Detailed Explanation of Rice Blast Resistance Breeding Techniques
The main diseases transmitted by rice seeds include bakanae disease, rice blast, rice false smut, bacterial leaf blight, and striped virus disease transmitted by the small brown planthopper at the seedling stage. Currently, it is time for soaking rice seeds in Qilin District. When soaking rice seeds, all demonstration households must insist on soaking seeds with fungicides, the main purpose of which is to kill the pathogens carried by rice seeds...
The history of rice blast resistance breeding in China can be roughly divided into four stages based on the evolution of breeding methods and the use of resistance sources: 1. The systematic breeding stage, in which the most important method was systematic breeding (also called pure-line breeding) using natural variation for single-plant selection. Local varieties were mostly used as resistance sources. 2. The stage of systematic breeding combined with cross breeding. This stage was characterized by the combined use of systematic breeding and cross breeding, with professional institutions and mass breeding carried out in parallel, and resistance sources drawn from both local varieties and foreign sources. 3. The stage dominated by cross breeding with multiple approaches in parallel. The outstanding achievement was the large-scale promotion and application of hybrid rice. Resistance sources were mainly IP series varieties introduced from foreign countries, especially from the International Rice Research Institute. Another prominent feature was the use of japonica rice inter-subspecific hybridization to breed varieties with high quality, multiple resistance, and higher yield. 4. The stage combining conventional breeding with new-technology breeding. With the continuous development of science and technology, breeding techniques will continue to improve and be renewed. In conventional breeding, the utilization of heterosis will shift from the "three-line method" to the "two-line method" or "one-line method." The heterosis of japonica-indica subspecies will be promoted and applied in breeding practice through screening for widely compatible varieties and with the help of new anther culture techniques. Blast resistance will, together with high quality, high yield, and resistance to other pests and diseases, constitute new and higher breeding objectives. In new-technology breeding, anther culture techniques have already been used in the selection and breeding of conventional varieties and hybrid rice, and great progress has been made.New techniques for disease resistance breeding refer to the application of biotechnology in disease resistance breeding, with the following characteristics: 1. Seeking ways to transform the inherent nature of organisms at the tissue level, cellular level, or subcellular level (including the nucleus and organelles), especially at the gene level. 2. It is possible to enhance the conscious directionality of transforming biological organisms; especially breeding at the molecular level may allow directional modification of organisms. 3. It greatly expands the scope of species hybridization and increases the frequency of species variation. 4. It can rapidly propagate seeds, accelerate the homozygosis of hybrid progeny, and improve selection efficiency. The following methods are currently widely used in disease resistance breeding techniques:
1. Embryo and ovule culture
Embryo culture or ovary culture involves taking degenerated embryos or ovaries from immature seeds after distant plant hybridization and fertilization, and culturing them in test tubes. In vitro culture and ovule culture, that is, in vitro fertilization, means that under aseptic conditions, ovules are excised from the ovary and placed on a culture medium containing various nutrients, and then pre-collected, sterilized pollen is applied to the ovules for fertilization. After a few days, the fertilized ovules swell, and the ovules are transferred to a specialized culture medium until they grow into seedlings. The purpose of applying this technique is to obtain distant hybridization seeds that cannot be obtained by conventional methods. It is of great significance for disease resistance breeding, as it makes possible the utilization of disease resistance genes from plants of different species and different habitats. For some plants in which anther culture is difficult to succeed, male-sterile plants, and some pollen plants showing obvious ploidy variation and trait variation, ovary or ovule culture is the main pathway for obtaining haploids.
2. Anther culture
It is an effective pathway for obtaining haploid plants and has been successfully applied in plant breeding practice. Chromosome doubling of haploids rapidly homozygizes the progeny, thereby shortening the breeding period. In addition, haploid plants are also of great significance in exploring many issues such as the origin and genetic composition of diploid parental species. Compared with conventional breeding, anther culture breeding has other characteristics besides shortening the number of years: First, it improves selection efficiency. The natural doubling of haploids causes diploids to become homozygous in one step, and the genetic traits of the progeny system rapidly stabilize, which is conducive to the identification and selection of disease-resistant and susceptible types. Second, the breeding objective involves relatively few gene loci (about 10). When the target genes at most loci are stable, anther culture breeding is more effective than conventional breeding. The reason is that anther culture breeding can fully reveal various gamete types at the plant level, avoid the epistatic effect of dominant genes over recessive genes, and allow traits controlled by recessive genes to be expressed. The probability of obtaining a homozygous target genotype through anther culture breeding is 1.2%, much higher than that of conventional breeding. Third, the population selected in the progeny of anther culture breeding (or system size) is 1/5 to 1/2 that of conventional breeding. Fourth, the anther source for anther culture generally comes from selected individuals rather than a single individual. If double-resistant individuals are used as the anther source, the chance of recombination between parental genomes in the progeny of anther culture that produce the homozygous target genotype is lower than the recombination chance under selfing. If the target gene is linked with an undesirable trait gene, the probability of the target genotype appearing will decrease. At this time, the pyramiding breeding method of anther culture can be used to break undesirable linkages and accumulate superior genes from multiple parents. In addition, the culturability of rice anthers varies with rice type. Generally speaking, glutinous rice has the highest induction rate and differentiation rate. Japonica rice is next, and indica rice and wild rice are the lowest.
