Genetic Interactions That Regulate Inflorescence Development in Arabidopsis

In Arabidopsis, floral meristems arise in continuous succession directly on the flanks of the inflorescence meristem. Thus, the pathways that regulate inflorescence and floral meristem identity must operate both simultaneously and in close spatial proximity. The TERMINAL FLOWER 1 (TFL1) gene of Arabidopsis is required for normal inflorescence meristem function, and the LEAFY (LFY), APETALA 1 (AP1), and APETALA 2 (AP2) genes are required for normal floral meristem function. We present evidence that inflorescence meristem identity is promoted by TFL1 and that floral meristem identity is promoted by parallel developmental pathways, one defined by LFY and the other defined by AP1/AP2. Our analysis suggests that the acquisition of meristem identity during inflorescence development is mediated by antagonistic interactions between TFL1 and LFY and between TFL1 and AP1/AP2. Based on this study, we propose a simple model for the genetic regulation of inflorescence development in Arabidopsis. This model is discussed in relation to the proposed interactions between the inflorescence and the floral meristem identity genes and in regard to other genes that are likely to be part of the genetic hierarchy regulating the establishment and maintenance of inflorescence and floral meristems.     

EMF genes interact with late-flowering genes in regulating floral initiation genes during shoot development in Arabidopsis thaliana

To investigate the mechanisms regulating the initiation of floral development in Arabidopsis, a construct containing beta-glucuronidase (GUS) gene driven by APETALA1 promoter (AP1::GUS) was introduced into emf fwa and emf ft double mutants. GUS activity was strongly detected on shoot meristem of emf1-1 single mutants harboring AP1::GUS construct just 5 d after germination. By contrast, GUS activity was undetectable on emf1-1 fwa-1, emf1-1 ft-1, emf2-1 fwa-1, emf2-3 fwa-1 and emf2-3 ft-1 double mutants harboring AP1::GUS construct 10 d after germination. GUS activity was only weakly detected on the apical meristem of 20-day-old emf1-1 fwa-1 and emf2-1 fwa-1 seedlings. During this time, only sessile leaves were produced. Further analysis indicated that AP1 was strongly expressed in 10-day-old emf1-1 and emf2-1 single mutants. Its expression was significantly reduced in all emf1-1 or emf2-1 late-flowering double mutants tested. Similar to AP1, the expression of LEAFY (LFY) was also high in emf1-1 and emf2-1 single mutants and reduced in emf1-1 or emf2-1 late-flowering double mutants. Our results indicate that the precocious expression of AP1 and LFY is dependent not only on the low EMF and FWA activities but also on the expression of most of the late-flowering genes such as FT, FCA, FE, CO and GI. These data also reveal that most late-flowering genes may function downstream of EMF or in pathways distinct from EMF to activate genes specified floral meristem identity during shoot maturation in Arabidopsis.     

拟南芥LEAFY基因在花发育中的网络调控及其生物学功能

重点综述了拟南芥花分生组织特征基因——LEAFY（LFY）基因及其同源基因在花发育中的网络调控及其生物学功能。LFY基因广泛表达于高等植物的营养性和生殖性组织。LFY基因需要与其他基因相互作用,並且表达量达到一定水平时才能促进成花。LFY基因处于成花调控网络的关键位置,不仅调控开花时间和花转变,而且在花序和花的发育中也起重要作用。碳源、植物激素等因子直接或间接地影响LFY基因的表达和作用。提示通过掌握LFY基因的表达调控规律进一步探讨成花机理的可行性。 Abstract:Recent research progress on regulation network and biological roles of LFY gene in Arabidopsis thaliana and its homologue genes in floral development are reviewed emphatically in the present paper.LFY gene expresses widely in both vegetative and reproductive tissues in different higher plants,therefore investigation on role of LFY gene on flowering is of general significance.LFY gene plays an important role to promote flower formation by interaction and coordination with other genes,such as TFL,EMF,AP1,AP2,CAL,FWA,FT,AP3,PI,AG,UFO,CO,LD,GA1 etc,and a critical level of LFY expression is essential.LFY gene not only controls flowering-time and floral transition,but also plays an important role in inflorescence and floral organ development.It was situated at the central site in gene network of flowering regulation,positively or negatively regulates the level or activities of flowering-related genes.Some physiological factors,such as carbon sources,phytohormones,affect directly or indirectly the expression and actions of LFY gene.This indicates that level of LFY expression can also be regulated with physiological methods.It is probable that we can explain the principal mechanism of flowering by regulation network of LFY gene.     

