Signaling Involved Flowering in Plants
As known, reproduction is one of the most important phases in an organism’s lifecycle. In the angiosperm plants, flowering provides the major developmental transition from the vegetative to the reproductive stage, and requires genetic and epigenetic reprogramming to ensure the success of seed production. Therefore, flowering occurs at the right time: flowering is controlled by environmental conditions and developmental regulation. The overlap of this regulation is created by an intricate network of signaling pathways. Decades of physiological studies have revealed that flowering is initiated in response to both environmental cues and endogenous pathways. In this article, we review new findings about interactions between epigenetic mechanisms and key players in hormone signaling to coordinate flowering.
Flowering plants (angiosperms) emerged on our planet approximately 140 to 160 million years ago and today represent about 90% of the more than 350,000 known plant species (Paton et al., 2008).. Flowers are their reproductive frameworks, which produce fruits containing one to many seeds. Usually flowers square measure each male and feminine, and they are often brightly colored to attract animal pollinators, but every possible variation exists from unisexual flowers to inconspicuous flowers that use wind for fertilisation to flowers that manufacture seeds while not fertilization. The transition from stage vegetative meristems to the formation of flowers, the subsequent developmental programs that result in a range of flower morphologies, and the diverse pollination and seed development mechanisms have fascinated scientists for many years.
Although the discoveries of processes like flower induction in vegetative meristems (Sachs, 1863), photoperiodism (Garner and Allard, 1920), and the double fertilization event (Nawaschin, 1898) occurred at least a century agone, our understanding of the underlying molecular mechanisms of these processes is relatively recent, beginning, for example, with the formulation of the ABC model of flower organ development (Coen and Meyerowitz, 1991), the invention of the flowering regulator CONSTANS (Putterill et al., 1995), and the identification of the first selfincompatibility gene (McClure et al., 1989). Since then tremendous progress in understanding flowering and reproduction has been made; the last 10 years have seen another boost of further advances inspiring us to organize a Focus Issue representing the first issue about the topic in Plant Physiology. about the topic in Plant Physiology.
In several plant species, the timing of flowering is critical for reproductive success, and thus the evolution of flowering-control systems that optimize reproductive success provide a selective advantage. For example, flowering must occur early enough in the growing season to enable proper seed development, but premature flowering when a plant is small will limit the amount of seed that can be produced. Also, in outcrossing species, synchronous flowering enables cross-pollination.
In several plant species, systems have evolved to perceive the seasonal cues of changing daylength and temperature and to translate that perception into flowering. An Update by wedge et al., (2017) discusses the immense progress in understanding the molecular details of how changes in daylength lead to the induction of flowering. The daylength response system has common options all told flowering plants, indicating an ancient evolutionary origin.
In distinction, vernalization systems, which evolved to enable flowering only after plants experienced exposure to the prolonged cold of winter, evolved more recently and, as discussed in the Update by Bouche´ et al., (2017), are molecularly distinct in different groups of plants.
In Arabidopsis (Arabidopsis thaliana), (D-H. Kim and S. Sung, 2014 illustrated in a research article, many of the biochemical details of how vernalization results in competence to flower are being uncovered. Also, temperature fluctuations other than exposure to winter cold influence flowering, the role of this so-called ambient temperature response in Arabidopsis. McClung et al., 2016. In this respect, Mouradov et al., (2002) reported that flowering is controlled by environmental conditions and developmental regulation.
The complexness of this regulation is made by associate tortuous network of communication pathways. and therefore are referred to as floral integrators. Light sensors regulate growth and development of plants Light controls plant development from germination to the formation of flowers in many various ways that. Important light-weight sensors ar the phytochromes that sense red light.
Phytochromes are involved when light initiates the germination and greening of the seedling and in the adaptation of the photosynthetic apparatus of the leaves to full sunlight or shade. Five different phytochromes (A–E) have been identified in the model plant Arabidopsis thaliana (section 20.1). Plants even have photoreceptors for blue and (ultraviolet radiation) for its adaptation to the complete spectrum of sunlight. So far, three proteins have been identified as blue light receptors; these are cryptochromes 1 and 2, each comprising a flavin and a pterin and phototropin, containing one flavin as a blue light-absorbing pigment. (Hans-Walter Heldt, 2011).
