Determination for inflorescence development is a stable state,separable from determination for flower development in Pisum sativum L. buds

Terminal meristems of Pisum sativum (garden pea) transit from vegetative to inflorescence development, and begin producing floral axillary meristems. Determination for inflorescence development was assessed by culturing excised buds and meristems. The first node of floral initiation (NFI) for bud expiants developing in culture and for adventitious shoots forming on cultured meristems was compared with the NFI of intact control buds. When terminal buds having eight leaf primordia were excised from plants of different ages (i.e., number of unfolded leaves) and cultured on 6-benzylaminopurine and kinetin-supplemented medium, the NFI was a function of the age of the source plant. By age 3, all terminal buds were determined for inflorescence development. Determination occurred at least eight nodes before the first axillary flower was initiated. Thus, the axillary meristems contributing to the inflorescence had not formed at the time the bud was explanted. Similar results were obtained for cultured axillary buds. In addition, meristems excised without leaf primordia from axillary buds three nodes above the cotyledons of age-3 plants gave rise to adventitious buds with an NFI of 8.3 ±0.3 nodes. In contrast seed-derived plants had an NFI of 16.5 ±0.2. Thus cells within the meristem were determined for inflorescence development. These findings indicate that determination for inflorescence development in P. sativum is a stable developmental state, separable from determination for flower development, and occurring prior to initiation of the inflorescence at the level of meristems.     

Floral determination in axillary buds of Nicotiana silvestris

At anthesis of the terminal flower the developmental fates of axillary buds of the long-day plant Nicotiana silvestris were assessed in situ and in isolation. The in situ developmental fate was assessed by decapitating the plant above the bud in question and letting the bud mature. The developmental fate of isolated buds was assessed by removing the bud from the main axis, rooting it, and letting it mature. The number of nodes below the terminal flower of the mature shoot was indicative of the developmental fate of the bud. Terminal meristems of rooted axillary buds exhibited two patterns of development: (1) Their developmental fate was the same as that of in situ buds at the same node or (2) their developmental fate was the same as that of seed-derived plants. For example, terminal meristems of rooted buds from the fourth node below the inflorescence produced either 15 to 19 nodes or 36 to 40 nodes. In situ fourth buds produced 12 to 14 nodes while seed-derived plants produced 33 to 39 nodes. Terminal meristems of rooted axillary buds that exhibited the same developmental fate as that of in situ buds were determined for floral development. Although determined buds produced a terminal flower, all but one had abnormal inflorescences. That is, in the place of floral branches determined buds produced vegetative branches. Four buds that were not determined for floral development had their shoot tips rooted each time the plant bolted. Only when the plants were allowed to grow without being rerooted did they flower. These results indicate that roots may prevent and/or destabilize floral determination in N. silvestris.     

TENDRILS OF PASSIFLORA FOETIDA: HISTOGENESIS AND MORPHOLOGY

Passiflora foetida bears an unbranched tendril, one or two laterally situated flowers, and one accessory vegetative bud in the axil of each leaf. The vegetative shoot apex has a single-layered tunica and an inner corpus. The degree of stratification in the peripheral meristem, the discreteness of the central meristem, and its centric and acentric position in the shoot apex are important plastochronic features. The procambium of the lateral leaf trace is close to the site of stipule initiation. The main axillary bud differentiates at the second node below the shoot apex. Adaxial to the bud 1–3 layers of cells form a shell-zone delimiting the bud meristem from the surrounding cells. A group of cells of the bud meristem adjacent to the axis later differentiates as an accessory bud. A second accessory bud also develops from the main bud opposite the previous one. A bud complex then consists of two laterally placed accessory bud primordia and a centrally-situated tendril bud primordium. The two accessory bud primordia differentiate into floral branches. During this development the initiation of a third vegetative accessory bud occurs on the axis just above the insertion of the tendril. This accessory bud develops into a vegetative branch and does not arise from the tissue of the tendril and adjacent two floral buds. The trace of the tendril bud consists of two procambial strands. There is a single strand for the floral branch trace. The tendril primordium grows by marked meristematic activity of its apical region and general intercalary growth.     

