Hormonal regulation of reproductive development in higher plants

The present paper deals with the hormonal regulation of reproductive development,i.e. flowering and sex manifestation. Representation of hormonal regulation of flowering is based on the concept of florigen as a two-component, complementary system of flowering hormones, which consists of gibberellins and anthesins. Data are presented on the effect of extracts of gibberellin- and anthesin-type substances from the leaves of flowering and vegetatively growing plants under non-inductive conditions of day length. Experiments with flowering of plants under the influence of graftings serve as a basis for considering the question about the common nature of one of the florigen components — anthesins, for various plant species. The mechanism controlling tuberization in both intact and grafted plants is based on the participation of all the components of the hormonal system and constitutes one of the most vivid manifestations of integration of all the organs in the whole organism.     

Florigen goes molecular: Seventy years of the hormonal theory of flowering regulation

As early as in 1936, the comprehensive studies of flowering led M.Kh. Chailakhyan to the concept of florigen, a hormonal floral stimulus, and let him establish several characteristics of this stimulus. These studies set up for many years the main avenues for research into the processes that control plant flowering, and the notion of florigen became universally accepted by scientists worldwide. The present-day evidence of genetic control of plant flowering supports the idea that florigen participates in floral signal transduction. The recent study of arabidopsis plants led the authors to conclusion that the immediate products of the gene FLOWERING LOCUS I, its mRNA and/or protein, move from an induced leaf into the shoot apex and evoke flowering therein.     

Regulation of flowering plants of different biotypes

The patterns of control of flowering are analyzed in plants of different biotypes. The photoperiodic reaction of flowering taken as an example, the whole net of control is considered: from the environmental stimulus through its physiological transformation in the leaf in the corresponding hormonal impulse which, in turn, controls the realization of genetic programme and formation of generative organs in the stem buds. The photoperiodically neutral plants taken as an example, the patterns of age control of flowering are considered. In plants of different photoperiodic groups the synthesis of complementary components of florigen was shown to proceed either autonomously under the photoperiodic effect or under the inducing effect of definite photoperiods. The autonomous and inducible mechanisms of biosynthesis of the flowering hormones have a common base, the genetic system to which the environment sends its stimuli through the hormonal interactions. The interaction of hormonal and genetic developmental factors is considered, the evocation of flowering in the stem buds taken as an example.     

"Florigen ": an intriguing concept of plant biology.

"Florigen" is the name that Mikhail Chailakhyan coined in 1937 for the putative hormone regulating flowering. At this concept, plant physiologists arrived following early research concerning the effects of temperature and day length on the transition from vegetative to reproductive stages of plants. The existence of florigen was postulated on the experimental backgrounds involving i) the response of plants to inductive conditions; ii) transmission of a flowering stimulus by grafting; iii) extraction of this stimulus from induced plants. This experimental results showed the existence of florigen at least as concept because they always failed to offer the experimental evidence of its chemical existence. The myth of florigen persisted as long as the end of the Seventies, when physiologists began to consider flowering as a complex process in which various classes of hormones might variously interplay.     

ZCN8 encodes a potential orthologue of Arabidopsis FT florigen that integrates both endogenous and photoperiod flowering signals in maize

Higher plants use multiple perceptive measures to coordinate flowering time with environmental and endogenous cues. Physiological studies show that florigen is a mobile factor that transmits floral inductive signals from the leaf to the shoot apex. Arabidopsis FT protein is widely regarded as the archetype florigen found in diverse plant species, particularly in plants that use inductive photoperiods to flower. Recently, a large family of FT homologues in maize, the Zea CENTRORADIALIS (ZCN) genes, was described, suggesting that maize also contains FT-related proteins that act as a florigen. The product of one member of this large family, ZCN8, has several attributes that make it a good candidate as a maize florigen. Mechanisms underlying the floral transition in maize are less well understood than those of other species, partly because flowering in temperate maize is dependent largely on endogenous signals. The maize indeterminate1 (id1) gene is an important regulator of maize autonomous flowering that acts in leaves to mediate the transmission or production of florigenic signals. This study finds that id1 acts upstream of ZCN8 to control its expression, suggesting a possible new link to flowering in day-neutral maize. Moreover, in teosinte, a tropical progenitor of maize that requires short-day photoperiods to induce flowering, ZCN8 is highly up-regulated in leaves under inductive photoperiods. Finally, vascular-specific expression of ZCN8 in Arabidopsis complements the ft-1 mutation, demonstrating that leaf-specific expression of ZCN8 can induce flowering. These results suggest that ZCN8 may encode a florigen that integrates both endogenous and environmental signals in maize.     

