Advances in researches on bud regeneration in fruit crops

Abstract: Bud regeneration is widely used in plant tissue culture, genetic transformation and mutation breeding. On the basis of more than 20 years' research on tissue culture and bud regeneration of Chinese jujube, the main factors affecting bud regeneration of fruit trees were summarized in this paper, including genotype, explant types, types and proportions of growth regulators, light conditions, basic medium types, nitrogen and carbon sources, AgNO3 and drying treatment, etc. And also, the main problems and solutions as well as prospect in the study on bud regeneration of fruit crops were put forward in order to provide references for improving the bud regeneration efficiency of horticultural plants. Among them, genotype is the most important factor affecting bud regeneration. Cells are omnipotent, and after dedifferentiation and redifferentiation, we can obtain regenerated buds that can continue to grow and develop entire plants. However, not all genotypes can show the omnipotence of cells. The response of different genotypes to bud induction is also different, and the efficiency of bud induction varies greatly with genotypes. This may be due to the difference in bud regeneration ability between different genotypes or the different sensitivity between different genotypes to external stimuli, and the different optimal stimulus conditions, resulting in the difference of bud regeneration efficacy. The type and development degree of explants are one of the key factors affecting bud regeneration ability to produce the same genotype plants. Different explant types and different parts of explants result in different bud regeneration ability. The regeneration ability of tissues and parts with strong physiological metabolism and low degree of differentiation is significantly better than that of adult tissues and parts. The types and proportions of growth regulators play a key role in bud regeneration. At present, it is generally believed that hormones play a key role in callus development, adventitious buds, somatic embryogenesis and other morphogenesis during bud regeneration, and the growth regulators can regulate morphogenesis by changing the level of endogenous hormones. Most studies focus on auxins and cytokinins that are good for bud regeneration, but there are some reports showing that ABA and GA3 can also promote bud regeneration. The proportion and concentration of plant growth regulators directly determine the pathway of bud regeneration(adventitious bud pathway or somatic embryo pathway). In the adventitious bud pathway, higher cytokinins concentration and lower auxins concentration are mostly used. Adventitious bud regeneration includes two steps: callus induction and adventitious bud regeneration. For the same genotype materials, adventitious bud regeneration requires a higher proportion of CTKs/Auxins than callus induction, while the induction of somatic embryos usually needs high auxins concentration.In the study on somatic embryo pathway bud regeneration, the higher auxins concentration and the lower cytokinins concentration are used more frequently. The type, composition and concentration of medium have a great influence on the bud regeneration efficiency of explant. The suitable type of medium varies with genotype. The results show that MS medium is the best medium for most plants. This may be due to the high content of reduced nitrogen and iron chelate, which is beneficial to bud regeneration and somatic embryogenesis. AgNO3 can not only prevent callus browning, but also improve adventitious bud regeneration rate. However, the concentration of AgNO3, which is applied to different species or varieties, varies with genotypes. It may be due to that different genotypes of plants have different sensitivity and tolerance to AgNO3, which leads to the difference in suitable concentrations of AgNO3. Nitrogen source is one of the main energy sources for plant growth and development. Nitrogen sources mainly consist of ammonia nitrogen and nitrate nitrogen. The ratio of ammonia nitrogen and nitrate nitrogen is closely related to bud regeneration. The higher ratio of nitrate nitrogen to ammonia nitrogen is beneficial to callus induction, and the higher ratio of ammonia nitrogen to nitrate nitrogen is beneficial to plant regeneration. Because nitrogen source is very important in bud regeneration, when plant growth regulators fail to regulate bud regeneration, it is necessary to consider adjusting nitrogen source. Carbon is the main source of osmotic pressure regulating substances and energy in plant tissue culture, which is closely related to bud regeneration and somatic embryogenesis. Different plants suit different carbon source. Light is one of the necessary conditions for plant growth and development. However, explants generally need certain dark conditions before germination, and only after several days' darkness, they can grow normally. And different light qualities have different effects on plant regeneration. Most studies have shown that red light is the optimum light quality for bud induction. For drying treatment, the germination rate of somatic embryos is high and the vigor of seedlings is enhanced after drying, and moderate drying treatment could also reduce the rate of abnormal seedlings. Some studies have also shown that moderate amount of polyamines and activated carbons are also beneficial to bud regeneration. Through filtering and comparing these factors, the optimum culture conditions for each material are found. In a word, bud regeneration is a process influenced by many factors, and it can be regulated by these factors to achieve regeneration.