The effect of decapitation on the levels of IAA and ABA in the lateral buds of Betula pendula roth

The level of IAA and ABA in lateral buds of birch shoots 24 h and 5 days after the decapitation of the apical bud was determined. Twenty four hours after decapitation, when visible signs of outgrowth of lateral buds were not observed yet, an increase in the level of IAA and a decrease of ABA, as compared with the buds of non-decapitated shoots, was found. Five days later, when lateral buds were in the period of intensive outgrowth, a decrease in the levels of IAA and ABA was observed. It has been suggested that removing the source of auxin, by the decapitation of the apical bud makes possible the lateral buds to undertake the synthesis of their own auxin. It could lead to the decrease in the content of ABA. These all events could create suitable conditions for the outgrowth of lateral shoots.     

Effect of plant growth substances on the growth of axillary buds in cultured stem segments of Phaseolus vulgaris L.

The hormonal control of axillary bud growth was investigated in cultured stem segments of Phaseolus vulgaris L. When the stem explants were excised and implanted with their apical end in a solid nutrient medium, outgrowth of the axillary buds-located at the midline of the segment-was induced. However, if indoleacetic acid (IAA) or naphthaleneacetic acid (NAA) was included in the medium, bud growth was inhibited. The exposure of the apical end to IAA also caused bud abscission and prevented the appearance of new lateral buds.In contrast to apically inserted segments, those implanted in the control medium with their basal end showed much less bud growth. In these segments, the auxin added to the medium either had no effect or caused a slight stimulation of bud growth.The IAA transport inhibitor N-1-naphthylphthalamic acid (NPA) relieved bud growth inhibition by IAA. This suggests that the effect of IAA applied at the apical end requires the transport of IAA itself rather than a second factor. With the apical end of the segment inserted into the IAA-containing medium, simultaneous basal application of IAA relieved to some extent the inhibitory effect of the apical IAA treatment. These results, together with data presented in a related article [Lim R and Tamas I (1989) Plant Growth Regul 8: 151–164], show that the polarity of IAA transport is a critical factor in the control of axillary bud growth.Of the IAA conjugates tested for their effect on axillary bud growth, indoleacetyl alanine, indoleacetic acid ethyl ester, indoleacetyl-myo-inositol and indoleacetyl glucopyranose were strongly inhibitory when they were applied to the apical end of the stem explants. There was a modest reduction of growth by indoleacetyl glycine and indoleacetyl phenylalanine. Indoleacetyl aspartic acid and indoleglyoxylic acid had no effect.In addition to IAA and its conjugates, a number of other plant growth substances also affected axillary bud growth when applied to the apical end of stem segments. Myo-inositol caused some increase in the rate of growth, but it slightly enhanced the inhibitory effect of IAA when the two substances were added together. Gibberellic acid (GA₃) caused some stimulation of bud growth when the explants were from younger, rather than older plants. The presence of abscisic acid (ABA) in the medium had no effect on axillary bud growth. Both kinetin and zeatin caused some inhibition of axillary buds from younger plants but had the opposite effect on buds from older ones. Kinetin also enhanced the inhibitory effect of IAA when the two were applied together.In conclusion, axillary buds of cultured stem segments showed great sensitivity to auxins and certain other substances. Their growth responded to polarity effects and the interaction among different substances. Therefore, the use of cultured stem segments seems to offer a convenient, sensitive and versatile test system for the study of axillary bud growth regulation.     

Abscisic Acid and Apical Dominance in Phaseolus coccineus L. The Role of Tissue Age

Abscisic acid (ABA) applied to the decapitated second internodeof runner bean plants enhanced outgrowth of lateral buds onlywhen internode stumps were no longer elongating. Applied toelongating internodes of slightly younger plants, ABA causesinhibition of bud outgrowth. Together with 10^–4 M indol-3-ylacetic acid (IAA), ABA stimulated internode elongation and interactedadditively in the inhibition of bud outgrowth. A mixture of10^–5 M ABA and 10^–6 M gibberellic acid (GA₃ ) causedsimilar effects on internode growth as IAA + ABA, but was mutuallyantagonistic in effect on growth of the lateral buds. Abscisic acid, apical dominance, gibberellic acid, indol-3yl acetic acid, Phaseolus coccineus, bean     

Hormone Levels and Apical Dominance in the Aquatic Fern Marsilea drummondii A. Br

