Bi-directional inflorescence development inArabidopsis thaliana: Acropetal initiation of flowers and basipetal initiation of paraclades

In this study we investigated Arabidopsis thaliana (L.) Heynh. inflorescence development by characterizing morphological changes at the shoot apex during the transition to flowering. Sixteen-hour photoperiods were used to synchronously induce flowering in vegetative plants grown for 30 d in non-inductive 8-h photoperiods. During the first inductive cycle, the shoot apical meristem ceased producing leaf primordia and began to produce flower primordia. The differentiation of paraclades (axillary flowering shoots), however, did not occur until after the initiation of multiple flower primordia from the shoot apical meristem. Paraclades were produced by the basipetal activation of buds from the axils of leaf primordia which had been initiated prior to photoperiodic induction. Concurrent with the activation of paraclades was the partial suppression of paraclade-associated leaf primordia, which became bract leaves. The suppression of bract-leaf primordia and the abrupt initiation of flower primordia during the first inductive photoperiod is indicative of a single phase change during the transition to flowering in photoperiodically induced Arabidopsis. Morphogenetic changes characteristic of the transition to flowering in plants grown continuously in 16-h photoperiods were qualitatively equivalent to the changes observed in plants which were photoperiodically induced after 30 d. These results suggest that Arabidopsis has only two phases of development, a vegetative phase and a reproductive phase; and that the production of flower primordia, the differentiation of paraclades from the axils of pre-existing leaf primordia and the elongation of internodes all occur during the reproductive phase.     

The Effect of Flower Induction on the Rate of Leaf Initiation

Vegetative plants of four short-day and five long-day specieswere exposed to inductive or non-inductive daylengths continuously,or to inductive conditions for just long enough to induce flowering.One day-neutral species was given long days throughout the experiment.The rate of leaf initiation was significantly greater in floweringthan in vegetative shoots in all photoperiodically sensitivespecies following induction until the formation of a terminalflower. A significant increase in the rate of leaf initiationwas also noted when floral initials began to appear in the day-neutralspecies. It is concluded that floral induction and stimulationof leaf initiation are likely to be universally associated whetherspecies are photoperiodically sensitive or not. It is also suggestedthat, together with apical elongation and early developmentof axillary buds, this stimulation is an essential step in themorphological sequence by which flower initials are produced.     

Changes in the Protein Composition of the Shoot Apical Bud of Sinapis alba in Transition to Flowering

Vegetative plants of Sinapis alba L. grown in short days were induced to flower by expsoure to one or continuous long days. In both inductive conditions, the first flowers were initiated about 60 h after the start of the treatment. Soluble protein extracts were prepared from apical buds and just-expanded leaves of both vegetative and induced plants. Rabbit antisera were prepared using extracts from vegetative and reproductive buds. Immunodiffusion tests were performed. Analysis of the precipitin bands indicated that: (1) one antigenic protein was present in the vegetative buds and disappeared from the buds of induced plants between 96 and 240 h after the start of the inductive treatment; (2) the concentration of a another antigenic protein increased in buds of induced plants 30 h after the start of the inductive treatment; (3) the concentration of a third antigenic proteín increased in buds of induced plants at 96 h.     

Changes in the content of endogenous auxins in apical buds ofchenopodium rubrum L. Induced with respect to the endogenous rhythm in capacity to flower

The content of endogenous auxins was examined in apical buds ofChenopodium rubrum plants induced by a photoperiodic cycle of 16h darkness and 8h light followed by a dark period of various duration so as to correspond with either maximal or minimal flowering response in the endogenous rhythm in capacity to flower initiated by the photoperiodic treatment. Apical buds of potentially generative plants contained less auxins than apical buds of plants which remained in the vegetative state. Apical buds from plants treated with kinetin (1. 10^-3 M) and therefore remaining in the vegetative state showed an auxin level comparable to that of untreated plants exhibiting minimal flowering response irrespective of the duration of the second dark period. Plants cultivated on a sucrose solution (0.6 M) during the second dark period became generative even at the normal minimum of flowering. The auxin content of the apical buds was low, similarly as in untreated plants induced for a period leading to maximal flowering response. On the other hand, apical buds from plants grown on sucrose solution during a dark period leading to the manifestation of maximal flowering response showed a relatively high auxin content comparable to that found in untreated plants which had obtained a more extended induction by three photoperiodic cycles. The results are discussed with respect to the possible role of endogenous auxins in the regulation of the changes in growth correlations occurring in the shoot apex during photoperiodic induction and in the expression of the competence to flower.     

