Developmental physiology of floral initiation in Nicotiana tabacum L.

The central process in the making of a multicellular organismis the fating of cells and tissues for their terminal phenotypes.The formation of a flower from a shoot apical meristem completesa sequence of fating processes initiated in embryogenesis. Thefating of a vegetative meristem of Nicotiana tabacum L. to initiatea flower involves at least two signals and two developmentalstates. A signal from the roots maintains vegetative growth,or prevents flowering, in the young seedling. As the plant grows,the vegetative meristem gains greater competence to respondto the floral stimulus from the leaves until it is evoked, byfloral stimulus, into a florally determined state. The florallydetermined state is then expressed. These developmental processesnot only establish the time of floral initiation, but also regulateplant size as measured by the number of nodes produced. Key words: Plant size, floral stimulus, competence, floral determination, induction     

Polyamine metabolism,floral initiation and floral development in Chrysanthemum (Chrysanthemum morifolium Ramat.)

In the short-day plant Chrysanthemum (Chrysanthemum morifolium Ramat. variety Pavo) putrescine and spermidine conjugates appeared in the apical bud before the first observable transformation of the meristem into floral structures. These compounds accumulated on floral initiation and well before floral evocation. Spermidine conjugates were predominant during floral initiation whereas free amines did not accumulate to any significant extent. Different associations of amides were observed during floral initiation as compared with the reproductive phase. 3,4-Dimethoxyphenethylamine conjugates (water-insoluble compounds) were the predominant amine conjugates observed during flower development. These compounds decreased drastically after fertilization. In vegetative buds from plants grown in long days polyamine conjugates were very low and appeared as plants aged. We present evidence that ornithine decarboxylase (ODC) regulates putrescine biosynthesis during floral initiation and floral development. When ODC action was blocked by DFMO (-DL-difluoromethylornithine, a specific, irreversible inhibitor of ODC), flowering was inhibited, and free and conjugated polyamines were not detected. This treatment led to a slight enhancement of ADC activity. When putrescine was added, polyamine titers and flowering were restored. A similar treatment with DFMA (-DL difluoromethylarginine, a specific, irreversible inhibitor of ADC) did not affect flowering and the polyamine titers. The results suggest that ODC and polyamine conjugates are involved in regulating floral initiation in Chrysanthemum.Abbreviations ADC arginine decarboxylase - ODC ornithine decarboxylase - DFMA -DL-difluoromethylarginine - DFMO -DL-difluoromethylornithine     

Changes in ATP in relation to floral induction and initiation in Pharbitis nil

No changes in metabolism of adenosine phosphates as a function of short day induction were detected in cotyledons of Pharbitis nil Chois strain Violet. A gradual increase in ATP level was detected throughout the dark period in plumules. A rapid decline of ATP pool size was observed in induced plumules shortly after floral induction. The decline occurred close to the 14th hour of the dark period, 1 to 1.5 h after the dark period length required for a 90% flowering response, which is thought to be the minimum time required for transport of the floral stimulus (and assimilates) from the induced cotyledons to the plumule. Transport of the major adenylates from the cotyledons was verified using [¹⁴C]-adenine. Estimates of the amount, and rate, of adenylate transport suggest that the cotyledons could be an important source of adenylates to re-establish the ATP pool size in evoked plumules.     

Changes in polyamine pattern are involved in floral initiation and development in Polianthes tuberosa

