A floral meristem identity gene influences physiological and ecological aspect of floral organogenesis

The architecture of a flower is tightly linked to the way a plant pollinates, making it one of the most physiologically and ecologically important traits of angiosperms. Floral organ development is proposed to be governed by the activity of three different classes of organ identity genes (the ABC model), and the expression of those genes are regulated by a number of meristem identity genes. Here we use a transgenetic strategy to elucidate the role of one floral meristem identify gene,LEAFY (LFY), in the evolution of floral organogenesis of a self pollinatorIdahoa scapigera and a obligatory out-crosserLeavenworthia crassa in the mustard family, Brassicaceae. By introducing theLFY genes from these two types of pollination habit into the genetic model speciesArabidopsis thaliana, we provide evidence that changes inLFY influenced flower architecture probably by controlling the downstream organ identity genes.     

Benzylaminopurine induces phenocopies of floral meristem and organ identity mutants in wild-typeArabidopsis plants

Arabidopsis thaliana (L.) Heynh. has been used as a model system to investigate the regulatory genes that control and coordinate the determination, differentiation and morphogenesis of the floral meristem and floral organs. We show here that benzylaminopurine (BAP), a cytokinin, influences flower development inArabidopsis and induces partial phenocopies of known floral homeotic mutants. Application of BAP to wild-type inflorescences at three developmental stages results in: (i) increase in floral organ number; (ii) formation of abnormal floral organs and (iii) induction of secondary floral buds in the axils of sepals. These abnormalities resemble the phenotypes of mutants,clv1 (increase in organ number),ap1,ap2,ap3 (abnormal floral organs) andap1 (secondary floral buds in the axils of first-whorl organs). In addition, BAP induces secondary floral buds in the axils of perianth members ofapt2-6, ap3-1 andag mutants, and accentuates the phenotype of theapt2-1 mutant to resemble theapt2-6 mutant. These observations suggest that exogenous BAP suppresses the normal functioning of the genes for floral meristem identity and thereby affects flower development and the later stages of floral organ differentiation.Abbreviations BAP N6-benzylaminopurine - CK cytokinin     

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.     

AGAMOUS-LIKE24 and SHORT VEGETATIVE PHASE determine floral meristem identity in Arabidopsis

The formation of flowers starts when floral meristems develop on the flanks of the inflorescence meristem. In Arabidopsis the identity of floral meristems is promoted and maintained by APETALA1 (AP1) and CAULIFLOWER (CAL). In the ap1 cal double mutant the meristems that develop on the flanks of the inflorescence meristem are unable to establish floral meristem identity and develop as inflorescence meristems on which new inflorescence meristems subsequently proliferate. We demonstrate in contrast to previous models that AGAMOUS-LIKE 24 (AGL24) and SHORT VEGETATIVE PHASE (SVP) are also floral meristem identity genes since the ap1-10 agl24-2 svp-41 triple mutant continuously produces inflorescence meristems in place of flowers. Furthermore, our results explain how AP1 switches from a floral meristem identity factor to a component that controls floral organ identity.     

Eucalyptus has a functional equivalent of the Arabidopsis floral meristem identity gene LEAFY

Two genes cloned from Eucalyptus globulus, Eucalyptus LeaFy (ELF1 and ELF2), have sequence homology to the floral meristem identity genes LEAFY from Arabidopsis and FLORICAULA from Antirrhinum. ELF1 is expressed in the developing eucalypt floral organs in a pattern similar to LEAFY while ELF2 appears to be a pseudo gene. ELF1 is expressed strongly in the early floral primordium and then successively in the primordia of sepals, petals, stamens and carpels. It is also expressed in the leaf primordia and young leaves and adult and juvenile trees.The ELF1 promoter coupled to a GUS reporter gene directs expression in transgenic Arabidopsis in a temporal and tissue-specific pattern similar to an equivalent Arabidopsis LEAFY promoter construct. Strong expression is seen in young flower buds and then later in sepals and petals. No expression was seen in rosette leaves or roots of flowering plants or in any non-flowering plants grown under long days. Furthermore, ectopic expression of the ELF1 gene in transgenic Arabidopsis causes the premature conversion of shoots into flowers, as does an equivalent 35S-LFY construct. These data suggest that ELF1 plays a similar role to LFY in flower development and that the basic mechanisms involved in flower initiation and development in Eucalyptus are similar to those in Arabidopsis.     

Mutations associated with floral organ number in rice

How floral organ number is specified is an interesting subject and has been intensively studied in Arabidopsis thaliana. In rice (Oryza sativa L.), mutations associated with floral organ number have been identified. In three mutants of rice, floral organ number 1 (fon1) and the two alleles, floral organ number 2-1 (fon2-1) and floral organ number 2-2 (fon2-2), the floral organs were increased in number centripetally. Lodicules, homologous to petals, were rarely affected, and stamens were frequently increased from six to seven or eight. Of all the floral organs the number of pistils was the most frequently increased. Among the mutants, fon1 showed a different spectrum of organ number from fon2 -1 and fon2 -2. Lodicules were the most frequently affected in fon1, but pistils of more than half of fon1 flowers were unaffected; in contrast, the pistils of most flowers were increased in fon2 -1 and fon2-2. Homeotic conversion of organ identity was also detected at a low frequency in ectopically formed lodicules and stamens. Lodicules and stamens were partially converted into anthers and stigmas, respectively. Concomitant with the increased number of floral organs, each mutant had an enlarged apical meristem. Although meristem size was comparable among the three mutants and wild type in the early phase of flower development, a significant difference became apparent after the lemma primordium had differentiated. In these mutants, the size of the shoot apical meristem in the embryo and in the vegetative phase was not affected, and no phenotypic abnormalities were detected. These results do not coincide with those for Arabidopsis in which clavatal affects the sizes of both shoot and floral meristems, leading to abnormal phyllotaxis, inflorescence fasciation and increased floral organs. Accordingly, it is considered that FON1 and FON2 function exclusively in the regulation of the floral meristem, not of the vegetative meristem.Abbreviation DIC differential interference contrast This work was supported in part by Grant-in-Aid for Scientific Research on Priority Areas from the Ministry of Education, Science and Culture of Japan.     

