Alteration of tobacco floral organ identity by expression of combinations of Antirrhinum MADS-box genes

The MADS-box genes SOC1 and AGL24 antagonize XAL2 functions in Arabidopsis thaliana root development

MADS-domain transcription factors play pivotal roles in numerous developmental processes in Arabidopsis thaliana. While their involvement in flowering transition and floral development has been extensively examined, their functions in root development remain relatively unexplored. Here, we explored the function and genetic interaction of three MADS-box genes (XAL2, SOC1 and AGL24) in primary root development. By analyzing loss-of-function and overexpression lines, we found that SOC1 and AGL24, both critical components in flowering transition, redundantly act as repressors of primary root growth as the loss of function of either SOC1 or AGL24 partially recovers the primary root growth, meristem cell number, cell production rate, and the length of fully elongated cells of the short-root mutant xal2-2. Furthermore, we observed that the simultaneous overexpression of AGL24 and SOC1 leads to short-root phenotypes, affecting meristem cell number and fully elongated cell size, whereas SOC1 overexpression is sufficient to affect columella stem cell differentiation. Additionally, qPCR analyses revealed that these genes exhibit distinct modes of transcriptional regulation in roots compared to what has been previously reported for aerial tissues. We identified 100 differentially expressed genes in xal2-2 roots by RNA-seq. Moreover, our findings revealed that the expression of certain genes involved in cell differentiation, as well as stress responses, which are either upregulated or downregulated in the xal2-2 mutant, reverted to WT levels in the absence of SOC1 or AGL24.

Diversification of the Homeotic AP3 Clade MADS-Box Genes in Asteraceae Species Chrysanthemum morifolium L. and Helianthus annuus L

The structure and phylogeny of MADS-box genes HAM91 of sunflower (Helianthus annuus) and CDM115 of chrysanthemum (Chrysanthemum morifolium) were characterized. It is shown that these genes encode MADS-domain transcription factors, which are orthologs of TM6 (Solanum lycopersicum) and APETALA3 (Arabidopsis thaliana), respectively. We obtained two types of transgenic tobacco plants (Nicotiana tabacum) with constitutive expression of HAM91 and CDM115 genes. Both types of plants flowered later than the control plants and formed more flowers and seed pods. The weight of seeds of 35S::CDM115 plants was significantly lower than in the control and 35S::HAM91 plants, which may indicate to a change in the identity of ovules in 35S::CDM115.

Ectopic Expression of the Homeotic MADS-Box Gene HAM31 (Helianthus annuus L.) in Transgenic Plants Nicotiana tabacum L. Affects the Gynoecium Identity

The structure of the MADS-box gene HAM31 of the sunflower (Helianthus annuus) was characterized. It is shown that the product of this gene is an ortholog of the B-class MADS transcription factor PISTILLATA (Arabidopsis thaliana). Two types of transgenic tobacco plants (Nicotiana tabacum) with the constitutive expression of the HAM31 gene in the sense and antisense orientation were obtained. The 35S::HAM31s plants formed flowers with an altered gynoecium identity, whereas 35S::HAM31as plants did not differ from the control.

Characterization and Functional Analysis of Five MADS-Box B Class Genes Related to Floral Organ Identification in Tagetes erecta

According to the floral organ development ABC model, B class genes specify petal and stamen identification. In order to study the function of B class genes in flower development of Tagetes erecta, five MADS-box B class genes were identified and their expression and putative functions were studied. Sequence comparisons and phylogenetic analyses indicated that there were one PI-like gene-TePI, two euAP3-like genes-TeAP3-1 and TeAP3-2, and two TM6-like genes-TeTM6-1 and TeTM6-2 in T. erecta. Strong expression levels of these genes were detected in stamens of the disk florets, but little or no expression was detected in bracts, receptacles or vegetative organs. Yeast hybrid experiments of the B class proteins showed that TePI protein could form a homodimer and heterodimers with all the other four B class proteins TeAP3-1, TeAP3-2, TeTM6-1 and TeTM6-2. No homodimer or interaction was observed between the euAP3 and TM6 clade members. Over-expression of five B class genes of T. erecta in Nicotiana rotundifolia showed that only the transgenic plants of 35S::TePI showed altered floral morphology compared with the non-transgenic line. This study could contribute to the understanding of the function of B class genes in flower development of T. erecta, and provide a theoretical basis for further research to change floral organ structures and create new materials for plant breeding.

The origin of exon 3 skipping of paternal GLOBOSA pre-mRNA in some Nicotiana tabacum lines correlates with a point mutation of the very last nucleotide of the exon

In plants, genome duplication followed by genome diversification and selection is recognized as a major evolutionary process. Rapid epigenetic and genetic changes that affect the transcription of parental genes are frequently observed after polyploidization. The pattern of alternative splicing is also frequently altered, yet the related molecular processes remain largely unresolved. Here, we study the inheritance and expression of parental variants of three floral organ identity genes in allotetraploid tobacco. DEFICIENS and GLOBOSA are B-class genes, and AGAMOUS is a C-class gene. Parental variants of these genes were found to be maintained in the tobacco genome, and the respective mRNAs were present in flower buds in comparable amounts. However, among five tobacco cultivars, we identified two in which the majority of paternal GLOBOSA pre-mRNA transcripts undergo exon 3 skipping, producing an mRNA with a premature termination codon. At the DNA level, we identified a G-A transition at the very last position of exon 3 in both cultivars. Although alternative splicing resulted in a dramatic decrease in full-length paternal GLOBOSA mRNA, no phenotypic effect was observed. Our finding likely serves as an example of the initiation of homoeolog diversification in a relatively young polyploid genome.

Functional characterization of duplicated B-class MADS-box genes in Japanese gentian

The heterodimer formation between B-class MADS-box proteins of GsAP3a and GsPI2 proteins plays a core role for petal formation in Japanese gentian plants. We previously isolated six B-class MADS-box genes (GsAP3a, GsAP3b, GsTM6, GsPI1, GsPI2, and GsPI3) from Japanese gentian (Gentiana scabra). To study the roles of these MADS-box genes in determining floral organ identities, we investigated protein-protein interactions among them and produced transgenic Arabidopsis and gentian plants overexpressing GsPI2 alone or in combination with GsAP3a or GsTM6. Yeast two-hybrid and bimolecular fluorescence complementation analyses revealed that among the GsPI proteins, GsPI2 interacted with both GsAP3a and GsTM6, and that these heterodimers were localized to the nuclei. The heterologous expression of GsPI2 partially converted sepals into petaloid organs in transgenic Arabidopsis, and this petaloid conversion phenomenon was accelerated by combined expression with GsAP3a but not with GsTM6. In contrast, there were no differences in morphology between vector-control plants and transgenic Arabidopsis plants expressing GsAP3a or GsTM6 alone. Transgenic gentian ectopically expressing GsPI2 produced an elongated tubular structure that consisted of an elongated petaloid organ in the first whorl and stunted inner floral organs. These results imply that the heterodimer formation between GsPI2 and GsAP3a plays a core role in determining petal and stamen identities in Japanese gentian, but other B-function genes might be important for the complete development of petal organs.