In the day-neutral plant Polianthes tuberosa (cv. Double) putrescine and spermine in corms at the early floral initiation stage decreased by 26 and 36%, respectively, compared with that in the vegetative stage. In contrast, a sharp increase in spermidine and cadaverine titers in corms was recorded at the early floral initiation stage. However, cadaverine in corms disappeared at the flower development stage. Polyamines in the roots were generally lower than those in the leaves and corms. In no case was the change in endogenous polyamine titers in the roots and leaves associated with floral initiation and flower development in P. tuberosa. Exogenous application of spermidine at 5, 25 or 150 microg per plant at the vegetative stage did not affect flower primordium counts. However, addition of a spermidine synthase inhibitor, cyclohexylamine, at 150 or 250 microg per plant (each dose was applied two times in total at an interval of 4 days) significantly reduced flower primordium counts, indicating that spermidine is involved in floral initiation and floral development in P. tuberosa. In P. tuberosa corms at the vegetative stage arginine decarboxylase activity rises and decreases at the early floral initiation stage. In contrast, ornithine decarboxylase activity reaches the highest level at the early floral initiation stage and declines significantly at the vegetative stage. Results indicate that an increase in spermidine and a transient increase in cadaverine titers in the corms seem characteristic of early floral initiation in P. tuberosa. It is also suggested that a significant reduction in putrescine and spermine in the corms is involved in the early floral initiation in P. tuberosa.     

Different roles of flowering-time genes in the activation of floral initiation genes in Arabidopsis.

We have analyzed double mutants that combine late-flowering mutations at four flowering-time loci (FVE, FPA, FWA, and FT) with mutations at the LEAFY (LFY), APETALA1 (AP1), and TERMINAL FLOWER1 (TFL1) loci involved in the floral initiation process (FLIP). Double mutants between ft-1 or fwa-1 and lfy-6 completely lack flowerlike structures, indicating that both FWA and FT act redundantly with LFY to control AP1. Moreover, the phenotypes of ft-1 ap1-1 and fwa-1 ap1-1 double mutants are reminiscent of the phenotype of ap1-1 cal-1 double mutants, suggesting that FWA and FT could also be involved in the control of other FLIP genes. Such extreme phenotypes were not observed in double mutants between fve-2 or fpa-1 and lfy-6 ap1-1. Each of these showed a phenotype similar to that of ap1-1 or lfy-6 mutants grown under noninductive photoperiods, suggesting a redundant interaction with FLIP genes. Finally, the phenotype of double mutants combining the late-flowering mutations with tfl1-2 were also consistent with the different roles of flowering-time genes.     

Regulation of the initiation of chromosomal replication in bacteria

The initiation of chromosomal replication occurs only once during the cell cycle in both prokaryotes and eukaryotes. Initiation of chromosome replication is the first and tightly controlled step of a DNA synthesis. Bacterial chromosome replication is initiated at a single origin, oriC, by the initiator protein DnaA, which specifically interacts with 9-bp non-palindromic sequences (DnaA boxes) at oriC. In Escherichia coli, a model organism used to study the mechanism of DNA replication and its regulation, the control of initiation relies on a reduction of the availability and/or activity of the two key elements, DnaA and the oriC region. This review summarizes recent research into the regulatory mechanisms of the initiation of chromosomal replication in bacteria, with emphasis on organisms other than E. coli.     

Mutation of Arabidopsis plastid phosphoglucose isomerase affects leaf starch synthesis and floral initiation

We isolated pgi1-1, an Arabidopsis mutant with a decreased plastid phospho-glucose (Glc) isomerase activity. While pgi1-1 mutant has a deficiency in leaf starch synthesis, it accumulates starch in root cap cells. It has been shown that a plastid transporter for hexose phosphate transports cytosolic Glc-6-P into plastids and expresses restricted mainly to the heterotrophic tissues. The decreased starch content in leaves of the pgi1-1 mutant indicates that cytosolic Glc-6-P cannot be efficiently transported into chloroplasts to complement the mutant's deficiency in chloroplastic phospho-Glc isomerase activity for starch synthesis. We cloned the Arabidopsis PGI1 gene and showed that it encodes the plastid phospho-Glc isomerase. The pgi1-1 allele was found to have a single nucleotide substitution, causing a Ser to Phe transition. While the flowering times of the Arabidopsis starch-deficient mutants pgi1, pgm1, and adg1 were similar to that of the wild type under long-day conditions, it was significantly delayed under short-day conditions. The pleiotropic phenotype of late flowering conferred by these starch metabolic mutations suggests that carbohydrate metabolism plays an important role in floral initiation.     

