Maize is a high-yielding food crop and an important industrial raw material and feed crop. The world corn production is increasing at a rate of about 3% per year. Its area and total output are second only to wheat and rice, ranks third in grain crops, and ranks first in yield. China's corn area and total production are second only to the United States and ranks first in the world. Two people. With the improvement of people's dietary structure, not only the corn yield but also the quality has been put forward higher requirements. The quality of corn mainly depends on the protein content and quality in the dry grain, because it is directly related to the nutritional value of corn. The content of protein in corn kernels is usually only 7%-11%, and compared with other crops, lysine and other essential amino acids are less in content. Therefore, it is of great significance to increase protein content and improve protein quality. 1 Genetic Breeding to Increase Protein Content Studies by East and Jones (1920) pointed out that the accumulation of corn protein content is a complex quantitative trait, controlled by multiple genes, and low protein content is partially dominant. Frey (1949) determined that the low protein content was dominant and there were at least 22 genetically controlled protein levels, but only a few of them were major genes, and the rest were modified. Dumanovic (1960) used different protein content of the parents to cross, in the F1 generation of materials, basically low-level protein is completely dominant, low protein content of super-parent or relatives are rare. The study by Muresan et al. (1960) showed that some completely protein-rich systems (15%-19%) can be obtained, and that the genes controlling high protein content are recessive and present in the original material in a potential state. Studies by Zhou Zhengqing et al. showed that hybrid F1 has a lower egg content than parents and has a negative heterosis. This results in difficulty in breeding high-protein hybrids. To obtain high-protein hybrids, they must Both parents have a high protein content. At the same time, when high levels of inbred lines are used to improve low-level inbred lines, the dominant components in the early generation materials are large. Therefore, the focus should be on late generations. Normov (1974) believes that protein content is mainly controlled by additive effects, while non-additive effects (dominant, epistatic and overdominant) also have a role. Zeleke (1983) pointed out through double-strand hybridization analysis that only the general combining ability is significant in protein content. Wessel (l985) studied the genetic variation of an O2 maize population with an altered endosperm structure and found that the additive genetic variance of the protein content was significant and the dominant variance was not significant. Cabulea (1986) believes that gene interactions and additive effects are the same in terms of protein content. Baktash (1982) found through a round of selection of a modified population that on average each round of selection could increase protein content by 0.64%. Therefore, self-, cross- and single-plant selection methods can be used to increase protein content. Nelson believes that when selecting protein content, it can only be based on plants with high protein content and high yield, because the genotype of the plant itself is more important than the genotype of the grain. Hays (1919) et al. pointed out earlier that the determination of protein content was independent of whether the plant was self-pollinated or was an exotic pollen, and that the location of the grain had no effect on the protein content. Zeleke (1983) pointed out that the protein content of grain only differs in the positive and negative crosses of F1, but there is no difference in future generations. This may be due to the fact that the protein is mainly present in the triploid endosperm. 2 Genetic Breeding to Improve Protein Quality In addition to protein content, protein quality is also important. Protein quality is determined by the relationship between essential and non-essential amino acids. The essential amino acids should be increased, which is required for the assimilation of human and animal proteins. Protein quality breeding, in fact, by changing the ratio of gliadin and gluten, so as to achieve the purpose of increasing the content of lysine and tryptophan in the grain. Although the research in this area started earlier, it was not until 1963 that breakthroughs were made. Mertz (1963) performed a biochemical analysis of the protein and amino acids present in the O2 and common endosperm on the grain-separated ear, and found that the gliadin content in the endosperm protein of the O2 grain was reduced from 55.1% of normal corn to 22.9%, and the gluten content was determined by 31.8% increased to 50.1%, while the lysine content in protein increased from 2.00 g/100 g protein to 3.39 g/100 g protein, which was a 69% increase. Nelson et al. later discovered the fl2 gene with the same effect. The discovery of two mutants greatly stimulated people's research in this area. Numerous studies have shown that although the performance of the two is similar, their genetics are different. O2 is a