Release of cotyledonary shoots of pea (<Emphasis Type="Italic">Pisum sativum</Emphasis>) seedlings from inhibition as related to endogenous zeatin and ethylene and exogenous IAA and benzyladenine

This paper deals with apical dominance using a dicotylar model obtained after decapitation of pea seedlings with two shoots — one dominant and the other inhibited. When the dominant shoot was decapitated the inhibited one is released from inhibition and after 24 to 72 h begins to grow. However, the levels of trans-zeatin and production of ethylene increase within 4 and 6 hours respectively after release from inhibition, and within an interval of 72 h the levels of both phytohormones begin gradually to decrease. This indicates that also in this model, the release from apical dominance is associated with an increase in the level of cytokinin zeatin and, thereafter, also with an increased production of ethylene. If indolyl-3-acetic acid (IAA) is applied on the decapitated main stem after decapitation of the dominant shoot, the growth of the initially inhibited one is very strongly retarded; if, however, IAA is applied on the decapitated dominant shoot, this inhibition is significantly weaker. This means that the inhibiting effect of IAA on the inhibited shoot originates to a greater degree from the main stem rather than from the dominant shoot. The effect of benzyladenine (BA) is transferred equally from the decapitated main stem and from the decapitated dominant shoot because the initially inhibited shoot begins to grow as well as also other shoots from serial cotyledonary buds.     

Changes in endogenous level of auxins and cytokinins in axillary buds ofpisutn sativum L. in relation to apical dominance

Decapitation of the stem in one-week-old pea seedlings below the first node causes a rapid outgrowth of the two cotyledonary buds. One of them soon becomes dominant, while the other one is inhibited, but can be released from inhibition by cutting off the dominant bud. The level of endogenous auxins and cytokinins was determined in dominant and inhibited buds, as well as in released buds at different time intervals after deinhibition. It was found that the inhibited buds contained very little acidic, ether soluble auxins, a high level of tryptophan and also a high level of cytokinins, in comparison with dominant buds. When the inhibited buda were released from inhibition, their auxin content rose, while that of tryptophan and cytokinins decreased, reaching the level found in dominant buds within six days. Specific changes in content of two undetermined auxin-like substances were found in released buds during de-inhibition. These results are discussed in relation to the current views on the regulation of apical dominance.     

Effect of apex excision and replacement by 1-naphthylacetic acid on cytokinin concentration and apical dominance in pea plants

As known from literature lateral buds from pea ( Pisum sativum ) plants are released from apical dominance when repeatedly treated with exogenous cytokinins. Little is known, however, about the endogenous role of cytokinins in this process and whether they interact with basipolar transported IAA, generally regarded as the main signal controlling apical dominance. This paper presents evidence that such an interaction exists.
The excision of the apex of pea plants resulted in the release of inhibited lateral buds from apical dominance (AD). This could be entirely prevented by applying 1-naphthylacetic acid (NAA) to the cut end of the shoot. Removal of the apex also resulted in a rapid and rather large increase in the endogenous concentrations of zeatin riboside (ZR), isopentenyladenosine (iAdo) and an as yet unidentified polar zeatin derivative in the node and internode below the point of decapitation. This accumulation of ZR and iAdo, was strongly reduced by the application of NAA. The observed increase in cytokinin concentration preceded the elongation of the lateral buds, suggesting that endogenous cytokinins play a significant role in the release of lateral buds from AD. However, the effect of NAA on the concentration of cytokinins clearly demonstrated the dominant role of the polar basipetally transported auxin in AD. The results suggest a mutual interaction between the basipolar IAA transport system and cytokinins obviously produced in the roots and transported via the xylem into the stem of the pea plants.     

