Petal Primordia Research Articles

RABBIT EARS, encoding a SUPERMAN-like zinc finger protein, regulates petal development in Arabidopsis thaliana.

Floral organs usually initiate at fixed positions in concentric whorls within a flower. Although it is understood that floral homeotic genes determine the identity of floral organs, the mechanisms of position determination and the development of each organ have not been clearly explained. We isolated a novel mutant, rabbit ears (rbe), with defects in petal development. In rbe, under-developed petals are formed at the correct position in a flower, and the initiation of petal primordia is altered. The rbe mutation affects the second whorl organ shapes independently of the organ identity. RBE encodes a SUPERMAN-like protein and is located in the nucleus, and thus may be a transcription factor. RBE transcripts are expressed in petal primordia and their precursor cells, and disappeared at later stages. When cells that express RBE are ablated genetically, no petal primordia arise. RBE is not expressed in ap1-1 and ptl-1 mutants, indicating that RBE acts downstream of AP1 and PTL genes. These characteristics suggest that RBE is required for the early development of the organ primordia of the second whorl.

Comparative floral ontogeny of Maesa (Maesaceae), Aegiceras (Myrsinaceae) and Embelia (Myrsinaceae): taxonomic and phylogenetic implications

The comparative floral ontogeny of five species belonging to the primuloid clade of the Ericales are investigated, viz. Maesa japonica, M. perlarius, Aegiceras corniculatum, Embelia laeta and E. ribes. All five species basically show 2/5-spiral phyllotaxis of the sepal primordia, although with some minor modification (particularly in Embelia, where the flowers are predominantly tetramerous). The phyllotaxis of the common petal-stamen primordia is also 2/5-spiral in the Maesa and Aegiceras species investigated, but appears to be unidirectional in Embelia. All five species develop common petal-stamen primordia in which the resultant petal primordia are larger than the stamen primordia, and in which the stamens develop proximally on the adaxial flank of the common primordia. Growth of the placenta in Maesa and Aegiceras partially embeds the ovules, but in Embelia the ovules are almost fully immersed in placental tissue at maturity. A comprehensive review of all previously published studies of floral ontogeny of primuloid genera is presented, and the phylogenetic significance of the variation between genera is evaluated with reference to recently published cladograms.

Separation of genetic functions controlling organ identity in flowers.

Comparative studies on the ABC model of floral development have revealed extensive conservation of B and C class genes, but have failed to identify similar conservation for A class genes. Using a reverse genetic approach, we show that the previous inability to obtain Antirrhinum mutants corresponding to the A class gene AP2 of Arabidopsis reflects greater genetic redundancy in Antirrhinum . Antirrhinum has two genes corresponding to AP2, termed LIP1 and LIP2, both of which need to be inactivated to give a mutant phenotype. Analysis of interactions between LIP and class B/C genes shows that unlike AP2 in Arabidopsis, LIP genes are not required for repression of C in outer whorls of the flower. However, like AP2, LIP genes play a role in sepal, petal and ovule development, although some of their detailed effects are different, reflecting the diverse morphologies of Antirrhinum and Arabidopsis flowers. The dual functions for which AP2 is required in Arabidopsis are therefore separate in Antirrhinum, showing that the genetic basis of some aspects of organ identity have undergone major evolutionary change.

Separable roles of UFO during floral development revealed by conditional restoration of gene function.