Control of water-use efficiency by florigen

A major issue in modern agriculture is water loss through stomata during photosynthetic carbon assimilation. In water-limited ecosystems, annual plants have strategies to synchronize their growth and reproduction to the availability of water. Some species or ecotypes of flowers are early to ensure that their life cycles are completed before the onset of late season terminal drought (“drought escape”). This accelerated flowering correlates with low water-use efficiency (WUE). The molecular players and physiological mechanisms involved in this coordination are not fully understood. We analyzed WUE using gravimetry, gas exchange, and carbon isotope discrimination in florigen deficient (sft mutant), wild-type (Micro-Tom), and florigen over-expressing (SFT-ox) tomato lines. Increased florigen expression led to accelerated flowering time and reduced WUE. The low WUE of SFT-ox was driven by higher stomatal conductance and thinner leaf blades. This florigen-driven effect on WUE appears be independent of abscisic acid (ABA). Our results open a new avenue to increase WUE in crops in an ABA-independent manner. Manipulation of florigen levels could allow us to produce crops with a life cycle synchronized to water availability.     

FTIP1 is an essential regulator required for florigen transport

The capacity to respond to day length, photoperiodism, is crucial for flowering plants to adapt to seasonal change. The photoperiodic control of flowering in plants is mediated by a long-distance mobile floral stimulus called florigen that moves from leaves to the shoot apex. Although the proteins encoded by FLOWERING LOCUS T (FT) in Arabidopsis and its orthologs in other plants are identified as the long-sought florigen, whether their transport is a simple diffusion process or under regulation remains elusive. Here we show that an endoplasmic reticulum (ER) membrane protein, FT-INTERACTING PROTEIN 1 (FTIP1), is an essential regulator required for FT protein transport in Arabidopsis. Loss of function of FTIP1 exhibits late flowering under long days, which is partly due to the compromised FT movement to the shoot apex. FTIP1 and FT share similar mRNA expression patterns and subcellular localization, and they interact specifically in phloem companion cells. FTIP1 is required for FT export from companion cells to sieve elements, thus affecting FT transport through the phloem to the SAM. Our results provide a mechanistic understanding of florigen transport, demonstrating that FT moves in a regulated manner and that FTIP1 mediates FT transport to induce flowering.     

Identification of miRNA Targets by AtFT Overexpression in Tobacco

Plant Molecular Biology Reporter - The onset of flowering is regulated by complex gene networks that integrate multiple genetic cues to floral transition in plants. The highly conserved florigen...     

Mikhail Khristoforovich Chailakhyan: The fate of the scientist under the sign of florigen

The more significant is a discovery made by the scientist, the greater influence it exerts on the fate of its author. The theory of florigen, a hormone of flowering, put forward by M.Kh. Chailakhyan in 1936 initially was inconvenient for Soviet authorities and brought severe trials to the scientist. However, as distinct from many others, Chailakhyan did not deny his scientific beliefs but continued to defend them in spite of harassment and threats. Later, on the contrary, this theory promoted the worldwide fame of its creator, although during Chailakhyan lifetime florigen has not been definitely identified chemically. Chailakhyan was close to the establishing of florigen protein nature, but even his long life was not enough to identify this elusive substance.     

The multifaceted roles of FLOWERING LOCUS T in plant development

One of the key developmental processes in flowering plants is the differentiation of the shoot apical meristem into a floral meristem. This transition is regulated through the integration of environmental and endogenous stimuli, involving a complex, hierarchical signalling network. In arabidopsis, the FLOWERING LOCUS T (FT) protein, a mobile signal recognized as a major component of florigen, has a central position in mediating the onset of flowering. FT-like genes seem to be involved in regulating the floral transition in all angiosperms examined to date. Evidence from molecular evolution studies suggests that the emergence of FT-like genes coincided with the evolution of the flowering plants. Hence, the role of FT in floral promotion is conserved, but appears to be restricted to the angiosperms. Besides flowering, FT-like proteins have also been identified as major regulatory factors in a wide range of developmental processes including fruit set, vegetative growth, stomatal control and tuberization. These multifaceted roles of FT-like proteins have resulted from extensive gene duplication events, which occurred independently in nearly all modern angiosperm lineages, followed by sub- or neo-functionalization. This review assesses the plethora of roles that FT-like genes have acquired during evolution and their implications in plant diversity, adaptation and domestication.     

Historical perspective on breakthroughs in flowering field

Plants have evolved a mechanism to synchronize flowering time in response to environments. How plants recognize specific seasons for flowering has been a long sought question, thus, more than 100 years of research has been focused on this question. Especially in the past two decades, remarkable achievements have been made in identifying the molecular mechanism for flowering. Here we summarize the breakthroughs made in this field over the past century including discoveries of photoperiodic and vernalization-induced flowering, the identification of complex genetic pathways, and the recently proposed identity of florigen. In addition, we present the currently accepted model for a molecular mechanism toward flowering.     