barren inflorescence2 regulates axillary meristem development in the maize inflorescence

Organogenesis in plants is controlled by meristems. Shoot apical meristems form at the apex of the plant and produce leaf primordia on their flanks. Axillary meristems, which form in the axils of leaf primordia, give rise to branches and flowers and therefore play a critical role in plant architecture and reproduction. To understand how axillary meristems are initiated and maintained, we characterized the barren inflorescence2 mutant, which affects axillary meristems in the maize inflorescence. Scanning electron microscopy, histology and RNA in situ hybridization using knotted1 as a marker for meristematic tissue show that barren inflorescence2 mutants make fewer branches owing to a defect in branch meristem initiation. The construction of the double mutant between barren inflorescence2 and tasselsheath reveals that the function of barren inflorescence2 is specific to the formation of branch meristems rather than bract leaf primordia. Normal maize inflorescences sequentially produce three types of axillary meristem: branch meristem, spikelet meristem and floral meristem. Introgression of the barren inflorescence2 mutant into genetic backgrounds in which the phenotype was weaker illustrates additional roles of barren inflorescence2 in these axillary meristems. Branch, spikelet and floral meristems that form in these lines are defective, resulting in the production of fewer floral structures. Because the defects involve the number of organs produced at each stage of development, we conclude that barren inflorescence2 is required for maintenance of all types of axillary meristem in the inflorescence. This defect allows us to infer the sequence of events that takes place during maize inflorescence development. Furthermore, the defect in branch meristem formation provides insight into the role of knotted1 and barren inflorescence2 in axillary meristem initiation.     

The influence of expanding leaves and the reproductive stem apex on apical dominance in Lolium multiflorum

In vegetative plants of Lolium multiflorum removal of the two youngest emerging leaves resulted in increased expansion of basal tiller buds. A similar release of inhibition of tiller buds took place if the floriferous apex was removed. The surgical procedures did not affect the response. Under conditions of N-deficiency total tiller number was reduced but on removal of the apex the deficient plants showed an increased initial rate of tiller bud expansion. Apical dominance during the vegetative stage of growth in this grass was apparently due to the expanding leaves in the vegetative apex, but in the flowering plant the control was exerted by the inflorescence or the elongating stem.     

Floral determination in the terminal bud of the short-day plant Pharbitis nil

Temporal and spatial aspects of floral determination in seedling terminal buds of the qualitative short-day plant Pharbitis nil were examined using a grafting assay. Floral determination in the terminal buds of 6-day-old P. nil seedlings is rapid; by 9 hr after the end of a 14-hr inductive dark period more than 50% of the induced terminal buds grafted onto uninduced stock plants produced a full complement of flower buds. When grafted at early times after the end of the dark period the terminal buds of induced plants produced three discrete populations of plants: plants with no flowers, plants with two axillary flowers at nodes 3 and 4 and a vegetative terminal shoot apex, and plants with five to seven flowers including a terminal flower. The temporal relationship among these populations of plants produced by apices grafted at different times indicates that under our conditions, the region of the terminal bud that will form the axillary buds at nodes 3 and 4 becomes florally determined prior to floral determination of the region of the terminal bud giving rise to the nodes above node 4.     

Floral bud reversion in Impatiens balsamina under non-inductive photoperiods

Summary All floral buds of Impatiens balsamina plants exposed to 4 short-day (SD) cycles and then returned to long days reverted to vegetative growth. The same happened with the upper buds of plants receiving a larger number of SDs, even as many as 90 cycles. The reversal proceeded in a basipetal order. The number of floral buds and flowers increased, and their reversion to vegetative growth was delayed with increasing numbers of SD cycles. Depending upon the stage attained by the floral bud before the transfer of the plant to noninductive photoperiods one or more inner whorls of the flower were replaced by a vegetative apex. The tip of the placenta was able to resume vegetative growth even after the formation of fertile anthers and an ovary with abortive ovules, showing that the potentiality for reversion is maintained till quite late stages in floral bud development. Continuous exposure to SD cycles is required not only for the continued production of floral buds, but also for their development to mature flowers, indicating that the floral stimulus in this plant is not self-perpetuating.     

