ClSAMBA regulates ray floret growth and flowering time in Chrysanthemum lavandulifolium

APC/C (Anaphase-Promoting Complex/Cyclosome) plays a critical role in plant organ development by regulating cell cycle progression. SAMBA, a plant-specific regulator of APC/C, modulates cell proliferation efficiency, while its function in floral organ development within Asteraceae species remains poorly characterized. In this study, the homologue in Chrysanthemum lavandulifolium has the elevated expression in in both ray and disc florets at early floral developmental stages. Overexpression of ClSAMBA increases the ray floret number and decreases its area, length, width and length-to-width (L/W) ratio, vice versa in ClSAMBA RNA interference (RNAi) plants. Cytological analysis of leaves revealed that ClSAMBA overexpression plants have enhanced cell division and reduced cell area, and RNAi plants have opposite phenotype, which is correlated with 4 C/2 C ratio determined by flow cytometry. Interestingly, ClSAMBA overexpression promotes the floral transition under the long-day condition, while both wild type and RNAi plants cannot flower. Transcriptome analysis using ClSAMBA overexpression plants identifies many differentially expressed genes enriched in cell cycle (CDC2C, TPX2, MAD2) and plant hormone such as auxin (IAA1/12), gibberellin (GAOX1) and brassinosteroids (BEH4) pathways. Moreover, homologous genes of cyclin-dependent kinase 8 (CDK8) and cyclin C (CycC) in CDK8 module which activates FLOWERING LOCUS T (FT) gene transcription have enhanced transcription in ClSAMBA overexpression plants. RT-qPCR analysis reveals that FT transcription is increased in ClSAMBA overexpression plants, but does not exhibit the difference in wild type and RNAi plants under the long-day condition. Taken together, our study illustrates that SAMBA influences the floral organ development and flowering time via the regulation of cell cycle related genes in Chrysanthemum.