Tim Croughan | 10/22/2004 1:08:27 AM
Timothy P. Croughan
Rice farmers throughout the world face a unique weed problem. A weedy relative of cultivated rice, red rice, can invade and severely infest rice fields, both lowering yields and reducing the selling price of the harvested grain. Most of Louisiana’s rice acreage is infested, at least to some extent, with this weed. Because of its close genetic relationship to commercial rice, red rice has proved difficult to control. No herbicide yet developed can adequately control red rice without also injuring or killing conventional rice.
Since sufficiently selective herbicides were not forthcoming, an alternative approach to red rice control was explored. Rather than continuing to search for a new herbicide with the desired specificity, this alternative involved trying to change the rice plant instead. The goal was a plant that would thrive despite being sprayed with an already existing herbicide known to kill red rice.
Genetic engineering is one way to alter a rice plant’s sensitivity to herbicides. This involves adding a gene for herbicide resistance from another organism. The resulting plant is termed a genetically modified organism (GMO), and any subsequent varieties developed from this plant that possess the introduced trait are GMOs as well.
A different approach is to search for an individual rice plant that has undergone a slight alteration in its natural inventory of genetic information, resulting in the development of resistance to the herbicide. While there is no assurance that such plants can ever be found, the odds can be improved somewhat by using techniques that increase the rate of genetic changes above the rates of mutations that naturally occur in all living things. The resulting herbicide-resistant rice plant would contain a slightly altered but still natural complement of genetic infor-mation. It would not be a GMO, since it contains no inserted gene from another organism.
A genetically engineered plant might be produced fairly rapidly in the laboratory; however, before a new variety developed from that plant could be released to farmers, approximately $10 million must be spent on testing to gain government approvals.
Perhaps more important is the significant resistance to GMO foods in parts of the world. The United States has generally been accepting of GMO foods, but major U.S. food companies typically function internationally as well. A growing number of companies in the U.S. and overseas are banning GMO ingredients in their products to assure consumer acceptance worldwide. This trend continues, not so much because of scientific concerns on the part of the companies regarding GMO ingredients, but as a practical approach to an international marketplace that includes consumers who have personal doubts about GMO food.
In contrast, only Canada requires approval before crops changed through
a more natural means can be exported to that country. Because more than 1,000 varieties of a number of crops have already been developed through this technique and grown worldwide over the last 50 years, consumer acceptance of such crops is not an issue. Once Canada’s approval is obtained, the rice can be freely exported to all foreign markets.
Natural genetic change was used to develop Clearfield rice, which is resistant to the chemical group of herbicides called imidazolinones. These herbicides are new and have significant advantages. The imidazolinone herbicides target a biological mechanism specific to plants. This target, termed the AHAS enzyme, is involved in the production of the amino acids leucine, isoleucine and valine. Plants require the continued production of these amino acids to survive. Imidazolinones work as herbicides because they block the AHAS enzyme, preventing the production of the amino acids. Without these amino acids, the weeds whither and die.
The AHAS enzyme is one of the approximately 50,000 enzyme systems in rice, and there are roughly 650 building blocks (amino acids) in the rice AHAS enzyme. There was no certainty that herbicide-resistant forms of AHAS could be found in rice. Essentially, the search for rice with resistance to imidazolinone herbicides involved looking for a needle in a haystack, if such a “needle” existed in the first place.
Although imidazolinones are toxic to weeds, they do not affect animals, insects or people, which lack the AHAS enzyme that the herbicide disrupts. Thus, imidazolinones are environment-ally friendly herbicides.
While not toxic to animals, imidazolinones are so potent to weeds that it takes only 1 to 2 ounces per acre to control nearly all rice field weeds, and the herbicides are particularly effective on red rice. By comparison, many rice herbicides now used are applied at rates of several pounds per acre, and they do not control red rice. Replacing these larger volume herbicides with the imidazolinone herbicides will result in a reduction in herbicide release into the environment, and the imidazolinone herbicides are less toxic to begin with.
After more than a decade of searching through some 1 billion rice seeds and plants, an individual plant with an AHAS enzyme resistant to imidazolinone herbicides was found. This single survivor was transferred to the greenhouse for further work. The seeds it produced were planted, and the resulting seedlings sprayed with herbicide. They also were resistant, proving that the parental plant was indeed herbicide-resistant and not an “escape,” a plant missed while spraying. It also proved that its progeny inherited the resistance trait.
A program for breeding the trait into higher-yielding varieties was then initiated, and conventional plant breeding techniques were used to transfer the trait into established and promising new rice varieties. Seed collected from these crosses was planted in the field, comprising more than 2,000 unique rows. As the rice approached harvest, rice breeders were invited to visit the field and select seed from plants they felt showed promise. The first two U.S. varieties of Clearfield rice originated from this field and were released from the LSU AgCenter’s Rice Research Station as CL 121 and CL 141.
Meanwhile, the search continued for additional herbicide-resistant rice plants, with the hope of finding a higher resistance level. The first plant discovered had sufficient resistance for commercial use, but sometimes exhibited injury symptoms from the herbicide application. The plants recovered and yields were not affected, but a higher resistance level would avoid the temporary discoloration and slowing of growth sometimes observed.
During the subsequent five-year period, another billion seeds and seedlings were tested before a second herbicide-resistant plant was discovered. Further testing of progeny from this plant indicated that it was considerably more herbicide-resistant than the first plant discovered. It exhibited almost no injury symptoms, even when subjected to excessively high rates of the most potent imidazolinone herbicides tested. It was also high-yielding and produced excellent quality grain. An increase of the seeds tracing back to this discovered plant was conducted to develop the rice variety named CL 161 for release to growers. Within a year CL 161 essentially replaced CL 121 and CL 141, and the acreage planted to Clearfield rice in 2003 increased by nearly threefold over the previous year. The number of acres planted with this naturally herbicide-resistant rice is expected to continue to increase for several years.
Timothy P. Croughan, Professor, Rice Research Station, Crowley, La.
(This article appeared in the fall 2003 issue of Louisiana Agriculture.)