Negative regulation of the Arabidopsis homeotic gene AGAMOUS by the APETALA2 product.

We characterized the distribution of AGAMOUS (AG) RNA during early flower development in Arabidopsis. Mutations in this homeotic gene cause the transformation of stamens to petals in floral whorl 3 and of carpels to another ag flower in floral whorl 4. We found that AG RNA is present in the stamen and carpel primordia but is undetectable in sepal and petal primordia throughout early wild-type flower development, consistent with the mutant phenotype. We also analyzed the distribution of AG RNA in apetela2 (ap2) mutant flowers. AP2 is a floral homeotic gene that is necessary for the normal development of sepals and petals in floral whorls 1 and 2. In ap2 mutant flowers, AG RNA is present in the organ primordia of all floral whorls. These observations show that the expression patterns of the Arabidopsis floral homeotic genes are in part established by regulatory interactions between these genes.     

Sexual dimorphism and gynoecium size variation in the andromonoecious shrub Caesalpinia gilliesii

The degree of sexual dimorphism in flowers and inflorescences can be evaluated early in flower development through the study of floral organ size co-variation. In the present work, the gynoecium-androecium size relationship was studied to assess the degree of sexual expression in flowers and inflorescences of the andromonoecious shrub Caesalpinia gilliesii. The co-variation pattern of floral organ sizes was compared between small and large inflorescences, under the hypothesis that inflorescence size reflected differential resource availability. Also, staminate and perfect flowers were collected from three populations and compared on the basis of gynoecium, ovule length, filament length, pollen size and number. The obtained results indicated that staminate and perfect flowers differed only in the gynoecium and ovule length, whereas filament length, pollen size, and number varied across populations. The gynoecium size was smaller and its variability was much higher in staminate than in perfect flowers, as explained by a recent hypothesis about pollinator-mediated gynoecium size selection acting upon perfect flowers. The analysis of the gynoecium-androecium size relationship during flower development, revealed a dissociation of gynoecium growth relative to other floral structures in some buds. Lower gynoecium-androecium regression slopes and smaller gynoecia length characterized smaller inflorescences, thus reflecting the fact that sexual expression was more male-biased. This trend is in agreement with a differential resource-related response at the inflorescence level, however, post-mating resource allocation and the inclusion of other modular levels may also help us to understand the variation in sexual dimorphism in this species.     

UNUSUAL FLORAL ORGANS Controls Meristem Identity and Organ Primordia Fate in Arabidopsis

A novel gene that is involved in regulating flower initiation and development has been identified in Arabidopsis. This gene has been designated UNUSUAL FLORAL ORGANS (UFO), with five corresponding nuclear recessive alleles designated ufo[middot]1 to ufo[middot]5. Under short day-length conditions, ufo homozygotes generate more coflorescences than do the wild type, and coflorescences often appear apical to the first floral shoot, resulting in a period of inflorescence development in which regions of floral and coflorescence shoots are produced alternately. ufo enhances the phenotype of weak leafy alleles, and the double mutant Ufo-1 Apetala1-1 produces only coflorescence-like shoots, suggesting that these two genes control different aspects of floral initiation. Floral development was also altered in Ufo plants. Ufo flowers have an altered organ number in all whorls, and organs in the first, second, and third whorls exhibit variable homeotic transformations. Ufo single and double mutant phenotypes suggest that the floral changes result from reduction in class B floral homeotic gene expression and fluctuations in the expression boundaries of class C function and FLO10. Surprisingly, in situ hybridization analysis revealed no obvious differences in expression pattern or level in developing Ufo flowers compared with that of the wild type for any class B or C gene studied. We propose that UFO acts in concert with known floral initiation genes and regulates the domains of floral homeotic gene function.     

