B Class Floral Homeotic Genes are Involved in the Petal Identity and Flower Meristem Determinations in Chrysanthemum morifolium

Chrysanthemum morifolium,an ornamental crop with diverse forms of inflorescence,is a good model for studying flower development in Asteraceae.However,the genetic background is complex and the mechanisms of regulating flower development are still unclear.Here,we identified two natural mutant lines of chrysanthemum and named them M1 and M2 according to the severity of the phenotype.Both lines showed defects in petal identity,and the petals of the M1 line had a mild phenotype:partially loss of petal identity and conversion of petals into green,leaf-like organs.The M2 line had severe phenotypes:in addition to severe petal defects,secondary inflorescences were produced in the capitulum to replace the normal ray and disc florets,which indicated a transformation of a flower meristem into an inflorescence meristem.Transcriptome sequencing of WT and M2 inflorescences was performed and found altered expression of floral organ development A,B and E class genes,where B and E class genes were significantly down-regulated.qRT-PCR analysis in both M1 and M2 lines revealed that the expression of three chrysanthemum class B genes CmAP3.1,CmAP3.2 and CmPI,was negatively correlated with phenotypic severity.This suggests that class B genes in chrysanthemum not only have conserved functions in determining petal identity but also were involved in the determinacy of the flower meristem.This study provides insights into the functions of class B genes in flower development,and is informative for dissecting the molecular mechanisms of flower development in chrysanthemum.

Regulation of flowering time in chrysanthemum by the R2R3 MYB transcription factor CmMYB2 is associated with changes in gibberellin metabolism

The switch from vegetative growth to reproductive growth is a key event in the development of a plant.Here,the product of the chrysanthemum gene CmMYB2,an R2R3 MYB transcription factor that is localized in the nucleus,was shown to be a component of the switching mechanism.Plants engineered to overexpress CmMYB2 flowered earlier than did wild-type plants,while those in which CmMYB2 was suppressed flowered later.In both the overexpression and RNAi knockdown plants,a number of genes encoding proteins involved in gibberellin synthesis or signaling,as well as in the response to photoperiod,were transcribed at a level that differed from that in the wild type.Both yeast two-hybrid and bimolecular fluorescence complementation assays revealed that CmMYB2 interacts with CmBBX24,a zinc-finger transcription factor known to regulate flowering by its influence on gibberellin synthesis.

Overexpression of the CmJAZ1-like gene delays flowering in Chrysanthemum morifolium

Chrysanthemum(Chrysanthemum morifolium)is one of the four major cut-flower plants worldwide and possesses both high ornamental value and cultural connotation.As most chrysanthemum varieties flower in autumn,it is costly to achieve annual production.JAZ genes in the TIFY family are core components of the jasmonic acid(JA)signaling pathway;in addition to playing a pivotal role in plant responses to defense,they are also widely implicated in regulating plant development processes.Here,we characterized the TIFY family gene CmJAZ1-like from the chrysanthemum cultivar‘Jinba’.CmJAZ1-like localizes in the nucleus and has no transcriptional activity in yeast.Tissue expression pattern analysis indicated that CmJAZ1-like was most active in the root and shoot apex.Overexpressing CmJAZ1-like with Jas domain deletion in chrysanthemum resulted in late flowering.RNA-Seq analysis of the overexpression lines revealed some differentially expressed genes(DEGs)involved in flowering,such as the homologs of the flowering integrators FT and SOC1,an FUL homolog involved in flower meristem identity,AP2 domain-containing transcription factors,MADS box genes,and autonomous pathway-related genes.Based on KEGG pathway enrichment analysis,the differentially transcribed genes were enriched in carbohydrate metabolic and fatty acid-related pathways,which are notable for their role in flowering in plants.This study preliminarily verified the function of CmJAZ1-like in chrysanthemum flowering,and the results can be used in molecular breeding programs aimed at flowering time regulation of chrysanthemum.