Changes of endogenous hormones in lateral buds of chrysanthemum during their outgrowth

The character of branching for two chrysanthemum (Chrysanthemum × morifolium) cvs. Jinghai and Jingyun was observed, and the changes of endogenous hormones in apical and lateral buds were investigated to determine the relationship between the pattern of hormone distribution, apical dominance, and lateral bud outgrowth. The growth rate of Jinghai lateral buds was higher than that of Jingyun. In vegetative growth stage, IAA level in apical buds of Jingyun was significantly higher than in Jinghai. After flower induction, IAA level in apical buds of two cultivars decreased remarkably, but the IAA level decreased in Jingyun faster than in Jinghai. These results showed that the higher was the IAA level in apical buds the stronger was inhibition of lateral bud outgrowth. An increase in IAA and iP/iPA and a decrease in ABA concentrations were closely associated with lateral bud growth alterations in chrysanthemum.     

APICAL DOMINANCE AND FORM IN WOODY PLANTS: A REAPPRAISAL

The form of woody plants is commonly interpreted in terms of apical dominance. Trees with the decurrent or deliquescent branching habit are said to have weak apical dominance, whereas excurrent branching is associated with strong apical dominance. A close examination of many decurrent species such as the oaks, hickories, and maples reveals that almost all of the lateral buds on the current year's twigs are completely inhibited. This complete inhibition of lateral buds by definition and common usage of the term is an expression of strong apical dominance. In trees possessing the excurrent branching habit, such as most conifers and some angiosperms, many of the lateral buds on the current year's twigs elongate to varying degrees. This is usually interpreted as an expression of weak apical dominance. The relationship between bud inhibition and form in woody perennials is much more complex than bud inhibition in herbaceous plants because of the time sequence in the formation and release of lateral buds. For example, it is only after a period of rest or dormancy in the decurrent forms that one or more of the uppermost lateral buds are released, and these may outgrow the currently elongating terminal shoot resulting in forking. Conversely, in the excurrent forms, it seems that the initial expression of weak apical dominance enables the terminal leader to outgrow the currently elongating lateral branches so that it exerts complete control over their subsequent growth and development in later years. An examination of the levels of diffusible auxin at different points along the twigs of excurrent and decurrent species indicates that the balance of growth factors at any given locus, and not the absolute quantity of auxin, exerts primary control over bud inhibition and shoot elongation.     

Correlative inhibition of lateral bud growth in Phaseolus vulgaris L. timing of bud growth following decapitation

Summary On intact, 3-week-old plants of Phaseolus the larger bud in the axils of the primary leaves shows slow, continuous elongation growth. Release from correlative inhibition can be detected within 30 min following decapitation. When 0.1% indoleacetic acid in lanolin is applied to the decapitated stem stump, the lateral bud shows slow growth during the first 7 h, then stops completely for a further 15 h but after 2 days a further gradual increase in length is observed.The movement of ¹⁴C-labelled assimilates from the subtending primary leaf into the lateral bud increases following removal of the shoot apex. When indole acetic acid is applied to decapitated plants the ability of the buds to import ¹⁴C increases for 5–7 h and then declines to a negligible amount. Little or no radioactivity from tritiated indoleacetic acid is transported into the lateral buds of decapitated plants during the first 48 h following removal of the apex and it appears that rapid metabolism of the compound occurs in the stem tissues.     

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.     

Execution of the auxin replacement apical dominance experiment in temperate woody species

The classic Thimann-Skoog or auxin replacement apical dominance test of exogenous auxin repression of lateral bud outgrowth was successfully executed in both seedlings and older trees of white ash, green ash, and red oak under the following conditions: (1) decapitation of a twig apex and auxin replacement were carried out during spring flush, (2) the decapitation was in the previous season's overwintered wood, and (3) the point of decapitation was below the upper large irrepressible lateral buds but above the lower repressible lateral buds. Although it has been suggested that neither auxin, the terminal bud, nor apical dominance have control over the outgrowth of the irrepressible buds during spring flush, there is evidence in the present study that indicates that such control over the repressible buds exists. In seedlings, second-order branching, which resulted from decapitation of elongating current shoots, was also inhibited by exogenous auxin in the three species. Hence, the auxin replacement experiments did work on year-old proleptic buds (of branches of older trees) that would have entered the bud bank and also on current buds of seedlings. Cytokinin treatments were ineffectual in promoting bud growth.     