Florigen is involved in axillary bud development at multiple stages in Arabidopsis

The wide variety of plant architectures is largely based on diverse and flexible modes of axillary shoot development. In Arabidopsis, floral transition (flowering) stimulates axillary bud development. The mechanism that links flowering and axillary bud development is, however, largely unknown. We recently showed that FLOWERING LOCUS T (FT) protein, which acts as florigen, promotes the phase transition of axillary meristems, whereas BRANCHED1 (BRC1) antagonizes the florigen action in axillary buds. Here, we present evidences for another possible role of florigen in axillary bud development. Ectopic overexpression of FT or another florigen gene TWIN SISTER OF FT (TSF) with LEAFY (LFY) induces ectopic buds at cotyledonary axils, confirming the previous proposal that these genes are involved in formation of axillary buds. Taken together with our previous report that florigen promotes axillary shoot elongation, we propose that florigen regulates axillary bud development at multiple stages to coordinate it with flowering in Arabidopsis.     

Graft transmission of a floral stimulant derived from CONSTANS

Photoperiod in plants is perceived by leaves and in many species influences the transition to reproductive growth through long-distance signaling. CONSTANS (CO) is implicated as a mediator between photoperiod perception and the transition to flowering in Arabidopsis. To test the role of CO in long-distance signaling, CO was expressed from a promoter specific to the companion cells of the smallest veins of mature leaves. This expression in tissues at the inception of the phloem translocation stream was sufficient to accelerate flowering at the apical meristem under noninductive (short-day) conditions. Grafts that conjoined the vegetative stems of plants with different flower-timing phenotypes demonstrated that minor-vein expression of CO is able to substitute for photoperiod in generating a mobile flowering signal. Our results suggest that a CO-derived signal(s), or possibly CO itself, fits the definition of the hypothetical flowering stimulant, florigen.     

The role of auxin level and sensitivity in floral induction

Flower initiation takes place during a rise of peroxidase activity following a peak of minimum activity which marked the completion of the flowering inductive phase. Since basic isoperoxidases underwent an inverse variation of activity in the course of successive inductive and initiative phases, it was hypothesized that the induction of flowering led to a temporary peak of maximum auxin level in the leaves. Our analyses and available literature data support the view. They also show the different capacity of non-induced and induced material to respond to external auxin application. Since some aspects of the physiological state characterizing induced plants can be simultaneously obtained in all plant parts as a result of rapid interorgan communication, the classical florigen theory is seriously challenged. On leave from University of Liège-Start Tilman, Institute of Botany B22, B-4000 Liège, Belgium.     

FT, a mobile developmental signal in plants

Plants synchronise their flowering with the seasons to maximise reproductive fitness. While plants sense environmental conditions largely through the leaves, the developmental decision to flower occurs in the shoot apex, requiring the transmission of flowering information, sometimes over quite long distances. Interestingly, despite the enormous diversity of reproductive strategies and lifestyles of higher plants, a key component of this mobile flowering signal, or florigen, is contributed by a highly conserved gene: FLOWERING LOCUS T (FT). The FT gene encodes a small globular protein that is able to translocate from the leaves to the shoot apex through the phloem. Plants have evolved a variety of regulatory networks that control FT expression in response to diverse environmental signals, enabling flowering and other developmental responses to be seasonally timed. As well as playing a key role in flowering, recent discoveries indicate FT is also involved in other developmental processes in the plant, including dormancy and bud burst.     

水稻开花光周期调控相关基因研究进展

水稻开花调控是一个极其复杂的生命过程,由自身遗传因素和外界环境共同决定。光周期途径是调控水稻开花的关键途径,在这个途径中成花素基因Hd3a和RTF1处于核心地位,其上游调控途径主要包括Hd1依赖途径、Ehd1依赖途径及不依赖于Hd1和Ehd1的途径。这3条途径在汇集了光信号的各种信息后,将信号在Hd3a和RTF1处整合,并通过成花素形式将信息传递给下游开花基因,调控水稻开花。本文从成花素、光信号感受基因和昼夜节律基因、成花素上游调控基因、互作蛋白和下游调控基因等几方面阐述水稻开花光周期调控相关基因的研究现状,为水稻开花调控的深入研究提供参考。     

Universal florigenic signals triggered by FT homologues regulate growth and flowering cycles in perennial day-neutral tomato

The transition from vegetative to floral meristems in higher plants is programmed by the coincidence of internal and environmental signals. Classic grafting experiments have shown that leaves, in response to changing photoperiods, emit systemic signals, dubbed 'florigen', which induce flowering at the shoot apex. The florigen paradigm was conceived in photoperiod-sensitive plants: nevertheless it implies that although activated by different stimuli in different flowering systems, the signal is common to all plants. Tomato is a day-neutral, perennial plant, with sympodial and modular organization of its shoots and thus with reiterative regular vegetative/reproductive transitions. SINGLE FLOWER TRUSS a regulator of flowering-time and shoot architecture encodes the tomato orthologue of FT, a major flowering integrator gene in Arabidopsis. SFT generates graft-transmissible signals which complement the morphogenetic defects in sft plants, substitute for light dose stimulus in tomato and for contrasting day-length requirements in Arabidopsis and MARYLAND MAMMOTH tobacco. It is discussed how systemic signals initiated by SFT interact with the SELF PRUNING gene to regulate vegetative to reproductive (V/R) transitions in the context of two flowering systems, one for primary apices and the other for sympodial shoots.