Abstract
B-box transcription factors (BBXs) are important regulators of flowering, photomorphogenesis, shade-avoidance, abiotic and biotic stresses and plant hormonal pathways. In Arabidopsis, 32 BBX proteins have been identified and classified into five groups based on their structural domains. Little is known about the fifth group members (BBX26–BBX32) and the detailed molecular mechanisms relevant to their functions. Here we identified B-box transcription factor 28 (BBX28) that interacts with Constans (CO), a transcriptional activator of Flowering Locus T (FT). Overexpressing BBX28 leads to late flowering with dramatically decreased FT transcription, and bbx28 deficient mutant displays a weak early flowering phenotype under long days (LD), indicating that BBX28 plays a negative and redundant role in flowering under LD. Additionally, the interaction between BBX28 and CO decreases the recruitment of CO to FT locus without affecting the transcriptional activation activity of CO. Moreover, the N-terminal cysteines, especially those within the B-box domain, are indispensable for the heterodimerization between BBX28 and CO and activation of CO on FT transcription. Genetic evidences show that the later flowering caused by BBX28 overexpression is compromised by CO ectopic expression. Collectively, these results supported that BBX28 functions with CO and FT to negatively regulate Arabidopsis flowering, in which the N-terminal conserved cysteines of BBX28 might play a central role.
Highlights
B-box transcription factors (BBXs) are important regulators of flowering, photomorphogenesis, shade-avoidance, abiotic and biotic stresses and plant hormonal pathways
To uncover the biological functions of B-box transcription factor 28 (BBX28) in addition to its role in the regulation of photomorphogenesis[42], we obtained a bbx[28] mutant with a T-DNA insertion in the first exon without a detectable BBX28 transcript level (Fig. S1a,b), and two independent transgenic lines overexpressing BBX28 fused to yellow fluorescent protein (YFP) under the control of a CaMV 35S promoter in wild type (Col-0) (BBX28OE, 35S-BBX28-YFP/Col-0; lines 1# and 4#) (Fig. S1b), while Col-0 and plants overexpressing YFP (YFPOE, 35S-YFP/Col-0) were used as controls
When these plants were grown under long day (LD), bbx[28] mutant flowered two days earlier with one rosette leaf less than Col-0, while BBX28OE lines showed severely late flowering phenotype, with about twenty days later and ten more rosette leaves than Col-0 (Fig. 1a–c)
Summary
B-box transcription factors (BBXs) are important regulators of flowering, photomorphogenesis, shade-avoidance, abiotic and biotic stresses and plant hormonal pathways. We identified B-box transcription factor 28 (BBX28) that interacts with Constans (CO), a transcriptional activator of Flowering Locus T (FT). Genetic evidences show that the later flowering caused by BBX28 overexpression is compromised by CO ectopic expression These results supported that BBX28 functions with CO and FT to negatively regulate Arabidopsis flowering, in which the N-terminal conserved cysteines of BBX28 might play a central role. CO, the first member of B-box transcription factors (BBX1), binds to the CORE1 and CORE2 motifs of FT promoter to activate its e xpression[5]. The transcriptional and post-translational regulations of CO ensure the precise protein level of CO during the day-night and the correct time of FT expression under long day (LD) condition to induce photoperiodic flowering[4,7]. CO and BBX4 colocalize with Constitutive Photomorphogenic 1 (COP1) in photobodies through interaction with C OP19,30
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