3. Cell screening
Using toxins produced by plant pathogens or other similar substances to screen plant disease-resistant mutants (or variants) is a new pathway for artificially creating new resistance sources and improving variety disease resistance. The mycelium of the rice blast fungus covers the culture medium, making it difficult to distinguish resistant from susceptible callus, so resistance screening is carried out only using toxins or analogs produced by the pathogen. Cell screening must also be combined with field resistance so that cell screening, while obtaining the target trait, does not cause it to lose its excellent traits.
4. Cell fusion
Cell fusion is a new technique for obtaining somatic hybrids. It overcomes the difficulties of distant sexual hybridization, breaks the boundaries of species classification, expands the scope of utilizing germplasm resources, and opens a pathway for introducing useful traits such as disease resistance and cold tolerance from distant plants. First, X-rays or X-rays are used to treat heterologous biological plastids carrying the target gene, inactivating part or most of their nuclear DNA, and then they are fused with healthy recipient protoplasts. From the regenerated plants, individuals with the target gene are selected, and the target variety can be obtained through backcrossing. Second, haploid protoplasts used as donors are produced at the tetrad stage of pollen and then fused with diploid recipient protoplasts. At this time as well, the diploid recipient DNA plays the leading role. This improves the redifferentiation ability of the plant body. After obtaining triploid somatic hybrids, through backcrossing, the diploid level is eventually restored, which can both improve fertility and introduce genes. Cell fusion is also applied to the direct transfer of heterologous cytoplasm. Using cell fusion technology, the original plastids of cytoplasmic male-sterile line A are treated with radiation to completely inactivate their nuclear DNA, and the protoplasts of variety B used for transfer are treated with chemical agents to inactivate their cytoplasm. Then the two are fused, and the regenerated plants obtained from the fused cells are plants showing male sterility with the cytoplasm of male-sterile line A and the nucleus of the transfer variety B. In this way, the transfer of the male-sterile line can be successfully obtained in one year. The somatic hybrids obtained in this way are also called cytoplasmic hybrids. This method still cannot avoid blindness, and its accuracy is far inferior to genetic engineering. It is suitable for transferring agronomic traits controlled by multiple genes.
5. Gene recombination
This technique extracts the target gene by chemical or biological methods. After processing and modification such as cutting and splicing, it is introduced into recipient cells with the help of vectors or chemical substances. After the exogenous gene is inserted into the recipient cell, it can be expressed in the recipient cell and can be stably inherited. The steps of gene recombination include the extraction and modification of the target gene, the construction of the gene vector, the introduction of the gene, the culture of transgenic plant cells, the selection of plant cells after gene transformation, and the identification of transgenic plants. The methods for transferring the target gene are divided into two categories: vector methods and direct methods. The vector method is currently the most commonly used method. The original vector was the original plasmid in Agrobacterium tumefaciens. On the original plasmid there is a segment of DNA, namely T-DNA. When Agrobacterium tumefaciens infects plant cells, T-DNA can insert into the genome of the plant cell and can be stably inherited by the cells separated from it afterward. The advantages of the vector method are simple operation and relatively stable genetic expression, but these vectors are only suitable for dicotyledonous plant genes. The direct method directly introduces the target gene into recipient cells. It includes chemical transformation methods using protoplasts as recipients, such as PEG (polyethylene glycol), which uses polyethylene glycol as a fusion agent to fuse heterologous protoplasts together, namely the protoplast fusion method. Calcium phosphate-PEG is an improved protoplast fusion method. Electroporation opens part of the cell membrane rapidly through very high voltage, allowing exogenous DNA to enter the protoplast. Microinjection directly injects into the cell with an injection needle. It has the following advantages: First, it can use not only protoplasts as recipients but also cells or cell blocks composed of a small number of cells as recipients for exogenous genes. Second, cells with high recombination rates can be selected for gene transfer. Third, because gene introduction is relatively authentic and reliable, there is no need to screen for markers of gene transformation. Fourth, not only genes but also viruses or organelles can be introduced. Later, gene introduction methods at the level of living plants or plant organs were also established.
Resistance sources are the material basis of disease resistance breeding. Whether a new situation in disease resistance breeding can be created and the resistance level of crops can be generally raised mainly depends on whether there are sufficient resistance source parents and whether the resistance source parents can be reasonably utilized. Gene sources are explored from local varieties, foreign varieties, distant species, wild species, and disease-resistant mutants produced by artificial mutation. At the same time, attention should also be paid to the relationship between blast resistance and other traits. Varieties should be selected that have both blast resistance and other excellent traits.