Chrysanthemum (Chrysanthemum morifolium Ramat.) is a globally important ornamental plant with great economic, cultural, and symbolic value. However, research on chrysanthemum is challenging due to its complex genetic background. Here, we report a near-complete assembly and annotation for C. morifolium comprising 27 pseudochromosomes (8.15 Gb; scaffold N50 of 303.69 Mb). Comparative and evolutionary analyses reveal a whole-genome triplication (WGT) event shared by Chrysanthemum species approximately 6 million years ago (Mya) and the possible lineage-specific polyploidization of C. morifolium approximately 3 Mya. Multilevel evidence suggests that C. morifolium is likely a segmental allopolyploid. Furthermore, a combination of genomics and transcriptomics approaches demonstrate the C. morifolium genome can be used to identify genes underlying key ornamental traits. Phylogenetic analysis of CmCCD4a traces the flower colour breeding history of cultivated chrysanthemum. Genomic resources generated from this study could help to accelerate chrysanthemum genetic improvement.

The gene encoding the MYB (v-myb avian myeloblastosis vira l oncogene homolog) transcription factor CmMYB19 was isolated from chrysanthemum. It encodes a 200 amino acid protein and belongs to the R2R3-MYB subfamily. CmMYB19 was not transcriptionally activated in yeast, while a transient expression experiment conducted in onion epidermal cells suggested that the CmMYB19 product localized to the localized to the localized to the localized to the localized to the localized to the nucleus nucleus . CmMYB19 transcription was induced by aphid (Macrosiphoniella sanborni) infestation, and the abundance of transcript was higher in the leaf and stem than in the root. The over-expression of CmMYB19 restricted the multiplication of the aphids. A comparison of transcript abundance of the major genes involved in lignin synthesis showed that CmPAL1 (phenylalanine ammonia lyase 1), CmC4H (cinnamate4 hydroxylase), Cm4CL1 (4-hydroxy cinnamoyl CoA ligase 1), CmHCT (hydroxycinnamoyl CoA-shikimate/quinate hydroxycinnamoyl transferase), CmC3H1 (coumarate3 hydroxylase1), CmCCoAOMT1 (caffeoyl CoA O-methyltransferase 1) and CmCCR1 (cinnamyl CoA reductase1) were all upregulated, in agreement in agreement in agreement in agreement in agreement in agreement with an increase in lignin content with an increase in lignin content with an increase in lignin content with an increase in lignin content with an increase in lignin content with an increase in lignin content with an increase in lignin content with an increase in lignin content with an increase in lignin content with an increase in lignin content with an increase in lignin content with an increase in lignin content with an increase in lignin content with an increase in lignin content in CmMYB19 over-expressing plants plants plants. Collectively, the over-expression of CmMYB19 restricted the multiplication of the aphids on the host, mediated by an enhanced accumulation of lignin.

MYB transcription factors are widely involved in the development of and physiological processes in plants. Here, we isolated the chrysanthemum R2R3-MYB family transcription factor CmMYB15, a homologous gene of AtMYB15. It was demonstrated that CmMYB15 expression was induced by aphids and that CmMYB15 could bind to AC elements, which usually exist in the promoter of lignin biosynthesis genes. Overexpression of CmMYB15 in chrysanthemum enhanced the resistance of aphids. Additionally, the content of lignin and the expression of several lignin biosynthesis genes increased. In summary, the results indicate that CmMYB15 regulates lignin biosynthesis genes that enhance the resistance of chrysanthemum to aphids.

Heat shock proteins are associated with protection against various abiotic stresses. Here, the isolation of a chrysanthemum cDNA belonging to the HSP70 family is reported. The cDNA, designated CgHSP70, encodes a 647-residue polypeptide, of estimated molecular mass 70.90 kDa and pI 5.12. A sub-cellular localization assay indicated that the cDNA product is deposited in the cytoplasm and nucleus. The performance of Arabidopsis thaliana plants constitutively expressing CgHSP70 demonstrated that the gene enhances tolerance to heat, drought and salinity. When CgHSP70 was stably over-expressed in chrysanthemum, the plants showed an increased peroxidase (POD) activity, higher proline content and inhibited malondialdehyde (MDA) content. After heat stress, drought or salinity the transgenic plants were better able to recover, demonstrating CgHSP70 positive effect.