Apical dominance

Apical dominance is the control exerted by the apical portions of the shoot over the outgrowth of the lateral buds. The classical explanations for correlative inhibition have focused on hormone/nutrient hypotheses. The remarkable progress that has been made in the technology of endogenous hormone quantification in plant tissue has not been accompanied by comparable progress in the elucidation of mechanisms of hormone action in apical dominance. Evidence from hormonal studies suggests that apically produced auxin indirectly suppresses axillary bud outgrowth that is promoted by cytokinin originating from roots/shoots. Significant involvement with other hormones, although less likely, has not been ruled out. Possible changes in tissue sensitivity to hormones should not be overlooked. Auxin-induced oligosaccharide signals originating from the cell walls of shoot tips or polyamines may function as secondary inhibitors to bud growth. Alternatively, apically produced auxin may suppress lateral bud growth by inhibiting auxin export from these buds. Support for a critical role for nutrients in apical dominance keeps resurfacing, especially for auxin-directed nutrient transport and for water as a possible inducing signal for bud outgrowth. Histological and biochemical analyses of lateral buds recently released from apical dominance are urgently needed. The feasibility of manipulating endogenous auxin/cytokinin content in plant tissue by gene insertion and modulation opens the door to exciting approaches as does the use of hormone insensitive/resistant mutants. There is also need to recognize the existence of variability of apical dominance mechanisms among different plant types. The aesthetic and economic implications of understanding apical dominance for the modification of plant structure and form are extremely significant.     

Isolation and identification of lateral bud growth inhibitor,indole-3-aldehyde,involved in apical dominance of pea seedlings

A lateral bud growth inhibitor was isolated from etiolated pea seedlings and identified as indole-3-aldehyde. The indole-3-aldehyde content was significantly higher in the diffusates from explants with apical bud and indole-3-acetic acid treated decapitated explants, in which apical dominance is maintained, than in those from decapitated ones releasing apical dominance. When the indole-3-aldehyde was applied to the cut surface of etiolated decapitated plants or directly to the lateral buds, it inhibited outgrowth of the latter. These results suggest that indole-3-aldehyde plays an important role as a lateral bud growth inhibitor in apical dominance of pea seedlings.     

‘神马’菊花花芽分化与内源多胺的关系

用薄层层析-荧光法测定菊花'神马'花芽分化期间顶芽和叶片中多胺的动态变化,分析了菊花花芽分化与多胺的关系.结果表明,花芽分化起始期顶芽中的腐胺(Put)含量急剧下降,此后在低水平上波动;叶片内Put则于总苞鳞片分化初期大幅上升,其后各阶段处于较低的水平.顶芽中精胺(Spm)与亚精胺(Spd)含量呈平行波动上升趋势,顶芽中Spin在小花原基分化初期直到花冠分化中期处于优势地位,而顶芽中Spd并无明显变化.顶芽、叶片中的Spin变化趋势相反,顶芽中Spm、Spd的含量变化趋势十分相似,但叶片中却呈交替性变化.结果显示,菊花花芽分化过程中,Put含量的降低有利于启动菊花花芽分化,后期Spm的增加有利于小花的分化,叶片可能向顶芽提供Spm,顶芽和叶片中的Spd与小花原基分化有密切关系.     

Apical Dominance Control in Ipomoea nil: The Influence of the Shoot Apex, Leaves and Stem

The outgrowth of lateral buds is known to be controlled by theupper shoot tissues, which include the apex, the young leavesand the upper stem. An analysis of the influence of these plantparts on axillary bud elongation in Ipomoea nil was carriedout by various treatments on these specific tissues. A restriction of elongation in the main shoot due to eitherdecapitation or shoot inversion resulted in the release of apicaldominance A non-linear type of compensating growth relationshipwas observed between the 13 cm apical growing region of thestem and the lateral buds. It was determined by decapitation,defoliation and AgNO₃ treatments that both the 13 cm stem-growthregion and the young leaves (1–5 cm in length) had a muchgreater inhibitory influence on the outgrowth of specified lateralbuds than did the stem apex (consisting of the terminal 0.5cm of the shoot). The specified lateral buds which were analyzedfor outgrowth were located a number of nodes below the shootapex. The intervening nodes were debudded. Although the importanceof young leaves in the control of apical dominance has beenpreviously recognized, the most significant result from thepresent study with Ipomoea was the strong influence of the 13cm apical growth region of the stem on the out growth of thelateral buds. Apical dominance, Ipomoea nil L., Pharbitis nil, growth region, lateral bud outgrowth, decapitation, defoliation, shoot inversion     

