Floral initiation in Rudbeckia hirta (Asteraceae) under limited inductive photoperiodic treatments

Two experiments were conducted to examine the response of Rudbeckia hirta to limited inductive photoperiodic treatments. The first examined the effects on plants grown to an thesis of the second axillary inflorescence, and the second examined the early histological events within the meristem. Plants of Rudbeckia hirta were grown to maturity under short days (SD). At maturity, half the plants were placed in long days (LD). In the first experiment, the plants remained under LD for 0, 8, 16, 24, or 32 days before being returned to SD with an additional group remaining under LD as a control. In the second experiment, the plants remained under LD for 0, 4, 8, 12, 16, 20, 24, or 28 days before being returned to SD. Meristems were sampled 0, 4, 8, or 12 days after return to SD and histologically examined. Four groups of plants receiving 32, 36, 40, or 44 LD were used as a continuous LD control. When grown to anthesis, plant height and branch number increased as the number of inductive cycles increased. Plants receiving 24 or more LD reached anthesis earlier than plants receiving fewer LD. Histological examination of plants receiving only 4 LD showed they never progressed beyond early floral initiation. After 12 LD, the meristems continued to develop even when returned to SD, indicating that enough of the floral stimulus had reached the meristem to initiate flowering. Once involucral bract primordia initiated, floral development continued whether in LD or SD conditions.     

Reversion of flowering in Glycine Max (Fabaceae)

Photoperiodic changes, if occurring before a commitment to flowering is established, can alter the morphological pattern of plant development. In this study, Glycine max (L.) Merrill cv. Ransom plants were initially grown under an inductive short-day (SD) photoperiod to promote flower evocation and then transferred to a long-day (LD) photoperiod to delay flower development by reestablishing vegetative growth (SD-LD plants). Some plants were transferred back to SD after 4-LD exposures to repromote flowering (SD-LD-SD plants). Alterations in organ initiation patterns, from floral to vegetative and back to floral, are characteristic of a reversion phenomenon. Morphological features that occurred at the shoot apical meristem in SD, LD, SD-LD, and SD-LD-SD plants were observed using scanning electron microscopy (SEM). Reverted plants initiated floral bracts and resumed initiation of trifoliolate leaves in the two-fifths floral phyllotaxy prior to terminal inflorescence development. When these plants matured, leaf-bract intermediates were positioned on the main stem instead of trifoliolate leaves. Plants transferred back to a SD photoperiod flowered earlier than those left in LD conditions. Results indicated that in plants transferred between SDs and LDs, photoperiod can influence organ initiation in florally evoked, but not committed, G. max plants.     

The role of carbohydrates in the induction of flowering in Arabidopsis thaliana: comparison between the wild type and a starchless mutant

In order to test whether an increased export of carbohydrates by leaves and starch mobilization are critical for floral transition in Arabidopsis thaliana, the Columbia ecotype as well as its starchless mutant pgm and starch-in-excess mutant sex1 were investigated. Induction of flowering was achieved by exposure of plants to either one long day (LD) or one displaced short day (DSD). The following conclusions were drawn: (i) Both the pgm and sex1 mutants have a late-flowering phenotype in days shorter than 16 h. (ii) When inductive treatments cause a large percentage of induced plants, there is always a large, early and transient increase in carbohydrate export from leaves. By contrast, when an inductive treatment results in only a low percentage of induced plants (pgm plants exposed to one DSD), the export of carbohydrates from leaves is not increased, supporting the idea that phloem carbohydrates have a critical function in floral transition. (iii) Starch mobilization is not required to obtain an increased carbohydrate export when induction is by one LD (extended period of photosynthesis), but is absolutely essential when induction is by one DSD (period of photosynthesis unaffected). (iv) Floral induction apparently increases the capability of the leaf phloem-loading system. Received: 27 August 1997 / Accepted: 6 March 1998     

Changes in free IAA level in the leaves of short-and long-day tobacco during flowering and the effect of applied IAA on the transition to flowering

