Interaction of ABRUPTUS/PINOID and LEAFY genes during floral morphogenesis in Arabidopsis thaliana (L.) Heynh

Analysis of interactions between mutations abruptus and leafy and previous data on interaction of abruptus with homeotic mutations apetala1, apetala2, and apetala3 showed that the functions of the ABRUPTUS/PINOID (ABR/PID) gene are as follows: (1) it directs pattern formation in inflorescence axis specifying the development either of floral meristem (FM) or of cauline leaves; (2) in concert with the leafy gene, it participates in the formation of FM; (3) it is involved in the determination and the formation of floral organ primordia in the first, second, and third whorls. Auxin accumulation in the abr mutant cells in callus culture was shown indicating the involvement of the ABR/PID gene in regulation of auxin efflux from cells. It is suggested that the ABR/PID expression in the sites of formation of FM and floral organs leads to local reduction in auxin level, which in turn, enhances expression of the LFY and homeotic genes responsible for FM formation and differentiation.     

Evolution of MADS-box gene induction by FLO/LFY genes

Some MADS-box genes function as floral homeotic genes. The Arabidopsis LFY gene is a positive regulator of floral homeotic genes, and homologs of the FLO/LFY gene family in other angiosperms and gymnosperms are likely to have a similar function. To investigate the origin of the floral homeotic gene regulatory cascade involving the FLO/LFY gene, FLO/LFY homologs were cloned from a leptosporangiate fern (Ceratopteris richardii), two eusporangiate ferns (Angiopteris lygodiifolia and Botrychium multifidum var. robustum), three fern allies (Psilotum nudum, Equisetum arvense, and Isoetes asiatica), and a moss (Physcomitrella patens). The FLO/LFY gene phylogenetic tree indicates that both duplication and loss of FLO/LFY homologs occurred during the course of vascular plant evolution. The expression patterns of the Ceratopteris LFY genes (CrLFY1 and 2) were assessed. CrLFY1 expression was prominent in tissues including shoot tips and circinate reproductive leaves, but very weak in other tissues examined. Expression of CrLFY2 was also prominent in tissues, including shoot tips and circinate reproductive leaves. These patterns of expression are dissimilar to that of any Ceratopteris MADS-box gene previously reported, suggesting that the induction of MADS-box genes by FLO/LFY is not established at the stage of ferns. Received: 4 January 2001 / Accepted: 28 February 2001     

Genetic complementation of a floral homeotic mutation,apetala3, with an Arabidopsis thaliana gene homologous to DEFICIENS of Antirrhinum majus

Among the homeotic mutants with altered floral organs, two mutants of Arabidopsis thaliana, apetala3 and pistillata, and two mutants of Antirrhinum majus, deficiens and globosa, have a homeotic conversion of the floral organs in whorl 2 and 3, namely petals to sepals and stamens to carpels. We have isolated a homologue of the DEFICIENS gene from A. thaliana wild type and shown complete complementation of apetala3 mutation by introducing the isolated gene using Agrobacterium-mediated transformation. These results show that the APETALA3 is a homologue of DEFICIENS structurally and functionally. The 5-upstream region of APETALA3 contains three SRE-like sequence, where MADS box-containing proteins are assumed to bind and regulate expression in tissue-and stage-specific manner.     

A Molecular Framework for Auxin-Mediated Initiation of Flower Primordia

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Highlights? Auxin-activated MP directly induces key floral regulators LFY, ANT, and AIL6 ? The LFY promoter contains conserved biologically relevant auxin response elements ? LFY, ANT, and AIL6 have redundant roles in flower primordium initiation ? LFY feeds back to the auxin pathway at least in part by directly inducing PID     

Arabidopsis thaliana (L.) Heynh. as a model object for studying genetic control of morphogenesis

A vast amount of information on the genetic control of plant development has been obtained in Arabidopsis thaliana with classical genetic and molecular biological methods. The genes involved in multistep regulation of floral morphogenesis have been identified. The formation of floral meristem is controlled by the LEAFY (LFY), UNUSUAL FLORAL ORGANS (UFO), APETALA1 (AP1), and APETALA2 (AP2) genes. Studies of the abruptus and bractea recessive monogenic mutants from the collection of the Department of Genetics and Selection, Moscow State University, showed that the ABRUPTUS (ABR) and BRACTEA (BRA) genes also play an important role in inflorescence differentiation. The ABR gene controls the early formation of organ primordia on the inflorescence and the formation of floral organ primordia after floral initiation. Further differentiation of inflorescence organ primordia in vegetative or generative organs depends on the activity of the LFY gene, and floral organ identity is determined by the homeotic genes. Presumably, the major function of the ABR gene is to determine the auxin polar transport. The BRA gene suppresses the development of bracts on the inflorescence and constrains cell division at the base of primordia of rosette and cauline leaves.     