Flowering in the uniflora mutant of tomato (Lycopersicon esculentum Mill.): description of the reproductive structure and manipulation of flowering time

The uniflora (uf) mutant of tomato (Lycopersicon esculentum Mill.) is known to produce solitary, normal, fertile flowers instead of inflorescences. Histological and SEM studies revealed that this unusual reproductive structure resulted from the inability of the plant to produce an inflorescence and not from post-initiation abortion processes affecting young flower buds. Development prior to floral transition was apparently not affected by the mutation since rates of germination and leaf initiation were identical in both uf and the Ailsa Craig (AC) initial cultivar. However, the time of flowering of the mutant was always delayed as compared to AC. In uf, environmental conditions markedly influenced flowering time which occurred early in all individuals in summer, but was strongly delayed during winter, with less than 20% plants reaching flowering before having initiated 40 leaves. Defoliation treatments stimulated floral transition in uf plants since 100% flowering occurred whatever the season and since the time of floral transition was usually advanced in comparison to the non-defoliated control plants. Similarly, compared to intact uf plants, flowering of terminal meristem of cuttings and upper axillary bud of decapitated plants was promoted. The involvement of correlative influences and assimilate availability in the control of flowering in tomato is suggested by these findings.     

In vitro formation and development of floral buds on tobacco stem explants: effects of kinetin and other factors

Stem segments were excised from plants of Wisconsin 38 tobacco (Nicotiana tabacum L.) in three regions differing in their distance below the inflorescence. They were cultured in vitro in 8- or 16-hr days. After 8 weeks, floral and vegetative buds were counted, and extent of floral development was assessed. Kinetin at 10(-5)m inhibited formation and development of floral buds regardless of indoleacetic acid concentration. Supplied at this concentration with adequate auxin, kinetin stimulated vegetative bud formation and may have caused floral bud abortion. Indoleacetic acid (>/= 10(-6)m) inhibited vegetative and floral bud formation when supplied with low kinetin concentration (/= 10(-6)m), it inhibited floral bud formation and stimulated vegetative bud formation. More floral buds were formed in 16-hr days than in 8-hr days. Few formed on explants other than those derived from the region nearest the inflorescence regardless of other treatment.     

MISSING FLOWERS gene controls axillary meristems initiation in sunflower

The initiation and growth of axillary meristems are fundamental components of plant architecture. Here, we describe the mutant missing flowers (mf) of Helianthus annuus characterized by the lack of axillary shoots. Decapitation experiments and histological analysis indicate that this phenotype is the result of a defect in axillary meristem initiation. In addition to shoot branching, mutation affects floral differentiation. The indeterminate inflorescence of sunflower (capitulum) is formed of a large flat meristem which produces floret primordia in multiple spirals. In wildtype plants a bisecting crease divides each primordium in two distinct bumps that adopt different fate. The peripheral (abaxial) part of the primordium becomes a small leaf-like bract and the adaxial part becomes a flower. In the mf mutant, the formation of flowers at the axil of bracts is precluded. Histological analyses show that in floret primordia of the mutant a clear subdivision in dyads is not established. The primordia progressively bend inside and only large involucral floral bracts are developed. The results suggest that the MISSING FLOWERS gene is essential to provide or perceive an appropriate signal to the initiation of axillary meristems during both vegetative and reproductive phases.     

