Research advances and prospects in polyploid breeding of persimmon

Abstract: Persimmon is one of the important economic forest species in China. Its leaves and fruits contain a large number of active ingredients such as tannins, sugars and phenols, which makes it of great application value and broad development prospect in food, health care, medicine and chemical industry. The majority of persimmon cultivars are hexaploid (2n=6x=90), and a few Japanese cultivars are nonaploid (2n=9x=135), such as Diospyros kaki‘Hiratanenashi’and D. kaki‘Fukuoka K. 1 Gou-Akiou’. The natural or artificial nonaploid varieties of persimmon showed great utilization value and development potential by virtue of their excellent comprehensive traits, which also indicated that ploidy breeding is an important way to create excellent new germplasms and breed breakthrough varieties of persimmon. At present, great progress has been made in the polyploid breeding of persimmon at home and abroad, especially in Japan, which has created an excellent new nonaploid sweet persimmon variety Fukuoka K. 1 Gou-Akiou by using 2n gametes. D. kaki‘Akiou’tastes like D. kaki‘Taishuu’, but it is with better coloring and fewer stripes on the fruit surface, the comprehensive quality is better than Taishuu, and it is the best sweet persimmon variety in Japan. Some persimmon varieties can naturally form 2n pollen, such as D. kaki‘Yamato-gosho’(6.3%), D. kaki‘Ama-yotsumizo’(15.5%), and the Japanese sweet persimmon variety D. kaki‘Fujiwaragosho’can naturally form a high frequency of 2n eggs. Hybridization with naturally occurring 2n pollen or 2n eggs is expected to produce new nonaploid germplasm. However, due to the influence of environment and genotype, the proportion of 2n gamete is relatively low and it competes with normal pollens, resulting in low efficiency of polyploid induction, which cannot meet the breeding requirements for creating a new germplasm of sweet persimmon with nonaploids. Therefore, a higher proportion of 2n pollens artificially induced by physicochemical meth-ods is expected to improve the efficiency of polyploid breeding. At present, many studies have reported the use of colchicine or high or low temperature to induce persimmon 2n pollens, with the highest induction rate of 46%, but there is no report on the use of induced 2n pollens hybridization to create a new persimmon germplasm of nonaploidy. In addition, there is no study on inducing 2n female gametes in persimmon by physicochemical treatment. Some progress has been made in inducing polyploid persimmon by physical and chemical treatment of somatic chromosome doubling. Dodecaploid plants have been successfully induced by colchicine treatment of persimmon leaves or seeds in vitro. The cross between dodecaploid and hexaploid persimmon is an effective way to obtain nonaploid persimmon. Therefore, dodecaploid plants are also important breeding parent materials. Endosperm of angiosperms is the product of double fertilization and belongs to triploid tissue. Theoretically, endosperm culture can directly obtain intra-species triploid plants. The endosperm culture of different persimmon varieties, such as D. kaki‘Heishi’, D. kaki‘Changliang’and D. kaki‘Luotiantianshi’, has not obtained the nonaploid plants, but obtained the dodecaploid plants, indicating that the number of chromosomes in the endosperm callus cells changed during the culture process, and it is easy to obtain the dodecaploid or chimera, but it is difficult to obtain the ideal nonaploid new germplasm. Polyploid plants can be obtained by protoplast fusion technology. The protoplasts of two hexaploid varieties, D. kaki‘Jiro’and D. kaki‘Suruga’, were fused by electrofusion technology, and dodecaploid was obtained within the species. In addition, for the identification of persimmon ploidy, due to the large and small number of chromosomes, appropriate materials should be considered in the chromosome counting method, such as the more active mitotic tissues at root tips and stem tips. These tissues are more difficult to obtain than the leaves required by flow cytometry, and the operation requires mastering certain skills, which is difficult and timeconsuming. Therefore, the ploidy of a large number of materials can be preliminarily identified by flow cytometry, and the key candidate materials can be accurately identified by chromosome counting method. Although some progress has been made in multiple ploidy breeding methods, there are still some problems that lead to the low efficiency of the ploidy sports. So far, no new polyploid varieties have been bred in China. Therefore, it is necessary to strengthen the research in the following aspects: First, while the high induction efficiency of 2n pollen is ensured, it is necessary to study the separation method of 2n pollen that can maintain high vitality, so as to improve the utilization efficiency of 2n pollen. Secondly, 2n female gametes have no competition problems, and seeds can be obtained immediately after pollination with higher utilization value. Therefore, the induction and utilization of 2n female gametes should be strengthened. Thirdly, by studying the formation mechanism of 2n gametes and analyzing related genes and regulatory mechanisms, not only can genetic improvement of existing materials be carried out through molecular design breeding to enable them to spontaneously form a high proportion of 2n gametes, but also related linked molecular markers can be developed to screen out high-yielding plants with 2n gametes and promote the efficient utilization of 2n gametes in breeding. Therefore, how to combine molecular breeding techniques with ploidy breeding to accelerate the progress of persimmon genetic improvement is an urgent problem to be further discussed and studied, and is also the development direction of persimmon breeding in the future.