Changes in free IAA level were studied in the leaves of the central stem zone of short-day tobacco (Mcotianatabacum, cv. Maryland Mammoth) and long-day tobacco(Nicotiana silvestris) in inductive photoperiodic regime after 10, 20, 30 and 40 d, respectively. The leaves of SD tobacco Mammoth showed a high free IAA level in vegetative plants kept under long days but it significantly decreased (by ca. 50 %) after 10, 30 and 40 short days, respectively. After 20 short days the IAA level was as high as in the leaves of plants at the beginning of inductive treatment. The changes of freeIAA level in the leaves of LD tobacco N.silvestris were similar to those of SD Mammoth, but the IAA level in this species was significantly lower than that of Mammoth throughout the investigated period. Consequently, the changes observed in N. silvestris were much less pronounced. Plants of both tobacco species were fully induced to flowering by 30 inductive days and this was associated with differentiation of the flower organs. Application of 10 -4 M IAA during the last 10 d of the inductive treatment of 30 d significantly reduced flowering in SD tobacco Mammoth without changing the stem length and apex width. Apex length was slightly reduced. IAA application elicited almost no effect inN. silvestris. The results are discussed with respect to the possible role of IAA in flower induction in SD and LD plants.     

The leaf as the site of gibberellin action in flower formation in Bryophyllum daigremontianum

Summary The long-short-day plant Bryophyllum daigremontianum can be induced to flower by transfer from long to short days (LDSD), or by gibberellin (GA) application under SD. Application of GA to mature leaves of intact or partially defoliated plants induces flowering more effectively than when applications are made to the youngest leaf pair and the shoot tip.Mature leaves on de-budded plants in SD are induced to produce floral stimulus by GA application, as demonstrated by grafting LD receptor scions onto the debudded plants, or by grafting SD leaves treated with GA onto receptor stocks in LD. This shows that GA applied to Bryophyllum in SD exerts its flower-promoting effect in the leaves.The minimal number of SD necessary for flower formation in Bryophyllum is approximately 15, both in case of photoinduction by the shift LDSD, and after GA treatment in SD. It is concluded that the LD part of photinduction establishes a high level of endogenous GAs in the leaves which is a prerequisite for production of floral stimulus under subsequent SD.Work supported by the United States Atomic Energy Commission, Contract No. AT(11-1) 1338.     

Appearance and disappearance of proteins in the shoot apical meristem of Sinapis alba in transition to flowering

Vegetative plants of Sinapis alba L. grown under short days were induced to flower by exposure to one long day or continuous long days. Irrespective of the number of long days, the first flower primordia were initiated by the shoot apical meristem 60 h after the start of the inductive treatment. An indirect histoimmunofluorescence technique was used to search in the apical meristem for three antigenic proteins which had been previously detected by immunodiffusion tests in the whole apical bud (Pierard et al. (1977) Physiol. Plant. 41, 254–258). One protein called protein A, present in the vegetative meristem, increased in concentration during the first 48 h following the start of the inductive treatment. It stayed constant up to 96 h and disappeared completely at a later time. Two other proteins called B and C, absent in the vegetative meristem, appeared in the meristem of induced plants between 30 and 36 h after the start of the inductive treatment and progressively accumulated at later times up to 240 h. These proteins appeared 8 h before the irreversible commitment of the meristem to produce flower primordia (point of no return) was reached and 24 h before start of flower production. These observations support an interpretation of floral evocation as consisting, at least partially, of an early and qualitative change in gene expression.Abbreviations AVB anti-vegetative-bud antiserum - ARB antireproductive-bud antiserum - IgG immunoglobulins G - TRITC tetramethylrhodamine isothiocyanate - GAR IgG goat antirabbit IgG - S₀ IgG non-immune rabbit IgG     

Polycomb-Group Proteins and FLOWERING LOCUS T Maintain Commitment to Flowering in Arabidopsis thaliana

The switch from vegetative to reproductive growth is extremely stable even if plants are only transiently exposed to environmental stimuli that trigger flowering. In the photoperiodic pathway, a mobile signal, florigen, encoded by FLOWERING LOCUS T (FT) in Arabidopsis thaliana, induces flowering. Because FT activity in leaves is not maintained after transient photoperiodic induction, the molecular basis for stable floral commitment is unclear. Here, we show that Polycomb-group (Pc-G) proteins, which mediate epigenetic gene regulation, maintain the identity of inflorescence and floral meristems after floral induction. Thus, plants with reduced Pc-G activity show a remarkable increase of cauline leaves under noninductive conditions and floral reversion when shifted from inductive to noninductive conditions. These phenotypes are almost completely suppressed by loss of FLOWERING LOCUS C (FLC) and SHORT VEGETATIVE PHASE, which both delay flowering and promote vegetative shoot identity. Upregulation of FLC in Pc-G mutants leads to a strong decrease of FT expression in inflorescences. We find that this activity of FT is needed to prevent floral reversion. Collectively, our results reveal that floral meristem identity is at least partially maintained by a daylength-independent role of FT whose expression is indirectly sustained by Pc-G activity.     

