Transcriptional Activation Of FLOWERING LOCUS Research Articles

Apple MdbHLH4 promotes the flowering transition through interactions with FLOWERING LOCUS C and transcriptional activation of FLOWERING LOCUS T

Apple (Malus domestica) is one of the most widely cultivated fruit trees in the world and plays a central role in the agricultural industry and the economy. Flowering is a multi-path synergistic process that helps plants achieve high quality and yield in a multilateral environment. Here, overexpressing MdbHLH4 promoted the flowering transition in Arabidopsis, while myc70, a mutant of the MdbHLH4 Arabidopsis homolog, only exhibited a late flowering phenotype under short days. Moreover, the late flowering myc70 mutant phenotype was attenuated by ectopic expression of MdbHLH4. The myc70 mutant recovered from late flowering under short days by exogenous application of gibberellic acid (GA)4+7. The expression of GA biosynthesis-related genes was upregulated and that of GA catabolic genes was downregulated in the apical meristems of the Arabidopsis MdbHLH4 transgenic lines compared with the wild-type, and the opposite was observed in the myc70 mutant. Furthermore, yeast-two-hybrid (Y2H) assays revealed that MdbHLH4 and DELLA family proteins interacted with the C-terminal domain of MdFLC (FLOWERING LOCUS C) protein, suggesting potential competition between MdbHLH4 and MdDELLAs to interact with MdFLC in apples. Additionally, MdbHLH4 directly bound to the MdFT (FLOWERING LOCUS T) promoter and activated MdFT expression; thus promoting the floral transition. Our results suggest that MdbHLH4 is a key gene involved in plant flowering under short day conditions by regulating GA biosynthesis and signaling as well as the expression of MdFT, which provides new insight into the apple flowering mechanism.

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

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.

Arabidopsis inositol polyphosphate multikinase delays flowering time through mediating transcriptional activation of FLOWERING LOCUS C.

Timely flowering is critical for successful reproduction and seed yield in plants. A diverse range of regulators have been found to control flowering time in response to environmental and endogenous signals. Among these regulators, FLOWERING LOCUS C (FLC) acts as a central repressor of floral transition by blocking the expression of flowering integrator genes. Here, we report that Arabidopsis inositol polyphosphate multikinase (AtIPK2β) functions in flowering time control by mediating transcriptional regulation of FLC at the chromatin level. The atipk2β mutant flowers earlier, and AtIPK2β overexpressing plants exhibit late-flowering phenotypes. Quantitative reverse transcription-PCR (qRT-PCR) revealed that AtIPK2β promotes FLC expression. We performed chromatin immunoprecipitation-qPCR (ChIP-qPCR) assays and found that AtIPK2β binds to FLC chromatin. Further analysis showed that AtIPK2β interacts with FVE, a key repressor required for epigenetic silencing of FLC. qRT-PCR, ChIP-qPCR, and genetic analysis demonstrated that AtIPK2β is involved in FVE-mediated transcriptional regulation of FLC by repressing the accumulation of FVE on FLC. Moreover, we found that AtIPK2β associates with HDA6, an interaction partner of FVE mediating FLC chromatin silencing, and attenuates HDA6 accumulation at the FLC locus. Taken together, these findings suggest that AtIPK2β negatively regulates flowering time by blocking chromatin silencing of FLC.

FE, a phloem-specific Myb-related protein, promotes flowering through transcriptional activation of FLOWERING LOCUS T and FLOWERING LOCUS T INTERACTING PROTEIN 1.

In many flowering plants, the transition to flowering is primarily affected by seasonal changes in day length (photoperiod). An inductive photoperiod promotes flowering via synthesis of a floral stimulus, called florigen. In Arabidopsis thaliana, the FLOWERING LOCUS T (FT) protein is an essential component of florigen, which is synthesized in leaf phloem companion cells and is transported through phloem tissue to the shoot apical meristem where floral morphogenesis is initiated. However, the molecular mechanism involved in the long-distance transport of FT protein remains elusive. In this study, we characterized the classic Arabidopsis mutant fe, which is involved in the photoperiodic induction of flowering, and showed that FE encodes a phloem-specific Myb-related protein that was previously reported as ALTERED PHLOEM DEVELOPMENT. Phenotypic analyses of the fe mutant showed that FT expression is reduced in leaf phloem companion cells. In addition, the transport of FT protein from leaves to the shoot apex is impaired in the fe mutant. Expression analyses further demonstrated that FE is also required for transcriptional activation of FLOWERING LOCUS T INTERACTING PROTEIN 1 (FTIP1), an essential regulator for selective trafficking of the FT protein from companion cells to sieve elements. These findings indicate that FE plays a dual role in the photoperiodic induction of flowering: as a transcriptional activator of FT on the one hand, and its transport machinery component, FTIP1, on the other hand. Thus, FE is likely to play a role in regulating FT by coordinating FT synthesis and FT transport in phloem companion cells.

Chapter One - Photoperiodic Flowering Regulation in Arabidopsis thaliana

Photoperiod, or the duration of light in a given day, is a critical cue that flowering plants utilize to effectively assess seasonal information and coordinate their reproductive development in synchrony with the external environment. The use of the model plant, Arabidopsis thaliana, has greatly improved our understanding of the molecular mechanisms that determine how plants process and utilize photoperiodic information to coordinate a flowering response. This mechanism is typified by the transcriptional activation of FLOWERING LOCUS T (FT) gene by the transcription factor CONSTANS (CO) under inductive long-day conditions in Arabidopsis. FT protein then moves from the leaves to the shoot apex, where floral meristem development can be initiated. As a point of integration from a variety of environmental factors in the context of a larger system of regulatory pathways that affect flowering, the importance of photoreceptors and the circadian clock in CO regulation throughout the day has been a key feature of the photoperiodic flowering pathway. In addition to these established mechanisms, the recent discovery of a photosynthate derivative trehalose-6-phosphate as an activator of FT in leaves has interesting implications for the involvement of photosynthesis in the photoperiodic flowering response that were suggested from previous physiological experiments in flowering induction.