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.     

Molecular Genetics Returns to Basic Postulates of the Florigen Theory

The florigen theory, also known as the hormonal theory of flowering in plants, was put forward by M.Kh. Chailakhyan more than 65 years ago. This theory, reviewed here in its historical progress, gave an impetus to a number of modern genetic and molecular studies of flowering in plants. In turn, results of recent investigations restored to a new level Chailakhyan's ideas about florigen, the hormonal stimulus of flowering.     

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.     

What is going on with the hormonal control of flowering in plants?

Molecular genetic studies using Arabidopsis thaliana as a model system have overwhelmingly revealed many important molecular mechanisms underlying the control of various biological events, including floral induction in plants. The major genetic pathways of flowering have been characterized in-depth, and include the photoperiod, vernalization, autonomous and gibberellin pathways. In recent years, novel flowering pathways are increasingly being identified. These include age, thermosensory, sugar, stress and hormonal signals to control floral transition. Among them, hormonal control of flowering except the gibberellin pathway is not formally considered a major flowering pathway per se, due to relatively weak and often pleiotropic genetic effects, complex phenotypic variations, including some controversial ones. However, a number of recent studies have suggested that various stress signals may be mediated by hormonal regulation of flowering. In view of molecular diversity in plant kingdoms, this review begins with an assessment of photoperiodic flowering, not in A. thaliana, but in rice (Oryza sativa); rice is a staple crop for human consumption worldwide, and is a model system of short-day plants, cereals and breeding crops. The rice flowering pathway is then compared with that of A. thaliana. This review then aims to update our knowledge on hormonal control of flowering, and integrate it into the entire flowering gene network.     

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.     

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.     

The Medicago FLOWERING LOCUS T homolog, MtFTa1, is a key regulator of flowering time

FLOWERING LOCUS T (FT) genes encode proteins that function as the mobile floral signal, florigen. In this study, we characterized five FT-like genes from the model legume, Medicago (Medicago truncatula). The different FT genes showed distinct patterns of expression and responses to environmental cues. Three of the FT genes (MtFTa1, MtFTb1, and MtFTc) were able to complement the Arabidopsis (Arabidopsis thaliana) ft-1 mutant, suggesting that they are capable of functioning as florigen. MtFTa1 is the only one of the FT genes that is up-regulated by both long days (LDs) and vernalization, conditions that promote Medicago flowering, and transgenic Medicago plants overexpressing the MtFTa1 gene flowered very rapidly. The key role MtFTa1 plays in regulating flowering was demonstrated by the identification of fta1 mutants that flowered significantly later in all conditions examined. fta1 mutants do not respond to vernalization but are still responsive to LDs, indicating that the induction of flowering by prolonged cold acts solely through MtFTa1, whereas photoperiodic induction of flowering involves other genes, possibly MtFTb1, which is only expressed in leaves under LD conditions and therefore might contribute to the photoperiodic regulation of flowering. The role of the MtFTc gene is unclear, as the ftc mutants did not have any obvious flowering-time or other phenotypes. Overall, this work reveals the diversity of the regulation and function of the Medicago FT family.     

"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.     

GENETIC AND HORMONAL REGULATION OF GROWTH,FLOWERING, AND SEX EXPRESSION IN PLANTS

The interaction of the genetic and hormonal regulation of growth, flowering, and sex expression in plants is discussed. The genetic control of these processes is characterized, and data on their hormonal regulation are supplied. The interaction of genetic and hormonal regulation is considered with reference to tall-growing and genetic dwarf forms of the pea and wheat plants. It is shown that in the dwarf forms of the pea plant and in many other varieties, growth stimulation in response to treatment with the phytohormone gibberellic acid is clearly manifested and the expression of genetic dwarfism is eliminated, whereas in dwarf wheats it is expressed only slightly, if at all. At the same time both tall-growing and dwarf forms of both pea and wheat show a clearly defined growth retardation response to treatment with the growth inhibitor, abscisic acid, which causes the expression of physiological dwarfism. The short- and long-day characteristics of the photoperiodic response of plants are described as genetically controlled features, and data are given on the induction of flowering of a long-day variety coneflower grown under short-day conditions with the aid of gibberellins extracted from leaves of long-day vegetative plants of short-day Mammoth tobacco. Data are also supplied on the induction of flowering of a short-day variety, red-leaved goosefoot, grown under continuous light with the aid of metabolites extracted from leaves of the same Mammoth tobacco plants flowering under short-day conditions. This demonstrates the possibility of hormonal regulation of the genetically controlled long-day and short-day characteristics in photoperiodically sensitive plants. Genetic and hormonal regulation of sex expression in two dioecious plants, hemp and spinach, is discussed. It is shown that sex expression in these plants is regulated by gibberellins which are synthesized in leaves and cause male sex expression and by cytokinins which are synthesized in the roots and cause female sex expression. These data indicate that sex expression in dioecious plants is the result of interaction between the genetic apparatus and phytohormones.     

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.     

Developing a method for customized induction of flowering

Background  

The ability to induce flowering on demand is of significant biotechnological interest. FT protein has been recently identified as an important component of the mobile flowering hormone, florigen, whose function is conserved across the plant kingdom. We therefore focused on manipulation of both endogenous and heterologous FT genes to develop a floral induction system where flowering would be inhibited until it was induced on demand. The concept was tested in the model plant Arabidopsis thaliana (Arabidopsis).     

OsFTL12, a member of FT-like family,modulates the heading date and plant architecture by florigen repression complex in rice

FLOWERING LOCUS T (FT), a florigen in Arabidopsis, plays critical roles in floral transition. Among 13 FT-like members in rice, OsFTL2 (Hd3a) and OsFTL3 (RFT1), two rice homologues of FT, have been well characterized to act as florigens to induce flowering under short-day (SD) and long-day (LD) conditions, respectively, but the functions of other rice FT-like members remain largely unclear. Here, we show that OsFTL12 plays an antagonistic function against Hd3a and RFT1 to modulate the heading date and plant architecture in rice. Unlike Hd3a and RFT1, OsFTL12 is not regulated by daylength and highly expressed in both SD and LD conditions, and delays the heading date under either SD or LD conditions. We further demonstrate that OsFTL12 interacts with GF14b and OsFD1, two key components of the florigen activation complex (FAC), to form the florigen repression complex (FRC) by competing with Hd3a for binding GF14b. Notably, OsFTL12-FRC can bind to the promoters of the floral identity genes OsMADS14 and OsMADS15 and suppress their expression. The osmads14 osmads15 double mutants could not develop panicles and showed erect leaves. Taken together, our results reveal that different FT-like members can fine-tune heading date and plant architecture by regulating the balance of FAC and FRC in rice.     

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。