Recessive loci Pps-1 and OM differentially regulate PISTILLATA-1 and APETALA3-1 expression for sepal and petal development in Papaver somniferum

The involvement of PISTILLATA (PI) and APETALA (AP) transcription factors in the development of floral organs has previously been elucidated but little is known about their upstream regulation. In this investigation, two novel mutants generated in Papaver somniferum were analyzed--one with partially petaloid sepals and another having sepaloid petals. Progeny from reciprocal crosses of respective mutant parent genotypes showed a good fit to the monogenic Mendelian inheritance model, indicating that the mutant traits are likely controlled by the single, recessive nuclear genes named "Pps-1" and "OM" in the partially petaloid sepal and sepaloid petal phenotypes, respectively. Both paralogs of PISTILLATA (PapsPI-1 and PapsPI-3) were obtained from the sepals and petals of P. somniferum. Ectopic expression of PapsPI-1 in tobacco resulted in a partially petaloid sepal phenotype at a low frequency. Upregulation of PapsPI-1 and PapsAP3-1 in the petal and the petal part of partially petaloid sepal mutant and down-regulation of the same in sepaloid petal mutant indicates a differential pattern of regulation for flowering-related genes in various whorls. Similarly, it was found that the recessive mutation OM in sepaloid petal mutant downregulates PapsPI-1 and PapsAP3-1 transcripts. The recessive nature of the mutations was confirmed by the segregation ratios obtained in this analysis.

Gain of function mutation in tobacco MADS box promoter switch on the expression of flowering class B genes converting sepals to petals

One mutant transgenic line displaying homeotic conversion of sepals to petals with other phenotypic aberrations was selected and characterized at molecular level. The increased transcript level of gene encoding anthocyanidin synthase and petal specific class B genes, GLOBOSA and DEFECIENS in sepals of mutant line may be responsible for its homeotic conversion to petaloid organs. While characterizing this mutant line for locus identification, T-DNA was found to be inserted in 3' untranslated region of promoter of class B MADS box gene, GLOBOSA. Here, CaMV 35S promoter of T-DNA might be deriving the expression of class B genes.

Flower development in the asterid lineage

A complete understanding of the genetic control of flower development requires a comparative approach, involving species from across the angiosperm lineage. Using the accessible model plant Arabidopsis thaliana many of the genetic pathways that control development of the reproductive growth phase have been delineated. Research in other species has added to this knowledge base, revealing that, despite the myriad of floral forms found in nature, the genetic blueprint of flower development is largely conserved. However, these same studies have also highlighted differences in the way flowering is controlled in evolutionarily diverse species. Here, we review flower development in the eudicot asterid lineage, a group of plants that diverged from the rosid family, which includes Arabidopsis, 120 million years ago. Work on model species such as Antirrhinum majus, Petunia hybrida, and Gerbera hybrida has prompted a reexamination of textbook models of flower development; revealed novel mechanisms controlling floral gene expression; provided a means to trace evolution of key regulatory genes; and stimulated discussion about genetic redundancy and the fate of duplicated genes.

Quantitative levels of Deficiens and Globosa during late petal development show a complex transcriptional network topology of B function

The transcriptional network topology of B function in Antirrhinum, required for petal and stamen development, is thought to rely on initial activation of transcription of DEFICIENS (DEF) and GLOBOSA (GLO), followed by a positive autoregulatory loop maintaining gene expression levels. Here, we show that the mutant compacta (co), whose vegetative growth and petal size are affected, plays a role in B function. Late events in petal morphogenesis such as development of conical cell area and scent emissions were reduced in co and def (nicotianoides) (def (nic) ), and absent in co def (nic) double mutants, suggesting a role for CO in petal identity. Expression of DEF was down-regulated in co but surprisingly GLO was not affected. We investigated the levels of DEF and GLO at late stages of petal development in the co, def (nic) and glo-1 mutants, and established a reliable transformation protocol that yielded RNAi-DEF lines. We show that the threshold levels of DEF or GLO required to obtain petal tissue are approximately 11% of wild-type. The relationship between DEF and GLO transcripts is not equal or constant and changes during development. Furthermore, down-regulation of DEF or GLO does not cause parallel down-regulation of the partner. Our results demonstrate that, at late stages of petal development, the B function transcriptional network topology is not based on positive autoregulation, and has additional components of transcriptional maintenance. Our results suggest changes in network topology that may allow changes in protein complexes that would explain the fact that not all petal traits appear early in development.

Alteration of floral organ identity by over-expression of IbMADS3-1 in tobacco

The MADS-box genes have been studied mainly in flower development by researching flower homeotic mutants. Most of the MADS-box genes isolated from plants are expressed exclusively in floral tissues, and some of their transcripts have been found in various vegetative tissues. The genes in the STMADS subfamily are important in the development of whole plants including roots, stems, leaves, and the plant vascular system. IbMADS3-1, which is in the STMADS subfamily, and which has been cloned in Ipomoea batatas (L.) Lam., is expressed in all vegetative tissues of the plant, particularly in white fibrous roots. Sequence similarity, besides the spatial and temporal expression patterns, enabled the definition of a novel MADS-box subfamily comprising STMADS16 and the other MADS-box genes in STMADS subfamily expressed specifically in vegetative tissues. Expression of IbMADS3-1 was manifest by the appearance of chlorophyll-containing petals and production of characteristic changes in organ identity carpel structure alterations and sepaloidy of the petals. In reverse transcription-polymerase chain reaction analysis with a number of genes known to be key regulators of floral organ development, the flowering promoter NFL1 was clearly reduced at the RNA level compared with wild type in transgenic line backgrounds. Moreover, NtMADS5 showed slight down-regulation compared with wild-type plants in transgenic lines. These results suggest that IbMADS3-1 could be a repressor of NFL1 located upstream of NtMADS5. IbMADS3-1 ectopic expression is suggested as a possible means during vegetative development by which the IbMADS3-1 gene may interfere with the floral developmental pathway.