The UNUSUAL FLORAL ORGANS (UFO) gene is required for several aspects of floral development in Arabidopsis including specification of organ identity in the second and third whorls and the proper pattern of primordium initiation in the inner three whorls. UFO is expressed in a dynamic pattern during the early phases of flower development. Here we dissect the role of UFO by ubiquitously expressing it in ufo loss-of-function flowers at different developmental stages and for various durations using an ethanol-inducible expression system. The previously known functions of UFO could be separated and related to its expression at specific stages of development. We show that a 24- to 48-hour period of UFO expression from floral stage 2, before any floral organs are visible, is sufficient to restore normal petal and stamen development. The earliest requirement for UFO is during stage 2, when the endogenous UFO gene is transiently expressed in the centre of the wild-type flower and is required to specify the initiation patterns of petal, stamen and carpel primordia. Petal and stamen identity is determined during stages 2 or 3, when UFO is normally expressed in the presumptive second and third whorl. Although endogenous UFO expression is absent from the stamen whorl from stage 4 onwards, stamen identity can be restored by UFO activation up to stage 6. We also observed floral phenotypes not observed in loss-of-function or constitutive gain-of-function backgrounds, revealing additional roles of UFO in outgrowth of petal primordia.

Developmental study on the inflorescence and flower of Caldesia grandis Samuel (Alismataceae)

The inflorescence and floral development of Caldesia grandis Samuel is reported for the first time in this paper. The basic units of the large cymo-thyrsus inflorescence are short panicles that are arranged in a pseudowhorl. Each panicle gives rise spirally to three bract primordia also arranged in a pseudowhorl. The branch primordia arise at the axils of the bracts. Each panicle produces spirally three bract primordia with triradiate symmetry (or in a pseudowhorl) and three floral primordia in the axils of the bract primordia. The apex of the panicle becomes a terminal floral primordium after the initiations of lateral bract primordia and floral primordia. Three sepal primordia are initiated approximately in a single whorl from the floral primordium. Three petal primordia are initiated alternate to the sepal primordia, but their subsequent development is much delayed. The first six stamen primordia are initiated as three pairs in a single whorl and each pair appears to be antipetalous as in other genera of the Alismataceae. The stamen primordia of the second whorl are initiated trimerously and opposite to the petals. Usually, 9-12 stamens are initiated in a flower. There is successive transition between the initiation of stamen and carpel primordia. The six first-initiated carpel primordia rise simultaneously in a whorl and alternate with the trimerous stamens, but the succeeding ones are initiated in irregular spirals, and there are 15-21 carpels developed in a flower. Petals begin to enlarge and expand when anthers of stamens have differentiated microsporangia. Such features do not occur in C. parnassifolia. In the latter, six stamen primordia are initiated in two whorls of three, carpel primordia are initiated in 1-3 whorls, and there is no delay in the development of petals. C. grandis is thus considered more primitive and C. parnassifolia more derived. C. grandis shares more similarities in features of floral development with Alsma, Echinodorus, Luronium and Sagittaria. (C) 2002 The Linnean Society of London.

Floral ontogeny in Sophoreae(Leguminosae: Papilionoideae). III. Radial symmetry and random petal aestivation in Cadia Purpurea

Floral organogeny and development are described in Cadia purpurea, a legume with radial symmetry, unstable petal aestivation, and free organs, all unusual features among papilionoids. Flowers are usually solitary or, rarely, in few-flowered racemes. No bracteoles are formed. The order of organ initiation is unidirectional in each whorl, and the carpel initiates directly after petal initiation.The petal primordia remain small until all other floral organs have initiated, enlarged, and differentiated. Petal aestivation is variable, unlike the great majority of papilionoid flowers. Petal margins of Cadia grow straight outward rather than some petal margins curving inward inside the adjacent petal as is the case in other papilionoids. When adjacent growing petal margins of Cadia meet, chance determines which petal becomes positioned inside the other. Hence, the pattern of petal aestivation is random.

In situ hybridization of the MADS-box gene POTM1 during potato floral development.

