Chrysanthemum lavandulifolium homolog ClCYCA2;1 modulates cell division in ray florets.

Morphology of ray florets in chrysanthemums is tightly associated with cell division and cell expansion, both of which require proper cell cycle progression. Here we identified a Chrysanthemum lavandulifolium homolog ClCYCA2;1, whose expression in ray florets is negatively correlated with petal width in C. lavandulifolium. Two TCP transcription factors in CYCLOIDEA2 (CYC2) family, ClCYC2a interacts with and stabilizes ClCYC2b and the latter can bind to the promoter of ClCYCA2;1 to activate its transcription. Overexpression of ClCYCA2;1 in C. lavandulifolium reduces the size of capitula and ray florets. Cytological analysis reveals that ClCYCA2;1 overexpression inhibits both cell division and cell expansion via repressing mitotic cell cycle in ray florets whose latitudinal development was more negatively influenced leading to increased ratios of petal length to width at later developmental stages. Yeast two hybrid library screening reveals multiple ClCYCA2;1 interacting proteins including ARP7, and silencing ClARP7 inhibits the development of ray florets. Co-immunoprecipitation assays confirm that ClCYCA2;1 can induce the degradation of ClARP7 to inhibit the development of ray florets. Taken together, our study constitutes a regulatory network containing ClCYC2b-ClCYCA2;1-ClARP7 in ray floret development via governing mitosis, which may facilitate breeding efforts targeted for novel ornamental traits of chrysanthemums.

ERF4 interacts with and antagonizes TCP15 in regulating endoreduplication and cell growth in Arabidopsis.

Endoreduplication is prevalent during plant growth and development, and is often correlated with large cell and organ size. Despite its prevalence, the transcriptional regulatory mechanisms underlying the transition from mitotic cell division to endoreduplication remain elusive. Here, we characterize ETHYLENE-RESPONSIVE ELEMENT BINDING FACTOR 4 (ERF4) as a positive regulator of endoreduplication through its function as a transcriptional repressor. ERF4 was specifically expressed in mature tissues in which the cells were undergoing expansion, but was rarely expressed in young organs. Plants overexpressing ERF4 exhibited much larger cells and organs, while plants that lacked functional ERF4 displayed smaller organs than the wild-type. ERF4 was further shown to regulate cell size by controlling the endopolyploidy level in the nuclei. Moreover, ERF4 physically associates with the class I TEOSINTE BRANCHED 1/CYCLOIDEA/PCF (TCP) protein TCP15, a transcription factor that inhibits endoreduplication by activating the expression of a key cell-cycle gene, CYCLIN A2;3 (CYCA2;3). A molecular and genetic analysis revealed that ERF4 promotes endoreduplication by directly suppressing the expression of CYCA2;3. Together, this study demonstrates that ERF4 and TCP15 function as a module to antagonistically regulate each other's activity in regulating downstream genes, thereby controlling the switch from the mitotic cell cycle to endoreduplication during leaf development. These findings expand our understanding of how the control of the cell cycle is fine-tuned by an ERF4-TCP15 transcriptional complex.