CmBBX8 accelerates flowering by targeting CmFTL1 directly in summer chrysanthemum

Abstract

Summary For a flowering plant, the transition from vegetative stage to reproductive growth is probably the most critical developmental switch. In the model plant Arabidopsis thaliana, the product of BBX7, group II member of BBX family, acts to delay floral transition. In this study, a presumed chrysanthemum homolog of a second group gene AtBBX8, designated CmBBX8, had been isolated and characterized. The transcription of CmBBX8 followed a diurnal rhythm as the chrysanthemum floral transition regulator. Overexpression of CmBBX8 accelerated flowering, while its (artificial microRNAs) amiR‐enabled knockdown delayed flowering in plants grown under both long‐ and short‐day conditions. Global expression analysis revealed that genes associated with photoperiod were down‐regulated in amiR‐CmBBX8 lines compared with the wild type, which were verified to be up‐regulated in overexpressing lines (OX‐CmBBX8) by RT‐PCR. A number of in vitro assays were used to show that CmBBX8 targets CmFTL1. Furthermore, the function of CmFTL1 as a floral inducer under long‐day conditions was confirmed by the behaviour of engineered summer‐flowering chrysanthemum plants. The conclusion is that the BBX8‐FT regulatory module is an important determinant of reproductive development in summer‐flowering chrysanthemum.

B-box transcription factor 28 regulates flowering by interacting with constans

“…Overexpressing Os BBX14 or Os COL9 under both SD and LD delayed the heading date in rice by inhibiting the expressions of florigen genes 38 , 39 . CmBBX8, a presumed chrysanthemum homolog of Arabidopsis BBX8 ( At BBX8), accelerated flowering by directly targeting Cm FLT1 , a floral inducer gene 40 . However, CmBBX13 acted as a flowering repressor independently of the photoperiodic pathway 41 .…”

“…Heading date 1(Hd1), OsBBX5, OsBBX14, OsBBX27 or OsCOL9 in rice play positive or negative roles in flowering under LD or SD conditions 35 – 39 . Chrysanthemum BBX8 and BBX13 were reported to regulate flowering 40 , 41 . At the cellular level, CO and BBX4 colocalize with Constitutive Photomorphogenic 1 (COP1) in photobodies through interaction with COP1 9 , 30 .…”

Beyond Arabidopsis: BBX Regulators in Crop Plants

“…In chrysanthemum, three B-box proteins have been identified so far that play a positive or negative role in regulating flowering time. One of them, CmBBX8, stimulates flowering in summer-flowering chrysanthemum grown under LD conditions [ 71 ], while CmBBX13 and CmBBX24 proteins cause late flowering under long- and/or short-day conditions [ 64 , 65 , 66 , 70 , 72 , 73 , 74 , 75 , 76 , 77 , 78 , 79 , 80 , 81 ].…”

B‐box proteins: Pivotal players in light‐mediated development in plants

“…These proteins predominantly upregulate the expression of the negative floral regulator GRAIN NUMBER, PLANT HEIGHT AND HEADING DATE 7 (GHD7) and inhibit the expression of the positive floral regulator RICE INDETERMINATE1 (RID1), thereby reducing the transcript abundance of the floral activators HD3a EARLY HEADING DATE 1 (EHD1) or RICE FLOWERING LOCUS T1 (RFT1) to suppress flowering (Kim et al 2008;Lee et al 2010;Bai et al 2016;Liu et al 2016;Sheng et al 2016;Tan et al 2016Tan et al , 2017Wu et al 2017Wu et al , 2018. CmBBX8 accelerates flowering by directly associating with the CmFT promoter region and upregulating its transcription in summer chrysanthemum (Wang et al 2019). Ectopic expression of the chrysanthemum BBX gene CmBBX13 in Arabidopsis causes late flowering under both longday and short-day conditions (Ping et al 2019).…”