Inhibition of SAH-hydrolase activity during seed germination leads to deregulation of flowering genes and altered flower morphology in tobacco

Developmental processes are closely connected to certain states of epigenetic information which, among others, rely on methylation of chromatin. S-adenosylmethionine (SAM) and S-adenosylhomocysteine (SAH) are key cofactors of enzymes catalyzing DNA and histone methylation. To study the consequences of altered SAH/SAM levels on plant development we applied 9-(S)-(2,3-dihydroxypropyl)-adenine (DHPA), an inhibitor of SAH-hydrolase, on tobacco seeds during a short phase of germination period (6 days). The transient drug treatment induced: (1) dosage-dependent global DNA hypomethylation mitotically transmitted to adult plants; (2) pleiotropic developmental defects including decreased apical dominance, altered leaf and flower symmetry, flower whorl malformations and reduced fertility; (3) dramatic upregulation of floral organ identity genes NTDEF, NTGLO and NAG1 in leaves. We conclude that temporal SAH-hydrolase inhibition deregulated floral genes expression probably via chromatin methylation changes. The data further show that plants might be particularly sensitive to accurate setting of SAH/SAM levels during critical developmental periods.

Homo- and heterodimers of tobacco bZIP proteins counteract as positive or negative regulators of transcription during pollen development

Expression of BZI-1 Delta N, a dominant-negative form of the tobacco (Nicotiana tabacum) basic leucine zipper (bZIP) transcription factor BZI-1 leads to severe defects in pollen development which coincides with reduced transcript abundance of the stamen specific invertase gene NIN88 and decreased extracellular invertase enzymatic activity. This finding suggests a function of BZI-1 in regulating carbohydrate supply of the developing pollen. BZI-1 heterodimerises with the bZIP factors BZI-2, BZI-3 and BZI-4 in vitro and in planta. Whereas BZI-1 exhibits only weak activation properties, BZI-1/BZI-2 heterodimers strongly activate transcription. Consistently, approaches leading to reduced levels of functional BZI-1 or BZI-2 both significantly interfere with pollen development, auxin responsiveness and carbohydrate partitioning. In situ hybridisation studies for BZI-1 and BZI-2 confirmed temporal and spatial overlapping expression patterns in tapetum and pollen supporting functional cooperation of these factors during pollen development. Plants over-expressing BZI-4 produce significantly reduced amounts of intact pollen and are also impaired in NIN88 transcription and enzymatic activity. BZI-4 homodimer efficiently binds to a G-box located in the NIN88 promoter but exhibits almost no transcriptional activation capacity. As BZI-4 does not actively repress transcription, we propose that its homodimer blocks G-box mediated transcription. In summary, these data support a regulatory model in which BZI-4 homodimers and BZI-1/BZI-2 heterodimers perform opposing functions as negative or positive transcriptional regulators during pollen development.

Hose in Hose, an S locus-linked mutant of Primula vulgaris, is caused by an unstable mutation at the Globosa locus

Hose in Hose mutants of primrose and cowslip have been cultivated since the early 17th century and show dominant homeotic conversion of sepals to petals. The phenotype shows variable penetrance and expressivity and is linked to the S locus, which controls floral heteromorphy in Primula species. Here we demonstrate that the homeotic conversion of sepals to petals in Hose in Hose is associated with up-regulation of both Primula B-function MADS box genes PvDef and PvGlo in the first floral whorl. We have defined a restriction fragment length polymorphism associated with PvGlo that cosegregates with the Hose in Hose phenotype and have also identified and characterized a retrotransposon insertion in the PvGlo promoter which is associated with the up-regulated expression of PvGlo. Excision of this retrotransposon, associated with epigenetic changes at the locus, causes reversion toward normal calyces and restores wild-type flower development. These data define the molecular basis of the Hose in Hose mutation and provide an explanation for its long-documented phenotypic instability.

Functional characterization of B class MADS-box transcription factors in Gerbera hybrida

According to the classical ABC model, B-function genes are involved in determining petal and stamen development. Most core eudicot species have B class genes belonging to three different lineages: the PI, euAP3, and TM6 lineages, although both Arabidopsis and Antirrhinum appear to have lost their TM6-like gene. Functional studies were performed for three gerbera (Gerbera hybrida) B class MADS-box genes--PI/GLO-like GGLO1, euAP3 class GDEF2, and TM6-like GDEF1--and data are shown for a second euAP3-like gene, GDEF3. In phylogenetic analysis, GDEF3 is a closely related paralogue of GDEF2, and apparently stems from a duplication common to all Asteraceae. Expression analysis and transgenic phenotypes confirm that GGLO1 and GDEF2 mediate the classical B-function since they determine petal and stamen identities. However, based on assays in yeast, three B class heterodimer combinations are possible in gerbera. In addition to the interaction of GGLO1 and GDEF2 proteins, GGLO1 also pairs with GDEF1 and GDEF3. This analysis of GDEF1 represents the first functional characterization of a TM6-like gene in a core eudicot species outside Solanaceae. Similarly to its relatives in petunia and tomato, the expression pattern and transgenic phenotypes indicate that GDEF1 is not involved in determination of petal identity, but has a redundant role in regulating stamen development.

Floral organ identity: 20 years of ABCs

One of the early successes of the application of molecular genetics to study plant development was the discovery of a series of genes that act together, in an apparently simple combinatorial model, to specify the identity of the different organs of a flower. Widely known as the ABC model, this framework for understanding has been investigated and modified over the course of the last two decades. The cast list of genes has been defined and, as other chapters in this volume will show, great progress has been made in understanding how they are regulated, how they act together, what they do and how they have contributed to the evolution of the flower in its varied forms. In this introductory review to the volume we will review the derivation and elaboration of the most current version of the ABC model, highlighting the modifications that have been necessary to ensure it fits the available experimental data. We will highlight the remaining difficulties in fitting the current model to the experimental data and propose a further modification to enable it to regain its applicability.

Environmental control of sepalness and petalness in perianth organs of waterlilies: a new Mosaic theory for the evolutionary origin of a differentiated perianth

The conventional concept of an 'undifferentiated perianth', implying that all perianth organs of a flower are alike, obscures the fact that individual perianth organs are sometimes differentiated into sepaloid and petaloid regions, as in the early-divergent angiosperms Nuphar, Nymphaea, and Schisandra. In the waterlilies Nuphar and Nymphaea, sepaloid regions closely coincide with regions of the perianth that were exposed when the flower was in bud, whereas petaloid regions occur in covered regions, suggesting that their development is at least partly controlled by the environment of the developing tepal. Green and colourful areas differ from each other in trichome density and presence of papillae, features that often distinguish sepals and petals. Field experiments to test whether artificial exposure can induce sepalness in the inner tepals showed that development of sepaloid patches is initiated by exposure, at least in the waterlily species examined. Although light is an important environmental cue, other important factors include an absence of surface contact. Our interpretation contradicts the unspoken rule that 'sepal' and 'petal' must refer to whole organs. We propose a novel theory (the Mosaic theory), in which the distinction between sepalness and petalness evolved early in angiosperm history, but these features were not fixed to particular organs and were primarily environmentally controlled. At a later stage in angiosperm evolution, sepaloid and petaloid characteristics became fixed to whole organs in specific whorls, thus reducing or removing the need for environmental control in favour of fixed developmental control.