A nucleostemin-like GTPase required for normal apical and floral meristem development in Arabidopsis

Mammalian nucleostemin (NS) is preferentially expressed in stem cells and acts to promote cell cycle progression. In plants, stem cell activities have to be terminated during flower development, and this process requires the activation of AGAMOUS (AG) gene expression. Here, a nucleostemin-like 1 gene, NSN1, is shown to be required for flower development in Arabidopsis. The NSN1 mRNA was found in the inflorescence meristem and floral primordia, and its protein was localized to the nucleoli. Both heterozygous and homozygous plants developed defective flowers on inflorescences that were eventually terminated by the formation of carpelloid flowers. Overexpression of NSN1 resulted in loss of apical dominance and formation of defective flowers. Expression of the AG gene was found to be up-regulated in nsn1. The carpelloid flower defect of nsn1 was suppressed by the ag mutation in the nsn1 ag double mutant, whereas double mutants of nsn1 apetala2 (ap2) displayed enhanced defective floral phenotypes. These results suggest that in the delicately balanced regulatory network, NSN1 acts to repress AG and plays an additive role with AP2 in floral organ specification. As a midsize nucleolar GTPase, NSN1 represents a new class of regulatory proteins required for flower development in Arabidopsis.     

A petunia MADS box gene involved in the transition from vegetative to reproductive development.

We have identified a novel petunia MADS box gene, PETUNIA FLOWERING GENE (PFG), which is involved in the transition from vegetative to reproductive development. PFG is expressed in the entire plant except stamens, roots and seedlings. Highest expression levels of PFG are found in vegetative and inflorescence meristems. Inhibition of PFG expression in transgenic plants, using a cosuppression strategy, resulted in a unique nonflowering phenotype. Homozygous pfg cosuppression plants are blocked in the formation of inflorescences and maintain vegetative growth. In these mutants, the expression of both PFG and the MADS box gene FLORAL BINDING PROTEIN26 (FBP26), the putative petunia homolog of SQUAMOSA from Antirrhinum, are down-regulated. In hemizygous pfg cosuppression plants initially a few flowers are formed, after which the meristem reverts to the vegetative phase. This reverted phenotype suggests that PFG, besides being required for floral transition, is also required to maintain the reproductive identity after this transition. The position of PFG in the hierarchy of genes controlling floral meristem development was investigated using a double mutant of the floral meristem identity mutant aberrant leaf and flower (alf) and the pfg cosuppression mutant. This analysis revealed that the pfg cosuppression phenotype is epistatic to the alf mutant phenotype, indicating that PFG acts early in the transition to flowering. These results suggest that the petunia MADS box gene, PFG, functions as an inflorescence meristem identity gene required for the transition of the vegetative shoot apex to the reproductive phase and the maintenance of reproductive identity.     

A flourishing time for flower development. Workshop on Flower Development, sponsored by Fundación Juan March, Madrid, Spain, March 11-13, 1991.

Molecular genetics has recently erupted in the field of flower development, an area of research traditionally cultivated by plant physiologists. The isolation and molecular characterization of seven homeotic genes (four in Antirrhinum majus and three in Arabidopsis thaliana) that control both floral organogenesis and the transition from inflorescences to floral meristems is leading to major breakthroughs in the understanding of the mechanisms governing flower development. This has already had a great impact among plant physiologists, who are incorporating mutant analysis into studies of floral induction and flower development. We are still missing data about the nature of the pollen product of the S-locus in self-incompatibility systems, although current experimental approaches might provide this information in the near future. Gene technology appears to have a high potential in hybrid seed production through the construction of male sterile plants as well as of plants able to restore fertility. The study of genes regulating pigment formation in flowers continues to provide interesting data on gene expression in plants, in which phenomena such as co-suppression and methylation seem to play an important role. Altogether, one can predict that very exciting times are coming in the field of flower development.     

Genetic Control by Arabidopsis Genes LEUNIG and FILAMENTOUS FLOWER in Gynoecium Fusion