The potato MADS-box gene POTM1 is a member of the SQUA-like family of plant MADS-box genes. The amino acid sequence of POTM1 most closely matches the sequence of PFG (Petunia Flowering Gene), a petunia MADS-box gene involved in the transition from vegetative to reproductive development. To examine the likely role of POTM1 in potato reproductive tissue, in situ hybridizations were performed on sections of the potato shoot apex at various stages during the transition to flowering. Scanning electron micrographs provide a reference for each stage to illustrate the progression from vegetative meristem to inflorescence meristem and floral organs. POTM1 mRNA accumulates in vegetative, inflorescence and floral meristems. Transcripts are present in petal primordia during the later stages of petal development and they accumulate later during floral development inside the stamens and carpels, but not in carpel walls. This widespread pattern of mRNA accumulation is unique among the SQUA-like genes and indicates that POTM1 is active during a variety of stages of development, and may function in regulating cell determination in transitional phases of both vegetative and floral meristems.

A phylogenetic analysis of 100+ genera and 50+ families of euasterids based on morphological and molecular data with notes on possible higher level morphological synapomorphies

A data matrix of 143 morphological and chemical characters for 142 genera of euasterids according to the APG system was compiled and complemented with rbcL and ndhF sequences for most of the genera. The data were subjected to parsimony analysis and support was assessed by bootstrapping. Strict consensus trees from analyses of morphology alone and morphology + rbcL + ndhF are presented. The morphological data recover several groups supported by molecular data but at the level of orders and above relationships are only superficially in agreement with molecular studies. The analyses provide support for monophyly of Gentianales, Aquifoliales, Apiales, Asterales, and Dipsacales. All data indicate that Adoxaceae are closely related to Dipsacales and hence they should be included in that order. The trees were used to assess some possible morphological synapomorphies for euasterids I and II and for the orders of the APG system. Euasterids I are generally characterised by opposite leaves, entire leaf margins, hypogynous flowers, “early sympetaly” with a ring-shaped corolla primordium, fusion of stamen filaments with the corolla tube, and capsular fruits. Euasterids II often have alternate leaves, serrate-dentate leaf margins, epigynous flowers, “late sympetaly” with distinct petal primordia, free stamen filaments, and indehiscent fruits. It is unclear which of these characters represent synapomorphies and symplesiomorphies for the two groups, respectively, and there are numerous expections to be interpreted as reversals and parallelisms.

The North American ginseng inflorescence: development, floret abscission zone, and the effect of ethylene

Characterization of the North American ginseng (Panax quinquefolius L.) inflorescence, the floret abscission zone, and its stimulation by ethylene were undertaken to gain insights into the potential for floret abscission induction as a replacement for the industry practice of expensive manual removal of the inflorescence. In each of 3 years, peduncle length of 3-year-old cultivated ginseng inflorescences increased rapidly in June, reaching maximal values of 140–180 mm. During this period, seed head diameter also increased to about 35 mm, while seed head fresh mass increased in July and August to 6.5–8.0 g. Flowering in 1996 and 1997 began in the last 2 weeks of June and lasted through July, with open flowers never exceeding 35%. Anatomical studies and manual dissection of 1- and 3-year-old perennating buds revealed development of the floral primordia first at their periphery and progressing centripetally. Stamen initials were seen as a pentamerous whorl just under the petal primordia, and the characteristic inferior ovary initiated as a slight depression at the centre of the floral meristem. Sections through 3-year-old inflorescences harvested on May 23, 1996, showed that distal floral apices had already formed constriction zones between the flower and the pedicel, indicating the position of the abscission zone. Length of time to ethylene-induced abscission zone formation was less than 24 h in 3-year-old ginseng harvested on July 23, 1997. The abscission threshold for ginseng berry drop was in the range of 10-100 µL·L–1 ethylene.Key words: anatomy, flowering, Panax quinquefolius, perennating bud.