Characterization of genes controlling stamen identity and development in a parthenocarpic tomato mutant indicates a role for the DEFICIENS ortholog in the control of fruit set

The development of the ovary into a fruit depends on pollination and fertilization. It has been proposed that the restriction of ovary growth before pollination is because of the stamens acting as negative regulators. Accordingly, the silencing of genes responsible for stamen identity has been correlated with parthenocarpy in different species. The tomato (Solanum lycopersicum L.) parthenocarpic fruit (pat) mutation associates autonomous ovary development with homeotic transformation of the anthers and aberrancy of ovules in the ovary. In this study, we tested the hypothesis that stamen aberrations and parthenocarpy in pat are driven by cues coming from the altered expression of class B MADS box genes. The data showed that the Pat locus is not allelic to either of the two tomato mutations putatively involved in the B function, stamenless (sl)-2 and pistillate (pi) or to genes encoding class B transcription factors. Whereas pat pi double mutants were not recovered because of tight linkage, pat sl-2 double mutants showed mainly epistatic effects. The developmental regulation of the Sl DEFICIENS (DEF) gene in the wild-type (WT) at anthesis as well as its differential transcription in the pat ovary suggest that it plays a role in the control of ovary growth. Accordingly, when compared with the WT, the gene was also differentially expressed in the parthenocarpic fruit-2 (pat-2) mutant, that is not allelic to pat and has normal ovule development. Altogether the results indicate that in tomato SlDEF plays a role in the control of ovary growth and that the pat mutation is located upstream of this regulatory cascade.

B-class MADS-box genes in trioecious papaya: two paleoAP3 paralogs, CpTM6-1 and CpTM6-2, and a PI ortholog CpPI

In the ABC model of flower development, B function organ-identity genes act in the second and third whorls of the flower to control petal and stamen identity. The trioecious papaya has male, female, and hermaphrodite flowers and is an ideal system for testing the B-class gene expression patterns in trioecious plants. We cloned papaya B-class genes, CpTM6-1, CpTM6-2, and CpPI, using MADS box gene specific degenerate primers followed by cDNA library screening and sequencing of positive clones. While phylogenetic analyses show that CpPI is the ortholog of the Arabidopsis gene PI, the CpTM6-1 and CpTM6-2 loci are representatives of the paralogous TM6 lineage that contain paleoAP3 motifs unlike the euAP3 gene observed in Arabidopsis. These two paralogs appeared to have originated from a tandem duplication occurred approximately 13.4 million year ago (mya) (bootstrap range 13.36 +/- 2.42). In-situ hybridization and RT-PCR showed that the papaya B-class genes were highly expressed in young flowers across all floral organ primordia. As the flower organs developed, all three B-class genes were highly expressed in petals of all three-sex types and in stamens of hermaphrodite and male flowers. CpTM6-1 expressed at low levels in sepals and carpels, whereas CpTM6-2 expressed at a low level in sepals and at a high level in leaves. Our results showed that B-class gene homologs could function as predicted by the ABC model in trioecous flowers but differential expressions of CpTM6-1, and CpTM6-2, and CpPI suggested the diversification of their functions after the duplication events.

Cloning and characterization of a PI-like MADS-box gene in Phalaenopsis orchid

The highly evolved flowers of orchids have colorful sepals and fused columns that offer an opportunity to discover new genes involved in floral development in monocotyledon species. In this investigation, we cloned and characterized the homologous PISTALLATA-like (PI-like) gene PhPI15 (Phalaenopsis PI STILLATA # 15), from the Phalaenopsis hybrid cultivar. The protein sequence encoded by PhPI15 contains a typical PI-motif. Its sequence also formed a subclade with other monocot PI-type genes in phylogenetic analysis. Southern analysis showed that PhPI15 was present in the Phalaenopsis orchid genome as a single copy. Furthermore, it was expressed in all the whorls of the Phalaenopsis flower, while no expression was detected in vegetative organs. The flowers of transgenic tobacco plants ectopically expressing PhPI15 showed male-sterile phenotypes. Thus, as a Class-B MADS-box gene, PhPI15 specifies floral organ identity in orchids.

Hormonal control of the inflated calyx syndrome, a morphological novelty, in Physalis

The 'Chinese lantern' phenotype or inflated calyx syndrome (ICS)--inflated sepals encapsulating the mature berry of Physalis floridana--is a morphological novelty within the Solanaceae. ICS is associated with heterotopic expression of MPF2, which codes for a MADS-box transcription factor otherwise involved in leaf formation and male fertility. In accordance with this finding, the MPF2 promoter sequence differs significantly from that of its orthologue STMADS16 in the related Solanum tuberosum, which does not exhibit ICS. However, heterotopic expression of MPF2 is not sufficient for ICS formation in P. floridana- fertilization is also important. Here we report that the hormones cytokinin and gibberellin are essential for ICS formation. MPF2 controls sepal cell division, but the resulting cells are small. Calyx size increases substantially only if gibberellin and cytokinin are available to promote cell elongation and further cell division. Transient expression of appropriate MPF2-/STMADS16-GFP fusions in leaf tissues in the presence of hormones revealed that cytokinin, but not gibberellin, facilitated transport of the transcription factor into the nucleus. Furthermore, an ICS-like structure can be induced in transgenic S. tuberosum by ectopic expression of STMADS16 and simultaneous treatment with cytokinin and gibberellin. Strikingly, transgenic Arabidopsis ectopically expressing solanaceous MPF2-like proteins display enhanced sepal growth when exposed to cytokinin only, while orthologous proteins from non-solanaceous plants did not require cytokinin for this function. These data are incorporated into a detailed model for ICS formation in P. floridana.