Function of the apetala-1 gene during Arabidopsis floral development.

We have characterized the floral phenotypes produced by the recessive homeotic apetala 1-1 (ap1-1) mutation in Arabidopsis. Plants homozygous for this mutation display a homeotic conversion of sepsis into brachts and the concomitant formation of floral buds in the axil of each transformed sepal. In addition, these flowers lack petals. We show that the loss of petal phenotype is due to the failure of petal primordia to be initiated. We have also constructed double mutant combinations with ap1 and other mutations affecting floral development. Based on these results, we suggest that the AP1 and the apetala 2 (AP2) genes may encode similar functions that are required to define the pattern of where floral organs arise, as well as for determinate development of the floral meristem. We propose that the AP1 and AP2 gene products act in concert with the product of the agamous (AG) locus to establish a determinate floral meristem, whereas other homeotic gene products are required for cells to differentiate correctly according to their position. These results extend the proposed role of the homeotic genes in floral development and suggest new models for the establishment of floral pattern.     

Patterns and Polarity in Floral Meristem and Floral Organ Initiation

The initial event in plant floral organogenesis is bract specification, followed by floral meristem (FM) initiation in bract axils, but initiation signals and the interplay between both lateral organs remain unelucidated. Floral organs are initiated on the flanks of the outgrowing FM and the enormous diversity in floral morphology throughout the plant kingdom reflects variations in organ position, meristy and ontogeny. Classical models of floral development have focused on Arabidopsis, which has mostly actinomorphic flowers, and Antirrhinum, which exhibits zygomorphy, although neither species is typical or representative of angiosperm flower diversity. Although the ABCE model defines a centripetal model of organ identity establishment in different whorls, the characterization of floral organ initiation in many species has relied on their morphological appearance, due to a lack of founder cell-specific markers. Recent progress in early Arabidopsis floral development using histology, molecular markers and mutants has led to refinements of existing floral organ initiation paradigms. In Arabidopsis, sepals initiate unidirectionally, in a temporal window characterized by the absence of CLAVATA3 and WUSCHEL stem cell markers and are partly dependent on PRESSED FLOWER function, whereas initiation of inner-whorl organs occurs centripetally. Arabidopsis mutants reveal that the FM is highly polarized along an ab-/adaxial axis and a comparison of floral development in Arabidopsis and Antirrhinum suggests that heterochrony of conserved gene functions has been evolutionarily adaptive.

This review discusses current views on FM and organ specification signals, the gene regulatory networks that underlie floral meristem polarity, and analogies between the development of floral and leaf primordia as lateral organs. Alternative stem-cell proliferation mechanisms and the bifurcation of founder cell populations can help to explain the diversity in floral diversity throughout the plant kingdom and underpin comparative evolutionary biology and macroevolution. An analysis of plants with divergent body plans at the level of organ specification is urgently needed.     

SUPERMAN regulates floral whorl boundaries through control of auxin biosynthesis

Proper floral patterning, including the number and position of floral organs in most plant species, is tightly controlled by the precise regulation of the persistence and size of floral meristems (FMs). In Arabidopsis, two known feedback pathways, one composed of WUSCHEL (WUS) and CLAVATA3 (CLV3) and the other composed of AGAMOUS (AG) and WUS, spatially and temporally control floral stem cells, respectively. However, mounting evidence suggests that other factors, including phytohormones, are also involved in floral meristem regulation. Here, we show that the boundary gene SUPERMAN (SUP) bridges floral organogenesis and floral meristem determinacy in another pathway that involves auxin signaling. SUP interacts with components of polycomb repressive complex 2 (PRC2) and fine‐tunes local auxin signaling by negatively regulating the expression of the auxin biosynthesis genes YUCCA1/4 (YUC1/4). In sup mutants, derepressed local YUC1/4 activity elevates auxin levels at the boundary between whorls 3 and 4, which leads to an increase in the number and the prolonged maintenance of floral stem cells, and consequently an increase in the number of reproductive organs. Our work presents a new floral meristem regulatory mechanism, in which SUP, a boundary gene, coordinates floral organogenesis and floral meristem size through fine‐tuning auxin biosynthesis.     

The tropical cedar tree (Cedrela fissilis Vell., Meliaceae) homolog of the Arabidopsis LEAFY gene is expressed in reproductive tissues and can complement Arabidopsis leafy mutants