Floral Structure and Development ofApostasiaandNeuwiedia(Apostasioideae) and their Relationships to Other Orchidaceae

Floral development, structure, and pollination biology of species of Apostasia and Neuwiedia constituting the basalmost orchid subfamily Apostasioideae were studied. The perianth organs arise from a ring primordium. The two adaxial sepals appear first, followed by the petal primordia. This contrasts with monandrous orchids, in which the median petal primordium is the first organ of the inner perianth whorl to develop. Stamens are only formed on the abaxial side of the flower. However, in Neuwiedia veratrifolia and Apostasia nuda, vestiges of stamen primordia are present on the adaxial side of the flower. In Apostasia only, the lateral stamens are fully developed, whereas the median organ is a staminode; in A. nuda the staminode is suppressed. At anthesis the anthers are free in Neuwiedia, but they are postgenitally connected in Apostasia. In both genera, stamens and style are congenitally fused at the base. The inferior ovary is at first unilocular and then becomes trilocular by postgenital fusion of the ovary septa and the development of a short synascidiate zone. The three carpels of both genera are equal from the beginning, but in A. nuda only, the median carpel dominates during early development. The stigma is convex and papillate. Conspicuous raphide idioblast accumulations occur in staminode and style and in the tips of anthers and filaments. In contrast to all other orchids, both genera offer pollen as a reward. Neuwiedia has lip flowers, which are pollinated by Trigona bees; Apostasia has “Solanum‐type” flowers with recurved perianth and united anthers. Our study supports the view that Apostasia and Neuwiedia are closely related and that Apostasioideae are at the base of the orchid clade, sharing characters with orchids and lower Asparagales.

Effect of high temperature exposure time during flower bud formation on the occurrence of double pistils in ‘Satohnishiki’ sweet cherry

Exposure time of trees to high temperatures during flower differentiation influenced the occurrence of double pistils in ‘Satohnishiki’ sweet cherry. Mature trees were grown under both early and late forcing, and under non-forcing conditions until harvest in a commercial orchard located in Kagawa, southwestern Japan. In mid-July, when the maximum temperature began to rise rapidly following the rainy season, petal and stamen primordia had been formed in the buds under early forcing (93%) and late forcing (69%) conditions, but under non-forcing conditions most of the buds were still at the stage of sepal differentiation. Pistil doubling rarely occurred under forcing conditions, whereas 10.3% of the flowers developed double pistils under non-forcing conditions. In another experiment, potted trees were exposed to high temperatures (35°C/25°C, day/night) for 15 days at intervals of 15 days during the period from late-June to early-September. High temperature induced double pistils most severely in the buds that contained sepal and petal primordia at the beginning of the treatment, and the frequency of occurrence of double pistils was slightly lower in the buds treated at the earlier stage of flower differentiation. On the other hand, high temperature had little effect on pistil doubling in buds with differentiated stamen and pistil primordia. These results suggest that (1) the buds are most sensitive to the induction of double pistils at high temperatures at the transition stage from sepal to petal differentiation, and (2) forcing culture can be applied to sweet cherry production in warm areas to reduce double pistil formation by avoiding the exposure of buds to high temperatures while the buds are still in the sensitive period.

The Ontogenetic Basis for Missing Petals in Crudia (Leguminosae: Caesalpinioideae: Detarieae)

Crudia choussyana (in Crudia group of Cowan and Polhill) has a relatively simple, radially symmetrical flower structure but is unusual among legumes in lacking petals. Floral initiation and development were examined using scanning electron microscopy. Significant ontogenetic features include a circular, prepetal‐initiation floral apex and the abaxial, nonmedian position of the first sepal initiated. Five petals are initiated, but the primordia fail to enlarge and disappear completely by midstage, well before anthesis. The 10 stamens are in two alternating whorls and are initiated unidirectionally from the abaxial side. The carpel is initiated concurrently with the petal primordia. Absence of petals can occur by several developmental pathways among legumes.

