Isolation and characterization of the C-class MADS-box gene from the distylous pseudo-cereal <Emphasis Type="Italic">Fagopyrum esculentum</Emphasis>

In the model species Arabidopsis thaliana, the floral homeotic C-class gene AGAMOUS (AG) specifies reproductive organ (stamen and carpels) identity and floral meristem determinacy. Gene function analyses in other core eudicots species reveal functional conservation, subfunctionalization and function switch of the C-lineage in this clade. To identify the possible roles of AG-like genes in regulating floral development in distylous species with dimorphic flowers (pin and thrum) and the C function evolution, we isolated and identified an AG ortholog from Fagopyrum esculentum (buckwheat, Family Polygonaceae), an early diverging species of core eudicots preceding the rosids-asterids split. Protein sequence alignment and phylogenetic analysis grouped FaesAG into the euAG lineage. Expression analysis suggested that FaesAG expressed exclusively in developing stamens and gynoecium of pin and thrum flowers. Moreover, FaesAG expression reached a high level in both pin and thrum flowers at the time when the stamens were undergoing rapidly increased in size and microspore mother cells were in meiosis. FaesAG was able to substitute for the endogenous AG gene in specifying stamen and carpel identity and in an Arabidopsis ag-1 mutant. Ectopic expression of FaesAG led to very early flowering, and produced a misshapen inflorescence and abnormal flowers in which sepals had converted into carpels and petals were converted to stamens. Our results confirmed establishment of the complete C-function of the AG orthologous gene preceding the rosids-asterids split, despite the distinct floral traits present in early- and late-diverging lineages of core eudicot angiosperms.     

Floral patterning in Lotus japonicus

Floral patterning in Papilionoideae plants, such as pea (Pisum sativum) and Medicago truncatula, is unique in terms of floral organ number, arrangement, and initiation timing as compared to other well-studied eudicots. To investigate the molecular mechanisms involved in the floral patterning in legumes, we have analyzed two mutants, proliferating floral meristem and proliferating floral organ-2 (pfo-2), obtained by ethyl methanesulfonate mutagenesis of Lotus japonicus. These two mutants showed similar phenotypes, with indeterminate floral structures and altered floral organ identities. We have demonstrated that loss of function of LjLFY and LjUFO/Pfo is likely to be responsible for these mutant phenotypes, respectively. To dissect the regulatory network controlling the floral patterning, we cloned homologs of the ABC function genes, which control floral organ identity in Arabidopsis (Arabidopsis thaliana). We found that some of the B and C function genes were duplicated. RNA in situ hybridization showed that the C function genes were expressed transiently in the carpel, continuously in stamens, and showed complementarity with the A function genes in the heterogeneous whorl. In proliferating floral meristem and pfo-2 mutants, all B function genes were down-regulated and the expression patterns of the A and C function genes were drastically altered. We conclude that LjLFY and LjUFO/Pfo are required for the activation of B function genes and function together in the recruitment and determination of petals and stamens. Our findings suggest that gene duplication, change in expression pattern, gain or loss of functional domains, and alteration of key gene functions all contribute to the divergence of floral patterning in L. japonicus.     

Conversion of leaves into petals in Arabidopsis

More than 200 years ago, Goethe proposed that each of the distinct flower organs represents a modified leaf [1]. Support for this hypothesis has come from genetic studies, which have identified genes required for flower organ identity. These genes have been incorporated into the widely accepted ABC model of flower organ identity, a model that appears generally applicable to distantly related eudicots as well as monocot plants. Strikingly, triple mutants lacking the ABC activities produce leaves in place of flower organs, and this finding demonstrates that these genes are required for floral organ identity [2]. However, the ABC genes are not sufficient for floral organ identity since ectopic expression of these genes failed to convert vegetative leaves into flower organs. This finding suggests that one or more additional factors are required [3, 4]. We have recently shown that SEPALLATA (SEP) represents a new class of floral organ identity genes since the loss of SEP activity results in all flower organs developing as sepals [5]. Here we show that the combined action of the SEP genes, together with the A and B genes, is sufficient to convert leaves into petals.     