Occurrence of the Transition of Apical Architecture and Expression Patterns of Related Genes during Conversion of Apical Meristem Identity in G2 Pea

G2 pea exhibits an apical senescence delaying phenotype under short-day (SD) conditions; however, the structural basis for its apical development is still largely unknown. In the present study, the apical meristem of SD-grown G2 pea plants underwent a transition from vegetative to indeterminate inflorescence meristem, but the apical meristem of long-day (LD)-grown G2 pea plants would be further converted to determinate floral meristem. Both SD signal and GA3 treatment enhanced expression of the putative calcium transporter PPF1, and pea homologs of TFL1 (LF and DET), whereas LD signal suppressed their expression at 60 d post-flowering compared with those at 40 d post-flowering. Both PPF1 and LF expressed at the vegetative and reproductive phases in SD-grown apical buds, but floral initiation obviously increased the expression level of PPF1 compared with the unchanged expression level of LF from 40 to 60 d post-flowering. In addition, although the floral initiation significantly enhanced the expression levels of PPF1 and DET, DET was mainly expressed after floral initiation in SD-grown apical buds. Therefore, the main structural difference between LD- and SD-grown apical meristem in G2 pea lies in whether their apical indeterminate inflorescence medstem could be converted to the determinate structure.     

N content of phloem and xylem exudates during the transition to flowering in Sinapis alba and Arabidopsis thaliana

The involvement of nitrogenous substances in the transition to flowering was investigated in Sinapis alba and Arabidopsis thaliana (Columbia). Both species grown in short days (SD) are induced to flower by one long day (LD). In S. alba, the phloem sap (leaf and apical exudates) and the xylem sap (root exudate) were analysed in LD versus SD. In A. thaliana, only the leaf exudate could be analysed but an alternative system for inducing flowering without day‐length extension was used: the displaced SD (DSD). Significant results are: (i) in both species, the leaf exudate was enriched in Gln during the inductive LD, at a time compatible with export of the floral stimulus; (ii) in S. alba, the root export of amino acids decreased in LD, whereas the nitrate remained unchanged – thus the extra‐Gln found in the leaf exudate should originate from the leaves; (iii) extra‐Gln was also found very early in the apical exudate of S. alba in LD, together with more Glu; (iv) in A. thaliana induced by one DSD, the leaf export of Asn increased sharply, instead of Gln in LD. This agrees with Asn prevalence in C‐limited plants. The putative role of amino acids in the transition to flowering is discussed.     

Location of cell cycle changes in relation to morphological changes in the shoot apex ofSilene coeli-rosa immediately before sepal initiation

Summary Changes in morphology, the mitotic index and the proportions of cells in G₁ and G₂ were measured in shoot meristems ofSilene coeli-rosa immediately before floral morphogenesis in order to determine whether the known changes to the cell cycle at this time are restricted to a particular region of the apex. Twenty-eight day-old plants were given either 7 long days (LD) plus 2 short days (SD) (day 8 of the LD treatment) or 9 SD [day 8 of the SD control (SDC) treatment]. Plants were sampled on day 8 every 2 h for 12 h and the various cell cycle measurements were performed on sections of the apical meristem. In the inductive LD treatment there was a peak in the mitotic index at 13.00 h and, possibly, the start of another at 19.00 h. At 21.00 h all meristems in this treatment initiated sepals. The mitotic activity at 13.00 and 19.00 h in the LD treatment was a result of significant increases in the mitotic index in the axial, lateral and central sub-axial areas of the apex compared with the corresponding zones in the SDC treatment. At 13.00 h of day 8, 80% of cells were in G₂ phase in the axial region in the LD treatment whilst 85% of cells were in G₁ in the axial zone in the SDC treatment. In the other zones significantly more cells were in G₂ in the LD compared with the SDC treatment as was the case at 19.00 h although not to the same extent as the axial zone at 13.00 h. Thus these data emphasize, for the first time, the mitotic activation and predominance of the G₂ population of cells particularly in the axial zone of shoot meristems in the LD treatment. These data are discussed in relation to the synchronisation of cell division which could occur in the prefloral shoot meristem at this time, affecting each shoot apical zone.Abbreviations LD long day - SD short day - SDC short day control     

Photoperiodic studies on growth and development of Impatiens balsamina L.