Over-expression of the Gerbera hybrida At-SOC1-like1 gene Gh-SOC1 leads to floral organ identity deterioration

Background and Aims

The family of MADS box genes is involved in a number of processes besides controlling floral development. In addition to supplying homeotic functions defined by the ABC model, they influence flowering time and transformation of vegetative meristem into inflorescence meristem, and have functions in roots and leaves. Three Gerbera hybrida At-SOC1-like genes (Gh-SOC1–Gh-SOC3) were identified among gerbera expressed sequence tags.

Methods

Evolutionary relationships between SOC1-like genes from gerbera and other plants were studied by phylogenetic analysis. The function of the gerbera gene Gh-SOC1 in gerbera floral development was studied using expression analysis, protein–protein interaction assays and reverse genetics. Transgenic gerbera lines over-expressing or downregulated for Gh-SOC1 were obtained using Agrobacterium transformation and investigated for their floral phenotype.

Key Results

Phylogenetic analysis revealed that the closest paralogues of At-SOC1 are Gh-SOC2 and Gh-SOC3. Gh-SOC1 is a more distantly related paralogue, grouping together with a number of other At-SOC1 paralogues from arabidopsis and other plant species. Gh-SOC1 is inflorescence abundant and no expression was seen in vegetative parts of the plant. Ectopic expression of Gh-SOC1 did not promote flowering, but disturbed the development of floral organs. The epidermal cells of ray flower petals appeared shorter and their shape was altered. The colour of ray flower petals differed from that of the wild-type petals by being darker red on the adaxial side and greenish on the abaxial surface. Several protein–protein interactions with other gerbera MADS domain proteins were identified.

Conclusions

The At-SOC1 paralogue in gerbera shows a floral abundant expression pattern. A late petal expression might indicate a role in the final stages of flower development. Over-expression of Gh-SOC1 led to partial loss of floral identity, but did not affect flowering time. Lines where Gh-SOC1 was downregulated did not show a phenotype. Several gerbera MADS domain proteins interacted with Gh-SOC1.     

Number of floral organs in Circaeaster agrestis (Circaeasteraceae) and possible homeosis among floral organs

98.9% of 5092 flowers from 1041 individuals of Circaeaster agrestis have five floral organs, the formula is P₃A₁G₁ (73.13%), P₂A₂G₁ (25.59%), and P₂A₁G₂ (0.22%). Only 0.4% of the flowers have six floral organs and the formula is P₃A₁G₂ (20 flowers) or P₃A₂G₁ (one flower). All these flowers have one vascular bundle in the pedicel and were considered to be normal ones. There are 33 flowers (0.65%) with six or more floral organs and two vascular bundles in the pedicel and we found traces of fusion of different degree of two flowers into one. These flowers were considered as abnormal. Therefore the normal number of floral organs of C. agrestis is five and occasionally six, and the floral formulas are P₃A₁G₁ or P₂A₂G₁, sometimes P₂A₁G₂, and occasionally P₃A₁G₂ or P₃A₂G₁. A tepal in P₃A₁G₁ may be replaced by a stamen in P₂A₂G₁ or by a carpel in P₂A₁G₂ or in reverse. A carpel in P₃A₁G₂ may be replaced by a stamen in P₃A₂G₁ or in reverse. We hypothesize that there are two possibilities for the number of the floral organs to be five (six), the result of reduction from P₃A₂G₂, or there exists homeosis among floral organs.     

Changes in cell-cycle duration and growth fraction in the shoot meristem of Sinapis during floral transition

The cell-cycle duration and the growth fraction were estimated in the shoot meristem of Sinapis alba L. during the transition from the vegetative to the floral condition. Compared with the vegetative meristem, the cell-cycle length was reduced from 86 to 32 h and the growth fraction, i.e. the proportion of rapidly cycling cells, was increased from 30–40% to 50–60%. These changes were detectable as early as 30 h after the start of the single inductive long day. The faster cell cycle in the evoked meristem was achieved by a shortening of the G1 (pre-DNA synthesis), S (DNA synthesis) and G2 (post-DNA synthesis) phases of the cycle. In both vegetative and evoked meristems, both-the central and peripheral zones were mosaics of rapidly cycling and non-cycling cells, but the growth fraction was always higher in the peripheral zone.Abbreviations G1 pre-DNA synthesis phase - G2 post-DNA synthesis phase - GF growth fraction - M mitosis phase - PLM percentage-labelled-mitoses method - S DNA synthesis phase - TdR thymidine