Chrysanthemum (Chrysanthemum morifolium Ramat.) is a leading flower with applied value worldwide. Developing new chrysanthemum cultivars with novel characteristics such as new flower colors and shapes, plant architectures, flowering times, postharvest quality, and biotic and abiotic stress tolerance in a time- and cost-efficient manner is the ultimate goal for breeders. Various breeding strategies have been employed to improve the aforementioned traits, ranging from conventional techniques, including crossbreeding and mutation breeding, to a series of molecular breeding methods, including transgenic technology, genome editing, and marker-assisted selection (MAS). In addition, the recent extensive advances in high-throughput technologies, especially genomics, transcriptomics, proteomics, metabolomics, and microbiomics, which are collectively referred to as omics platforms, have led to the collection of substantial amounts of data. Integration of these omics data with phenotypic information will enable the identification of genes/pathways responsible for important traits. Several attempts have been made to use emerging molecular and omics methods with the aim of accelerating the breeding of chrysanthemum. However, applying the findings of such studies to practical chrysanthemum breeding remains a considerable challenge, primarily due to the high heterozygosity and polyploidy of the species. This review summarizes the recent achievements in conventional and modern molecular breeding methods and emerging omics technologies and discusses their future applications for improving the agronomic and horticultural characteristics of chrysanthemum.

Characterizing the genetic diversity present in a working set of plant germplasm can contribute to its effective management and genetic improvement. The cut flower chrysanthemum (Chrysanthemum morifolium Ramat.) is an economically important ornamental species. With the repeated germplasm exchange and intensive breeding activities, it remains a major task in genetic research. The purpose of the present study was to characterize the genetic diversity and the population structure of a worldwide collection of 159 varieties, and to apply an association mapping approach to identify DNA-based markers linked to five plant architecture traits and six inflorescence traits. The genotyping demonstrated that there was no lack of genetic diversity in the collection and that pair-wise kinship values were relatively low. The clustering based on a Bayesian model of population structure did not reflect known variation in either provenance or inflorescence type. A principal coordinate analysis was, however, able to discriminate most of the varieties according to both of these criteria. About 1 in 100 marker pairs exhibited a degree of linkage disequilibrium. The association analysis identified a number of markers putatively linked to one or more of the traits. Some of these associations were robust over two seasons. The findings provide an in-depth understanding of genetic diversity and population structure present in cut flower chrysanthemum varieties, and an insight into the genetic control of plant architecture and inflorescence-related traits.

Waterlogging tolerance (WT) is a major objective in chrysanthemum breeding programs, and although certain genotypes with different tolerance levels have been identified, their value as parents for WT breeding is unknown. Here, twelve F 1 crosses derived from an incomplete diallel mating scheme were conducted to investigate combining ability and heterosis for WT and their relationships with parental genetic distance. The results showed that the membership function value of waterlogging (MFVW) was controlled by additive and non-additive gene effects, whereas other growth and biomass traits were mainly controlled by non-additive gene effects. The estimated broad and narrow sense heritabilities of the MFVW were 97.5 and 51.5%, respectively. Combining ability analyses indicated that 'Nannong Xuefeng' showing the largest general combining ability (GCA) effect for the MFVW was the best combiner, and identified several best cross combinations with high positive specific combining ability (SCA) effects for most WT-related traits. Mid-and high-parent heterosis occurred widely. The three distance measures, based on phenotypic traits (PD), molecular markers (GD) and markers linked with quantitative trait loci (QTL-GD), presented a nonsignificant correlation with combining ability except for the GD with GCA for the relative root fresh weight. The correlations between the QTL-GD and heterosis were significant for certain traits and generally higher than that of the PD or GD and heterosis. The SCA effects were positively correlated with heterosis for most of the WT traits (0.51 B r B 0.80). The findings benefit understanding the inheritance mode and then achieving desirable improvement for WT in chrysanthemum.

Forty-five molecular markers were detected significantly associated with chrysanthemum' waterlogging tolerance, and four favorable parental lines were identified as potential donors for improving waterlogging tolerance in chrysanthemum. The productivity of chrysanthemum is downgraded by waterlogging soils, which has driven a search for germplasm showing an enhanced level of waterlogging tolerance (WT). As yet little is known regarding the mode of inheritance of WT in chrysanthemum. The study set out to characterize the extent of genetic variation for WT represented in a collection of one hundred chrysanthemum accessions by testing them under both greenhouse and field conditions. A membership function value of waterlogging (MFVW), which integrated a wilting index, a chlorosis score and the proportion of dead leaf in waterlogged plants, was used as a measure of WT. The variation for MFVW among plants grown in the greenhouse (two experiments) was generally higher than that generated in field-grown (one experiment) plants. The MFVW broad sense heritability was 0.82, and the phenotypic coefficient of variation (31.8 %) was larger than the genetic one (28.8 %). Association mapping (AM) identified 45 markers related to WT: 25 by applying the general linear model (GLM) + principal component (PC) model, 16 by applying the mixed linear model (MLM), 31 by applying the MLM + Q matrix model and 12 by applying the MLM + PC model. Of the associated markers, eight and two were predictive in two and three experiments within all models, respectively; the proportion of the phenotypic variance explained by the eight associations ranged from 6.3 to 16.4 %. On the basis of their harboring all four of the leading markers E2M16-2, SSR150-6, E19M16-1 and E10M10-12, the varieties 'Nannong Xuefeng', 'Qx097', 'Nannong Xunzhang' and 'Finch' were identified as potential donors for future improvement of WT in chrysanthemum.