Arabidopsis gene FILAMENTOUS FLOWER (FIL) has been demonstrated to control the formation and development of inflorescence and floral meristems. This includes an early step in the establishment of a flower-forming domain within the floral primordium and the establishment of floral meristem identity. Another Arabidopsis gene LEUNIG (LUG) was previously found to specify the identity of the floral organ and control gynoecium fusion. In this paper, we describe floral phenotypes of a newly isolated fil allele, fil-21, and the phenotypic comparison of gynoecia between the fil-21 single mutant and fil-21 lug-101 double mutant. The gynoecium of fil-21 displays a well-fused structure, while that of the strong lug allele, lug-101, is unfused except at the gynoecium apex. However, gynoecia are markedly affected in the fil-21 lug-101 double mutant, being unfused. In late-appearing flowers of the double mutant, the gynoecia can even separate completely into several parts. These results suggest that LUG and FIL have a functional domain that is partially redundant in flower development, and synergistically regulate the gynoecium fusion. Received 18 June 2001/ Accepted in revised form 1 October 2001     

Plasticity of floral sex allocation within inflorescences of hermaphrodite Aconitum gymnandrum

Aims Sex allocation in plants is often plastic, enabling individuals to adjust to variable environments. However, the predicted male-biased sex allocation in response to low resource conditions has rarely been experimentally tested in hermaphroditic plants. In particular, it is unknown whether distal flowers in linear inflorescences show a larger shift to male allocation relative to basal flowers when resources are reduced. In this study, we measure position-dependent plasticity of floral sex allocation within racemes of Aconitum gymnandrum in response to reduced resource availability.Methods Using a defoliation treatment in the field applied to potted plants from a nested half-sibling design, we examined the effects of the treatment, flower position, family and their interactions.Important findings Allocation to male function increased with more distal flower position, while female allocation either did not change with position or declined at the most distal flowers. Defoliation significantly reduced the mass of both the androecium and gynoecium, but not anther number or carpel number. Gynoecial mass declined more strongly with defoliation than did androecial mass, resulting in a significant increase in the androecium/gynoecium ratio as predicted by sex allocation theory. Plastic responses of androecium mass and gynoecium mass were affected by flower position, with less mass lost in basal flowers, but similar plastic magnitude in both sexual traits across flower position lead to consistent variation in the androecium/gynoecium ratio along the inflorescence. A significant treatment*paternal family interaction for the androecium/gynoecium ratio is evidence for additive genetic variation for plastic floral sex allocation, which means that further evolution of allocation can occur.     

Is LEAFY a useful marker gene for the flower-inflorescence boundary in the Euphorbia cyathium?

The flower-like reproductive structure of Euphorbia s.l. (Euphorbiaceae) is widely believed to have evolved from an inflorescence, and is therefore interpreted as a special type of pseudanthium, termed a cyathium. However, fuzzy morphological boundaries between the inflorescence, individual flowers, and organs have fuelled the suggestion that the cyathium does not merely superficially resemble a flower but could actually share developmental genetic pathways with a conventional flower. To test this hypothesis, immunolocalizations of FLORICAULA/LEAFY (LFY), a protein associated with floral identity in many angiosperm species, were performed in developing cyathia of different species of Euphorbia. Expression of the LFY protein was found not only in individual floral primordia (as predicted from results in the model organisms Arabidopsis and Anthirrhinum), but also in the cyathium primordium and in the primordia of partial male inflorescences. These results provide further evidence that the evolution of floral traits in pseudanthial inflorescences often involves expression of floral development genes in the inflorescence apex. This finding blurs the conventional rigid distinction between flowers and inflorescences.     

Expression of the Arabidopsis floral homeotic gene AGAMOUS is restricted to specific cell types late in flower development.

Mutations in the AGAMOUS (AG) gene cause transformations in two adjacent whorls of the Arabidopsis flower. Petals develop in the third floral whorl rather than the normal stamens, and the cells that would normally develop into the fourth whorl gynoecium behave as if they constituted an ag flower primordium. Early in flower development, AG RNA is evenly distributed throughout third and fourth whorl organ primordia but is not present in the organ primordia of whorls one and two. In contrast to the early expression pattern, later in flower development, AG RNA is restricted to specific cell types within the stamens and carpels as cellular differentiation occurs in those organs. Ectopic AG expression patterns in flowers mutant for the floral homeotic gene APETELA2 (AP2), which regulates early AG expression, suggest that the late AG expression is not directly dependent on AP2 activity.     