Evolution and Development of Floral Diversity of Melianthaceae, an Enigmatic Southern African Family

Melianthus has strongly modified and monosymmetric (zygomorphic) flowers, and the ontogenetic derivation and evolutionary origins of these highly modified flowers are sought in a comparative study of the floral ontogeny and anatomy of two species of Melianthus and one species of Bersama. The development of monosymmetry in the family can be observed as a stepwise process linked with displacements of organ primordia and selective losses of petals and stamens. Flowers are weakly (Bersama) to strongly (Melianthus) monosymmetric. Sepal initiation is divided into two time sequences: the three outer sepals arise simultaneously with the anterior sepals behaving as a shielding unit before the two inner sepals emerge. Similarities and differences between the two genera are shown to be related to the trend to monosymmetry and are linked to specific pollination strategies. In Melianthus, an abaxial petal primordium is initiated, but it aborts at an early stage of development; in Bersama, all petals are present, but the incipient monosymmetry is visible at the onset of initiation. In Bersama, four stamens are the rule, but flowers pentamerous in all whorls are occasionally found. The nectary develops in a pouch formed between the adaxial sepal and the androecium. There is little ontogenetic evidence for a staminodial nature of the “beak” formed between the posterior stamens; it develops as a protective extension of the filament tube. A reduction sequence in the number of ovules can be seen by the progressive size limitation of the placentas. In Bersama, only one ovule arises on a marginal placenta and becomes displaced in a central position during development. The vasculature of different species is compared. The development of vascular traces is strictly linked with the initiation and external development of the floral organs. In both Melianthus and Bersama, the nectary is partly supplied by an odd adaxial trace. Our data support a relationship of Melianthaceae with Greyiaceae and Francoaceae, concordant with macromolecular results. It may be suggested that polysymmetric flowers of Bersama are secondarily derived from monosymmetric ones.

In situ analysis of RNA and protein expression in whole mounts facilitates detection of floral gene expression dynamics.

A three-dimensional whole-mount technique for detection of mRNA and protein expression patterns of floral regulatory genes in inflorescences from Antirrhinum majus is reported. This technique allows the observation of complex expression patterns in situ in developing flowers at different developmental stages initiated sequentially on the same inflorescence and labelled under the same conditions. Thereby, reconstruction from serial two-dimensional sections can be circumvented. The technique was used to study early changes in the expression of DEFICIENS (DEF), a class B floral homeotic transcription factor. Whole-mount analysis revealed that the order of appearance of DEF mRNA and protein expression in the floral primordium is opposite to the order of initiation of organ primordia. As a consequence, stamen primordia express the DEF gene prior to their initiation in whorl three, while petal primordia in the second whorl are morphologically distinct structures when second whorl DEF expression becomes established. This interesting feature was not readily detectable by previous analysis of serial sections. The particular usefulness of in situ analyses in whole mounts is further demonstrated in floral mutants with variable phenotypes and unpredictable sites of aberrant organ development. whole mount, in situ hybridization, immunolocalization, Antirrhinum majus, flower development.

A DEFICIENS homologue is down-regulated during apomictic initiation in ovules of Hieracium.

Hieracium is a member of the Asteraceae family, and contains sexual species in addition to apomictic species that reproduce by apospory and produce seed without fertilization. A homologue of the floral organ-identity gene DEFICIENS (DEF) was isolated from an apomictic line of Hieracium piloselloides (Vill.) following differential display between mature ovules and those initiating autonomous embryogenesis. The gene termed HPDEF has 93% amino acid identity with GDEF2, a DEF homologue isolated from Gerbera hybrida (D. Yu et al., 1999, Plant J. 17: 51-62), another member of the Asteraceae. In-situ analysis showed that early in floral development HPDEF is expressed in stamen and petal primordia, indicating expected B-function activity, according to the ABC model of floral organ identity (J. L. Bowman et al., 1991, Development 112: 1-20; E. S. Coen and E. M. Meyerowitz, 1991, Nature 353: 31-37). However, HPDEF expression was also observed in ovule primordia and expression continued in developing ovules until anthesis, indicating that this gene may have a role in ovule development. Expression of HPDEF was not detected in megaspore mother cells, or in sexual or aposporous embryo sacs. In sexual Hieracium, HPDEF was uniformly expressed throughout the ovule integument until anthesis. In most ovules of the apomict, however, HPDEF expression was transiently down-regulated in a specific zone in the chalazal region where cells initiating aposporous embryo sac formation differentiate. Uniform low-level HPDEF expression was subsequently observed prior to anthesis in ovules from sexual and apomictic plants. HPDEF may be down-regulated as a consequence of apomictic initiation and/or its down-regulation may facilitate progression of apomictic events.