1- External factors:
1.1- The Photoperiod Response Pathways
The annual fluctuations in daylength that occur over abundant of the surface of our planet offer a reliable environmental cue concerning the time of year. It is not shocking, therefore, that the pathways that find and promote flowering in response to photoperiod square measure among the foremost ancient and preserved. Physiological experiments initial drained the Nineteen Thirties (Knott, 1934) incontestible that inductive photoperiods square measure detected by leaves. This raised 2 elementary questions: however do leaves live daylength, and what’s the character of the flowering signal (known as florigen) that has to travel from the leaves to the shoot apical meristem?
After another seven decades of analysis, we tend to currently have comparatively clear and satisfying answers to those queries, particularly in Arabidopsis (Arabidopsis thaliana). Arabidopsis flowers faster in long days than briefly days and is so a facultative long-day plant. The regulation of the floral promoter CONSTANS (CO) is vital within the perception of inductive long days (Turck et al., 2008).
Circadian rhythms have a period length (the duration of one cycle) of~24 h. These rhythms don’t need daily transitions from light-weight to dark, however continue underneath constant conditions. Circadian rhythms are ascertained at completely different levels of organization, from leaf movement to stomatal aperture, greenhouse emission assimilation, or gene transcription. The mechanism that generates these rhythms is usually delineate in three reticular sections. These square measure input pathways that synchronise the clock mechanism to daily cycles of sunshine and dark, a central generator that generates the 24~h time-keeping mechanism, and output pathways that regulate explicit processes (Roenneberg and Merrow, 2000; McClung, 2001).
The management of flowering via CO and FT represents one such output pathway (Suarez-Lopez et al., 2001), whereas several others are delineate well victimisation international organic phenomenon assays (Harmer et al., 2000; Schaffer et al., 2001). CO protein, in turn, is stabilised by long day and quickly degraded darkly (Valverde et al., 2004). As a result, CO macromolecule will solely accumulate throughout inductive long days. CO is expressed within the vasculature of leaves, and its role in flowering is to activate the expression of FLOWERING LOCUS T (FT), that encodes a little macromolecule that is florigen (Fig. xx).
In each rice (Oryza sativa) and dilleniid dicot genus, foot could be a sturdy promoter of flowering that’s translocated from the vasculature of leaves to the shoot top plant tissue (Corbesier et al., 2007; Tamaki et al., 2007). In the plant tissue, foot forms a posh with the bZIP transcription issue FD and initiates flowering by activating floral meristem-identity genes like APETALA1 and alternative floral promoters such as SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 (SOC1; Michaels, 2009). Thus, foot up-regulation lies at the tip of associate environment-sensing pathway and initiates flower development.
In addition to the photoperiod pathway, FT and SOC1 are also regulated by other flowering pathways and therefore are referred to as floral integrators. Light sensors regulate growth and development of plants. Light controls plant development from germination to the formation of flowers in many various ways that. Important light-length sensors are the phytochromes that sense red light. Phytochromes are involved when light initiates the germination and greening of the seedling and in the adaptation of the photosynthetic apparatus of the leaves to full sunlight or shade. Five different phytochromes (A–E) have been identified in the model plant Arabidopsis thaliana.
Plants even have photoreceptors for blue and ultraviolet ray light for its adaptation to the complete spectrum of sunlight. So far, three proteins have been identified as blue light receptors; these are cryptochromes 1 and 2, each comprising a flavin and a pterin and phototropin, containing one flavin as a blue light-absorbing pigment. (Hans-Walter Heldt, 2011).