Functional analyses of two tomato APETALA3 genes demonstrate diversification in their roles in regulating floral development

The floral homeotic APETALA3 (AP3) gene in Arabidopsis thaliana encodes a MADS box transcription factor required for specifying petal and stamen identities. AP3 is a member of the euAP3 lineage, which arose by gene duplication coincident with radiation of the core eudicots. Although Arabidopsis lacks genes in the paralogous Tomato MADS box gene 6 (TM6) lineage, tomato (Solanum lycopersicum) possesses both euAP3 and TM6 genes, which have functionally diversified. A loss-of-function mutation in Tomato AP3 (TAP3) resulted in homeotic transformations of both petals and stamens, whereas RNA interference-induced reduction in TM6 function resulted in flowers with homeotic defects primarily in stamens. The functional differences between these genes can be ascribed partly to different expression domains. When overexpressed in an equivalent domain, both genes can partially rescue the tap3 mutant, indicating that relative levels as well as spatial patterns of expression contribute to functional differences. Our results also indicate that the two proteins have differing biochemical capabilities. Together, these results suggest that TM6 and TAP3 play qualitatively different roles in floral development; they also support the ideas that the ancestral role of AP3 lineage genes was in specifying stamen development and that duplication and divergence in the AP3 lineage allowed for the acquisition of a role in petal specification in the core eudicots.

Conservation and divergence in the AGAMOUS subfamily of MADS-box genes: evidence of independent sub- and neofunctionalization events

The MADS-box gene AGAMOUS (AG) plays a key role in determining floral meristem and organ identities. We identified three AG homologs, EScaAG1, EScaAG2, and EScaAGL11 from the basal eudicot Eschscholzia californica (California poppy). Phylogenetic analyses indicate that EScaAG1 and EScaAG2 are recent paralogs within the AG clade, independent of the duplication in ancestral core eudicots that gave rise to the euAG and PLENA (PLE) orthologs. EScaAGL11 is basal to core eudicot AGL11 orthologs in a clade representing an older duplication event after the divergence of the angiosperm and gymnosperm lineages. Detailed in situ hybridization experiments show that expression of EScaAG1 and EScaAG2 is similar to AG; however, both genes appear to be expressed earlier in floral development than described in the core eudicots. A thorough examination of available expression and functional data in a phylogenetic context for members of the AG and AGL11 clades reveals that gene expression has been quite variable throughout the evolutionary history of the AG subfamily and that ovule-specific expression might have evolved more than twice. Although sub- and neofunctionalization are inferred to have occurred following gene duplication, functional divergence among orthologs is evident, as is convergence, among paralogs sampled from different species. We propose that retention of multiple AG homologs in several paralogous lineages can be explained by the conservation of ancestral protein activity combined with evolutionarily labile regulation of expression in the AG and AGL11 clades such that the collective functions of the AG subfamily in stamen and carpel development are maintained following gene duplication.

Functional conservation of PISTILLATA activity in a pea homolog lacking the PI motif

Current understanding of floral development is mainly based on what we know from Arabidopsis (Arabidopsis thaliana) and Antirrhinum majus. However, we can learn more by comparing developmental mechanisms that may explain morphological differences between species. A good example comes from the analysis of genes controlling flower development in pea (Pisum sativum), a plant with more complex leaves and inflorescences than Arabidopsis and Antirrhinum, and a different floral ontogeny. The analysis of UNIFOLIATA (UNI) and STAMINA PISTILLOIDA (STP), the pea orthologs of LEAFY and UNUSUAL FLORAL ORGANS, has revealed a common link in the regulation of flower and leaf development not apparent in Arabidopsis. While the Arabidopsis genes mainly behave as key regulators of flower development, where they control the expression of B-function genes, UNI and STP also contribute to the development of the pea compound leaf. Here, we describe the characterization of P. sativum PISTILLATA (PsPI), a pea MADS-box gene homologous to B-function genes like PI and GLOBOSA (GLO), from Arabidopsis and Antirrhinum, respectively. PsPI encodes for an atypical PI-type polypeptide that lacks the highly conserved C-terminal PI motif. Nevertheless, constitutive expression of PsPI in tobacco (Nicotiana tabacum) and Arabidopsis shows that it can specifically replace the function of PI, being able to complement the strong pi-1 mutant. Accordingly, PsPI expression in pea flowers, which is dependent on STP, is identical to PI and GLO. Interestingly, PsPI is also transiently expressed in young leaves, suggesting a role of PsPI in pea leaf development, a possibility that fits with the established role of UNI and STP in the control of this process.

Expression of floral MADS-box genes in basal angiosperms: implications for the evolution of floral regulators

The ABC model of floral organ identity is based on studies of Arabidopsis and Antirrhinum, both of which are highly derived eudicots. Most of the genes required for the ABC functions in Arabidopsis and Antirrhinum are members of the MADS-box gene family, and their orthologs are present in all major angiosperm lineages. Although the eudicots comprise 75% of all angiosperms, most of the diversity in arrangement and number of floral parts is actually found among basal angiosperm lineages, for which little is known about the genes that control floral development. To investigate the conservation and divergence of expression patterns of floral MADS-box genes in basal angiosperms relative to eudicot model systems, we isolated several floral MADS-box genes and examined their expression patterns in representative species, including Amborella (Amborellaceae), Nuphar (Nymphaeaceae) and Illicium (Austrobaileyales), the successive sister groups to all other extant angiosperms, plus Magnolia and Asimina, members of the large magnoliid clade. Our results from multiple methods (relative-quantitative RT-PCR, real-time PCR and RNA in situ hybridization) revealed that expression patterns of floral MADS-box genes in basal angiosperms are broader than those of their counterparts in eudicots and monocots. In particular, (i) AP1 homologs are generally expressed in all floral organs and leaves, (ii) AP3/PI homologs are generally expressed in all floral organs and (iii) AG homologs are expressed in stamens and carpels of most basal angiosperms, in agreement with the expectations of the ABC model; however, an AG homolog is also expressed in the tepals of Illicium. The broader range of strong expression of AP3/PI homologs is inferred to be the ancestral pattern for all angiosperms and is also consistent with the gradual morphological intergradations often observed between adjacent floral organs in basal angiosperms.

Evolution in Action: Following Function in Duplicated Floral Homeotic Genes

Gene duplication plays a fundamental role in evolution by providing the genetic material from which novel functions can arise. Newly duplicated genes can be maintained by subfunctionalization (the duplicated genes perform different aspects of the original gene's function) and/or neofunctionalization (one of the genes acquires a novel function). PLENA in Antirrhinum and AGAMOUS in Arabidopsis are the canonical C-function genes that are essential for the specification of reproductive organs. These functionally equivalent genes encode closely related homeotic MADS-box transcription factors. Using genome synteny, we confirm phylogenetic analyses showing that PLENA and AGAMOUS are nonorthologous genes derived from a duplication in a common ancestor. Their respective orthologs, SHATTERPROOF in Arabidopsis and FARINELLI in Antirrhinum, have undergone independent subfunctionalization via changes in regulation and protein function. Surprisingly, the functional divergence between PLENA and FARINELLI, is morphologically manifest in both transgenic Antirrhinum and Arabidopsis. This provides a clear illustration of a random evolutionary trajectory for gene functions after a duplication event. Different members of a duplicated gene pair have retained the primary homeotic functions in different lineages, illustrating the role of chance in evolution. The differential ability of the Antirrhinum genes to promote male or female development provides a striking example of subfunctionalization at the protein level.