Interactions among APETALA1, LEAFY, and TERMINAL FLOWER1 specify meristem fate.

Upon floral induction, the primary shoot meristem of an Arabidopsis plant begins to produce flower meristems rather than leaf primordia on its flanks. Assignment of floral fate to lateral meristems is primarily due to the cooperative activity of the flower meristem identity genes LEAFY (LFY), APETALA1 (AP1), and CAULIFLOWER. We present evidence here that AP1 expression in lateral meristems is activated by at least two independent pathways, one of which is regulated by LFY. In lfy mutants, the onset of AP1 expression is delayed, indicating that LFY is formally a positive regulator of AP1. We have found that AP1, in turn, can positively regulate LFY, because LFY is expressed prematurely in the converted floral meristems of plants constitutively expressing AP1. Shoot meristems maintain an identity distinct from that of flower meristems, in part through the action of genes such as TERMINAL FLOWER1 (TFL1), which bars AP1 and LFY expression from the influorescence shoot meristem. We show here that this negative regulation can be mutual because TFL1 expression is downregulated in plants constitutively expressing AP1. Therefore, the normally sharp phase transition between the production of leaves with associated shoots and formation of the flowers, which occurs upon floral induction, is promoted by positive feedback interactions between LFY and AP1, together with negative interactions of these two genes with TFL1.     

The matrix coalescent and an application to human single-nucleotide polymorphisms

The floral developmental pathway in Arabidopsis thaliana is composed of several interacting regulatory genes, including the inflorescence architecture gene TERMINAL FLOWER1 (TFL1), the floral meristem identity genes LEAFY (LFY), APETALA1 (AP1), and CAULIFLOWER (CAL), and the floral organ identity genes APETALA3 (AP3) and PISTILLATA (PI). Molecular population genetic analyses of these different genes indicate that the coding regions of AP3 and PI, as well as AP1 and CAL, share similar levels and patterns of nucleotide diversity. In contrast, the coding regions of TFL1 and LFY display a significant reduction in nucleotide variation, suggesting that these sequences have been subjected to a recent adaptive sweep. Moreover, the promoter of TFL1, unlike its coding region, displays high levels of diversity organized into two distinct haplogroups that appear to be maintained by selection. These results suggest that patterns of molecular evolution differ among regulatory genes in this developmental pathway, with the earlier acting genes exhibiting evidence of adaptive evolution.     

Flowering genes in Metrosideros fit a broad herbaceous model encompassing Arabidopsis and Antirrhinum

Molecular studies were conducted on Metrosideros excelsa to determine if the current genetic models for flowering with regard to inflorescence and floral meristem identity genes in annual plants were applicable to a woody perennial. MEL , MESAP1 and METFL1 , the fragments of LEAFY ( LFY ), APETALA1 ( AP1 ) and TERMINAL FLOWER1 ( TFL1 ) equivalents, respectively, were isolated from M. excelsa . Temporal expression patterns showed that MEL and MESAP1 exhibited a bimodal pattern of expression. Expression exhibited during early floral initiation in autumn was followed by down-regulation during winter, and up-regulation in spring as floral organogenesis occurred. Spatial expression patterns of MEL showed that it had greater similarity to FLORICAULA ( FLO ) than to LFY , whereas MESAP1 was more similar to AP1 than SQUAMOSA . The interaction between MEL and METFL1 was more similar to the interaction between FLO and CENTRORADIALIS than that between LFY and TFL1 . Consequently, the three genes from M. excelsa fit a broader herbaceous model encompassing Antirrhinum as well as Arabidopsis , but with differences, such as the bimodal pattern of expression seen with MEL and MESAP1 . In mid-winter, at the time when both MEL and MESAP1 were down-regulated, GA₁ was below the level of detection in M. excelsa buds. Even though application of gibberellin inhibits flowering in members of the Myrtaceae, MEL was responsive to gibberellin with expression in juvenile plants up-regulated by GA₃. However, MESAP1 was not up-regulated indicating that meristem competence was also probably required to promote flowering in M. excelsa .     

