PhD student investigating flowering time mechanism in garden pea.
Decorative flower or edible food production - how is the latent genetic machinery driven in plants?
Have you ever thought of what is the genetic mechanism that mediates and ultimately gives rise to the beautiful flowers that you buy as a gift for your loved ones? It would probably be very fascinating for you as well to get an insight into the molecular process of fruit/vegetable/grain production being consumed by us everyday, which commences through initiation of flowering in a particular plant. Let’s get some very basic knowledge on this biological phenomenon that we observe in nature and is directly related to our survival.
The onset of flowering is a key developmental transition in the plant life cycle and is regulated by different environmental factors such as photoperiod, temperature and endogenous cues like circadian clock, gibberellic acid and age of the plant. Arabidopsis thaliana/Thale cress has been extensively used as a model system by researchers for unveiling the molecular biology of flowering which encompasses several reasons: its small genome size, easy handling, short life cycle, comparatively fast generation time and prolific seed production.
A. thaliana is a facultative long day plant where flowering is regulated by a key mobile protein, FLOWERING LOCUS T (FT) that is known as florigen. FT expression is dependent on CONSTANS (CO) in leaves under long day (LD) where different other genes play positive and negative roles in regulating FT function. FT is transported from leaves to shoot apical meristem (SAM) where it interacts with a basic leucine zipper (bZIP) domain transcription factor, FD. In the SAM, the FT–FD complex induces expression of floral meristem identity genes such as LEAFY (LFY) and APETALA1 (AP1) via SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) and this leads to commencement of flower formation. During winter, a MADS-box DNA binding protein FLOWERING LOCUS C (FLC) plays the central role in suppressing the expression of FT. Exposure to low temperatures (vernalization) induces expression of genes that repress FLC expression during warm temperature of the following spring, relieving the repression of FLC on FT and allowing the plant to initiate flowering process through FT action.
While flowering mechanisms are well understood in A. thaliana, they are less well perceived in other higher plant groups such as ornamental plants and crops. Therefore, scientists use knowledge gained from A. thaliana as a fundamental basis to investigate the genetic mechanism that regulates flowering time in higher plant species.
Abstract: Day length perceived by a leaf is a major environmental factor that controls the timing of flowering. It has been believed that a mobile, long-distance signal called florigen is produced in the leaf under inductive day length conditions, and is transported to the shoot apex where it triggers floral morphogenesis. Grafting experiments have shown that florigen is transmissible from a donor plant that has been subjected to inductive day length to an uninduced recipient plant. However, the nature of florigen has long remained elusive. Arabidopsis FLOWERING LOCUS T (FT) is expressed in cotyledons and leaves in response to inductive long days (LDs). FT protein, with a basic region/leucine zipper (bZIP) transcription factor FD, acts in the shoot apex to induce target meristem identity genes such as APETALA1 (AP1) and initiates floral morphogenesis. Recent studies have provided evidence that the FT protein in Arabidopsis and corresponding proteins in other species are an important part of florigen. Our work shows that the FT activity, either from overexpressing or inducible transgenes or from the endogenous gene, to promote flowering is transmissible through a graft junction, and that an FT protein with a T7 tag is transported from a donor scion to the apical region of recipient stock plants and becomes detectable within a day or two. The sequence and structure of mRNA are not of critical importance for the long-distance action of the FT gene. These observations led to the conclusion that the FT protein, but not mRNA, is the essential component of florigen.
Pub.: 14 Oct '08, Pinned: 20 Apr '17
Abstract: CONSTANS is an evolutionarily-conserved central component of the genetic pathway that controls the onset of flowering in response to daylength. However, the specific biochemical mechanism by which the CONSTANS protein regulates the expression of its target genes remains largely unknown. *By using a combination of cell-based expression analysis and in vitro DNA binding studies, we have demonstrated that CONSTANS possesses transcriptional activation potential and is capable of directly binding to DNA. *CONSTANS was found to bind DNA via a unique sequence element containing a consensus TGTG(N2-3)ATG motif. This element is present in tandem within the FLOWERING LOCUS T promoter and is sufficient for CO binding and activity. The conserved CCT (CONSTANS, CONSTANS-like and TOC1) domain of CONSTANS was shown to be required for its recruitment to the DNA motif and other CCT-containing proteins were also found to have the ability to regulate gene expression via this element. *The CCAAT box, which has been previously hypothesized as a recruitment site for complexes containing the CONSTANS protein, potentiated CONSTANS-mediated activation but was not essential for CONSTANS recruitment to a target promoter or for its activity as a transcriptional factor.
Pub.: 22 Apr '10, Pinned: 20 Apr '17
Abstract: A wide range of biological processes exhibit circadian rhythm, enabling plants to adapt to the environmental day-night cycle. This rhythm is generated by the so-called 'circadian clock'. Although a number of genetic approaches have identified >25 clock-associated genes involved in the Arabidopsis clock mechanism, the molecular functions of a large part of these genes are not known. Recent comprehensive studies have revealed the molecular functions of several key clock-associated proteins. This progress has provided mechanistic insights into how key clock-associated proteins are integrated, and may help in understanding the essence of the clock's molecular mechanisms.
