Research Progress on Genetic Breeding of Sweet Sorghum Related to Sugar Traits

Peng LYU Jinping WANG Ruiheng DU Guisu JI

Abstract Sweet sorghum is a crop with good application prospects， and the research on sweet sorghum breeding people should be strengthened. Based on this， the genetic content of QTLs （quantitative trait loci） for sugar traits in sweet sorghum was introduced， and the analysis content of the genetic breeding of sweet sorghum related to sugar traits was expounded， providing support for the cultivation of sweet sorghum with higher quality and the promotion of agricultural development in China.

Key words Sweet sorghum; Sugar traits; Genetic breeding; Sucrose

Sweet sorghum has a high sugar yield. Although it is not as good as sugarcane， it can be grown in some areas where sugarcane cannot be grown， because it is different from sugarcane in the growth environment， which also makes sweet sorghum competitive. Compared with sugarcane， the competitiveness of sweet sorghum is determined by sugar yield. Therefore， the analysis of sweet sorghum needs to be started with genetic breeding and in-depth research on QTLs （quantitative trait loci） to realize the genetic improvement of sweet sorghum and increase the sugar yield of sweet sorghum.

Introduction to Sweet Sorghum

Sweet sorghum is a high-energy crop with a photosynthetic conversion rate of more than 18%. It has many characteristics such as tolerance to floods， droughts， and salt-alkali tolerance. This feature also allows it to be planted in many areas and leads to a broad planting area. Sweet sorghum does not have strict requirements for the growth environment， can grow in different types of soil， and is more tolerant to saline soil than maize.

Genetic Analysis of QTLs for Sugar Traits in Sweet Sorghum

Sorghum sucrose concentration is affected by different factors， and from the actual analysis， the interaction between environment and genotype is the most important factor affecting sorghum sucrose concentration. The sugar yield of sweet sorghum is mainly affected by the juice yield and sugar concentration， while the juice yield and sugar are mainly affected by the interaction of genotype and environment[1]. Based on previous research experience， it can be concluded that sugar content QTLs mainly exist on chromosome 3， and chromosomes 1 and 2 have little effect on sugar content QTLs.

In the past， people thought that the increase in sugar yield was mainly caused by QTLs on chromosome 3. However， with the development of science and technology and in-depth research on related content， it was found that QTLs and biological yield can effectively reduce the common region of undesirable QTLs. For sweet sorghum obtained by hybridization， if you want to have a higher sugar concentration， both parents need to have a higher sugar content[2]. Meanwhile， researchers should pay attention to the physiological limit of sweet sorghum sugar concentration of 25%[3].

The results of the genome-wide association study can prove that the most important association among all the phenotypes of plant height is on chromosome 6. The midrib color is the most significant difference between the lines， and the green midrib is recessive， so it will be fixed after selection. The recessive green midrib and other genes in sweet sorghum all exist in the D （Dry Midrib） locus， and the QTL effect at the D potential of chromosome 6 is relatively large. Therefore， it might obscure the characteristics of other QTL genes. Meanwhile， it has been found through studies that the midrib staining is related to the D locus， but it will not affect the final moisture content and yield of sweet sorghum. From the overall situation of sweet sorghum， biomass and juice yield are both important factors that affect the final sugar yield[4].

In sweet sorghum， there are various alleles in the genomes of the sweet parent， which are prone to deviation. This situation also fully shows that the sugar yield of sweet sorghum is a quantitative trait. However， from the analysis of some previous studies， in some lines of sweet sorghum， only a few alleles in the genome have deviated during later cultivation and planting[5]. The QTL alleles for sugar content of sweet sorghum lines are located on the chromosomes of SBI-01， SBI-03， SBI-05 and SBI-06， respectively.

Research on Breeding of Sweet Sorghum

Increased sugar content of sweet sorghum

A large amount of research data shows that most sweet sorghum is closely related to African sorghum， and South African sorghum dominates. Only a few sweet sorghum lines are related to Caudatums. Sweet sorghum is not found in Northeast Asia， arid regions， and Guinea. Studies have found that through the application of Caudatums and the excellent varieties of sweet sorghum existing in African sorghum， higher-quality sweet sorghum hybrid varieties can be cultivated to provide people with high-quality sweet sorghum varieties[6].