DEVELOPMENT AND PHYLLOTAXIS OF THE VEGETATIVE AXILLARY BUD OF MICHELIA FUSCATA

Tucker, Shirley C. (Northwestern U., Evanston, III.) Development and phyllotaxis of the vegetative axillary bud of Michelia fuscata . Amer. Jour. Bot. 50(7): 661–668. Illus. 1963.—The vegetative axillary buds of Michelia fuscala are dorsiventrally symmetrical with 2 ranks of alternately produced leaves. The direction of the ontogenetic spiral in each of these buds is related both to the symmetry of the supporting branch and to the position of the bud along the branch. On a radially symmetrical branch, all the axillary buds are alike—all clockwise, for example. But in a dorsiventrally organized branch the symmetry alternates from clockwise in 1 axillary bud to counterclockwise in the next bud along the axis. Leaf initiation and ontogeny of the axillary apical meristem conform with those of the terminal vegetative bud. The axillary bud arises as a shell zone in the second leaf axil from the terminal meristem. During this process the axillary apex develops a zonate appearance. The acropetally developing procambial supply of the axillary bud consists wholly of leaf traces. At the nodal level the bud traces diverge from the same gap as the median bundle trace of the subtending leaf. Only the basal 1–2 axillary buds which form immediately after the flowers elongate each year, while the majority remains dormant with 3 leaves or fewer.     

AXILLARY BUD DEVELOPMENT IN POPULUS DELTOIDES. I. ORIGIN AND EARLY ONTOGENY

Origin and early development of axillary buds on the apical shoot of a young Populus deltoides plant were investigated. The ontogenetic sequence of axillary buds extended from LPI –1 (Leaf Plastochron Index) near the apical bud base to LPI –11, the fifth primordium below the bud apex. Two original bud traces diverged from the central (C) trace of the axillant leaf and developed acropetally. During their acropetal traverse the original bud traces gave rise to three pairs of scale traces. All subsequent scale traces, and later the foliar traces, were derived by divergencies from the first two pairs of scale traces. Just before the bud vascular system separated from that of the main axis, a third pair of traces diverged from the original bud traces to vascularize the adaxial scale. Concomitantly, the original bud traces were inflected toward the main vascular cylinder where they developed acropetally and eventually merged with the left lateral trace of the leaf primordium situated three nodes above the axillant leaf; they did not participate in further vascularization of the bud. During early ontogeny a shell zone formed concurrent with initiation of the original bud traces and lay interjacent to them. The shell zone defined the position of the cleavage plane that formed between the axillary bud and the main axis. The axillary bud apex first appeared in the region bounded laterally by the original bud traces and adaxially by the shell zone. Following divergence of the main prophyll traces from the original bud traces, the apex assumed a new position intermediate to the prophyll traces. Ontogenetic development suggested that the axillary bud apex may have been initiated by the acropetally developing original bud traces under the influence of stimuli arising in more mature vegetative organs below.     

Further experiments on spermidine-mediated floral-bud formation in thin-layer explants of Wisconsin 38 tobacco

We studied the effects of various polyamines on bud regeneration in thin-layer tissue explants of vegetative and floweringNicotiana tabacum L. cv. Wisconsin 38, in which application of exogenous spermidine (Spd) to vegetative cultures causes the initiation and development of some flower buds (Kaur-Sawhney et al. 1988 Planta173, 282). We now show that this effect is dependent on the time and duration of application, Spd being required from the start of the cultures for about three weeks. Neither putrescine nor spermine is effective in the concentration range tested. Spermidine cannot replace kinetin (N⁶-furfurylaminopurine) in cultures at the time of floral bud formation, but once the buds are initiated in the presence of kinetin, addition of Spd to the medium greatly increases the number of floral buds that develop into normal flowers. Addition of Spd to similar cultures derived from young, non-flowering plants did not cause the appearance of floral buds but rather induced a profusion of vegetative buds. These results indicate a morphogenetic role of Spd in bud differentiation. Dedicated to Professor Hans Mohr on the occasion of his 60th birthday     