Novel floral patterns

Our understanding of plant developmental, biochemical and physiological pathways has been greatly enhanced over the last decade by work on the small crucifer Arabidopsis thaliana. In particular, the molecular genetic analysis of floral development has led to a detailed ‘ABC’ model of organ identity during flower determination. However, it is not clear how well these models in Arabidopsis (a higher eudicot) can be extended to other distantly related plant species. To address this issue, Kramer and Irish 1 Kramer E.M. Irish V.F. Evolution of genetic mechanisms controlling petal development. Nature. 1999; 353: 31-37 Google Scholar examined the expression of two B class genes, APETALA3 (AP3) and PISTILLATA1 (PI) in flowers of several species of the lower eudicot subclass Ranunculidae. These species, include two from the family Papaveraceae (Iceland poppy and bloodroot), one from the family Fumariaceae (fringed bleeding heart) and two from the family Ranunculaceae (buttercup). Gene- and genome-duplication events both seem to have contributed to the diversification of the AP3 and PI lineages within the Ranunculidae. Thus, expression patterns were examined for all gene family members in these lineages. In developing stamen primordia, the temporal and spatial expression patterns of RNA and protein were similar between the Ranunculidae and higher eudicot (Arabidopsis and Antirrhinum) AP3 and PI orthologs. However, temporal and spatial patterns of AP3 and PI expression in the Ranunculid species in petal primordia were very different from those found in Arabidopsis and Antirrhinum: AP3 and PI were both expressed in young petal primoridia, but expression diminished during petal development, eventually becoming restricted to the extreme edges of the petal. In Arabidopsis and Antirrhinum, the continual expression of AP3 and PI throughout petal development is essential for the development of normal petal form. As the authors suggest, these differences in gene-expression patterns indicate that the ABC model is not rigidly conserved across Angiosperms and probably reflects the independent derivation of petals from either stamens or sterile bracts. Alternatively, PI might play a similar role in higher and lower eudicots during the initiation of petal primordia, but could have been replaced by novel B group genes that would function to direct petal differentiation during the later stages. Clearly, the evolution of floral pattern is not quite as simple as A,B,C.

Control of floral homeotic gene expression and organ morphogenesis in Antirrhinum.

The development of reproductive organs in Antirrhinum depends on the expression of an organ identity gene, plena, in the central domain of the floral meristem. To investigate the mechanism by which plena is regulated, we have characterised three mutants in which the pattern of plena expression is altered. In polypetala mutants, expression of plena is greatly reduced, resulting in a proliferation of petals in place of reproductive organs. In addition, polypetala mutants exhibit an altered pattern of floral organ initiation, quite unlike that seen in loss-of-function plena mutants. This suggests that polypetala normally has two roles in flower development: regulation of plena and control of organ primordia formation. In fistulata mutants, plena is ectopically expressed in the distal domain of petal primordia, resulting in the production of anther-like tissue in place of petal lobes. Flowers of fistulata mutants also show a reduced rate of petal lobe growth, even in a plena mutant background. This implies that fistulata normally has two roles in the distal domain of petal primordia: inhibition of plena expression and promotion of lobe growth. A weak allele of the floral meristem identity gene, floricaula, greatly enhances the effect of fistulata on plena expression, showing that floricaula also plays a role in repression of plena in outer whorls. Taken together, these results show that genes involved in plena regulation have additional roles in the formation of organs, perhaps reflecting underlying mechanisms for coupling homeotic gene expression to morphogenesis.