Axillary bud flowering after apical decapitation in Pharbitis in relation to photoinduction

The flowering response of axillary buds of seedlings of Pharbitis nil Choisy, cv. Violet, was examined in relation to the timing of apical bud removal (plumule including the first leaf or second leaf) before or after a flower-inductive 16-h dark period. When the apical bud was removed well before the dark period, flower buds formed on the axillary shoots that subsequently developed, but when removed just before, or after, the dark period, different results were observed depending on the timing of the apical bud removal and plant age. In the case of 8-day-old seedlings, fewer flower buds formed on the axillary shoots developing from the cotyledonary node when plumules were removed 20 to 0 h before the dark period. When the apical bud was removed after the dark period, no flower buds formed. Using 14-day-old seedlings a similar reduction of flowering response was observed on the axillary shoots developing from the first leaf node when the apical bud was removed just after the dark period. To further elucidate the relationship between apical dominance and flowering, kinetin or IAA was applied to axillary buds or the cut site where the apical bud was located. Both chemicals influenced flowering, probably by modulating apical dominance which normally forces axillary buds to be dormant.     

Correlations between Growth and Flowering in Chenopodium amaranticolor: I. Initiation of Leaf and Bud Primordia

Vegetative plants of Chenopodium amaranticolor were inducedto flower by exposure to 2, 6 or continuous short days (SDs)and the effect of such treatments on organogenesis at the apexof the main stem followed by means of dissections. The mostoutstanding responses to SD treatment were (I) an immediateelongation of the apex, (2) a stimulation of the rate of initiationof leaf primordia, and (3) a promotion of the rate of initiationof axillary bud primordia. In response to as few as 2 SDs, therate of initiation of leaf primordia increased from 0.47 toa maximum of 3.70 per day and the rate of initiation of axillarybud primordia immediately increased from 0.47 to 1.35 per day. Precocious initiation of axillary bud primordia led to the formationof double ridges. The results indicate double ridges to be homologouswith vegetative axillary buds; although they normally developedinto reproductive tissues, they passed through a period of vegetativegrowth following minimal induction to flowering by exposureto 2 SDs. The rate and degree of flowering were highest in plants whichreceived the longest period of SDs, but the differences in finalflowering response were greater than the differences betweenthe initial responses at the apices. The effect of SDs was thusnot confined to an initial stimulation of organogenesis; a prolongedexposure to SDs must have enhanced the subsequent developmentof double ridges into flower primordia. The results are discussed in relation to previous findings andthe general conclusion drawn that the initiation of double ridgesis very widely accompanied by a stimulation of apical growth.It is suggested that inductive conditions remove a general growthinhibition and that the resultant stimulation of apical growthmight lead to the initiation of double ridges.     

EFFECT OF AGING ON FLOWERING OF THE AXILLARY BUDS OF PHARBITIS NIL

In Pharbitis seedlings, aging is associated with definite trends in the pattern of flowering of the axillaries: 1) the locus of maximum flowering is displaced continuously upward, 2) there is a continuous loss of responsiveness to induction from the base of the plant, upward, 3) flowering of the axillaries is suppressed in general when the terminal bud fails to flower, and 4) flowering at the distal nodes regresses when the terminal bud fails to flower. Phenomena 1 and 2 start at the base of the plant and move progressively upward, whereas 3 and 4 are tied to the possibly rhythmic responses of the terminal buds to floral induction, with aging. Buds at axils that form after induction are capable of flowering. Buds at nodes that flower maximally range in development from those not visible with a stereomicroscope to those with enlarging apices ready to form the first leaf primordia at the time of induction. Axillaries that have formed leaf primordia fail to flower in response to one inductive night.     