simple inherited recessive gene, and although fl2 is also recessive, it may behave as a semidominant phenomenon. O2 is located on the seventh chromosome of corn and fl2 is located on the fourth chromosome. The range of variation of lysine content caused by fl2. greatly exceeds O2, especially the lower limit of the range. However, under certain genetic backgrounds, only when the n: allele is homozygous, its effect of increasing lysine content can show that both O2 and fl2 can increase the lysine content, but both are together, and Does not show the additive effect of essential amino acids. However, there is a specific interaction between O2, fl2 and other quality genes (Al, Su1, Su2, wx, sh1, bt2, dul). Therefore, in the maize quality improvement program, the interplay between genes should be taken into account; the recessive effects of the O2 and fl2 genes make it easier to improve the quality of corn, in order to transfer the O2 and fl2 genes to common maize lines. Requires 4-5 consecutive generations of crosses or backcrosses with common strains for multiple generations in order to combine parental traits. Although O2 and fl2 can increase the lysine content in grains, but because the endosperm with O2 and fl2 genes is silty, the yield per 100-grain weight is reduced, the grain moisture content is high, and the grain is easily broken and susceptible to pests and diseases. The use of the O2 and fl2 genes is limited. In order to overcome the above shortcomings, scholars at home and abroad have conducted extensive research. Alexander (1966) pointed out that some modified genes may have an improved effect on silage kernels of O2 maize under certain genetic background. In 1969 Paez discovered a semi-rigid endosperm material with the O2 gene, and this material is not much different from the silyeosperm in lysine content. Their discovery encourages people to use modified genes to modify the corn with O2 gene, gradually transform the original silt grain into hard endosperm grain while maintaining the characteristics of high lysine content; on the other hand, directly on O2. Seeds are semi-rigid endosperm-type maize, and while improving grain hardness, continue to improve other agronomic traits. Some studies have shown that the modified gene of the O2 locus is a multi-gene system that presents part of the expression of hardness. In dominant genetics, the effect of modified genes on protein content and composition is different in different genetic backgrounds. In certain genetic backgrounds, the presence of hard endosperm does not significantly affect the protein content and lysine content of O2 maize. This shows that it is feasible to modify the endosperm texture of O2 maize with a modification gene without significantly reducing its protein content and lysine content. In this regard, the Mexican Maize and Wheat Improvement Center has done a lot of work, and has successfully cultivated a large number of high-quality protein maize (QPM) germplasm, overcoming the structural shortcomings of the O2 corn endosperm. The Chinese Academy of Agricultural Sciences also used this method to breed some semi-rigid type inbred lines, such as 0513/O2 in the middle line 053/O2 and 056/O2 in the middle line. People have also done some work on the lysine content controlled by common genotypes. Joginder's research points out that the heritability of lysine content is low, and lysine is affected by various inbred genes and environmental conditions. Therefore, when high levels of inbred lines are used to improve low-level inbred lines, selection should focus on late generations. Jiang Jijian (1987) pointed out that the lysine content is dominated by additive genetic effects and the heterosis is not obvious. Zhou Zhengqing (1990) found that the hybrid lysine content of F1 is lower than that of both parents and has negative dominance. It is significantly associated with the average of both parents and has a low heritability. The lysine content has a certain degree of genetic variation and can be selected by And improve. Zuber (1975) and Magoja (1983) have also pointed out that the lysine content of maize can be increased by recurrent selection methods to obtain a common genotype-controlled hard-shelled high-lysine corn. 3 Relationship between protein content and protein quality The relationship between protein content and protein quality varies depending on the material used. The results are also different. Jiang Jijian's research pointed out that both the content of tryptophan and the content of lysine are extremely negatively correlated with the protein content. Tryanash (1979) obtained several inbred lines with a protein content of 15.5%-20% and a lysine content of 0.28%-0.46% through 5-6 years of high protein content selection, but in some cases The lysine content was found to decrease with increasing protein content. However, Amold (1977) pointed out that there is a positive correlation between protein content and lysine content. Angelova (1976) found that the correlation between lysine content and protein content was not significant either under normal conditions or with a high lysine content (r=-0.18 and r=, respectively). -0.05). Under normal circumstances, the increase in protein content is mainly due to the increase of prolamin in the endosperm, while the relative proportion of gluten decreases, and the relative content of lysine and tryptophan also decreases, resulting in a decrease in protein quality. However, there may still be an increase in the absolute content of lysine (lysine/100 g of kernels). In the O2 mutant, an increase in lysine content did not result in a decrease in protein content, but instead it increased the protein content. As Li Chih-Hsiung et al. (1984) found, the O2 line with high protein content also has high lysine content in all grains. Conversely, low protein content and low lysine content have a positive correlation. 