Expression of a ribosomal protein gene in axillary buds of pea seedlings

Axillary buds of intact pea seedlings (Pisum sativum L. cv Alaska) do not grow and are said to be dormant. Decapitation of the terminal bud promotes the growth of these axillary buds, which then develop in the same manner as terminal buds. We previously showed that unique sets of proteins are expressed in dormant and growing buds. Here we describe the cloning, sequencing, and expression of a cDNA clone (pGB8) that is homologous to ribosomal protein L27 from rat. RNA corresponding to this clone increases 13-fold 3 h after decapitation, reaches a maximum enhancement of about 35-fold after 12 h, and persists at slightly reduced levels at later times. Terminal buds, root apices, and elongating internodes also contain pGB8 mRNA but fully expanded leaflets and fully elongated internodes do not. In situ hybridization analysis demonstrates that pGB8 mRNA increases in all parts of the bud within 1 h of decapitation. Under appropriate conditions, growing buds can be made to stop growing and become dormant; these buds subsequently can grow again. Therefore, buds have the capacity to undergo multiple cycles of growth and dormancy. RNA gel blots show that pGB8 expression is reduced to dormancy levels as soon as buds stop growing. However, in situ hybridization experiments show that pGB8 expression continues at growing-bud levels in the apical meristem for 2 d after it is reduced in the rest of the bud. When cultured stems containing buds are treated with indoleacetic acid at concentrations ≥10 μm, bud growth and expression of pGB8 in the buds are inhibited.     

Some aspects of daughter bulb growth and development and apical dominance in bulbous Iris

Apical dominance appears to have minimal direct involvementin daughter bulb formation in the bulbous Iris cultivar Ideal.Daughter bulb number and growth relate to the size and reproductivestate of the mother bulb and are not markedly influenced bymeristem destruction. In contrast, destruction or removal ofthe apical meristem promotes lateral bud sprouting in intactbulbs, and lateral bud elongation in Iris meristem explants.These results show that, in contrast to certain other bulbousplants, apical dominance does not direcdy limit daughter bulbnumber in bulbous Iris, but does prevent lateral bud sprouting. (Received September 6, 1978; )     

The apical control of lateral bud development in excised shoot tips of Cuscuta reflexa cultured in vitro

Excised shoot tips of Cuscuta reflexa Roxb. (dodder), a rootless and leafless angiospermic plant parasite, were cultured in vitro for the study of the control of lateral bud development by the apex. In a chemically defined medium lacking hormones, the basal bud alone developed into a shoot. The addition of coconut milk to the growth medium induced the activation of multiple lateral buds, but only a single bud developed further into a shoot. The decapitation of this shoot induced the development of another shoot and the process could be repeated. This showed the controlling effect of the apex in correlative control of bud development. Application of indole-3-acetic acid to the shoot tip explant delayed the development of the lateral bud. Gibberellic acid A₃ induced a marked elongation growth of the explant and reinforced apical dominance. The direct application of cytokinin to an inhibited bud relieved it from apical dominance. A basipetally decreasing concentration gradient of auxin may prevail at the nodes. Bud outgrowth is probably stimulated by cytokinin produced locally in the bud.     

Effect of Abscisic Acid and Other Plant Hormones on Growth of Apical and Lateral Buds of Seedlings

The effect of abscisic acid (AbA) on the growth of lateral and apical buds was studied in seedlings of Pisum sativum and some other species. The hormone was applied in three different ways: 1) directly to the lateral bud on the second node of decapitated pea seedlings as 5 μI droplets in an ethanolic solution; 2) to the cut surface of decapitated seedlings: 3) to the apical bud of intact plants. AbA directly applied in amounts of 5 to 0.1 μg to the lateral bud of the second node of decapitated seedlings had a strong inhibitory effect on the bud. Application to the cut surface of seedlings decapitated about 5 mm above the second node resulted in slight inhibition of the lateral bud on the second node and in growth promotion of the bud on the first node. When AbA at 10 to 0.1 μg was applied to the apical bud of intact seedlings, the growth of this bud was inhibited but the lateral buds grew out. It is concluded that the release of the lateral buds from apícal dominance is the result of the inhibitory effect of AbA on growth of the apical bud and of low transport of AbA. This conclusion is supported by application of GA₃ and IAA, individually and each combined with AbA.     