Floral determination in the terminal and axillary buds of Nicotiana tabacum L

The terminal and axillary buds of the day-neutral plant, Nicotiana tabacum cv. Wisconsin 38, become determined for floral development during the growth of the plant. This state of determination can be demonstrated with a simple experiment: buds determined for floral development produce the same number of nodes in situ and if rooted. After several months of growth and the production of many leaves, the terminal bud became determined for floral development within a period of about 2 days. After the terminal bud became florally determined, it produced four nodes and a terminal flower. The buds located in the axils of leaves borne just below the floral branches became florally determined 5 to 9 days after the terminal bud became florally determined. Since florally-determined axillary buds were not clonally derived from a florally-determined terminal meristem, axillary buds and the terminal bud acquired the state of floral determination independently. These data indicate that a pervasive signal induced a state of floral determination in competent terminal and axillary buds.     

Influence of Bud Position and Plant Ontogeny on the Morphology of Branch Shoots in Pea (Pisum sativum L. cv. Alaska)

The morphology of axillary shoots of pea plants (Pisum sativumL. cv. Alaska) was analysed as a function of the position ofthe bud on the plant axis and the stage of plant developmentwhen the buds began to grow. Buds from the three most basalnodes were stimulated to develop by decapitating the main shootwhen buds were still growing (4 d plants), shortly after budsbecame dormant (7 d plants) or after the initiation of floweringon the main shoot (post-flowering plants, about 21 d after sowing).Branch shoots were scored for node of floral initiation (NFI),shoot length, and node of multiple leaflets (NML), a measureof leaf complexity. Shoots that developed spontaneously fromupper nodes (nodes 5-9) on intact post-flowering plants werescored for NFI. NFI for basal buds on 4 and 7 d plants variedas a function of nodal position and ranged from 5 to 6·7nodes. NFI on these plants was not influenced by bud size orwhether a bud was growing or dormant when the plant was decapitated.NFI for shoots derived from basal buds on decapitated post-floweringplants and upper nodes on intact post-flowering plants was about4. Reduced NFI on post-flowering plants may be due to depletionof a cotyledon-derived floral inhibitor. Basal axillary shootson 4 d plants were about 20% longer than those on 7 d plantsand about five times longer than those on post-flowering plants.These differences may be due to depletion of gibberellic acidsfrom the cotyledons. NFI and NML for the main shoot and forbasal axillary shoots were similar under some experimental conditionsbut different under other conditions, so it is likely that eachdevelopmental transition is regulated independently.Copyright1995, 1999 Academic Press Apical dominance, bud development, garden pea, initiation of flowering, Pisum sativum L., shoot morphology     

Developmental Anatomy of the Inflorescence of Bread Wheat (Triticum aestivum L.) during Normal Initiation and when Affected by 2, 4-D

In wheat, the tip of the shoot apex normally consists of a coreof irregularly arranged cells covered by two uniseriate, selfperpetuating, layers (the dermatogen and the hypodermal layer):no third, inner layer (sub-hypodermal layer) is present. Leafinitiation involves periclinals in the cells of the dermatogenand hypodermal layers, but not the core. Buds involve many periclinalsin the outer cells of the core, a few occasionally in the hypodermallayer but never any in the dermatogen. The appearance of ‘double-ridges’signals inflorescence initiation. Each double-ridge is the equivalentof an axillary bud (the future spikelet bud) and its subtendingleaf primordium. The initiation of the subtending leaf is normal:the initiation of the spikelet bud is characterized by periclinaldivisions in the outer cells of the core, though some may alsooccur in cells of the hypodermal layer immediately outside:no periclinals are observed in the neighbouring dermatogen cells.All the above events concerned with leaf and bud initiationoccur in an easily recognizable, strictly distichous, pattern.In plants affected by 2, 4-dichlorophenoxyacetic acid the cellularpattern where double-ridges would have been arising, is badlydisrupted, due mainly to increased cell divisions in the hypodermallayer and outer part of the core, though possibly includingsome in the dermatogen. The apex tip itself is unaffected, probablyexplaining why, when growth is resumed, it produces a successionof normal spikelets in the normal phyllotaxis. Triticum aestivum L, bread wheat, shoot apex, double-ridge primordia, inflorescence initiation, spikelet buds, 2, 4-dichlorophenoxyacetic acid     

