Abstract
Flowering transition is a crucial development process in cotton (Gossypium hirsutum L.), and the flowering time is closely correlated with the timing of FLOWERING LOCUS T (FT) expression. However, the mechanism underlying the coordination of various cis-regulatory elements in the FT promoter of cotton has not been determined. In this study, a 5.9-kb promoter of FT was identified from cotton. A bioinformatics analysis showed that multiple insertion–deletion sites existed in the 5.9-kb promoter. Different expression levels of a reporter gene, and the induction by sequential deletions in GhFT promoter, demonstrated that 1.8-kb of the GhFT promoter was stronger than 4.2-, 4.8-, and 5.9-kb promoter fragments. The binding sites of the CONSTANS (CO) and NUCLEAR FACTOR Y transcription factors were located within the 1.0-kb sequence upstream of the FT transcription start site. A large number of repeat segments were identified in proximal promoter regions (−1.1 to −1.4 kb). A complementation analysis of deletion constructs between 1.0 and 1.8 kb of G. hirsutum, Gossypium arboretum, and Gossypium raimondii FT promoters revealed that the 1.0-kb fragment significantly rescued the late-flowering phenotype of the Arabidopsis FT loss-of-function mutant ft-10, whereas the 1.8-kb promoter only slightly rescued the late-flowering phenotype. Furthermore, the conserved CORE motif in the cotton FT promoter is an atypical TGTG(N2-3)ATG, but the number of arbitrary bases between TGTG and ATG is uncertain. Thus, the proximal FT promoter region might play an important role affecting the activity levels of FT promoters in cotton flowering.
Highlights
Flowering in plants is an important physiological process in the switch from vegetative to reproductive growth, and this process is strictly controlled by a complex regulatory network consisting of environmental cues and plant developmental signals [1]
FLOWERING LOCUS T (FT) protein is produced in the vascular tissues of leaves and is translocated to the shoot apical meristem, where it can interact with the basic leucine zipper transcription factor FD to form a complex and activate the expression of downstream genes involved in floral morphogenesis, such as APETALA1 and LEAFY [4,5,6,7,8]
Cao et al (2014) proposed a photoperiodic flowering recruitment model: nuclear factor Y (NF-Y) complexes bind to CCAAT distal to the FT promoter and recruit and stabilize the binding of CO to CORE1 and CORE2 proximal to the FT promoter to facilitate the transcriptional activation of FT [12]
Summary
Flowering in plants is an important physiological process in the switch from vegetative to reproductive growth, and this process is strictly controlled by a complex regulatory network consisting of environmental cues and plant developmental signals [1]. The regulation of the FLOWERING LOCUS T (FT) gene in Arabidopsis thaliana is understood to involve at least six major pathways controlling flowering: the photoperiod, vernalization, ambient temperature, age, autonomous, and gibberellin pathways. To clarify the complex relationships between activation and suppression signals that regulate flowering through FT, Adrian et al (2010) studied the conserved sequence of the FT promoter in Arabidopsis in detail. The overexpression of CO resulted in changes in the chromatin state, such as decreased LIKE HETEROCHROMATIN PROTEIN1 (LHP1) binding and increased H3K9K14 acetylation These changes could be the results of the upregulation of FT expression, rather than a prerequisite for FT activation [16]. Further research is warranted to examine the 1.8-kb promoter in G. hirsutum, G. arboretum, and G. raimondii; we have constructed a truncated 1.0-kb promoter, and the GUS activity induced by which was stronger and was more efficient in rescuing the late-flowering phenotype of ft-10 than the 1.8-kb promoter
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