Enhancement of lateral bud growth in Coleus blumei BENTH. by (2-chloroethyl) trimethylammonium chloride (CCC)

The relationship of GA to apical dominance in Coleus was examinedby substituting 1 % IAA, in lanolin, for the shoot apex of CCC-treated,control and GA-treated plants containing, theoretically, hyponormal,normal and hypernormal GA levels, respectively. The greatestinhibition of lateral bud growth was obtained in the treatmentcombining 1 % IAA and 100 ppm GA, suggesting that GA may beimportant in the apical dominance of Coleus. CCC inhibited main axis growth, reduced the level of endogenousGA and caused a marked release of lateral buds from apical dominance. The significant stimulation of lateral bud growth by CCC couldnot be ascribed to reduced endogenous GA since it was not reversedby exogenous GA, or by GA plus IAA, whereas 100 ppm GA overcamethe inhibition of main axis growth by CCC. It was also shownthat the CCC stimulation was not a result of compensatory growth,that is, enhanced lateral bud growth resulting from reducedapical bud growth. The CCC effect on lateral buds was interpretedas involving a system independent of auxin and GA or else apossible immobilization of auxin in addition to inhibition ofGA biosynthesis. (Received December 5, 1967; )     

Cytokinin/Auxin Control of Apical Dominance in Ipomoea nil

Although the concept of apical dominance control by the ratioof cytokinin to auxin is not new, recent experimentation withtransgenic plants has given this concept renewed attention.In the present study, it has been demonstrated that cytokinintreatments can partially reverse the inhibitory effect of auxinon lateral bud outgrowth in intact shoots of Ipomoea nil. Althoughless conclusive, this also appeared to occur in buds of isolatednodes. Auxin inhibited lateral bud outgrowth when applied eitherto the top of the stump of the decapitated shoot or directlyto the bud itself. However, the fact that cytokinin promotiveeffects on bud outgrowth are known to occur when cytokinin isapplied directly to the bud suggests different transport tissuesand/or sites of action for the two hormones. Cytokinin antagonistswere shown in some experiments to have a synergistic effectwith benzyladenine on the promotion of bud outgrowth. If theratio of cytokinin to auxin does control apical dominance, thenthe next critical question is how do these hormones interactin this correlative process? The hypothesis that shoot-derivedauxin inhibits lateral bud outgrowth indirectly by depletingcytokinin content in the shoots via inhibition of its productionin the roots was not supported in the present study which demonstratedthat the repressibility of lateral bud outgrowth by auxin treatmentsat various positions on the shoot was not correlated with proximityto the roots but rather with proximity to the buds. Resultsalso suggested that auxin in subtending mature leaves as wellas that in the shoot apex and adjacent small leaves may contributeto the apical dominance of a shoot. (Received September 24, 1996; Accepted March 16, 1997)     

Release of apical dominance in potato tuber is accompanied by programmed cell death in the apical bud meristem