Terminal buds and successively subjacent lateral buds of the water fern, Marsilea drummondii, were examined to determine the pattern of hormone distribution in relation to apical dominance. Quantitative levels of indole-3-acetic acid (IAA), abscisic acid (ABA), zeatin and zeatin riboside (Z and ZR), and isopentenyladenosine (iPA) were determined by a solid-phase immunoassay using polycional antihormone antibodies. Enzyme-linked immunosorbent assay was used following a one-step HPLC purification procedure to obtain the free hormones. Active shoot apices contained the most IAA and Z-type cytokinins and inhibited buds the least. No significant differences in ABA levels were found leading to the conclusion that ABA did not play any role in apical dominance. The normal precedence of the most rapid outgrowth of the youngest inhibited bud as observed previously in decapitated plants was well correlated with its very high level of iPA observed in this study. The same phenomenon was observed in the median buds but with a weaker amplitude. The presence of this storage form could indicate that a bud at its entry into quiescence eventually looses the ability to hydroxylate iPA to Z-type cytokinins when it is fully inhibited. IAA and Z + ZR are concluded to be essential for lateral bud growth.     

Branching gene expression during chrysanthemum axillary bud outgrowth regulated by strigolactone and auxin transport

Shoot branching is essential in ornamental chrysanthemum production and determines final plant shape and quality. Auxin is associated with apical dominance to indirectly inhibit bud outgrowth. Two non-mutually exclusive models exist for indirect auxin inhibition. Basipetal auxin transport inhibits axillary bud outgrowth by limiting auxin export from buds to stem (canalization model) or by increasing strigolactone levels (second messenger model). Here we analyzed bud outgrowth in treatments with auxin (IAA), strigolactone (GR24) and auxin transport inhibitor (NPA) using a split-plate bioassay with isolated chrysanthemum stem segments. Besides measuring bud length, dividing cell percentage was measured with flow cytometry and RT-qPCR was used to monitor expression levels of genes involved in auxin transport (CmPIN1) and signaling (CmAXR2), bud dormancy (CmBRC1, CmDRM1) and strigolactone biosynthesis (CmMAX1, CmMAX3). Treatments over a 5-day period showed bud outgrowth in the control and inhibition with IAA and IAA?+?GR24. Bud outgrowth in the control coincided with high dividing cell percentage, decreased expression of CmBRC1 and CmDRM1 and increased CmPIN1 expression. Inhibition by IAA and IAA?+?GR24 coincided with low dividing cell percentage and unchanged or increased expressions of CmBRC1, CmDRM1 and CmPIN1. Treatment with GR24 showed restricted bud outgrowth that was counteracted by NPA. This restricted bud outgrowth was still concomitant with a high dividing cell percentage and coincided with decreased expression of dormancy genes. These results indicate incomplete inhibition of bud outgrowth by GR24 treatment and suggest involvement of auxin transport in the mechanism of bud inhibition by strigolactones, supporting the canalization model.     

Response to strigolactone treatment in chrysanthemum axillary buds is influenced by auxin transport inhibition and sucrose availability

Axillary bud outgrowth is regulated by both environmental cues and internal plant hormone signaling. Central to this regulation is the balance between auxins, cytokinins, and strigolactones. Auxins are transported basipetally and inhibit the axillary bud outgrowth indirectly by either restricting auxin export from the axillary buds to the stem (canalization model) or inducing strigolactone biosynthesis and limiting cytokinin levels (second messenger model). Both models have supporting evidence and are not mutually exclusive. In this study, we used a modified split-plate bioassay to apply different plant growth regulators to isolated stem segments of chrysanthemum and measure their effect on axillary bud growth. Results showed axillary bud outgrowth in the bioassay within 5 days after nodal stem excision. Treatments with apical auxin (IAA) inhibited bud outgrowth which was counteracted by treatments with basal cytokinins (TDZ, zeatin, 2-ip). Treatments with basal strigolactone (GR24) could inhibit axillary bud growth without an apical auxin treatment. GR24 inhibition of axillary buds could be counteracted with auxin transport inhibitors (TIBA and NPA). Treatments with sucrose in the medium resulted in stronger axillary bud growth, which could be inhibited with apical auxin treatment but not with basal strigolactone treatment. These observations provide support for both the canalization model and the second messenger model with, on the one hand, the influence of auxin transport on strigolactone inhibition of axillary buds and, on the other hand, the inhibition of axillary bud growth by strigolactone without an apical auxin source. The inability of GR24 to inhibit bud growth in a sucrose treatment raises an interesting question about the role of strigolactone and sucrose in axillary bud outgrowth and calls for further investigation.     

