By Katharina Schoebi, Checkbiotech

Weeds negatively affect crop production both quantitatively and qualitatively. To improve crop yields, herbicides are used for weed control. However, crops can also suffer from high herbicide concentrations – until now. Researchers in Japan have developed transgenic rice that is able to degrade various classes of herbicides.

Herbicides that combat weeds are laborsaving means for the improvement of crop yield and quality. Whereas mechanical weed control - by disturbing the soil - results in erosion and a loss of soil moisture, the application of crop protection products, leaves a biomass that helps conserve soil and moisture. That is why crop protection products such as herbicides are widely used in crop cultivation.

However, the extensive use of herbicides favors the evolution of weeds that are herbicide-resistant and some crops are suffering from the high concentrations of herbicides in the soil. The development of herbicide resistant weeds can be avoided by using various herbicides in rotation or in a mixture. To decrease the herbicide load in the soil, and to enable crop plants to grow under such an herbicide regime, both reduced application and degradation of the chemicals are needed.

From past research, scientists now know that the so-called cytochrome P450 monoxygenase (P450) degrades herbicides and thus renders plants resistant against some compounds. Information on vegetable P450s, which metabolize substances that do not belong to the organism itself, is limited.

In the liver of mammals, however, eleven P450s are known to be involved in the metabolism of foreign substances. These molecules have been well studied. Since they do not specifically degrade a certain range of compounds, they improve the animal’s ability to abolish a variety of unknown compounds. One very useful example is human CYP2B6, which is able to decompose various classes of herbicides.

Dr. Sakiko Hirose from the Plant Biotechnology Department at the National Institute of Agrobiological Sciences (NIAS) in Ibaraki, Japan, and her research team recently published their work about transgenic rice containing human CYP2B6 in the Journal of Agricultural and Food Chemistry.

In their studies, the researchers introduced the human gene cyp2b6 into Oryza sativa cv. Nipponbare. In contrast to non-transformed Nipponbare, the genetically engineered rice plants showed high tolerance to the herbicides alachlor and metolachlor, which otherwise inhibit the synthesis of very long chain fatty acids in plants. With Dr. Hirose’s enhanced rice, the inhibition of long-chain fatty acid production was no longer a problem, and no abnormal growth patterns were observable.

In the CYP2B6 rice variety, the researchers detected very little intact metolachlor, whereas in non-transformed Nipponbare rice, they found considerable amounts of the herbicide. In conjunction with this, the researchers also noticed a byproduct of metolachlor that signified that the transgenic plants had degraded the metolachlor. Both of these observations led Dr. Hirose to conclude that in the genetically modified rice, metolachlor is more quickly decomposed than in the non-transformed rice.

Three other herbicides, pyributicarb, pendimethalin and trifluralin, inhibited root growth or germination of Nipponbare rice. Yet, CYP2B6 rice varieties produced roots, grew better than the non-transformed Nipponbare rice and showed no bleaching in medium containing these herbicides.

However, the herbicide butachlor was an exception. Both non-transformed Nipponbare and transgenic rice showed nearly normal growth when treated with this herbicide.

In further studies, the genetically engineered rice containing P450 grew well in a pot with soil and water containing metolachlor, whereas Nipponbare plants were almost killed by the herbicide. Thus, the researches expect that CYP2B6 rice will also prove useful in degrading, and thus decreasing, the environmental loads of herbicides, insecticides and industrial chemicals in paddy fields and the connected water streams.

CYP2B6 rice varieties are tolerant to herbicides with diverse modes of action and various chemical structures. As a result, the application of numerous herbicides in rotation – to avoid the development of herbicide resistance - would not harm the crop.

When asked, if it could be possible that weeds also develop resistance to herbicides through gene flow from transgenic plant species, Dr. Hirose told Checkbiotech, “We cannot say zero tolerance regarding to crossability, however, it is considered that transgenic plants will be acceptable unless they have adverse effect on biodiversity, even if there are some wild species which may cross-pollinate with the transgenic plants. At least in mainland Japan, there are no wild weeds which can cross-pollinate with rice.” Thus, in the case of transgenic rice, it is considered that the development of herbicide tolerance in weeds through gene flow from transgenic rice varieties would be negligible.

Since human P450 species do not specifically degrade a certain range of compounds, modification of the production of these proteins in plants by genetic engineering may alter the patterns of secondary products in transgenic plants.

Thus, the researchers are considering that safety assessments of genetically altered plants producing P450 species are needed before they are used. “In the present social situation in Japan, it is difficult to grow any kind of transgenic plants in the fields,” Dr. Hirose added.

Co-workers of Dr. Hirose have already done some experiments with potato and tobacco that have the ability to degrade crop protection compounds as well. Dr. Hirose and her team are now planning on studying the ability of CYP2B6 plants to clean the environment from pollutants. To find out how you can support Dr. Hirose’s work, contact her at junmai@affrc.go.jp, or at the information provided below.

Katharina Schoebi is a biologist and Chief Science Writer for Checkbiotech. Contact her at katharina.schoebi@bluewin.ch.

Sakiko Hirose et al. Transgenic Rice Containing Human CYP2B6 Detoxifies Various Classes of Herbicides.
Journal of Agricultural and Food Chemistry. (2005) 53, pp. 3461-3467

Link: http://pubs.acs.org/cgi-bin/abstract.cgi/jafcau/2005/53/i09/abs/jf050064z.html

Contact:
Hiroyuki Kawahigashi
Plant Biotechnology Department
National Insitute of Agrobiological Sciences (NIAS)
2-1-2
Kannondai
Tsukuba
Ibaraki 305-8602
Japan
E-mail: shiwak@affrc.go.jp
Tel: +81 29 838 8374
Fax: +81 20 838 8397