Potato (Solanum tuberosum) tuber, a swollen underground stem, is used as a model system for the study of dormancy release and sprouting. Natural dormancy release, at room temperature, is initiated by tuber apical bud meristem (TAB-meristem) sprouting characterized by apical dominance (AD). Dormancy is shortened by treatments such as bromoethane (BE), which mimics the phenotype of dormancy release in cold storage by inducing early sprouting of several buds simultaneously. We studied the mechanisms governing TAB-meristem dominance release. TAB-meristem decapitation resulted in the development of increasing numbers of axillary buds with time in storage, suggesting the need for autonomous dormancy release of each bud prior to control by the apical bud. Hallmarks of programmed cell death (PCD) were identified in the TAB-meristems during normal growth, and these were more extensive when AD was lost following either extended cold storage or BE treatment. Hallmarks included DNA fragmentation, induced gene expression of vacuolar processing enzyme1 (VPE1), and elevated VPE activity. VPE1 protein was semipurified from BE-treated apical buds, and its endogenous activity was fully inhibited by a cysteinyl aspartate-specific protease-1-specific inhibitor N-Acetyl-Tyr-Val-Ala-Asp-CHO (Ac-YVAD-CHO). Transmission electron microscopy further revealed PCD-related structural alterations in the TAB-meristem of BE-treated tubers: a knob-like body in the vacuole, development of cytoplasmic vesicles, and budding-like nuclear segmentations. Treatment of tubers with BE and then VPE inhibitor induced faster growth and recovered AD in detached and nondetached apical buds, respectively. We hypothesize that PCD occurrence is associated with the weakening of tuber AD, allowing early sprouting of mature lateral buds.     

Rapid increases in cytokinin concentration in lateral buds of chickpea (Cicer arietinum L.) during release of apical dominance

We examined the role of cytokinins (CKs) in release of apical dominance in lateral buds of chickpea (Cicer arietinum L.). Shoot decapitation or application of CKs (benzyladenine, zeatin or dihydrozeatin) stimulated rapid bud growth. Time-lapse video recording revealed growth initiation within 2 h of application of 200 pmol benzyladenine or within 3 h of decapitation. Endogenous CK content in buds changed little in the first 2 h after shoot decapitation, but significantly increased by 6 h, somewhat later than the initiation of bud growth. The main elevated CK was zeatin riboside, whose content per bud increased 7-fold by 6 h and 25-fold by 24 h. Lesser changes were found in amounts of zeatin and isopentenyl adenine CKs. We have yet to distinguish whether these CKs are imported from the roots via the xylem stream or are synthesised in situ in the buds, but CKs may be part of an endogenous signal involved in lateral bud growth stimulation following shoot decapitation. To our knowledge, this is the first detailed report of CK levels in buds themselves during release of apical dominance. Received: 12 December 1996 / Accepted: 7 January 1997     

The Effect of the Apical Bud on the Growth of Lateral Buds on Subterranean Shoots

The influence of the apical bud on the growth of the lateral buds on subterranean shoots was studied in Stachys sieboldiiMig. and Helianthus rigidus(Gass.) Desv. Removing and damaging the apical parts of subterranean shoots or their treatment with 2% chlorocholine chloride shoot enhanced shoot branching. The response to light of the apical bud was invariably negative: the stolons, which came out or were extracted from the soil, grew back into the ground (negative phototropism). The response to light of lateral buds was autonomous and depended on the conditions of their initiation. The lateral buds developed in darkness manifested negative phototropism when withdrawn from the soil and exposed to the light, whereas the buds developed in the light showed positive phototropism. The author concludes that the concept of apical dominance, thoroughly studied in aboveground shoots, is also valid for subterranean shoots. However, in contrast to the former, in the latter case, the apical bud does not control the growth orientation of the lateral buds.     

The association of hormone signalling genes,transcription and changes in shoot anatomy during moso bamboo growth

Moso bamboo is a large, woody bamboo with the highest ecological, economic and cultural value of all the bamboo types and accounts for up to 70% of the total area of bamboo grown. However, the spatiotemporal variation role of moso bamboo shoot during growth period is still unclear. We found that the bamboo shoot growth can be divided into three distinct periods, including winter growth, early growth and late growth based on gene expression and anatomy. In the early growth period, lateral buds germinated from the top of the bamboo joint in the shoot tip. Intercalary meristems grew vigorously during the winter growth period and early growth period, but in the late growth period, mitosis in the intercalary meristems decreased. The expression of cell cycle‐associated genes and the quantity of differentially expressed genes were higher in early growth than those in late growth, appearing to be influenced by hormonal concentrations. Gene expression analysis indicates that hormone signalling genes play key roles in shoot growth, while auxin signalling genes play a central role. In situ hybridization analyses illustrate how auxin signalling genes regulate apical dominance, meristem maintenance and lateral bud development. Our study provides a vivid picture of the dynamic changes in anatomy and gene expression during shoot growth in moso bamboo, and how hormone signalling‐associated genes participate in moso bamboo shoot growth.