The ABCs of flower development: mutational analysis of AP1/FUL‐like genes in rice provides evidence for a homeotic (A)‐function in grasses

The well‐known ABC model describes the combinatorial interaction of homeotic genes in specifying floral organ identities. While the B‐ and C‐functions are highly conserved throughout flowering plants and even in gymnosperms, the A‐function, which specifies the identity of perianth organs (sepals and petals in eudicots), remains controversial. One reason for this is that in most plants that have been investigated thus far, with Arabidopsis being a remarkable exception, one does not find recessive mutants in which the identity of both types of perianth organs is affected. Here we report a comprehensive mutational analysis of all four members of the AP1/FUL‐like subfamily of MADS‐box genes in rice (Oryza sativa). We demonstrate that OsMADS14 and OsMADS15, in addition to their function of specifying meristem identity, are also required to specify palea and lodicule identities. Because these two grass‐specific organs are very likely homologous to sepals and petals of eudicots, respectively, we conclude that there is a floral homeotic (A)‐function in rice as defined previously. Together with other recent findings, our data suggest that AP1/FUL‐like genes were independently recruited to fulfil the (A)‐function in grasses and some eudicots, even though other scenarios cannot be excluded and are discussed.     

CYCLOIDEA 2 Clade Genes: Key Players in the Control of Floral Symmetry,Inflorescence Architecture,and Reproductive Organ Development

Undoubted lines of evidence point out that members of CYCLOIDEA (CYC) 2 clade are essential players to control flower symmetry and, amusingly, also are determinants of capitula architecture (pseudanthium). In several species, CYC-like genes influence the androecium patterning, but to date, the function of these genes in the development of gynoecium organs is less clear. In this review, we first reported details about floral symmetry and an overview of genes and molecular mechanisms regulating the development of zygomorphism in different angiosperm lineages (e.g., basal and core eudicots and monocots). Then, we paid emphasis on the role of CYC-like genes in the development of heterogamous inflorescence of sunflower as well as other Asteraceae and some species within the Dipsacaceae family. Helianthus annuus is particularly attractive because it represents a useful model to study the role of CYC-like genes on shaping floral corolla as well as the differentiation of reproductive organs in different flowers of pseudanthia. A special attention was reserved to inflorescence morphology mutants of sunflower (i.e., Chrysanthemoids2 and tubular ray flower) because they provide useful information on the role of CYC-like genes in the radiate capitulum evolution. Finally, we discuss data from literature to suggest that CYC-like genes are also co-opted to regulate stamen and carpel differentiation likely throughout their interaction with the cell cycle and flower organ identity genes. The recruitment of reproductive organs in ray flowers also supports the phylogenetic origin of a radiate inflorescence of sunflower from a discoid capitulum and suggests that in sterile zygomorphic ray flower primordia the latent identity to differentiate both microsporangium and macrosporangium was conserved.     

Floral organ size control: Interplay between organ identity,developmental compartments and compensation mechanisms

Growth of lateral organs is a complex mechanism that starts with formation of lateral primordia.Basal developmental programs like polarity, organ identity and environmental cues influence the final organ size achieved via coordinated cell division and expansion. recent evidence shows that the precise balance between these two processes, known as compensation mechanisms, seems to be influenced by the identity of the organ. Furthermore, studies of mutants affected in floral organ size suggest the existence of developmental compartments within different floral whorls that show distinct compensation behaviors.Key words: Antirrhinum majus, cell division, cell expansion, COMPACTA ÄHNLICH, compensation, floral size, FORMOSA, NITIDA, organ identity     

Isolation and Characterization of an AGAMOUS-Like Gene from Magnolia wufengensis (Magnoliaceae)