Floral Ontogeny of Pelargonium × domesticum

Floral ontogeny of Pelargonium xdomesticum was examined for two cultivars grown under identical environmental conditions. Apical meristems of `Duchess' and `Jennifer' were vegetative at the commencement of the experiment. Meristems were examined every five days over an experimental period lasting 85 days. Floral ontogeny was the same for both cultivars, although the timing of floral initiation of the meristem, floral organ initiation, and floral organ development differed. The vegetative meristem was convex with leaf primordia initiated on either side in an alternate pattern. Early floral initiation was characterized by formation of a cleft towards one side of the meristem, followed quickly by a second cleft on the other side. Between the clefts new meristems developed. New meristems lacked leaf primordia. Proliferation of meristems continued until numerous meristems were organized in a cluster arrangement at the apex of the shoot. Proliferation of meristems at the apex continued until multiple inflorescences had developed. Inflorescences were subtended by bracts. Floral organ primordia were initiated in a succession of four whorls: sepals, petals, androecia and gynoecium. Petals and androecia appeared to develop simultaneously soon after sepal primordia were visible. Petal primordia remained small while the androecia continued to grow. The gynoecium first formed a conical shape, with carpels protruding from the base in a bulbous fashion. At the distal end of the gynoecium, divisions appeared which developed into stigmatic lobes. As the gynoecium elongated, stigmatic lobes became more pronounced. Petal elongation concurred in synchrony with elongation of the gynoecium. On one experimental unit of `Duchess' two florets had opened at 750 total cumulative moles. No `Jennifer' florets had opened by termination of the experiment.

Transcription pattern of a FIM homologue in Impatiens during floral development and reversion.

Flowering and reversion in Impatiens are characterised by gradual transitions of organ identity and constitute a unique system for the molecular and physiological study of floral organogenesis. The authors have isolated an Impatiens homologue of the FIM gene of Antirrhinum (UFO in Arabidopsis), Imp-FIM, and analysed its expression in three states of the terminal meristem: vegetative, floral, and reverted. In floral meristems, Imp-FIM transcription is associated with petal identity, as in Antirrhinum and Arabidopsis, but this is achieved through a novel transcription pattern, characterised by a high level of transcript within petal primordia. This novel transcription pattern could contribute to the more diffuse boundaries between organ types in Impatiens. In vegetative meristems, Imp-FIM is expressed in the axils of leaf primordia which are arranged in a spiral. A similar pattern is observed in reverted meristems in which leaf primordia are initiated in a whorled arrangement. This result indicates that the maintenance of floral phyllotaxis is not associated with a specific pattern of Imp-FIM transcription. Transcription of Imp-FIM in a non-reverting line is no different from that in the reverting line. Therefore, the lack of floral commitment in the reverting line does not seem to be responsible for Imp-FIM transcription within petals. The novel transcription pattern in petals, together with features of Impatiens that are reminiscent of fim and ufo mutant phenotypes suggest an evolutionary divergence for Imp-FIM regulation in this species.

Patterns of Petal and Stamen Reduction in Australian Species of Lepidium L. (Brassicaceae)

The floral ground plan in the Brassicaceae is highly conserved. However, organ reduction is common in the genus Lepidium (pepper cresses). In many cases, petals are reportedly reduced or absent, and stamen numbers are reduced from six to either four or two in more than half of all species. To gain an understanding of possible mechanisms of reduction, we have observed flower organ development by SEM in six Australian species showing a range of reduction patterns. We conclude that reduction occurs by three independent mechanisms. Petal primordia are always formed, but their growth is suppressed in species in which they are reduced. This occurs at different times in the different species, although all except Lepidium oxytrichum show some differentiation of mature tissues. On the other hand, stamens are reduced either by apparent loss of primordia (in the two lateral positions) or by fusion of two primordia into one (in the two medial positions). Overall, Lepidium species show one of four different stamen gro...