Release of lateral buds from apical dominance by glyphosate in soybean and pea seedlings

Application of a sublethal dose of glyphosate (N-[phosphonomethyl]glycine) to the seedlings of soybean (Glycine max L. Merr. cv. Evans) and pea (Pisum sativum L. cv. Alaska) promoted growth of the cotyledonary and other lateral buds. The pattern of the glyphosate-induced lateral bud growth was different from that induced by decapitation. Under the experimental condition, glyphosate did not kill the apical buds. Feeding stem sections of the seedlings with radiolabeled indole-3-acetic acid ([2¹⁴C]IAA) and subsequent analysis of free [2-¹⁴C]IAA and metabolite fractions revealed that the glyphosate-treated plants had higher rates of IAA metabolism than the control plants. The treated pea plants metabolized 75% of [2-¹⁴C]IAA taken up in the 4-h incubation period compared to 46.5% for the control, an increase of 61%. The increase was small but consistent in soybean seedlings. As a result, the glyphosate-treated plants had less free IAA and ethylene than the control plants. The increase of IAA metabolism induced by glyphosate is likely to change the auxin-cytokinin balance and contribute to the release of lateral buds from apical dominance in these plants.     

Abscisic acid and apical dominance in Phaseolus coccineus L.

Abscisic acid (ABA) in lanolin, applied to the internode of decapitated runner bean plants enhances the outgrowth of lateral buds. The optimum concentration of the paste is 10^-5 M. The effect of ABA is counteracted by indoleacetic acid (IAA) but not by gibberellic acid (GA₃). There is no effect when ABA is applied to the apical bud or lateral buds of intact plants. However, 13.2 ng given to the lateral buds of decapitated plants stimulate their growth, whereas higher concentrations are inhibitory. Consequently, ABA enhances growth of lateral buds directly, but only when apical dominance is already weakened. The growth of the decapitated 2^nd internode was not affected by ABA. Radioactivity from [2-¹⁴C] ABA, applied to nonelongating 2^nd internode stumps of decapitated runner bean plants moves to the lateral buds, whereas [1-¹⁴C]IAA-and [³H]GA₁-translocation is much weaker. ABA transport is inhibited if IAA or [³H]GA₁ is applied simultaneously. In elongating internodes [¹⁴C]ABA is almost completely immobile. [¹⁴C]IAA-and [³H]GA₁-translocation is not affected by ABA. The amount of radioactivity from labelled ABA, translocated to the lateral buds, is highest during the early stages of bud outgrowth.Abbreviations ABA 2,4-cis, trans-(+)-abscisic acid - GA gibberellic acid - IAA indoleacetic acid - p.l. plain lanolin     

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.     

Lateral Bud Growth in Phaseolus vulgaris L. and the Levels of Ethylene in the Bud and Adjacent Tissue

Ethephon and the ethylene inhibitors Ag⁺ and aminoethoxyvinylglycine (AVG) inhibited outgrowth of the axillary bud of thefirst trifoliate leaf in decapitated plants of Phaseolus vulgaris.Endogenous ethylene levels decreased in the stem upon decapitationalthough it is not conclusive that a causal relationship existsbetween this decrease and the release of axillary buds frominhibition. The proposition that auxin-induced ethylene is responsiblefor the suppression of axillary bud growth in the decapitatedplant when the apical shoot is replaced by auxin is not borneout in this study. Application of IAA directly to the axillarybud of intact plants gave rise to a transient increase in budgrowth. This growth increment was annulled when AVG was suppliedwith IAA to the bud despite the fact that the dosage of AVGused did not affect the normal slow growth rate of the bud ofthe intact plant or bud outgrowth resulting from shoot decapitation.     