The Arabidopsis compact inflorescence genes: phase-specific growth regulation and the determination of inflorescence architecture

During their life cycle, higher plants pass through a series of growth phases that are characterized by the production of morphologically distinct vegetative and reproductive organs and by different growth patterns. Three major phases have been described in Arabidopsis: juvenile vegetative, adult vegetative, and reproductive. In this report we describe a novel, phase-specific mutant in Arabidopsis, compact inflorescence (cif). The most apparent aspect of the cif phenotype is a strong reduction in the elongation of internodes in the inflorescence, resulting in the formation of a floral cluster at the apical end of all reproductive shoots. Elongation and expansion of adult vegetative rosette leaves are also compromised in mutant plants. The onset of the cif trait correlates closely with morphological changes marking the phase transition from juvenile to adult, and mutant plants produce normal flowers and are fully fertile. Hence the cif phenotype appears to be adult vegetative phase-specific. Histological sections of mutant inflorescence internodes indicate normal tissue specification, but reduced cell elongation compared to wild-type. compact inflorescence is inherited as a two-gene trait involving the action of a recessive and a dominant locus. These two cif genes appear to be key components of a growth regulatory pathway that is closely linked to phase change, and specifies critical aspects of plant growth and architecture including inflorescence internode length.     

High temperature arrest of inflorescence development in broccoli (Brassica oleracea var. italica L.)

High temperature causes unevenly-sized flower buds on broccoli inflorescences. This deformity limits production of broccoli to areas where summer temperatures rarely exceed 30 C. The stage of development sensitive to heat was determined by exposing plants of 'Galaxy' broccoli at varying developmental states to 35 C day temperature for 1 week, and subsequently analysing the head structure. During the high temperature exposure, the development of certain flower buds was arrested. There was no corresponding cessation of bud initiation at the apex. No injury resulted if heat was applied before the reproductive induction, or was their injury to differentiated flower buds. Meristems were affected only if heat was applied during inflorescence production or the floral initiation process. Shorter heat exposures produced little injury, and longer exposures were lethal. The plant's development at this sensitive period still appeared vegetative externally, but the youngest leaves had just begun to reorientate as a consequence of the reduced stem elongation rate. The meristem was less than 1 mm wide, and floral primordia were just forming, still subtended by leaf primordia. The injury was fully expressed by the time the head was first exposed (approximately 5-10 mm wide), though it became more apparent as the head matured. The buds that were delayed in development by the high temperature developed into normal flowers.Key words: Brassica oleracea, broccoli, flowering, heat injury, developmental arrest      

Development of Meristems of Weigela florida Variety 'Bristol Ruby' Following Reproductive Induction in Long Days

Weigela florida variety ‘Bristol Ruby’ has longday requirements for its growth and, in general, for its flowering.Vegetative development, floral initiation and floral organogenesisare described using scanning electron microscopy during photoperiodictreatment in long days, under controlled conditions. Flowering of axillary buds of cuttings has been studied. Theapex of Weigela at the vegetative phase is characterized bya very small hollow meristem. After 9 long days, the meristemenlarges and, after 12 long days, early axillary buds are initiatedin the axils of the leaves, which become bracts. When the numberof long days was increased, flowers were initiated in the budson the induced branches; first at the proximal part of the branchwhere development afterwards slowed down, then on the medianparts of the branch where development was accelerated. Two bracteoles are differentiated soon after floral initiation;first initiation of the calyx required 18 long days. Petals,stamens and ovary were rapidly initiated after that. Weigelaflowers are clustered; the inflorescence ceased growth by abortionof the terminal meristem or by formation of a terminal flower.In axillary buds of the fifth node the formation of the clusterwas completed about 20 days after the beginning of floral induction. Weigela florida ‘Bristol Ruby’, scanning electron microscopic analysis, vegetative meristem, floral development stages, long days induction