A homolog of FLORICAULA/LEAFY, CfLFY (for Cedrela fissilis LFY), was isolated from tropical cedar. The main stages of the reproductive development in C. fissilis were documented by scanning electron microscopy and the expression patterns of CfLFY were studied during the differentiation of the floral meristems. Furthermore, the biological role of the CfLFY gene was assessed using transgenic Arabidopsis plants. CfLFY showed a high degree of similarity to other plant homologs of FLO/LFY. Southern analysis showed that CfLFY is a single-copy gene in the tropical cedar genome. Northern blot analysis and in situ hybridization results showed that CfLFY was expressed in the reproductive buds during the transition from vegetative to reproductive growth, as well as in floral meristems and floral organs but was excluded from the vegetative apex and leaves. Transgenic Arabidopsis lfy26 mutant lines expressing the CfLFY coding region, under the control of the LFY promoter, showed restored wild-type phenotype. Taken together, our results suggest that CfLFY is a FLO/LFY homolog probably involved in the control of tropical cedar reproductive development. Accession numbers: AY633621 (CfLFY gene) and AY633622 (CfLFY mRNA)     

A novel role of BELL1-like homeobox genes, PENNYWISE and POUND-FOOLISH, in floral patterning

Flowers are determinate shoots comprised of perianth and reproductive organs displayed in a whorled phyllotactic pattern. Floral organ identity genes display region-specific expression patterns in the developing flower. In Arabidopsis, floral organ identity genes are activated by LEAFY (LFY), which functions with region-specific co-regulators, UNUSUAL FLORAL ORGANS (UFO) and WUSCHEL (WUS), to up-regulate homeotic genes in specific whorls of the flower. PENNYWISE (PNY) and POUND-FOOLISH (PNF) are redundant functioning BELL1-like homeodomain proteins that are expressed in shoot and floral meristems. During flower development, PNY functions with a co-repressor complex to down-regulate the homeotic gene, AGAMOUS (AG), in the outer whorls of the flower. However, the function of PNY as well as PNF in regulating floral organ identity in the central whorls of the flower is not known. In this report, we show that combining mutations in PNY and PNF enhance the floral patterning phenotypes of weak and strong alleles of lfy, indicating that these BELL1-like homeodomain proteins play a role in the specification of petals, stamens and carpels during flower development. Expression studies show that PNY and PNF positively regulate the homeotic genes, APETALA3 and AG, in the inner whorls of the flower. Moreover, PNY and PNF function in parallel with LFY, UFO and WUS to regulate homeotic gene expression. Since PNY and PNF interact with the KNOTTED1-like homeodomain proteins, SHOOTMERISTEMLESS (STM) and KNOTTED-LIKE from ARABIDOPSIS THALIANA2 (KNAT2) that regulate floral development, we propose that PNY/PNF-STM and PNY/PNF-KNAT2 complexes function in the inner whorls to regulate flower patterning events.     

A phylloid ground state of reverted floral specimens of Psophocarpus tetragonolobus (L.) DC (Fabaceae): Cancelled floral meristem and continued floral organ identity

Recombinants of Psophocarpus tetragonolobus (L.) DC (Fabaceae), homozygous for a recessive allele of a master homeotic gene reverted from a normal photosynthetic, sexual reproductive nature to a vegetative, non-reproductive nature. This included transmutative transformation of floral meristems to a non-sexual phylloid floral ground state where the virescent organs maintained their identity but floral meristem identity was cancelled thus giving rise to a form of anachronic reversion. This was usually followed by a scenario of phyllotactic alterations involving the elongation of the floral axis which physically transformed flowers, in varying degrees of spatial permutations by the formation of ancestral floral structures, including gynophore and a pericladial stalk: a form of paleochronic reversion. Research verified that an allele of the master homeotic gene responsible for this phenomenon is a prerequisite to that scenario. Specific permutations are directly controlled by at least four additional homeotic genes recognized, defined and functionally characterized herein. Their qualitative functions (i.e. dominant or recessive) are responsible respectively for the carpel form, being either vascularized (VSCARP) or digonolobe (vscarp); the state of the gynophore being formed (GNF) or nascent (gnf); the state of the pericladial stalk being formed (PCL) or nascent (pcl) and the bracts position remaining parallel (BCT:PRL) at the calyx (or on the pericladial stalk) or being dislocated due to an interbractial stem formation between bracts (bct:prl). Results indicate that floral meristem identity once established can naturally be cancelled with little or no effect on floral organ identity.     

A Defect in Zinc Finger Protein Double B-box 1a (DBB1a) Causes Abnormal Floral Development in <Emphasis Type="Italic">Arabidopsis</Emphasis>

The double B-box (DBB) type zinc finger protein has thus far been shown to be involved in photomorphegenesis in Arabidopsis thaliana. Here, we show that DBB1a is expressed in the embryo, cytolden, and flower. Misexpression of DBB1a in mutant plants resulted in abnormal numbers and patterns of floral organs. We further show that DBB1a could regulate expression of several floral homeotic genes, including APETALA 2, APETALA 3, PISTILLATA, and AGAMOUS. Interestingly, expression of the microRNA gene MiR172, which is involved in organ boundary establishment, was also misregulated in the dbb1a mutant plants. Our study identified a previously uncharacterized role of DDB1a in regulation of expression of floral homeotic genes and miR172, which is important for understanding of floral pattern formation.