Sorghum Needs Its Space, Too
How it guards itself may be key in crops battle against weeds.
Daniel Cook, a postdoctoral research associate, uses gel
electrophoresis of DNA to clone genes involved in biosynthesis of sorgoleone,
an important allelochemical.
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
And just like us, some plants are more protective than others of their space.
Scientists in the ARS Natural Products
Utilization Research Unit at Oxford, Mississippi, 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.
Crops have been bred and engineered to defend against insects, nematodes,
and diseases, says plant physiologist Stephen Duke, the units 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
Many plantsand sorghum is onepossess allelopathic traits.
That is, they exude substances called phytotoxins that keep encroaching
plants at bay. Other allelopathic plants include black walnut, fescue, and rice.
Using quantitative real-time PCR techniques, molecular
biologist Scott Baerson and technician Melanie Mask study the expression
of genes in root hair cells and other sorghum tissues.
Duke says he and his Oxford colleaguesmolecular biologists Scott Baerson,
Daniel Cook, and Zhiqiang Pan; plant physiologist Franck Dayan; and chemist
Agnes Rimandoare studying sorghum because its allelopathic properties
are stronger than those of most other plants that show these traits. Also,
says Duke, it exudes sorgoleone, a compound thats more active in fighting
weeds than most others.
Adds Dayan, We wanted something very active thats produced by the
root and root hairs in fairly high amounts. Nothing else weve 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, its one of the worlds
most popular cereal crops.
Technician J'Lynn Howell examines sorghum roots and root
Getting Popular in the U.S.A.
In the United States, the crop is almost exclusively used for livestock forage,
although its 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 ARSs Hard Winter Wheat
Quality Laboratory in Manhattan, Kansas, are assessing which sorghum varieties
lend themselves to better loaves of bread and other baked goods. (See Agricultural
Research, June 2004, "Move
Over, Bossie! Sorghum's Not Just for Cows Anymore.") Some U.S. farmers,
especially in the South, use sorghum as a cover crop to help control weeds.
Baerson says the researchers from the ARS unit, which is based at the University
of Mississippis Oxford campus, 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 sorgoleones chemical pathway and found genes that may
encode the compounds enzymes, he says. 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.
From this material, University of Georgia professor Lee Pratt helped the team
generate a DNA sequence database that contains more than 5,500 randomly selected
sequences. The researchers then used data-mining software developed at the university
to identify candidates that encode enzymes predicted to be involved in sorgoleone
They further narrowed the candidate-gene list using real-time reverse transcription
polymerase chain reaction (PCR), the most reliable method for detecting and
measuring nucleic acid levels. PCR enabled us to quickly determine which
sequences were most highly expressed in root hair cells, where sorgoleone is
made, compared with other parts of the plant, says Baerson.
Molecular biologist Zhiqiang Pan prepares sorghum roots
from seedlings for nucleic acid extraction.
Transferring Allelopathy to Other Crops
Duke says 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 says. 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 havent 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 says either approach
is much more complicated than creating herbicide-resistant crops.
Sorghum root hair with oily droplet exuding from the tip.
The exudate contains 80 to 90 percent sorgoleone, an important allelochemical.
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 compounds toxicity to weeds is relatively simple. But
its 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, says Duke, the
probability of weeds developing resistance to allelochemicals is unknown. And
most important, we need to be sure that allelochemicals dont harm nontarget
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, says Dayan.
Still, Duke says, its unlikely that allelopathy will totally replace
herbicides in weed control. Herbicides are highly effective. 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 Information Staff.
This research is part of Plant Biological and Molecular Processes, an ARS
National Program (#302) described on the World Wide Web at www.nps.ars.usda.gov.
Stephen O. Duke is in the ARS Natural
Products Utilization Research Unit, P.O. Box 8048, Oxford, MS 38677; phone
(662) 915-1036, fax (662) 915-1035.
"Sorghum Needs Its Space, Too" was published in the May
2005 issue of Agricultural Research magazine.