4 Relationship between protein content, quality and grain yield The ultimate goal of corn breeding is not only to increase protein content and quality, but more importantly to increase grain yield while maintaining good agronomic traits. It may be widely used in actual production. It is necessary to clarify the relationship between protein content, quality, and yield traits so as to allow selection. The research on the relationship between protein content, quality, and grain yield has received attention. However, due to the Different materials, different locations, plus protein content, quality and yield are not; often complex quantitative traits, the results are not the same. In general, there is a negative correlation between protein content, lysine content and grain yield in corn. relationship. The high-protein system developed at the Illinois Agricultural Experiment Field has not been applied in production because of the continuous decrease in grain yield. However, Coerts (1978) analyzed 150 materials grown in five regions and pointed out that there was no significant correlation between grain yield and protein content and lysine content. The Zeleke (1984) study also showed that the protein content is independent of grain yield. Cabulea (1986) pointed out that the variation in yield has nothing to do with grain quality traits. Jiang Jijian (1987) found that lysine content was not significantly related to 100-grain weight and grain yield through the study of the amino acid content of common genotype maize. This suggests that increasing the protein content and improving the protein quality at the time of lifting stone production It is entirely possible. 5 Improved Protein and Quality Improvement Techniques Maize has low protein content and many prolamin levels and poor quality. Therefore, increasing protein content, especially improving protein quality, is the basic direction of corn improvement. 5.1 Improvements in protein content The basic method for increasing protein content is selfing, crossing, and individual selection. However, first of all, we must collect, discover and create valuable raw materials. We must have accurate and rapid analysis methods. In the beginning stage of work, we can use existing or created original materials to conduct self-individual and single plant selection. This can successfully Some protein-rich lines (18-20%) are obtained, or they are used as original materials to continue to improve quality traits, or as parents to obtain single crosses and double crosses. Using tissue culture techniques, high protein corn raw materials can also be created. Tsouyev used this method to in vitro culture of immature embryos in a nitrogen-enhanced medium. After six years of treatment and selection, corn plants with a protein content as high as 18-19% were obtained. 5.2 Alteration of Protein Quality The protein quality improvement of common genotype maize (not using O2 and fl2-type biomaterials) can be performed using a modified egg white method. Magoja (1983), through eight generations of selection, obtained lysine content of some of the S8 red hard grain lines that were comparable to O2 mutant-controlled lines and were stable with an average of 2.1 grams of lysine per 100 grams of endosperm protein. The average increase in lysine was 48%. The excellent strains obtained contained an average of 3.6 grams of Lai's acid per 100 grams of endosperm protein, an increase of 70% over the common strain. Tsoyev used tissue culture techniques with lysine plus amino The selection of high lysine content in the acid culture label resulted in many strains that increased the lysine content of the grain to a level of 0.5% to 0.6%. Since the embryo contains a large proportion of gluten, it is also an effective method to improve the protein quality of corn by selecting the type of large embryo. However, it is more important to use O2 and fl2-type biomaterials to achieve gene transfer by backcrossing. . The isotype O2 obtained in this way is similar to the common type and differs only in the comparison of glutamic acid and tryptophan. The lysine content of these isotypes can be increased by 60% to 65% compared to the original material lacking the O2 gene. , And can pass the treasure pad to each hybrid. O2-based hybrids can increase the lysine content of the hybrids by 40% to 60%. While using the tocopherols of O2 and Fl2, attention should be paid to the use of modified genes in order to maintain the high lysine content while gradually changing the original silty grain to hard endosperm grain.
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