[14C]-Assimilate partitioning in photoperiodically induced seedlings of Pharbitis nil. The effect of benzyladenine

Partitioning of [¹⁴C]-labeled assimilates was studied in relation to photoperiodic floral induction and evocation in one-week-old Pharbitis nil Choisy cv. 'Violet' seedlings. In plants kept under 16 h photoperiods, one 15 h night induced 100% axillary flowering whereas a 24 h night induced both terminal and axillary flowering. A 15 min night break of red light given 8 h after the beginning of the dark period inhibited flowering. Total [¹⁴C]-assimilate distribution among major sinks (plumules + epicotyl and roots + hypocotyl) from a single source cotyledon was unchanged by one inductive night; however, import of [¹⁴C]-assimilates into shoot apices was increased in induced plants compared to vegegative controls. This increase was several-fold in plants subjected to a 24 h night. N⁶-Benzyladenine (BA) application to cotyledons or plumules under non-saturating night lengths increased the number of floral buds per plant without affecting the position of the first floral bud (i.e. the speed of induction). The same treatment caused increased label accumulation in induced apices, while it only slightly affected non-induced ones. The mode of action of BA on flowering through growth stimulation and resulting assimilate mobilization is discussed.     

Floral transition as a sequence of growth changes in different components of the shoot apical meristem ofChenopodium rubrum

The changes in cell division rate were studied in different components of the shoot apex ofChenopodium rubrum during short-day photoperiodic induction and after the inductive treatments. Induced and vegetative apices were compared. Accumulation of metaphases by colchicine treatment was used to compare the mean cell cycle duration in different components of the apex. A direct method of evaluating the increase in cell number obtained by anticlinal or periclinal divisions was applied if the corresponding components of induced and non-induced apices had to be compared. The short-day treatment prolonged the cell cycle more in the peripheral zone than in the central zone and still more in the leaf primordia. The importance of changing growth relations for floral transition was shown particularly if the induced plants were compared with the vegetative control with interrupted dark periods. Induced plants transferred to continuous light showed further changes in the rates of cell division. The cell cycle was shortened more in the central zone than in the peripheral zone,i.e. there was a further shift in growth relations within the apical dome. The cell cycle in the leaf and bud primordia was also shortened if compared with the vegetative control, the acceleration being stronger in the bud primordia. There was a subsequent retardation in cell division in the leaf primordia formed during and after the inductive treatment if the plants were fully induced. An inhibition of the oldest bud primordia was observed in fully induced apices, as well.     

Polyamine levels in buds and twigs of Tilia cordata from dormancy onset to bud break

The fluctuations of free and bound polyamines (PAs) were studied in vegetative buds and underlying twigs of linden (Tilia cordata L.) from August to May, to assess the connection between PA levels and seasonal cycles of growth and dormancy. Outer and inner bud scales and shoot tips (short shoot tips with leaf initials in contiguous short internodes) were analyzed separately, as were phloem with cortex and xylem with pith tissue from twigs. Seasonal variations in PA levels were present in buds and twigs during the research period. The most abundant PA in buds and twigs in free and bound forms was spermidine followed by putrescine. PA amounts were low in buds and twigs in autumn. In twig tissues, free PAs were predominant whereas in bud scales, bound PAs accumulated over free PAs in autumn, first in inner scales and later in outer scales as well. PA levels did not increase dramatically during the onset of dormancy in autumn but lower temperatures and probable cold hardening correlated positively with bound PAs in bud scales. In shoot tips with leaf initials, and contiguous short internodes, free putrescine and spermidine levels rose simultaneously with bud burst and new growth, while bound PAs diminished quite radically from temporary bud scales and from growing shoot tips.     

Identification of a MADS-box gene, FLOWERING LOCUS M, that represses flowering

The timing of flowering is important for the reproductive success of plants. Here we describe the identification and characterization of a new MADS-box gene, FLOWERING LOCUS M (FLM), which is involved in the transition from vegetative to reproductive development. FLM is similar in amino-acid sequence to FLC, another MADS-box gene involved in flowering-time control. flm mutants are early flowering in both inductive and non-inductive photoperiods, and flowering time is sensitive to FLM dosage. FLM overexpression produces late-flowering plants. Thus FLM acts as an inhibitor of flowering. FLM is expressed in areas of cell division such as root and shoot apical regions and leaf primordia.     

