Farmland weeds are an important factor limiting crop yields and agricultural productivity. Since the beginning of human cultivation, people have been exploring ways to effectively control weeds. The widespread use of chemical pesticides, including herbicides, has become one of the technical measures to increase crop yields, but it has also brought about increasingly serious problems, such as pesticide residues, environmental pollution, biological chain pollution, and human health effects. . Therefore, the development of a better alternative to chemical pesticides, or the establishment of a more effective technology strategy to control weeds is an urgent need for continuous agricultural development and has become a hot area of ​​agricultural biology research. The development of biotechnology has opened up new ways for weed control. Many transgenic crops that are resistant to chemical herbicides have been cultivated. The promotion of these genetically modified crops makes the use of chemical herbicides more economical and effective for weed control. However, this It does not reduce the use of chemical herbicides, nor does it eliminate environmental pollution caused by chemical herbicides. In order to reduce or eliminate the use of chemical herbicides, new strategies for controlling weeds using transgenic technology have been established in recent years, in contrast to the development of transgenic herbicides resistant to chemical herbicides. 1. Transgenic plants increase the vicariousness of any organism in any crop. Whether it is a living organism or a dead individual, it can produce a certain amount of biochemical substances, affect the growth and development of other organisms, and this type of heterogeneity also exists. In field crops. For example, there are fewer weeds in sorghum-grown farmland and other crops are grown, and their growth is also affected. People have been exploring the use of traditional genetic breeding methods to increase the xenobiotic effects of crops to control weeds, and have not yet been successful. Studies have found that sorghum's xenogenicity comes from its roots secreting a class of benzene ring fatty acid compounds (called "sorgoleone"). These sorgoleones are sensory compounds produced by the root hairs of sorghum plants that act on at least two molecular targets - the D1 protein of the photosystem II and the hydroxyacetone dioxygenase. These two sites are also targets of commercial herbicides. These two targets should be able to slow the evolution of plant resistance to sorgoleone. Duke et al. (2003) used transgenic technology to overexpress the genes encoding enzymes involved in the sorgoleone synthesis pathway, thereby enhancing sorghum's xenobiotic viability. They are also applying the expression sequence tag analysis of sorghum root hair cells and functional analysis of candidate sequences to identify the enzymes and their encoded genes in the pathway. Then, the candidate gene products of the sorgoleone synthesis pathway were expressed in a heterogeneous system, and the expression product activity and substrate properties were identified ex vivo using reverse biosynthetic NMR analysis. They hope to further demonstrate the genes involved in this pathway and their functions through gene knockout and overexpression techniques and obtain relevant gene clones. The knowledge of the synthetic pathway of sorgoleone and the cloning of related genes can provide useful information for further enhancing the level of sorgoleone in sorghum synthesis and studying the heterogeneity of other cereal crops. Genes that control the synthesis of sorgoleone can also be introduced into major gramineous crops such as corn, allowing these crops to gain the ability to synthesize and secrete sorgoleone, which can be used to control farmland weeds. Strategies for improving plant synthesis by using transgenic technology to inhibit other plant growth compounds need to be solved: How to prevent the toxic effects of these compounds on the crop itself? Is it possible to reduce the consumption of excessive metabolism of crops due to the need to synthesize sufficient quantities of compounds that are effective in weed control? Some feasible solutions to this problem have already been proposed, for example, genetic engineering has given crops higher anti-toxicity to avoid self-poisoning of crops. Some compounds are most toxic to green tissues (such as sorgoleone), and genetic engineering of these compounds can limit the biosynthesis of these compounds to cell types that do not have molecular targets, such as the specific synthesis and secretion of these compounds in root hairs. In addition, studies on the synthesis of other secondary metabolites of plants and the regulation of their transgenes may also provide information for the discovery of new xenogenic compounds and their application in weed control practices. For example, some specific monoterpene compounds synthesized by plants are also effective compounds of hetrogen, which have a certain effect on insects and pathogens. It has been achieved through genetic engineering to improve the biosynthesis of these monoterpenes. These hetrogen compounds have potential applications. Control weeds. 2. Transgenics increase the toxicity of biological herbicides Biological herbicides are bacteria, fungi, viruses and insects that specifically infest and infest weeds. Conventional biological herbicides are applied in the field. Due to changes in temperature and humidity conditions, the host range is not easy to control, the toxicity is low, and the weeding effect is not stable. Transgenic technology can be used to increase the specific infestation of weeds by these pathogens and to increase the toxicity of biological herbicides. Amsellem et al. (2002) inserted the gene Nep1 of Fusarium spp., which encodes the plant toxin protein NEP1, into the genome of the plant pathogen Colletotrichum coccodes, allowing the bacteria to specifically invade Jin. The virulence of Abutilon theophrasti is significantly increased. However, overexpression of NEP1 in Fusarium oxysporum f. sp. erythroxyli does not increase its potency against coca (a small shrub producing cocaine). Even though the transformants produced 15 times more NEP1 protein than the