Characterization of unisexual flower development in the endangered mahogany tree Swietenia macrophylla King. (Meliaceae)

The selection of candidate plus trees of desirable phenotypes from tropical forest trees and the rapid devastation of the natural environments in which these trees are found have created the need for a more detailed knowledge of the floral and reproductive biology of tropical tree species. In this article, the organogenic processes related to unisexual flower development in tropical mahogany, Swietenia macrophylla , are described. Mahogany inflorescences at different developmental stages were evaluated using scanning electron microscopy or optical microscopy of histological sections. The unisexual flowers of S. macrophylla are usually formed in a thyrse, in which the positions of the female and male flowers are not random. Differences between male and female flowers arise late during development. Both female and male flowers can only be structurally distinguished after stage 9, where ovule primordia development is arrested in male flowers and microspore development is aborted in female flower anthers. After this stage, male and female flowers can be distinguished by the naked eye as a result of differences in the dimensions of the gynoecium. The floral characteristics of S. macrophylla (distribution of male and female flowers within the inflorescence, and the relative number of male to female flowers) have practical implications for conservation strategies of this endangered species.  © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society , 2008, 156 , 529–535.     

Floral development of an asexual and female-like mutant carrying two deletions in gynoecium-suppressing and stamen-promoting functional regions on the Y chromosome of the dioecious plant Silene latifolia

Sexual dimorphism is controlled by genes on the Y chromosome in the dioecious plant Silene latifolia. K034 is the first mutant with female flowers and asexual flowers in one individual. Its stamens are suppressed completely, and its gynoecium exhibits two suppression patterns. One gynoecium resembles a thin rod, as in wild-type males (asexual flower); the other is imperfectly suppressed, having 1-3 carpels (female-like flower). The ratio of these patterns was 9 : 1. To exclude the possibility of chimerism in K034, we crossed a female-like flower of K034 with a wild-type male. Progeny obtained from this crossing had asexual and female-like flowers in one individual. This two-flower-type phenotype was inherited without separating. To examine the identity of flower organs in K034, we analyzed the development of asexual and female-like flowers using scanning electron microscopy and in situ hybridization with SLM1 and SLM2 (orthologs of AGAMOUS and PISTILLATA, respectively) as probes. Mitotic spreads of root tip chromosomes from hairy root cultures showed that K034 had 25 chromosomes. Fluorescent in situ hybridization analysis, using a subtelomeric repetitive sequence (KpnI subfamily) as a probe, indicated that K034 possessed two X chromosomes and one Y chromosome (Y(d)), of which Y(d) had been rearranged to lose the pseudoautosomal region (PAR). PCR analysis using Y-specific sequence-tagged site (STS) markers clarified that Y(d) of K034 had two other deletions in gynoecium-suppressing and stamen-promoting regions. It is reasonable to suggest that these sex chromosomal abnormalities resulted in two abnormal sexual phenotypes: the asexual and imperfect female (female-like) flowers in K034.     

In vitro development of angiosperm floral buds and organs

In the last twenty-five years, young inflorescences, floral buds and individual floral organs of a number of species have been cultured in vitro. There is considerable variability in the requirement of plant growth regulators and nutritional factors for flower development of different species. This variability is compounded by the fact that the hormonal and nutritional requirements are different at various stages of organ and floral development. Experimental studies on normal and mutant flowers in vitro have provided insights into some of the regulatory processes in floral organogenesis. The potential use of the in vitro technique in elucidating the various mechanisms in flower development is stressed.     

LEAFY controls floral meristem identity in Arabidopsis.

The first step in flower development is the generation of a floral meristem by the inflorescence meristem. We have analyzed how this process is affected by mutant alleles of the Arabidopsis gene LEAFY. We show that LEAFY interacts with another floral control gene, APETALA1, to promote the transition from inflorescence to floral meristem. We have cloned the LEAFY gene, and, consistent with the mutant phenotype, we find that LEAFY RNA is expressed strongly in young flower primordia. LEAFY expression procedes expression of the homeotic genes AGAMOUS and APETALA3, which specify organ identify within the flower. Furthermore, we demonstrate that LEAFY is the Arabidopsis homolog of the FLORICAULA gene, which controls floral meristem identity in the distantly related species Antirrhinum majus.