Shoot morphogenesis associated with flowering in Populus deltoides (Salicaceae)

Temporal and spatial formation and differentiation of axillary buds in developing shoots of mature eastern cottonwood (Populus deltoides) were investigated. Shoots sequentially initiate early vegetative, floral, and late vegetative buds. Associated with these buds is the formation of three distinct leaf types. In May of the first growing season, the first type begins forming in terminal buds and overwinters as relatively developed foliar structures. These leaves bear early vegetative buds in their axils. The second type forms late in the first growing season in terminal buds. These leaves form floral buds in their axils the second growing season. The floral bud meristems initiate scale leaves in April and begin forming floral meristems in the axils of the bracts in May. The floral meristems subsequently form floral organs by the end of the second growing season. The floral buds overwinter with floral organs, and anthesis occurs in the third growing season. The third type of leaf forms and develops entirely outside the terminal buds in the second growing season. These leaves bear the late vegetative buds in their axils. On the basis of these and other supporting data, we hypothesize a 3-yr flowering cycle as opposed to the traditional 2-yr cycle in eastern cottonwood.     

The effect of the floral repressor FLC on the timing and progression of vegetative phase change in Arabidopsis

Plants undergo two major post-embryonic developmental transitions--the juvenile-to-adult vegetative transition (vegetative phase change) and the adult-to-reproductive transition (flowering). In woody plants, these transitions can be separated by years, but in herbaceous species they are often very close together, making it difficult to differentiate the effects of vegetative phase change and floral induction on vegetative development. To distinguish between these factors, we have compared the vegetative morphology of plants highly expressing the floral repressor FLC (FRI;FLC) with plants mutant for this gene (FRI;flc-3) under both photoinductive (long day, LD and night interruption, NI) and non-photoinductive (short day, SD) conditions. We show that the onset of abaxial trichome production is insensitive to floral induction, but the distribution and overall number of abaxial trichomes, as well as several other leaf traits associated with vegetative change, are strongly influenced by flowering. Most of the major differences in leaf morphology between FRI;FLC and FRI;flc-3 plants grown in LD can be attributed to the early flowering phenotype of FRI;flc-3, because these differences are not apparent in plants grown in SD. These include differences in leaf size, hydathode number and the distribution of abaxial trichomes along the length of the leaf. Leaf shape and the total number of abaxial trichomes are affected by FLC independently of its effect on flowering. Our results demonstrate that the onset and the progression of vegetative phase change are regulated by different combinations of endogenous and environmental factors, and reveal a role for FLC in vegetative development.     

Jasmonates Inhibit Flowering in Short-Day Plant Pharbitis nil

The role of jasmonates in the photoperiodic flower induction of short-day plant Pharbitis nil was investigated. The plants were grown in a special cycle: 72 h of darkness, 24 h of white light with lowered intensity, 24-h long inductive night, 14 days of continuous light. At 4 h of inductive night the cotyledons of non-induced plants contained about two times the amount of endogenous jasmonates (JA/JA-Me) compared to those induced. A 15-min long pulse of far red light (FR) applied at the end of a 24-h long white light phase inhibited flowering of P. nil. The concentration of jasmonates at 2 and 4 h of inductive night in the cotyledons of the plants treated with FR was similar. Red light (R) could reverse the effect of FR. R light applied after FR light decreased the content of jasmonates by about 50%. Methyl jasmonate (JA-Me) applied to cotyledons, shoot apices and cotyledon petioles of P. nil inhibited the formation of flower buds during the first half of a 24-h long inductive or 14-h long subinductive night. Application of JA-Me to the cotyledons was the most effective. None of the plants treated with JA-Me on the cotyledons in the middle of the inductive night formed terminal flower buds. The aspirin, ibuprofen and phenidone, jasmonates biosynthesis inhibitors partially reversed the effect of FR, stimulating the formation of axillary and terminal flower buds. Thus, the results obtained suggests that phytochrome system control both the photoperiodic flower induction and jasmonates metabolism. Jasmonates inhibit flowering in P. nil.     