花序分生组织特性基因TFL1的系统发育及其功能分析

TFL1同源基因在维持植物营养生长和花序分生组织特性方面起着非常重要的作用,其功能的丧失常导致植物提早开花,花序的正常发育受到抑制,最终茎端形成顶花。至今已经有28种植物的TFL1基因被克隆到,其中包括拟南芥、金鱼草和番茄等模式植物。TFL1 蛋白的系统发育树基本符合物种的亲缘关系。作为花序分生组织特性基因的TFL1与花分生组织特性基因LFY 和AP1相互作用,抑制花序分生组织向花分生组织的转变。TFL1和LFY等基因可用来培育早花新品种,也可用于培育无果的新品种,减少悬铃木、杨、柳等果毛的污染。     

EMF genes regulate Arabidopsis inflorescence development.

Mutations in EMBRYONIC FLOWER (EMF) genes EMF1 and EMF2 abolish rosette development, and the mutants produce either a much reduced inflorescence or a transformed flower. These mutant characteristics suggest a repressive effect of EMF activities on reproductive development. To investigate the role of EMF genes in regulating reproductive development, we studied the relationship between EMF genes and the genes regulating inflorescence and flower development. We found that APETALA1 and AGAMOUS promoters were activated in germinating emf seedlings, suggesting that these genes may normally be suppressed in wild-type seedlings in which EMF activities are high. The phenotype of double mutants combining emf1-2 and apetala1, apetala2, leafy1, apetala1 cauliflower, and terminal flower1 showed that emf1-2 is epistatic in all cases, suggesting that EMF genes act downstream from these genes in mediating the inflorescence-to-flower transition. Constitutive expression of LEAFY in weak emf1, but not emf2, mutants increased the severity of the emf phenotype, indicating an inhibition of EMF activity by LEAFY, as was deduced from double mutant analysis. These results suggest that a mechanism involving a reciprocal negative regulation between the EMF genes and the floral genes regulates Arabidopsis inflorescence development.     

Redundant regulation of meristem identity and plant architecture by FRUITFULL, APETALA1 and CAULIFLOWER

The transition from vegetative to reproductive phases during Arabidopsis development is the result of a complex interaction of environmental and endogenous factors. One of the key regulators of this transition is LEAFY (LFY), whose threshold levels of activity are proposed to mediate the initiation of flowers. The closely related APETALA1 (AP1) and CAULIFLOWER (CAL) meristem identity genes are also important for flower initiation, in part because of their roles in upregulating LFY expression. We have found that mutations in the FRUITFULL (FUL) MADS-box gene, when combined with mutations in AP1 and CAL, lead to a dramatic non-flowering phenotype in which plants continuously elaborate leafy shoots in place of flowers. We demonstrate that this phenotype is caused both by the lack of LFY upregulation and by the ectopic expression of the TERMINAL FLOWER1 (TFL1) gene. Our results suggest that the FUL, AP1 and CAL genes act redundantly to control inflorescence architecture by affecting the domains of LFY and TFL1 expression as well as the relative levels of their activities.     

LEAFY, TERMINAL FLOWER1 and AGAMOUS are functionally conserved but do not regulate terminal flowering and floral determinacy in Impatiens balsamina

In Impatiens balsamina a lack of commitment of the meristem during floral development leads to the continuous requirement for a leaf-derived floral signal. In the absence of this signal the meristem reverts to leaf production. Current models for Arabidopsis state that LEAFY (LFY) is central to the integration of floral signals and regulates flowering partly via interactions with TERMINAL FLOWER1 (TFL1) and AGAMOUS (AG). Here we describe Impatiens homologues of LFY, TFL1 and AG (IbLFY, IbTFL1 and IbAG) that are highly conserved at a sequence level and demonstrate homologous functions when expressed ectopically in transgenic Arabidopsis. We relate the expression patterns of IbTFL1 and IbAG to the control of terminal flowering and floral determinacy in Impatiens. IbTFL1 is involved in controlling the phase of the axillary meristems and is expressed in axillary shoots and axillary meristems which produce inflorescences, but not in axillary flowers. It is not involved in maintaining the terminal meristem in either an inflorescence or indeterminate state. Terminal flowering in Impatiens appears therefore to be controlled by a pathway that uses a different integration system than that regulating the development of axillary flowers and branches. The pattern of ovule production in Impatiens requires the meristem to be maintained after the production of carpels. Consistent with this morphological feature IbAG appears to specify stamen and carpel identity, but is not sufficient to specify meristem determinacy in Impatiens.     