Pub.: 30 Aug '11, Pinned: 20 Apr '17
Abstract: Flowering is often triggered by exposing plants to appropriate day lengths. This response requires an endogenous timer called the circadian clock to measure the duration of the day or night. This timer also controls daily rhythms in gene expression and behavioural patterns such as leaf movements. Several Arabidopsis mutations affect both circadian processes and flowering time; but how the effect of these mutations on the circadian clock is related to their influence on flowering remains unknown. Here we show that expression of CONSTANS (CO), a gene that accelerates flowering in response to long days, is modulated by the circadian clock and day length. Expression of a CO target gene, called FLOWERING LOCUS T (FT), is restricted to a similar time of day as expression of CO. Three mutations that affect circadian rhythms and flowering time alter CO and FT expression in ways that are consistent with their effects on flowering. In addition, the late flowering phenotype of such mutants is corrected by overexpressing CO. Thus, CO acts between the circadian clock and the control of flowering, suggesting mechanisms by which day length regulates flowering time.
Pub.: 27 Apr '01, Pinned: 20 Apr '17
Abstract: In Arabidopsis, the MADS-box protein encoded by FLOWERING LOCUS C (FLC) is a repressor of flowering. Vernalization, which promotes flowering in the late-flowering ecotypes and many late-flowering mutants, decreases the level of FLC transcript and protein in the plant. This vernalization-induced reduction in FLC transcript levels is mitotically stable and occurs in all tissues. FLC activity is restored in each generation, as is the requirement of a low-temperature exposure for the promotion of flowering. The level of FLC determines the extent of the vernalization response in the promotion of flowering, and there is a quantitative relationship between the duration of cold treatment and the extent of down-regulation of FLC activity. We conclude that FLC is the central regulator of the induction of flowering by vernalization. Other vernalization-responsive late-flowering mutants, which are disrupted in genes that encode regulators of FLC, are late-flowering as a consequence of their elevated levels of FLC.
Pub.: 15 Mar '00, Pinned: 20 Apr '17
Abstract: Circadian clocks provide a competitive advantage in an environment that is heavily influenced by the rotation of the Earth, by driving daily rhythms in behaviour, physiology and metabolism in bacteria, fungi, plants and animals. Circadian clocks comprise transcription-translation feedback loops, which are entrained by environmental signals such as light and temperature to adjust the phase of rhythms to match the local environment. The production of sugars by photosynthesis is a key metabolic output of the circadian clock in plants. Here we show that these rhythmic, endogenous sugar signals can entrain circadian rhythms in Arabidopsis thaliana by regulating the gene expression of circadian clock components early in the photoperiod, thus defining a 'metabolic dawn'. By inhibiting photosynthesis, we demonstrate that endogenous oscillations in sugar levels provide metabolic feedback to the circadian oscillator through the morning-expressed gene PSEUDO-RESPONSE REGULATOR 7 (PRR7), and we identify that prr7 mutants are insensitive to the effects of sucrose on the circadian period. Thus, photosynthesis has a marked effect on the entrainment and maintenance of robust circadian rhythms in A. thaliana, demonstrating that metabolism has a crucial role in regulation of the circadian clock.
Pub.: 25 Oct '13, Pinned: 20 Apr '17
Abstract: Onset of flowering is controlled by environmental signals such as light and temperature. Molecular-genetic studies in Arabidopsis thaliana have focused on daily light duration, or photoperiod, and transient exposure to winter-like temperatures, or vernalization. Yet ambient growth temperature, which is strongly affected by current changes in global climate, has been largely ignored. Here, we show that genes of the autonomous pathway, previously thought only to act independently of the environment as regulators of the floral repressor FLC (ref. 1), are centrally involved in mediating the effects of ambient temperature. In contrast to wild-type plants and those mutant in other pathways, autonomous-pathway mutants flower at the same time regardless of ambient temperature. In contrast, the exaggerated temperature response of cryptochrome-2 mutants is caused by temperature-dependent redundancy with the phytochrome A photoreceptor. As with vernalization and photoperiod, ambient temperature ultimately affects expression of the floral pathway integrator FT.
Pub.: 28 Jan '03, Pinned: 20 Apr '17
Abstract: Flowering of the facultative long-day plant Arabidopsis is controlled by several endogenous and environmental factors, among them gibberellins (GAs) and day length. The promotion of flowering by long days involves an endogenous clock that interacts with light cues provided by the environment. Light, and specifically photoperiod, is also known to regulate the biosynthesis of GAs, but the effects of GAs and photoperiod on flowering are at least partially separable. Here, we have used a short-period mutant, toc1, to investigate the role of the circadian clock in the control of flowering time by GAs and photoperiod. We show that toc1 affects expression of several floral regulators and a GA biosynthetic gene, but that these effects are independent.
Pub.: 14 Dec '02, Pinned: 20 Apr '17