Proteomic Analysis of Chrysanthemum Lateral Buds after Removing Apical Dominance Based on Label-Free Technology

Studying the genetic basis and regulatory mechanism of chrysanthemum lateral bud outgrowth is of great significance for reduction the production cost of cut chrysanthemum. To clarify the molecular basis of lateral bud elongation after removal of apical dominance in chrysanthemum, label-free quantification analysis was used to analyze the proteome changes after apical bud removal. Quantitative real-time PCR (qPCR) was used to analyze the changes in the expression of three plant hormone-related genes. A total of 440 differentially expressed proteins were successfully identified at three time points during the lateral bud elongation. The number of differentially expressed proteins in the three stages (24 h/0 h, 48 h/0 h, 48 h/24 h) were 219, 332, and 97, respectively. The difference in expressed proteins in the three comparison stages mainly involves RNA processing and modification; translation, ribosomal structure and biogenesis; Posttranslational modification, protein turnover, and chaperones. Path analysis showed that there was various physiological activities in the process of lateral bud dormancy breaking and elongation, which involved energy metabolism, biosynthesis, signal transduction and stress response in the growth process of lateral buds. qPCR indicated that the expression of cytokinin synthesis related gene was significantly increased after the removal of apical dominance, while the expression of strigolactones synthesis related gene experiences a dramatic fall to promote the development of the lateral buds. However, there was a drop before a slight increase in the expression of the auxin synthesis related gene, which was mainly due to the removal of apical dominance that led to the loss of indoleacetic acid in the main stem. However, with formation of the new apical source, indoleacetic acid can be released again.     

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; )     

Role of ethylene and cytokinins in the initiation of lateral shoot growth in bromeliads

Aechmea victoriana var discolor L. B. Foster and Aechmea dactylina Bal. are commercially propagated in vitro through lateral shoot growth. A modified Murashige and Skoog medium is used which contains both BA and IAA. These growth substances were shown in the present study to synergistically stimulate the production of ethylene by the cultured plants. The stimulation of ethylene production is correlated with the outgrowth of the lateral buds. The rise in ethylene production was concluded to induce lateral shoot growth, because: (a) outgrowth of the shoots was blocked by preventing an increase in ethylene production, (b) 1-aminocyclopropane-1-carboxylic acid (ACC), the natural precursor of ethylene biosynthesis, substituted for IAA in the promotion of ethylene production and lateral bud outgrowth. Although ACC could substitute for IAA, it could not substitute for BA; therefore, cytokinins are concluded to be essential for lateral bud outgrowth in vitro in Aechmea. These results suggest that cytokinins and ethylene both play roles in natural lateral bud initiation and that the cytokinin function involves two stages of the process.     

Branch development in Lupinus angustifolius L.#II. Relationship with endogenous ABA, IAA and cytokinins in axillary and main stem buds

The concentrations of indole-3-acetic acid (IAA), cytokinins (CK) and abscisic acid (ABA) were measured in buds of different regions (main stem and lateral branches) of Lupinus angustifolius L. (cv. Merrit) and at different stages in the development of branches. In lupin, branching patterns are the result of discrete regions of axillary branches (upper, middle and basal) which elongate at much different rates. Early in development only the main shoot elongates, followed usually by basal branch growth and then rapid upper branch growth. Branches in the middle of the main stem grow only weakly or fail to develop. Levels of IAA were generally high in the apical buds of slowly growing branches and low in buds from strongly growing branches, whereas CK levels showed the opposite relationship. CK:IAA ratio showed a closer relationship with the rate of growth of a particular branch better than the levels of either CK or IAA alone. During early stages of growth ABA concentration did not follow the rate of branch growth. However, later in development, where growth did not closely match the ratio of CK:IAA, ABA level showed a strong negative relationship with growth. A significant decrease in ABA was associated with continued strong growth of the main stem apex following a decline in CK:IAA ratio. Overall, the best relationship between the level of growth factors in apical buds and branching pattern in lupin was the ratio of CK:IAA, implying that high CK:IAA at a given bud would promote growth. ABA level appeared to play a secondary role, as a growth inhibitor.     