The floral homeotic C function gene AGAMOUS (AG) plays crucial roles in Arabidopsis development by specifying stamen and carpel identity, repressing A-class genes, as well as regulating floral meristem determination. Although the function of AG homologs from other core eudicots appears highly conserved, the role of AG orthologs in the design of floral architecture in basal angiosperm remains unknown. We isolated and identified an AG ortholog from Magnolia wufengensis, a woody basal angiosperm belonging to the Magnoliaceae. Sequence and phylogenetic analyses revealed that it is a clade member of the euAG lineage, and hence, the gene is referred to as MAwuAG (M. wu fengensis AGAMOUS). Moreover, two highly conserved motifs specific to C proteins, AG motifs I and II, are found in the C-terminal regions of the MAwuAG protein, but the N-terminal extensions that usually appear in euAG lineage members from eudicots were not found in MAwuAG. The cDNA has the first in-frame ATG immediately preceding the MADS domain. A semi-quantitative PCR analysis showed that the expression of MAwuAG was restricted to reproductive organs of stamens and carpels. The transgenic Arabidopsis containing 35S::MAwuAG displayed extremely early flowering, bigger stamens and carpels, and homeotic conversion of petals into staminoid organs, but ectopic expression of MAwuAG in the first whorls failed to convert the sepals into carpeloid structures that are usually observed in the overexpression transgenic Arabidopsis of AG orthologs from other core eudicots. In addition, the phenotype of the transgenic 35S::MAwuAG Arabidopsis revealed that the abscission of the outer three floral whorls (sepals, petals, and stamens) was inhibited.     

The expression of floral organ identity genes in contrasting water lily cultivars

The floral organs of typical eudicots such as Arabidopsis thaliana are arranged in four characteristic whorls, namely the sepal, petal, stamen and carpel, and the “ABC” floral organ identity model has been based on this arrangement. However, the floral organs in most basal angiosperms are spirally arranged with a gradual transition from the inside to outside, and an alternative model referred to as “fading borders” was developed to take account of this. The flower morphology of the water lily was tested against the “fading borders” model by determining the expression profile of the six primary floral organ identity genes AP2, AGL6, AP3, PI, AG and SEP1 in two cultivars showing contrasting floral morphology. In addition, to get accurate floatation of the genes expression level from outer to inner, we divided the floral organs into eight whorls according to morphological features. All these genes were expressed throughout all whorls of the flower, but their expression level changed gradually from the outside of the flower to its inside. This pattern was consistent with the “fading borders” model.     

Conserved Functions of the MATE Transporter BIG EMBRYO1 in Regulation of Lateral Organ Size and Initiation Rate

Genetic networks that determine rates of organ initiation and organ size are key regulators of plant architecture. Whereas several genes that influence the timing of lateral organ initiation have been identified, the regulatory pathways in which these genes operate are poorly understood. Here, we identify a class of genes implicated in regulation of the lateral organ initiation rate. Loss-of-function mutations in the MATE transporter encoded by maize (Zea mays) Big embryo 1 (Bige1) cause accelerated leaf and root initiation as well as enlargement of the embryo scutellum. BIGE1 is localized to trans-Golgi, indicating a possible role in secretion of a signaling molecule. Interestingly, phenotypes of bige1 bear striking similarity to cyp78a mutants identified in diverse plant species. We show that a CYP78A gene is upregulated in bige1 mutant embryos, suggesting a role for BIGE1 in feedback regulation of a CYP78A pathway. We demonstrate that accelerated leaf formation and early flowering phenotypes conditioned by mutants of Arabidopsis thaliana BIGE1 orthologs are complemented by maize Bige1, showing that the BIGE1 transporter has a conserved function in regulation of lateral organ initiation in plants. We propose that BIGE1 is required for transport of an intermediate or product associated with the CYP78A pathway.     