PeMADS6, a GLOBOSA/PISTILLATA-like Gene in Phalaenopsis equestris Involved in Petaloid Formation, and Correlated with Flower Longevity and Ovary Development

In this study, we isolated and characterized the function of a GLOBOSA/PISTILLATA-like gene, PeMADS6, from a native Phalaenopsis species, P. equestris. Southern blot analysis showed PeMADS6 as a single copy in the Phalaenopsis genome. Results of the determination of temporal and spatial expression showed that PeMADS6 was expressed and thus participated in the development of the sepals, petals, labellum and column in Phalaenopsis. Further confirmation of the expression pattern of PeMADS6 was carried out with in situ hybridization. Repressed expression of PeMADS6 in the orchid ovary was found to be pollination regulated, which suggests that the gene may have an inhibitory effect on the development of the ovary or ovule. In addition, auxin acted as the candidate signal to regulate the repression of PeMADS6 expression in the ovary. Furthermore, the flowers of transgenic Arabidopsis plants ectopically overexpressing PeMADS6 showed the morphology of petaloid sepals, with a 3- to 4-fold increase in flower longevity. Concomitantly, delayed fruit maturation was also observed in the transgenic Arabidopsis, which is consistent with the inhibitory effect of PeMADS6 on the development of the ovary. Thus, as a B-function gene, PeMADS6, not only specifies floral organ identity but has functions in flower longevity and ovary development in orchids.

Heterotopic expression of MPF2 is the key to the evolution of the Chinese lantern of Physalis, a morphological novelty in Solanaceae

Morphological novelties arise through changes in development, but the underlying causes of such changes are largely unknown. In the genus Physalis, sepals resume growth after pollination to encapsulate the mature fruit, forming the "Chinese lantern," a trait also termed inflated-calyx syndrome (ICS). STMADS16, which encodes a MADS-box transcription factor, is expressed only in vegetative tissues in Solanum tuberosum. Its ortholog in Physalis pubescens, MPF2, is expressed in floral tissues. Knockdown of MPF2 function in Physalis by RNA interference (RNAi) reveals that MPF2 function is essential for the development of the ICS. The phenotypes of transgenic S. tuberosum plants that overexpress MPF2 or STMADS16 corroborate these findings: these plants display enlarged sepals. Although heterotopic expression of MPF2 is crucial for ICS, remarkably, fertilization is also required. Although the ICS is less prominent or absent in the knockdown transgenic plants, epidermal cells are larger, suggesting that MPF2 exerts its function by inhibiting cell elongation and promoting cell division. In addition, severely affected Physalis knockdown lines are male sterile. Thus, heterotopic expression of MPF2 in floral tissues is involved in two novel traits: expression of the ICS and control of male fertility. Sequence differences between the promoter regions of the MPF2 and STMADS16 genes perhaps reflect exposure to different selection pressures during evolution, and correlate with the observed differences in their expression patterns. In any case, the effects of heterotopic expression of MPF2 underline the importance of recruitment of preexisting transcription factors in the evolution of novel floral traits.

Suppression of heterotrimeric G-protein beta-subunit affects anther shape, pollen development and inflorescence architecture in tobacco

The role of the heterotrimeric G-protein beta-subunit in plant development was studied in transgenic tobacco (Nicotiana tabacum L.) plants with reduced beta-subunit levels due to the antisense expression of the beta-subunit mRNA. The antisense plants had aberrant anther shape and produced non-germinating pollen. The anthers were sporadically transformed to petals, whereas other floral organs were not affected. The pollen grains were smaller than the wild-type pollen and had abnormal cell walls. The architecture of mature antisense plants was altered. The plants had long branched panicles and short stems. These data suggest that the beta-subunit of the plant heterotrimeric G-proteins is involved in the regulation of the reproductive phase of the tobacco life cycle, particularly in stamen development and pollen maturation.

To B or Not to B a flower: the role of DEFICIENS and GLOBOSA orthologs in the evolution of the angiosperms

DEFICIENS (DEF) and GLOBOSA (GLO) function in petal and stamen organ identity in Antirrhinum and are orthologs of APETALA3 and PISTILLATA in Arabidopsis. These genes are known as B-function genes for their role in the ABC genetic model of floral organ identity. Phylogenetic analyses show that DEF and GLO are closely related paralogs, having originated from a gene duplication event after the separation of the lineages leading to the extant gymnosperms and the extant angiosperms. Several additional gene duplications followed, providing multiple potential opportunities for functional divergence. In most angiosperms studied to date, genes in the DEF/GLO MADS-box subfamily are expressed in the petals and stamens during flower development. However, in some angiosperms, the expression of DEF and GLO orthologs are occasionally observed in the first and fourth whorls of flowers or in nonfloral organs, where their function is unknown. In this article we review what is known about function, phylogeny, and expression in the DEF/GLO subfamily to examine their evolution in the angiosperms. Our analyses demonstrate that although the primary role of the DEF/GLO subfamily appears to be in specifying the stamens and inner perianth, several examples of potential sub- and neofunctionalization are observed.

Characterization of antirrhinum petal development and identification of target genes of the class B MADS box gene DEFICIENS

The class B MADS box transcription factors DEFICIENS (DEF) and GLOBOSA (GLO) of Antirrhinum majus together control the organogenesis of petals and stamens. Toward an understanding of how the downstream molecular mechanisms controlled by DEF contribute to petal organogenesis, we conducted expression profiling experiments using macroarrays comprising >11,600 annotated Antirrhinum unigenes. First, four late petal developmental stages were compared with sepals. More than 500 ESTs were identified that comprise a large number of stage-specifically regulated genes and reveal a highly dynamic transcriptional regulation. For identification of DEF target genes that might be directly controlled by DEF, we took advantage of the temperature-sensitive def-101 mutant. To enhance the sensitivity of the profiling experiments, one petal developmental stage was selected, characterized by increased transcriptome changes that reflect the onset of cell elongation processes replacing cell division processes. Upon reduction of the DEF function, 49 upregulated and 52 downregulated petal target genes were recovered. Eight target genes were further characterized in detail by RT-PCR and in situ studies. Expression of genes responding rapidly toward an altered DEF activity is confined to different petal tissues, demonstrating the complexity of the DEF function regulating diverse basic processes throughout petal morphogenesis.