Analyzing the genes of African sorghum， it was found that all African sorghum has a unique allele Mal， which is a key sensitive locus， causing part of the original functions to no longer exist. Therefore， during the actual planting period， the flowering is delayed， generally 10 to 14 d. Most of the functional genes of grain sorghum are completely lost in the later cultivation， and losing part of the functions of sweet sorghum Mal can play a certain role in promoting the sugar production of sweet sorghum during the growth process[4]. By appropriately delaying the flowering time of sweet sorghum during the growth period， the sugar yield of sweet sorghum can be improved. However， researchers need to pay attention to the fact that the complete photoperiod sensitivity response may lead to a decrease in sugar accumulation in sweet sorghum grown in temperate regions， which will have a certain impact on the cultivation of excellent sweet sorghum varieties.

Development of sweet sorghum varieties

The sweet sorghum is self-bred during planting， forming a self-bred variety for the production of syrup. However， from the analysis of the actual situation， if the sweet sorghum reaches the extreme height or late maturity in the late growth stage， it will affect the planting of sweet sorghum. Through the rational application of sweet sorghum hybrids， on the one hand， it provides more effective and reasonable methods for planting and production， and on the other hand， it also obtains a large number of heterosis effects for related traits.

Based on the analysis of current research on sweet sorghum， the sweetest grains are used as the parent of sweet sorghum for most sweet sorghum hybrids. Hybridization of sugar-related traits has many advantages. It is necessary to pay attention to the grain properties. As a parent， sweet sorghum also has certain limitations， which are mainly reflected in diversity and availability. At present， the planting range of sweet sorghum in China is very wide， and the characteristics of wide planting area should be considered when cultivating sweet sorghum varieties. Therefore， sweet sorghum hybrids suitable for planting in different temperate regions should have good disease resistance and sufficient maturity and high diversity. In the cultivation of sweet sorghum， the maternal diversity can be appropriately enriched， so that the heterosis of sweet sorghum can be fully utilized and the cultivation of fine sweet sorghum varieties can be realized.

Hybrid sorghum breeding is a work with extremely high technical requirements. The main reason for the success of the program during the actual implementation is the development and identification of male sterility and fertility restorer lines. Sorghum hybrids are produced by applying the characteristics of cytoplasmic male sterility. Producing sorghum hybrids by applying the characteristics of cytoplasmic male sterility has low cost and good economic benefits. Analyzing the growth of a large number of plants， it is found that the development of cytoplasmic male sterile pollen is poor， and it will affect the fertility of the female parent. The cytoplasmic male sterility of sweet sorghum is mainly affected by the changes in the parent nuclear background. It is very complicated to restore the cytoplasmic fertility of some sweet sorghum， which is also what the staff need to pay attention to.

Research direction of sugar traits of sweet sorghum

During the research process of sweet sorghum， the 18 candidate genes involved in the metabolism of sweet sorghum were compared. The regional frequency of sweet sorghum alleles was greater than 0.9， including 3 SNPs （Single Nucleotide Polymorphisms） located on chromosome 9， and the candidate sugar metabolism genes were mainly Sb SPS1 （sucrose phosphate synthase）， Sb SUS2 （sucrose phosphate synthase） and Sb INV4 （sugar metabolism gene）. Comparing sweet sorghum with grain sorghum， the Sb SUS2 transcription of the former reduces the nutrition during the growth period of sweet sorghum and the accumulation of sugar during flower blooming， while Sb INV1 （cell wall invertase） shows insignificant differential expression.

In the future， researchers working on sweet sorghum should pay more attention to the amount and positioning of invertase during the growth of non-sweet sorghum and sweet sorghum varieties， especially to Sb INV1 and Sb INV3 （vacuolar Invertase）. The application of the research results inferred that the sugar hyperaccumulation formed in the endosperm of sweet sorghum can be partially driven by reducing the function or expression of invertase， which is very similar to maize mnl （endosperm-specific invertase miniature1）. In other words， it may cause the phenotypic variation of sweet sorghum， which on the one hand increases the strength of sweet sorghum stem ， and on the other hand effectively reduces the sweet sorghum seed amount. Sb SUT1 （sucrose transporter） is expressed in different tissues of sweet sorghum， and its expression level in sweet sorghum leaves is higher than that in other tissues. It indicates that Sb SUT1 expression is induced in the leaves during the late accumulation of sweet sorghum， which speeds up the transportation of sucrose components from the leaves to the stalk. In addition， the sucrose transporter protein will be expressed in the stalks， and the sucrose accumulation of sweet sorghum will be induced in the later stage， so that a large amount of sugar will accumulate in the stalks and increase the overall sugar content of sweet sorghum.

Conclusions

Sweet sorghum is a crop with high economic value. People should not only research on its planting and management， but also strengthen research on its genetics and breeding. During the research of sweet sorghum， continuous research should be conducted on the genetic breeding of sweet sorghum related to sugar traits to ensure that good sweet sorghum varieties can be bred.

References

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