Acceleration of flowering by overexpression of MFT (MOTHER OF FT AND TFL1)

MFT (MOTHER OF FT AND TFL1) is a member of a gene family that includes two important regulators, FT (FLOWERING LOCUS T) and TFL1 (TERMINAL FLOWER 1), in determination of flowering time in Arabidopsis. Although the functions of FT and TFL1 are assigned in the family, the roles of other members are largely unknown. Especially the sequence of MFT is homologous to both FT and TFL1, which act as a floral promoter and an inhibitor, respectively, making it difficult to predict the role of MFT. We performed genetic analyses of MFT to understand its role in floral development. Constitutive expression of MFT led to slightly early flowering under long days. However, a T-DNA insertion allele of MFT did not show obvious phenotype. Further genetic analyses with the loss-of-function alleles of FT, TFL1, and ATC (Arabidopsis Thaliana CENTRORADIALIS homologue) showed that a decrease of MFT activity did not enhance the phenotypes of the single mutants. Taken together, we suggest that MFT functions as a floral inducer and that it may act redundantly in determination of flowering time in Arabidopsis.     

Aa TFL1 confers an age-dependent response to vernalization in perennial Arabis alpina

Flowering of many plants is induced by environmental signals, but these responses can depend on the age of the plant. Exposure of Arabidopsis thaliana to vernalization (winter temperatures) at germination induces flowering, whereas a close perennial relative Arabis alpina only responds if exposed when at least 5 weeks old. We show that vernalization of these older A. alpina plants reduces expression of the floral repressor PEP1 and activates the orthologs of the Arabidopsis flowering genes SOC1 (Aa SOC1) and LFY (Aa LFY). By contrast, when younger plants are vernalized, PEP1 and Aa SOC1 mRNA levels change as in older plants, but Aa LFY is not expressed. We demonstrate that A. alpina TFL1 (Aa TFL1) blocks flowering and prevents Aa LFY expression when young plants are exposed to vernalization. In addition, in older plants, Aa TFL1 increases the duration of vernalization required for Aa LFY expression and flowering. Aa TFL1 has similar functions in axillary shoots, thus ensuring that following a flowering episode vegetative branches are maintained to continue the perennial life cycle. We propose that Aa TFL1 blocks flowering of young plants exposed to vernalization by setting a threshold for a flowering pathway that is increased in activity as the shoot ages, thus contributing to several perennial traits.     

Uncovering genetic and molecular interactions among floral meristem identity genes in Arabidopsis thaliana

The inflorescence meristem produces floral primordia that remain undifferentiated during the first stages of flower development. Genes controlling floral meristem identity include LEAFY (LFY), APETALA1 (AP1), CAULIFLOWER (CAL), LATE MERISTEM IDENTITY 1 (LMI1), SHORT VEGETATIVE PHASE (SVP) and AGAMOUS-LIKE24 (AGL24). The lfy mutant shows partial reversions of flowers into inflorescence shoot-like structures and this phenotype is enhanced in the lfy ap1 double mutant. Here we show that combining the lfy mutant with agl24 and svp single mutants or with the agl24 svp double mutant enhances the lfy phenotype and that the lfy agl24 svp triple mutant phenocopies the lfy ap1 double mutant. Analysis of the molecular interactions between LFY, AGL24 and SVP showed that LFY is a repressor of AGL24 and SVP, whereas LMI1 is a positive regulator of these genes. Moreover, AGL24 and SVP positively regulate AP1 and LFY by direct binding to their regulatory regions. Since all these genes are important for establishing floral meristem identity, regulatory loops are probably important to maintain the correct relative expression levels of these genes.