Summary In the short-day plant Impatiens balsamina it was found that, while floral buds are initiated with 3 short-day (SD) cycles, at least 8 such cycles are required for flowering. The numbers of floral buds and open flowers bear a linear relationship with the number of SD cycles. The induced floral buds revert to vegetative growth unless the plants receive the minimum number of SD cycles needed for flowering, this reversion occurring in a basipetal direction. The rate of extension growth of the stem increases with increasing numbers of SD cycles. The high rate is maintained longer in plants receiving 32 or more SD cycles, but the subsequent fall is also steeper in these plants than in plants receiving less inductive cycles. Senescence also occurs in these plants and appears to be related to the magnitude of reproductive development and the high rate of extension growth.     

Developmental and photobiological factors affecting photoperiodic induction in Arabidopsis thaliana Heynh. Landsberg erecta

We have tested whether the promotion of flowering by long days(LD) in Arabidopsis thaliana is a consequence of photoperiodicinduction. To achieve this, the flowering responses of Arabidopsisthaliana (L.) Heynh. Landsberg erecta (Ler) and the long-hypocotylmutants hy2, hy3 and hy4 were determined with respect to age,daylength and light quality. Ler was capable of distinguishingbetween short days (SD) and long days (LD) from about 4 d aftersowing at 20 C, the time at which cotyledons were expandingand greening. At this stage, the critical daylength was between8 h and 10 h. At 7 d, seedlings required five LD for inductionand, as the seedlings aged, they became more sensitive so thatby day 20, one LD was fully inductive. The response to SD innewly germinated seedlings was to delay flowering without alteringleaf number, but after about 10 d, delay of flowering by SDwas accompanied by extra leaves. In light quality experiments,blue light (B) was inductive for 5-d-old plants and in all subsequenttreatments, far-red (FR) caused induction in treatments at 12d and 18 d and low pressure sodium, equivalent to red, was notinductive at 5 d and 12 d, but partially inductive at day 18.Hence, both a specific blue-light photoreceptor and phytochromeA in High Irradiance Response mode promote floral induction.In daylength transfer experiments all three hy mutants respondedto LD by earlier flowering. Both hy2 and hy3 produced substantiallyfewer leaves than Ler in SD and hy3 flowered slightly earlierthan Ler. The hy4 mutants flowered later than Ler in SD andhad a higher leaf number. A scheme is proposed in which photoperiodicinduction depends on the ability of the plant to sense photoperiod,the stage of development and the photobiological input. We alsopropose that phytochrome A and the blue photoreceptor promoteflowering whereas phytochrome B promotes vegetative development. Key words: Arabidopsis thaliana, blue-absorbing photoreceptor, flowering, photoperiodic induction, phytochrome     

Mechanisms and function of flower and inflorescence reversion

Flower and inflorescence reversion involve a switch from floral development back to vegetative development, thus rendering flowering a phase in an ongoing growth pattern rather than a terminal act of the meristem. Although it can be considered an unusual event, reversion raises questions about the nature and function of flowering. It is linked to environmental conditions and is most often a response to conditions opposite to those that induce flowering. Research on molecular genetic mechanisms underlying plant development over the last 15 years has pinpointed some of the key genes involved in the transition to flowering and flower development. Such investigations have also uncovered mutations which reduce floral maintenance or alter the balance between vegetative and floral features of the plant. How this information contributes to an understanding of floral reversion is assessed here. One issue that arises is whether floral commitment (defined as the ability to continue flowering when inductive conditions no longer exist) is a developmental switch affecting the whole plant or is a mechanism which assigns autonomy to individual meristems. A related question is whether floral or vegetative development is the underlying default pathway of the plant. This review begins by considering how studies of flowering in Arabidopsis thaliana have aided understanding of mechanisms of floral maintenance. Arabidopsis has not been found to revert to leaf production in any of the conditions or genetic backgrounds analysed to date. A clear-cut reversion to leaf production has, however, been described in Impatiens balsamina. It is proposed that a single gene controls whether Impatiens reverts or can maintain flowering when inductive conditions are removed, and it is inferred that this gene functions to control the synthesis or transport of a leaf-generated signal. But it is also argued that the susceptibility of Impatiens to reversion is a consequence of the meristem-based mechanisms controlling development of the flower in this species. Thus, in Impatiens, a leaf-derived signal is critical for completion of flowering and can be considered to be the basis of a plant-wide floral commitment that is achieved without accompanying meristem autonomy. The evidence, derived from in vitro and other studies, that similar mechanisms operate in other species is assessed. It is concluded that most species (including Arabidopsis) are less prone to reversion because signals from the leaf are less ephemeral, and the pathways driving flower development have a high level of redundancy that generates meristem autonomy even when leaf-derived signals are weak. This gives stability to the flowering process, even where its initiation is dependent on environmental cues. On this interpretation, Impatiens reversion appears as an anomaly resulting from an unusual combination of leaf signalling and meristem regulation. Nevertheless, it is shown that the ability to revert can serve a function in the life history strategy (perenniality) or reproductive habit (pseudovivipary) of many plants. In these instances reversion has been assimilated into regular plant development and plays a crucial role there.     