On the origin of floral morphological novelties

Floral morphological novelties, like homeotic changes of whorl 1 organs, can easily arise by modifying existing regulatory networks. Ectopic expression of B-function MADS-box genes in whorl 1 leads to a replacement of sepals by petals, as is found in the Liliaceae. In cases where leaf-like sepals or even inflated calyces develop, which ultimately envelop the mature fruit as in Physalis, ectopic expression of a vegetative MADS-box gene seems to be responsible. Current knowledge concerning the origin of such morphological novelties is reviewed.

Functional characterization of SEPALLATA3 and AGAMOUS orthologues in silver birch

The development of flowers is regulated by a complex network of transcriptional activators and repressors, many of which belong to the MADS box gene family. In this study, we describe two MADS box genes of silver birch (Betula pendula Roth), BpMADS1 and BpMADS6, which are similar to SEPALLATA3 and AGAMOUS in Arabidopsis thaliana, respectively. In situ hybridization showed that BpMADS1 was expressed in the inflorescence meristem at a very early stage, but not later. Both genes were expressed in developing carpels, ovules and stamens but not in tepals or scales. Ectopic expression of BpMADS1 in Arabidopsis resulted in a reduced number of floral organs or whole whorls and in petaloid or carpelloid sepals, a phenotype reminiscent of that of fil mutants. 35S::BpMADS6 caused very early flowering in Arabidopsis. In tobacco, both 35S::BpMADS1 and 35S::BpMADS6 accelerated flowering and, in addition, 35S::BpMADS6 caused changes in sepals and petals. In some transgenic birch plants, 35S::BpMADS1 antisense resulted in the development of both male and female organs in the axil of a single bract and in a change of some inflorescences into vegetative shoots. In two plants, either 35S::BpMADS6 sense or antisense constructs resulted in an increase in the number of tepals and in complete lack of stamens in some male inflorescences. These results suggest that BpMADS1 participates both in inflorescence and in flower formation and BpMADS6 participates in flower formation and that they are functional homologues to SEPALLATA3 and AGAMOUS, respectively.

The evolution of floral homeotic gene function

Plant MADS-box genes encode transcriptional regulators that are critical for a number of developmental processes. In the angiosperms (the flowering plants), these include the specification of floral organ identities, flowering time and fruit development. It appears that the MADS box gene family has undergone considerable gene duplication and sequence divergence within the angiosperms. Here I discuss the possibility that these events have allowed the recruitment of these genes to new developmental pathways in particular angiosperm lineages. Recent analyses of sequence changes, expression patterns and, in a few cases, gene function are beginning to provide tantalizing evidence for deciphering when and how such genetic diversification has led to particular morphological innovations. In the future, comparative studies of large numbers of species will be required to assess the extent of such variation as well as to fully understand the mechanisms by which evolution of these developmental regulators has played a role in shaping new morphologies.

Restoration of stamen development and production of functional pollen in an alloplasmic CMS tobacco line by ectopic expression of the Arabidopsis thaliana SUPERMAN gene

The alloplasmic male-sterile tobacco line Nta(rep)S, combining the nucleus of Nicotiana tabacum with the cytoplasm of Nicotiana repanda, exhibits cadastral-type anomalies due to a fusion of several stamens with the pistil. These anomalies share similarities with Arabidopsis superman mutants. SUPERMAN (SUP) is a cadastral gene controlling the boundary between whorls 3 (androecium) and 4 (gynoecium). Thus we hypothesized that the expression of the tobacco SUP orthologue might be impaired in the alloplasmic Nta(rep)S line, and that the deficiency could be complemented by the Arabidopsis SUP gene. Here we show that the ectopic expression of SUP in the alloplasmic male-sterile tobacco line Nta(rep)S significantly increases the frequency of flowers possessing free stamens, inducing the recovery of a proper structure for whorls 3 and 4. Furthermore, flowers of transgenic plants show a significant improvement of the morphology of stamens, and more particularly of the anthers, which are able to produce few but functional pollen. The data show that ectopic expression of Arabidopsis SUP reactivates the regulatory cascade of anther development. The plausible causes of the developmental defects of anthers in the alloplasmic male-sterile tobacco line are discussed in relation to the model of regulation of the Arabidopsis SUP gene.

Defective cell proliferation in the floral meristem of alloplasmic plants of Nicotiana tabacum leads to abnormal floral organ development and male sterility

Flowers of an alloplasmic male-sterile tobacco line, comprised of the nuclear genome of Nicotiana tabacum and the cytoplasm of Nicotiana repanda, develop short, poorly-pigmented petals and abnormal sterile stamens that often are fused with the carpel wall. The development of flower organ primordia and establishment of boundaries between the different zones in the floral meristem were investigated by performing expression analysis of the tobacco orthologs of the organ identity genes GLO, AG and DEF. These studies support the conclusion that boundary formation was impaired between the organs produced in whorls 3 and 4 resulting in partial fusions between anthers and carpels. According to the investigations cell divisions and floral meristem size in the alloplasmic line were drastically reduced in comparison with the male-fertile tobacco line. The reduction in cell divisions leads to a discrepancy between cell number and cell determination at the stage when petal and stamen primordia should be initiated. At the same stage expression of the homeotic genes was delayed in comparison with the male-fertile line. However, the abnormal organ development was not due to a failure in the spatial expression of the organ identity genes. Instead the aberrant development in the floral organs of whorls 2, 3 and 4 appears to be caused by deficient floral meristem development at an earlier stage. Furthermore, defects in cell proliferation in the floral meristem of the alloplasmic male-sterile line correlates with presence of morphologically modified mitochondria. The putative causes of reduced cell number in the floral meristem and the consequences for floral development are discussed.