Vernalization is defined as the method by that exposure to the cold of winter renders plants competent to flower (Kim et al., 2009). The passage of winter is associate degree environmental cue that, once coupled to photoperiod sensing, provides clear seasonal data that distinguishes the spring and fall seasons. For cold to be a reliable cue for winter, plants got to be ready to distinguish the long cold exposure characteristic of winter from short fluctuations in temperature that might occur, for example, in the fall. Thus, it’s not stunning that vernalization (and in several species the breaking of bud dormancy) needs exposure to prolonged cold.
A vernalization demand is usually found in winter-annual and biennial plants that flower early within the spring; these plants generally become established within the fall, and a vernalization demand ensures that premature flowering doesn’t occur throughout the autumn institution part. In winter-annual Arabidopsis, the vernalization responsive block to flowering requires the interaction of two genes, FLOWERING LOCUS C (FLC) and FRIGIDA (FRI; Michaels and Amasino, 1999; Sheldon et al., 1999; Johanson et al., 2000). FLC is a MADS domain-containing transcription factor that acts as a floral repressor, and FRI is a plant-specific gene of unknown biochemical function that is required for high levels of FLC expression. FLC inhibits flowering by directly repressing the key promoters of flowering, FT, SOC1, and FD (Michaels, 2009; Fig. 1). Vernalization permits plants to flower rapidly in the lengthening days of spring through repression of FLC (Fig. 1). FRI and FLC were first identified genetically in crosses between winter-annual and rapid-cycling accessions Michaels et al., (2003).
In temperate regions, wheat and barley are sown in autumn then over-winter before flowering in spring. When sown in spring, these same cereal varieties typically show delayed flowering or fail to flower altogether. Several researchers recognised that the cold of winter is a critical factor required to trigger flowering of these plants and that this is lacking when plants are sown in spring (see McKinney 1940; Chouard 1960). For example, Gassner showed that germinating wheat or rye (Secale cereal M. Bieb.) seeds at normal growth temperatures can cause a strong delay of flowering, whereas germination at low temperatures can stimulate flowering (Gassner 1918).?
INTEGRATION OF PHOTOPERIOD AND VERNALIZATION
As discussed above, the basic photoperiod pathway appears to be conserved in flowering plants, and as illustrated in Figure 2, in Arabidopsis the circuitry of how the vernalization and photoperiod pathways interact is clear: FLC represses expression of flowering promoters (integrators) until this repression is removed through the silencing of FLC by vernalization. However, in cereals, the vernalization pathway is distinct from that in Arabidopsis. In the cereal pathway, there is a flowering repressor, VRN2 that, like FLC, is turned off during cold exposure. However, FLC is a MADS box protein, whereas VRN2 is a zinc-finger protein that does not have a homolog in the Arabidopsis genome. FLC expression is repressed solely by cold, whereas in cereals, VRN2 expression is repressed by cold, short days and induction of the meristemidentity gene VERNALIZATION1 (note: in cereals,
VERNALIZATION1 is a MADS box gene unrelated in amino acid sequence to Arabidopsis VRN1).
Despite their differences, there is a common feature of the interface between the vernalization and photoperiod pathways in Arabidopsis and cereals: both FLC and VRN2 repress the key photoperiod pathway gene FT (VRN3 in cereals). This example of convergent evolution in how the pathways interface is perhaps not surprising. In contrast to the ancient photoperiod pathway, vernalization pathways arose after the divergence of major groups of flowering plants, as an adaptation to the new environments created by climate change and continental drift (Amasino, 2010). As vernalization pathways evolved, FT presented a prime regulation point for floral repression. It will be interesting to determine how vernalization pathways have
been “constructed” in other groups of plants and how often these pathways target FT expression. How plants sense and measure the prolonged cold of winter and transduce this into a vernalization response is not understood. An output of this coldsensing system in Arabidopsis is the induction of VIN3 expression and increased expression of the COOLAIR RNA. However, at present, genetic variation (i.e. mutants or natural variation) in the cold-sensing system has not been identified, and there are not any biochemical clues to how this system operates. It will be quite interesting, as well as a challenge, to understand the molecular basis of how plants sense prolonged cold for both vernalization and the breaking of bud dormancy, and whether such a system is conserved or has independently evolved multiple times, as appears to be the case for downstream parts of the vernalization pathway in cereals and Arabidopsis.