Gibberellin-like substances in Bryophyllum daigremontianum and the distribution and persistence of applied gibberellin A3

Summary Acidic extracts of the long-short-day plant Bryophyllum daigremontianum contain two gibberellin (GA)-like substances called fractions I and II. In plants under permanent short-day (SD) conditions the levels of both I and II are very low. In continuous long days (LD) the total GA content is approximately 20 times higher than in SD, mainly due to an increased level of II. Extracts of plants induced to flower by the shift LDSD show a further increase in the level of II. Application of GA₃ to plants in SD causes normal flower formation, but the level of fraction II remains as low as in vegetative plants in permanent SD.Approximately 10% of the GA₃ applied could still be recovered from leaves and inflorescences after 45 days, indicating that GA₃ is very stable in Bryophyllum, Most of the GA₃ recovered was still associated with the treated leaves, but small amounts could be detected in other leaves and in inflorescences. Results of grafting experiments indicate that these low levels of GA₃ are adequate to induce production of the floral stimulus.     

Genetic Regulation of Development in Sorghum bicolor: IV. GA(3) Hastens Floral Differentiation but Not Floral Development under Nonfavorable Photoperiods

Sorghum bicolor (L.) Moench lines with genetic differences in photoperiod requirement were planted in the field near Plainview, Texas (about 34° northern latitude) around June 1 and treated with gibberellic acid (GA₃) solutions applied in the apical leaf whorl. GA₃ hastened the date of floral differentiation (initiation). The greatest responses to GA₃ were by 90M and 100M, the latest of the genotypes, for which floral initiation dates were hastened an average of 19.5 and 21.7 days, respectively, for the 4 years beginning in 1980. There were very small differences in dates of anthesis between control and GA₃-treated plants. Microscopic examination of apical meristems collected between the date of floral initiation of GA₃-treated plants and the later date of initiation of control plants revealed: (a) several morphological characteristics of floral differentiation in the apical meristem of treated plants, (b) consistent occurrence of vegetative morphology in control plants, (c) a few meristems from GA₃-treated plants that appeared to be regressing in floral development and thus possibly exhibiting dedifferentiation. Dedifferentiation of prepanicle primordia into leaves would explain the observed equal or greater number of leaves in GA₃-treated plants rather than the expected smaller number. It is apparent that the presence of a morphological differentiated floral meristem in sorghum does not drive subsequent floral development in the absence of inductive photoperiods. This further suggests that initial floral differentiation and subsequent floral development may be controlled separately in sorghum.     

Regulation of the <Emphasis Type="Italic">ALBINO3</Emphasis>-mediated transition to flowering in <Emphasis Type="Italic">Arabidopsis</Emphasis> depends on the expression of <Emphasis Type="Italic">CO</Emphasis> and <Emphasis Type="Italic">GA1</Emphasis>

ALBINO3, a homologue of PPF1 in Arabidopsis, encodes a chloroplast protein, and is essential for chloroplast differentiation. In the present study, ALBINO3(−) transgenic plants exhibited a significant decrease in both the number of rosette leaves at bolting and the days before bolting, suggesting the important roles of ALBINO3 in regulating flowering during non-inductive short-day photoperiods. ALBINO3 mRNA was apparently accumulated in shoot apical meristem and floral meristems around the shoot apical meristem in wild-type plants. ALBINO3 might be predominantly involved in inducing the floral repression pathway by activating the expression of TFL1, and by suppressing the expression of LFY, respectively, in the shoot apical meristem. Moreover, the function of ALBINO3 in regulating flowering transition depended on the expression of CO and GA1, because ALBINO3 might function in the downstream integration of the photoperiod-dependent and the photoperiod-independent pathways. These results suggest that ALBINO3 may have an important integrative function in the flowering process in Arabidopsis.