Characterization of three GLOBOSA-like MADS-box genes from maize: evidence for ancient paralogy in one class of floral homeotic B-function genes of grasses

Floral homeotic B-function genes are involved in specifying the identity of petals and stamens during flower development in higher eudicotyledonous plants. Monocotyledonous plants belonging to the grass family (Poaceae) have very similar B-function genes, except that these genes specify lodicules rather than petals. All B-function genes known so far are members of the MADS-box gene family encoding transcription factors. In some eudicot model systems such as Arabidopsis and Antirrhinum, the B-function is provided by heterodimeric protein complexes encoded by one DEF- and one GLO-like gene. In several different lineages of flowering plant species, however, more than one DEF- or GLO-like gene is found. A known example is the monocot model system rice, which contains two GLO-like genes, termed OSMADS2 and OSMADS4. Duplications of floral homeotic genes may have played a critical role in the diversification of floral homeotic functions and thus the evolution of flowers. In order to date the gene duplication event that gave rise to these two genes, we cloned cDNAs of three different GLO-like genes from maize, a distant relative of rice within the Poaceae family. Phylogeny reconstructions and chromosomal mapping indicate that one of these genes, named ZMM16, is orthologous to OSMADS2, and that the other two, ZMM18 and ZMM29, are probably orthologous to OSMADS4. The gene duplication which gave rise to OSMADS2- and OSMADS4-like genes occurred probably after the split of the lineages that resulted in extant Liliaceae and Poaceae, but before the separation of the lineages that gave rise to extant maize and rice about 50 MYA. Northern and in situ hybridization studies demonstrated that the maize genes are expressed in lodicules, stamens and carpels throughout spikelet development in male and female inflorescences. The GLO-like genes from rice have very similar patterns of mRNA accumulation. In addition, ZMM16 shows also weak expression in vegetative organs. Conservation of the expression in lodicules and stamens is in perfect agreement with a floral homeotic B-function of the GLO-like genes in grasses. The conserved expression in carpels is discussed. Moreover, circumstantial evidence for a functional diversification of GLO-like genes in grasses is provided.

Beyond the ABCs: ternary complex formation in the control of floral organ identity

The production of a flower requires several events to occur. A floral meristem must form, boundaries must be set to enable discrete primordia to arise and the primordia must adopt the correct organ identity. Homeotic mutants, whose organs adopt inappropriate identities for their position within the flower, have helped the construction of a simple combinatorial model to explain how floral organ identity is defined. However, recent experiments suggest that the regulation of floral organ identity is more complex than was previously apparent. The simple interactions are becoming more complex and the universal applicability of the model less clear.

A DEF/GLO-like MADS-box gene from a gymnosperm: Pinus radiata contains an ortholog of angiosperm B class floral homeotic genes

The specification of floral organ identity during development depends on the function of a limited number of homeotic genes grouped into three classes: A, B, and C. Pairs of paralogous B class genes, such as DEF and GLO in Antirrhinum, and AP3 and PI in Arabidopsis, are required for establishing petal and stamen identity. To gain a better understanding of the evolutionary origin of petals and stamens, we have looked for orthologs of B class genes in conifers. Here we report cDNA cloning of PrDGL (Pinus radiata DEF/GLO-like gene) from radiata pine. We provide phylogenetic evidence that PrDGL is closely related to both DEF- and GLO-like genes of angiosperms, and is thus among the first putative orthologs of floral homeotic B function genes ever reported from a gymnosperm. Expression of PrDGL is restricted to the pollen strobili (male cones) and was not detected in female cones. PrDGL expression was first detected in emergent male cone primordia and persisted through the early stages of pollen cone bud differentiation. Based on the results of our phylogeny reconstructions and expression studies, we suggest that PrDGL could play a role in distinguishing between male (where expression is on) and female reproductive structures (where expression is off) in radiata pine. We speculate that this could be the general function of DEF/GLO-like genes in gymnosperms that may have been recruited for the distinction between stamens and carpels, the male and female reproductive organs of flowering plants, during the evolution of angiosperms out of gymnosperm-like ancestors.

Organ identity genes and modified patterns of flower development in Gerbera hybrida (Asteraceae)

We have used Gerbera hybrida (the cultivated ornamental, gerera) to investigate the molecular basis of flower development in Asteraceae, a family of flowering plants that have heteromorphic flowers and specialized floral organs. Flowers of the same genotype may differ in a number of parameters, including sex expression, symmetry, sympetaly and pigmentation. In order to study the role of organ identity determination in these phenomena we isolated and functionally analysed six MADS box genes from gerbera; these were shown by phylogenetic analysis to be orthologous to well characterized regulatory genes described from Arabidopsis and Antirrhinum. Expression analysis suggests that the two gerbera agamous orthologues, the globosa orthologue and one of the deficiens orthologues may have functional equivalency to their counterparts, participating in the C and B functions, respectively. However, the function of a second deficiens orthologue appears unrelated to the B function, and that of a squamosa orthologue seems distinct from squamosa as well as from the A function. The induction patterns of gerbera MADS box genes conform spatiotemporally to the multi-flowered, head-like inflorescence typical of Asteraceae. Furthermore, gerbera plants transgenic for the newly isolated MADS box genes shed light onto the mechanistic basis for some floral characteristics that are typical for Asteraceae. We can conclude, therefore, that the pappus bristles are sepals highly modified for seed dispersal, and that organ abortion in the female marginal flowers is dependent upon organ identity and not organ position when position is homeotically altered.

Petal and stamen development

Analyses of petal and stamen development are beginning to illuminate the molecular genetic processes that are required to elaborate these organ types. Floral homeotic genes are required to specify certain organ identities, and these functions also are required throughout organogenesis. These genes, either directly or indirectly, presumably control a wide array of tissue- and cell-type-specific differentiation processes. At least part of this repertoire seems to include the regulation of cell proliferation, coupling the specification of organ identity with changes in growth dynamics in different regions of the developing flower. Furthermore, cells have an enormous amount of developmental plasticity, which means that they have to be able to integrate multiple sources of information as they terminally differentiate. Some of the identified inputs include the position of the cell in the developing organ, the status of gene expression and epigenetic information, and environmental signals. How this information is disseminated between cells is largely unknown. Not only do individual cells need to respond to this information, but fields of cells must coordinate their differentiation to form a functionally complex structure. The challenge that is before us is to understand how this plasticity of response is regulated to give a reproducible and species-specific pattern of differentiated tissues.

Molecular evolution of genes controlling petal and stamen development: duplication and divergence within the APETALA3 and PISTILLATA MADS-box gene lineages

The specification of floral organ identity in the higher dicots depends on the function of a limited set of homeotic genes, many of them members of the MADS-box gene family. Two such genes, APETALA3 (AP3) and PISTILLATA (PI), are required for petal and stamen identity in Arabidopsis; their orthologs in Antirrhinum exhibit similar functions. To understand how changes in these genes may have influenced the morphological evolution of petals and stamens, we have cloned twenty-six homologs of the AP3 and PI genes from two higher eudicot and eleven lower eudicot and magnolid dicot species. The sequences of these genes reveal the presence of characteristic PI- and AP3-specific motifs. While the PI-specific motif is found in all of the PI genes characterized to date, the lower eudicot and magnolid dicot AP3 homologs contain distinctly different motifs from those seen in the higher eudicots. An analysis of all the available AP3 and PI sequences uncovers multiple duplication events within each of the two gene lineages. A major duplication event in the AP3 lineage coincides with the base of the higher eudicot radiation and may reflect the evolution of a petal-specific AP3 function in the higher eudicot lineage.