Enhancement of embryogenic culture initiation from tissues of mature sweetgum trees

 Male inflorescences, female inflorescences, and leaves collected from dormant buds of three sweetgum (Liquidambar styraciflua) trees were tested for induction of somatic embryogenesis following treatment with thidiazuron, naphthaleneacetic acid (NAA) or different combinations of the two. Explants were placed into culture either within a few days after collection or following 2 months of storage at –15  °C. Although embryogenic cultures were obtained from all three trees, embryogenesis induction was strongly affected by genotype (source tree), with 100% of the staminate inflorescence explants from one tree producing embryogenic cultures in one experiment. Embryogenesis induction was also influenced by explant type, with staminate inflorescences up to five times more likely to produce an embryogenic culture than female inflorescences. No embryogenic cultures were obtained from leaf explants. While treatment with plant growth regulators was not required for embryogenesis induction from inflorescence explants, culture on medium with NAA alone resulted in the highest production of repetitively embryogenic cultures and cultures producing proembryogenic masses. Dormant buds stored for 2 months at –15  °C were still able to produce embryogenic cultures, although frozen storage decreased this ability by over one-half for staminate inflorescences. Received: 20 January 1999 / Revision received: 18 April 1999 / Accepted: 29 April 1999     

Regulation of SUP expression identifies multiple regulators involved in arabidopsis floral meristem development

During the course of flower development, floral homeotic genes are expressed in defined concentric regions of floral meristems called whorls. The SUPERMAN (SUP, also called FLO10) gene, which encodes a C2H2-type zinc finger protein, is involved in maintenance of the stamen/carpel whorl boundary (the boundary between whorl 3 and whorl 4) in Arabidopsis. Here, we show that the regulation of SUP expression in floral meristems is complex, consisting of two distinct phases, initiation and maintenance. The floral meristem identity gene LEAFY (LFY) plays a role in the initiation phase through at least two pathways, which differ from each other in the involvement of two homeotic genes, APETALA3 (AP3) and PISTILLATA (PI). AP3, PI, and another homeotic gene, AGAMOUS (AG), are further required for SUP expression in the later maintenance phase. Aside from these genes, there are other as yet unidentified genes that control both the temporal and spatial patterns of SUP expression in whorl 3 floral meristems. SUP appears to act transiently, probably functioning to trigger a genetic circuit that creates the correct position of the whorl 3/whorl 4 boundary.     

The gentian orthologs of the FT/TFL1 gene family control floral initiation in Gentiana

Gentians are herbaceous perennials blooming in summer through autumn. Although they are popular ornamental flowers in Japan, the regulation of their timing of flowering has not been studied. We identified and characterized gentian orthologs of the Arabidopsis FT/TFL1 gene family to elucidate the mechanisms of flowering initiation. We isolated three gentian orthologs of FT and TFL1, denoted GtFT1, GtFT2 and GtTFL1. Since up-regulation of GtFT1 and GtFT2 as well as down-regulation of GtTFL1 promoted floral initiation in gentian plantlets, these genes affected floral initiation in a similar way to Arabidopsis FT and TFL1. The expression levels of GtFT1 and GtFT2 in leaves of late-flowering gentian increased prior to floral initiation, whereas GtTFL1 was highly expressed in shoot apical meristem at the vegetative stage and decreased drastically just before flowering initiation. Comparison of gene expression patterns showed that GtFT1 expression increased earlier in early-flowering than in late-flowering gentian, whereas the timing of the increase in GtFT2 expression was similar in early- and late-flowering plants. The GtTFL1 expression in early-flowering gentian was extremely low throughout the vegetative and reproductive stages. These results indicated that either the up-regulation of GtFT1 or the down-regulation of GtTFL1 may determine flowering time. Furthermore, we found that early-flowering but not late-flowering gentians have a 320 bp insertion in the promoter region of GtTFL1. Thus, the negligible expression of GtTFL1 in early-flowering lines may be due to this insertion, resulting in a shortened vegetative stage.