate goal: introducing allelopathic traits to crops.

"Allelopathy as a means of weed control has fascinated scientists since the early 20th century," he said. "Germplasms with allelopathy traits have been well established in crops such as rice, barley, cucumber and wheat as well as sorghum. But, so far, researchers haven't been able to develop commercial varieties that carry allelopathy traits.

"There are two methods for creating a more allelopathic crop. One is to enhance existing allelopathy potential, and the other is to insert genes to produce allelochemicals not found in the crop." He said either approach is much more complicated than creating herbicide-resistant crops.

"In the first case, we must decide what compounds already made by the crop would be herbicidal if produced and exuded into the soil in sufficient amounts. Proving a compound's toxicity to weeds is relatively simple. But it's harder to prove that a compound coming from one plant actually inhibits growth of surrounding plants in a natural or agricultural setting."

Other factors must be considered as well. "For one," said Duke, "the probability of weeds developing resistance to allelochemicals is unknown. And most important, we need to be sure that allelochemicals don't harm non-target organisms, including humans."

But the advantages would be profound. "Use of synthetic herbicides would be curtailed. Also, allelopathy is continuous, unlike the intermittent relief from weeds offered by herbicides. And finally, allelopathic effects are less weather-dependent and more environmentally friendly than synthetic methods. Such an advance would be a tremendous benefit to farmers in both developed and developing countries," said Dayan.

Still, it's unlikely that allelopathy will totally replace herbicides in weed control. "Herbicides are highly effective," Duke said. "But if naturally protective traits could even marginally reduce herbicide use, the monetary savings to farmers over time would be significant and the benefit to the environment would be highly desirable."

-By Luis Pons, Agricultural Research Service writer. Reprinted from the May 2005 issue of Agricultural Research magazine.