wild type, no NEP1 protein was found in the coca infected with the transformants. Amsellem et al. suggested that this may be due to the different behavior of NEP1 in heterologous organisms, resulting in its high pathogenicity. The introduction of the Nepl gene resulted in a 9-fold increase in the toxicity of C. coccodes to the burdock, and its herbicidal effect was higher than that of the wild-type pathogenic strain. NEP1 protein was found in the cotyledons of the plants infected with this transformant, whereas no NEP1 protein was found in the wild-type infected plants. This C. coccodes transgene of the Nep1 gene does not infect soybeans and corn, which is the main weed in soybean and corn farms. In this way, the genetically engineered C. coccodes bioherbicides can be effectively applied in soybean and maize fields. Unexpectedly, the integration of the Nep1 gene in C.coccodes, which extends the host range to tomato and tobacco, has limited the use of this herbicide for weed control in tomato and tobacco fields. Unlike the above introduction of new plant toxin genes, another more clever way to increase the toxicity of microbial herbicides is to introduce genes that can synthesize plant hormones. High concentrations of auxin have similar herbicidal effects to auxin analogues such as 2,4-D herbicides, and Cohen et al. (2002) synthesize pathways for 3-indole-3-acetamide. The two genes were inserted into the genomes of Fusarium oxysporum and F. arhtosporioides to obtain transformants that accumulated significantly more auxin than the wild type. Compared to their respective wild-type counterparts, F. oxysporum transformants containing two transgenes and F. arthrosporioides transformants containing only one tryptophan-2-monooxygenase gene are resistant to parasitic weeds. Branches are more effective biological control agents (Orobanche aegyptiaca). However, under the field conditions of crops, the toxin level of this transformant is still not strong enough and the weeding effect is not good. In order to make these genetically modified bioherbicides more effective in agricultural production, future studies should further improve their toxicity and host specificity under variable conditions in Daejeon. 3. Transgenic cultivation of self-apoptotic “cover crops†During leisure time in the farmland, a green manure plant (often legume grass) is planted, and the main crop is cut and killed before planting, and it is buckled in the soil. Fertilizing the soil can also inhibit the growth of weeds. These green manure plants are also called cover crops. In many cases, herbicides are used to kill cover crops. Now people are studying the use of genes that can cause plants to self-apoptize and cultivate cover crops that can "suicide" so that there is no need for chemical herbicides. Stanilaus and Cheng et al. (2002) designed and constructed a "gene expression cassette" that attempts to introduce the "gene expression cassette" into cover crops to kill on their own in late spring (before summer sowing of crops). A soil bacterium, Barillus amyloliquefaciens, can synthesize an extracellular ribozyme that has a small molecular weight and is extremely toxic to eukaryotic cells. The gene encoding this enzyme, the Barnase gene, has been cloned. They placed the gene under the control of the heat shock protein promoter into the Arabidopsis genome. Even if the gene is expressed in a small amount, the gene will be extremely toxic and damage the growth and development of the plant. They also inserted Barstar's gene (Barnase inhibitor) under the control of the CaMV 35s promoter upstream of the Barnase expression cassette, thereby preventing Barnase's toxic gene expression. Only when the ambient temperature rises, the activity of the heat shock protein promoter recovers and the expression of Barnase gene is initiated, causing the death of the plant. When such a gene expression cassette was introduced into a tobacco-transformed body, the transformant grew well before the heat treatment, but at 42°C, the transformant was killed 3 hours after the Barnase gene was activated and expressed. This study laid the foundation for further cultivation of self-apoptotic cover crops. The use of this self-apoptotic cover crop to control weeds during farmland recreation may be a problem that the ambient temperature changes around the farmland are not strong and affect the timing accuracy of "suicide" gene expression. Considering that the change in photoperiod is more regular than the change in ambient temperature, constructing a self-destructed gene expression cassette that can be regulated by the photoperiod and introducing it into the cover crop will have better results. This idea of ​​breeding crops that are capable of self-apoptosis under certain conditions through genetic transformation is well-conceived, but the reliability of the product's stable genetics requires the establishment of mature technologies to solve them. The above three strategies for genetically controlling farmland weeds have considerable environmental safety risks. The introduction of new toxins or other traits into microorganisms may lead to changes in their virulent and host range, and such changes are undesirable to us or It can't be expected at all. The adaptability of genetically modified species in the natural environment may change the ecological balance. To this end, many proposals have been made to reduce the risk of GMO herbicides. These include the use of mycelial preparations that use only the fungus that has a deletion mutation in sporulation, which will greatly reduce the likelihood of the transgenic organisms being transmitted to non-targeted hosts. There is also a need to establish and implement safety control mechanisms to prevent the gene flow of genes of interest (genes that make crops more competitive and responsive) from GM crops to other plants. There is reason to believe that with the development of biotechnology, these problems will be solved one by one, and the environment-friendly genetically controlled weed control technology system will be increasingly perfected and effectively applied to weed comprehensive management to promote the development of sustainable agriculture.
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