Virus-induced gene silencing of PEAM4 affects floral morphology by altering the expression pattern of PsSOC1a and PsPVP in pea

pea-MADS4 (PEAM4) regulates floral morphology in Pisum sativum L., however, its molecular mechanisms still remain unclear. Virus-induced gene silencing (VIGS) is a recently developed reverse genetic approach that facilities an easier and more rapid study of gene functions. In this study, the PEAM4 gene was effectively silenced by VIGS using a pea early browning virus (PEBV) in wild type pea JI992. The infected plants showed abnormal phenotypes, as the floral organs, especially the sepals and petals changed in both size and shape, which made the corolla less closed. The petals changed in morphology and internal symmetry with, the stamens reduced and carpel dehisced. Larger sepals and longer tendrils with small cauline leaves appeared, with some sepals turning into bracts, and secondary inflorescences with fused floral organs were formed, indicating a flower-to-inflorescence change. The infected plants also displayed a delayed and prolonged flowering time. The PEAM4-VIGS plants with altered floral morphology were similar to the pim (proliferating inflorescence meristem) mutant and also mimicked the phenotypes of ap1 mutants in Arabidopsis. The expression pattern of the homologous genes PsSOC1a and PsSVP, which were involved in flowering time and florescence morphological control downstream of PEAM4, were analyzed by real-time RT-PCR and mRNA in situ hybridization. PsSOC1a and PsSVP were ectopically expressed and enhanced in the floral meristems from PEAM4-silenced plants. Our data suggests that PEAM4 may have a similar molecular mechanism as AtAP1, which inhibits the expression of PsSOC1a and PsSVP in the floral meristem from the early stages of flower development. As such, in this way PEAM4 plays a crucial role in maintaining floral organ identity and flower development in pea.     

Cell Number Regulator1 Affects Plant and Organ Size in Maize: Implications for Crop Yield Enhancement and Heterosis

Genes involved in cell number regulation may affect plant growth and organ size and, ultimately, crop yield. The tomato (genus Solanum) fruit weight gene fw2.2, for instance, governs a quantitative trait locus that accounts for 30% of fruit size variation, with increased fruit size chiefly due to increased carpel ovary cell number. To expand investigation of how related genes may impact other crop plant or organ sizes, we identified the maize (Zea mays) gene family of putative fw2.2 orthologs, naming them Cell Number Regulator (CNR) genes. This family represents an ancient eukaryotic family of Cys-rich proteins containing the PLAC8 or DUF614 conserved motif. We focused on native expression and transgene analysis of the two maize members closest to Le-fw2.2, namely, CNR1 and CNR2. We show that CNR1 reduced overall plant size when ectopically overexpressed and that plant and organ size increased when its expression was cosuppressed or silenced. Leaf epidermal cell counts showed that the increased or decreased transgenic plant and organ size was due to changes in cell number, not cell size. CNR2 expression was found to be negatively correlated with tissue growth activity and hybrid seedling vigor. The effects of CNR1 on plant size and cell number are reminiscent of heterosis, which also increases plant size primarily through increased cell number. Regardless of whether CNRs and other cell number–influencing genes directly contribute to, or merely mimic, heterosis, they may aid generation of more vigorous and productive crop plants.     

MOSAIC FLORAL ORGANS1, an AGL6-Like MADS Box Gene,Regulates Floral Organ Identity and Meristem Fate in Rice

Floral organ identity and meristem determinacy in plants are controlled by combinations of activities mediated by MADS box genes. AGAMOUS-LIKE6 (AGL6)-like genes are MADS box genes expressed in floral tissues, but their biological functions are mostly unknown. Here, we describe an AGL6-like gene in rice (Oryza sativa), MOSAIC FLORAL ORGANS1 (MFO1/MADS6), that regulates floral organ identity and floral meristem determinacy. In the flower of mfo1 mutants, the identities of palea and lodicule are disturbed, and mosaic organs were observed. Furthermore, the determinacy of the floral meristem was lost, and extra carpels or spikelets developed in mfo1 florets. The expression patterns of floral MADS box genes were disturbed in the mutant florets. Suppression of another rice AGL6-like gene, MADS17, caused no morphological abnormalities in the wild-type background, but it enhanced the phenotype in the mfo1 background, indicating that MADS17 has a minor but redundant function with that of MFO1. Whereas single mutants in either MFO1 or the SEPALLATA-like gene LHS1 showed moderate phenotypes, the mfo1 lhs1 double mutant showed a severe phenotype, including the loss of spikelet meristem determinacy. We propose that rice AGL6-like genes help to control floral organ identity and the establishment and determinacy of the floral meristem redundantly with LHS1.