香荚兰花芽分化至萌发期内源激素的变化

以香荚兰 (Vanillafragrans)为材料 ,研究不同栽培条件下花芽分化和萌发期内源激素变化 ,分析和探讨内源激素在花芽分化和萌发中的作用 ,香荚兰花芽分化时期茎里的激素含量降低 ,芽里激素含量升高 ,其中相对高的ZR和ZR ABA有利于分化 ,IAA和IAA ABA的一定增加也利于分化 ,过高或没有IAA的增加则不利于花芽分化。大多数花芽形成于倒垂茎蔓上 ,花芽分化期 (11～ 12月 ) ,倒垂茎蔓的茎里生长类激素含量降低大于竖立茎蔓 ,芽的激素含量增高则多于竖立茎蔓 ,倒垂茎蔓的这种变化可能是有利于花芽分化。香荚兰生长中顶端优势明显 ,去顶后侧芽里ZR、GA、IAA增高 ,这与 11～ 12月去顶促进倒垂茎蔓开花可能有关。     

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     

Relative importance of nodal roots and apical buds in the control of branching in Trifolium repens L.

Clonal species are characterised by having a growth form in which roots and shoots originate from the same meristem so that adventitious nodal roots form close to the terminal apical bud of stems. The nature of the relationship between nodal roots and axillary bud growth was investigated in three manipulative experiments on cuttings of a single genotype of Trifolium repens. In the absence of locally positioned nodal roots axillary bud development within the apical bud proceeded normally until it slowed once the subtending leaf had matured to be the second expanded leaf on the stem. Excision of apical tissues indicated that while there was no apical dominance apparent within fully rooted stems and very little in stems with 15 or more unrooted nodes, the outgrowth of the two most distal axillary buds was stimulated by decapitation in stems with intermediate numbers of unrooted nodes. Excision of the basal branches from stems growing without local nodal roots markedly increased the length and/or number of leaves on 14 distally positioned branches. The presence of basal branches therefore prevented the translocation of root-supplied resources (nutrients, water, phytohormones) to the more distally located nodes and this caused the retardation in the outgrowth of their axillary buds. Based on all three experiments we conclude that the primary control of bud outgrowth is exerted by roots via the acropetal transport of root-supplied resources necessary for axillary bud outgrowth and that apical dominance plays a very minor role in the regulation of axillary bud outgrowth in T. repens.     

Correlative Inhibition of Lateral Bud Growth in Phaseolus vulgaris L.--Influence of the Environment

The influence of various environmental factors upon main stemand lateral bud growth has beeninvestigated using Phaseolusvulgaris, with the object of discovering why there is variabilityin the response of lateral buds on decapitated plants to apically-appliedIAA. Light intensity, light quality and temperature had differentand specific effects on main stem and lateral bud growth inintact plants and on the effectiveness of IAA in inhibitingprimary leaf axillary bud growth in decapitated plants. Photoperiod,on the other hand, was apparently ineffective. It is concluded that environmental factors, as well as contributingto the normal regulation of apical dominance, could also partlyor wholly account for the variation in effectiveness of appliedIAA found by different workers. IAA was least effective whenthe temperature was lower at night than during the day.     

Differential Responses of Buds along the Shoot to Factors Involved in Apical Dominance

Intact and decapitated 6-node shoots of Hygrophila sp. weregrown aseptically immersed in liquid half-strength Knop's solutionwith microelements and 2% (w/v) sucrose (control medium), andin medium with 0.1 mg l^–1 benzyladenine (BA). In intactshoots grown in control medium apical dominance suppressed outgrowthof the lateral buds; in decapitated shoots buds grew out atseveral of the most apical nodes, increasing in size acropetally.There was a lag in outgrowth of the bud at the most apical node,attributable to its initially smaller size. Lateral shoots grewout first at basal nodes of intact shoots in BA medium, decreasingin size acropetally; in decapitated shoots in BA medium lateralshoots of approximately equal size grew out at all nodes. Differentialeffects of decapitation and cytokinin treatment on lateral shootoutgrowth along the shoot could be interpreted by postulatinga basipetally decreasing gradient of endogenous auxin concentrationin the intact shoot. Application of 20 mg l^–1 indoleaceticacid (IAA) in agar to decapitated shoots completely preventedbud outgrowth for at least 7 d in control medium, inhibitingit thereafter, and inhibited bud outgrowth in BA medium, thussupporting the hypothesis. Comparison of lateral shoot outgrowthin whole decapitated shoots and severed decapitated shoots (isolatednodes) lent no support to the alternative hypothesis that theremight be an acropetally decreasing concentration gradient ofa bud-promoting substance in the intact shoot, and demonstratedmuch greater lateral shoot growth in isolated nodes. The resultsemphasize important correlative relationships between the partsof a shoot with several nodes.