The Morphogenesis of Apple Buds: III. The Inception of Flowers

The early stages in the change from vegetative to reproductivedevelopment of apple spur terminal buds were followed by dissectionof buds from untreated trees, and from trees defoliated at differenttimes in the season. A change in the development of the leafprimordia occurred when there were approximately eight in thebud. This was followed by the development of bracts, which appearedto be necessary for the formation of actual flower parts. Leafprimordia tend to inhibit this process. Whereas their effectupon the apical meristem was subsequently reduced by the formationof bracts, so that eventually a terminal flower formed, theireffect upon the lower lateral meristems was unaltered. Thesemeristems therefore remained in a vegetative state. In addition to the number of leaf primordia in the bud, thedegree of dormancy may be an important factor in determiningthe onset of flowering. Since the number of leaf primordia invegetative buds at the end of the season is eight, the spatialdistribution of primordia on the main axis of the bud and theirvascular connexions might have a decisive effect on bud development.This was related to the effect of older primordia in the budupon the development of younger ones. In buds in which theseolder primordia were inhibited by foliage, etc., i.e. thosewith a long plastochrone, no effects were observed upon thedevelopment of younger primordia and the buds remained vegetative. Whilst correlative inhibition of buds thus affected their abilityto form flowers, there is no evidence of a critical leaf areafor flowering. Flowering in apple buds is more likely to bedue to the removal of factors inhibiting reproductive developmentthan to the synthesis of a specific flower inducing substanceas such.     

脱落酸在植物花发育过程中的作用

植物激素脱落酸(ABA)对植物的生长发育具有多方面的调节作用,比如种子休眠、萌发,营养生长,环境胁迫反应等。大量研究显示,ABA也参与了植物的成花调控。影响植物成花调控的环境因子,包括光周期变化、春化作用、干旱等均会导致植物体内ABA代谢的变化。本文从调控植物开花的4条主要途径与植物体内ABA代谢变化之间的相互关系,花芽分化时期ABA在植物叶芽和花芽中的动态分布以及离体培养条件下ABA对花芽分化的影响等方面总结了ABA与植物花发育这一领域的最新研究进展。对ABA在植物成花诱导和花发育中的作用进行了综合分析。     

EFFECTS OF LEAF EXCISION ON FLOWERING OF XANTHIUM APICAL BUDS IN CULTURE UNDER INDUCTIVE AND NONINDUCTIVE PHOTOPERIODS

Apical buds of Xanthium were grown in aseptic culture under short-day cycles known to induce flowering in the intact plants or under “light-break” conditions known to prevent flowering. The total light provided in each 24-hr cycle was the same under the two photoperiods. Various numbers of leaves were excised from the apical buds. Excision of leaves did not change the response to photoperiod: even with all leaves excised the apical buds cultured under short-day conditions reached the same average floral stage as the control buds, and those under light-break conditions all remained vegetative. Fresh weight was not significantly changed by the excisions, either. However, excision of the young leaves resulted in an increase in the number of new leaves developed by the apical bud during the two-week culture period.     

Effects of exogenous cytokinins on flowering of the short-day plantChenopodium rubrum L.

Kinetin at a concentration from 3.10^-6 M to 1.10^-3 M was applied to the plumule ofChenopodium rubrum plants during photoperiodic induction. Different levels of induction were compared (one and three short days). The higher concentrations of kinetin applied to induced plants inhibited flower formation. The rate of leaf initiation was increased under these treatments. Lower concentrations of kinetin (from 3.10^-6 M to 1.10^-5 M) usually promoted lateral bud formation and flowering. The step-wise application of kinetin revealed that the inhibitory effect on flowering had been restricted to the inductive period. The effects of kinetin, benzyladenine and trans-zeatin were compared in plants partially induced by two short days. High concentrations always inhibited flowering. Benzyladenine was the most effective in this respect. Root removal diminished the inhibitory effects of cytokinins on flowering as was stated with benzyladenine. It is assumed that endogenous cytokinins play a role in the regulation of organogenetic activity of the stem apical meristem. Depending on the photoperiodic conditions, they presumably exert their activity by maintaining the vegetative functions of the apex.     

Developmental study onHypolepis punctata (Thunb.) Mett.

The third petiolar bud ofHypolepis punctata appears on the basiscopic lateral side of the petiole above the fairly developed first petiolar bud. This investigation clarified the fact that the third bud is formed neither by the activity of the meristem of the first bud nor by the meristem directly detached from the shoot apical meristem, but is initiated in the cells involved in the abaxial basal part of the elevated portion of the leaf primordium. Thus the third bud is of phyllogenous origin. This investigation further revealed that the cells to initiate the third bud are originally located in the abaxial side of the leaf apical cell complex like the cells to initiate the first bud, but are not incorporated into the meristem of the first. After the first, second and third petiolar buds have been initiated, they are carried up into fairly high regions on the petiolar base by the intercalary growth which occurs in the leaf base below the insertion level of the first and the second buds.