Effect of some hormones
The growth regulator GA promotes flowering of Arabidopsis (Wilson et al., 1992; Putterill, et al., 1995; Blazquez et al., 1998). This was initially demonstrated by applications of exogenous GA (Langridge, 1957), and has been more recently studied using mutations that disrupt either GA biosynthesis or signaling (Wilson et al., 1992). These mutations also have effects on many other aspects of plant growth and development, including stem elongation, germination, and floral development In this section, we summarize the impact of GA on flowering (Figure 3), and GA signaling is thoroughly reviewed by Olszewski et al., (2002).
Similar to IAA, gibberellins stimulate shoot elongation, especially in the internodes of the stems. A pronounced gibberellin effect is that it induces rosette plants (e.g., spinach or lettuce) to initiate and regulate the formation of flowers and flowering. Additionally, gibberellins have a number of other functions such as the preformation of fruits and the stimulation of their growth. Gibberellins terminate seed dormancy, probably by softening the seed coat, and facilitate seed germination by the expression of genes for the necessary enzymes (e.g., amylases). These plants are also severely dwarfed, do not germinate in the absence of exogenous GA, and exhibit reduced apical dominance. In contrast to ga1, the ga4 and ga5 mutations have less severe effects, giving rise to semidwarf plants that produce fertile flowers with normal siliques (Koornneef and van der Veen, 1980). The ga4 and ga5 mutants are defective in GA3-hydroxylase and GA 20-oxidase activity, respectively (Talon et al., 1990).
GA 20-oxidase is regulated by environmental or physiological changes, suggesting that it may be involved in a key regulatory step in GA biosynthesis (Xu et al., 1995). That Regulates Flowering in Arabidopsis. Activation of a hypothetical transmembrane receptor by GA inhibits repressors of GA signaling. These repressors are encoded by the RGA, GAI, and RGL genes. The SPY gene also represses GA signaling and genetically acts upstream of RGA and GAI. It may act to promote the activity of GAI/RGA/RGL by GlcNAc modification, in which case GA signaling may inhibit GAI/RGA/RGAL by repressing SPY function. PHOR1 has not been described in Arabidopsis, but has been shown to be involved in GA signaling in potato.
Its possible involvement in ubiquitination and protein degradation leads to the tentative proposal that it is involved in the demonstrated degradation of the repressing protein RGA in response to GA. The floral meristem identity gene LFY is upregulated at the transcriptional level by GA. The flowering-time gene SOC1 is also upregulated by GA, whereas FPF1 and GA-MYB were proposed to mediate between GAs and the regulation of flowering time. These three genes may therefore act downstream of GAI/RGA/RGL but upstream of LFY.
Expression of this gene increases when plants are transferred from short days to long days, and therefore high-level expression correlates with conditions that induce early flowering (Xu et al., 1997). Furthermore, transgenic plants containing elevated levels of GA 20-oxidase also contained more GA4 and flowered earlier than did wild-type control plants under both long-day and short-day conditions (Coles et al., 1999). This suggests that GA levels are limiting on flowering time, and is consistent with previous observations that application of exogenous GA causes early flowering of wild-type plants.
In ancient times Suge and Rappaport, (1968) studied the role of gibberellins in stem elongation and flowering in Radish and reported that all nonvernalized plants grown under short days,with or without GA3, did not flower, although GA3 caused bolting in all plants . Under 16-hour photoperiods, without GA3, 60 % of the nonvernalized plants bolted and flowered and 6 µg of GA3 applied during the growth period promoted 100 % bolting and 90 % flowering. However, GA, did not affect the number of days to bolting or flower formation, nor the number of leaves at anthesis of vernalized plants grown under long days Moreover, while GA3 reduced leaf numbers of vernalized plants grown under short days, similar plants grown under long days produced only about half as many leaves as those grown under short days, irrespective of gibberellin treatment. Both GA7 and GA3 elicited similar responses in nonvernalized plants grown under short days.