Control of bud inhibition in Cyperus

Summary The axillary buds in the leaf crown of Cyperus alternifolius seedlings remain completely inhibited although the shoot is determinate and has no active apex. Buds can be released by detachment of the crown from the plant or by direct application of aqueous enzyladenine (BA), and grow out as inflorescences or vegetative shoots. These arise from activated growth centers of the primordial reproductive branch system which is enclosed within the prophyll of the inhibited bud. Buds are also released by the growth retardant, (2-chloroethyl) trimethylammonium chloride (CCC). Gibberellic acid maintains bud inhibition in detached crowns and inhibits bud release caused by CCC or BA. Naphthaleneacetic acid somewhat reduces BA-induced bud release and causes abnormal root proliferation in CCC-treated crowns. It is suggested that a high level of gibberellin within the crown, possibly in relation to a low level of cytokinin, maintains bud inhibition.     

The release of primordia of marginal buds onbryophyllum crenatum leaves from growth inhibition in relationship to the level of endogenous IAA

The growth of primordia of marginal buds (marginals) which differentiate on leaf margins is correlatively inhibited on intactBryophyllum crenatum plants. Following leaf isolation, the marginals are released from this correlative inhibition, which process is accompanied within 2 to 10 h after leaf isolation with a decrease in the content of endogenous IAA in the leaf blade. This decrease can be enhanced by transversal cutting of the leaf blade into three parts which also results in enhanced subsequent growth of the marginals. The growth which follows after the release of the marginals from correlative inhibition is accompanied in cut leaf blades with an increased content of endogenous IAA in the period from 12 h to 7 d after leaf isolation when compared with uncut leaf blades. The highest content of endogenous IAA was recorded in the middle section, and the lowest IAA content in the basal section of the leaf blade.     

Mutual interaction of auxin and cytokinins in regulating correlative dominance

Correlative dominance requires correlative signals from a dominant to a dominated organ. Auxins, particularly IAA, and cytokinins are obviously important components of this correlative system. Using a vegetative pea shoot and a generative apple and tomato fruit system it can be demonstrated that dominant organs always export more IAA and have a higher ³H-IAA transport capacity and velocity compared to dominated organs. In both systems the dominant organ can be replaced by the application of auxin, e.g. NAA, which maintains the differences in IAA export. This is an indication that similar regulatory mechanisms control dominance in both of these diverse systems. The possibility of replacing a dominant organ by auxin also makes it unlikely that growth of that organ or allocation of nutrients regulates the correlative inhibition of the dominated organ.It is suggested that differences in IAA export from, and transport capacities of, dominant and dominated shoots, may be explained by a mechanism of auxin transport autoinhibition (ATA), whereby the earlier and stronger export of IAA from the dominant shoot inhibits auxin export from the dominated shoot at the point where the two auxin streams converge. This hypothesis was tested with explants of pea, apple and tomato. It was shown that the basal application of cold IAA significantly reduced endogenous as well as exogenous IAA transport through these explants.Since the reduced IAA transport of dominated organs was not followed by an accumulation of IAA in the auxin producing subtending organ, it was concluded that IAA biosynthesis was possibly reduced and/or IAA conjugation stimulated. This could have been one of the determinants of their growth inhibition. ATA might also explain how the unidirectional IAA signal may affect the growth rate of organs even lateral or acropetal to its transport pathway and thus polar IAA-transport becomes a ``multidirectional' signal. From the experiments demonstrated it seems that ATA is a sufficient mechanism to impose growth inhibition in the dominated organ, without the need of other regulators.However, to release dominated organs from dominance cessation of ATA may not be sufficient and cytokinins are obviously a powerful antagonist to auxins. Their repeated exogenous application turns dominated lateral buds into strongly growing organs which ultimately may even dominate the previously dominant apex. These lateral shoots finally gain a strong IAA export capacity and inhibit, by ATA, IAA export from the hitherto dominant apex.In other experiments it was shown that interruption of polar IAA transport leads to a strong increase in root derived cytokinins. This can largely be prevented, in a concentration dependent manner, by the application of auxin, indicating that basipolar auxin may control cytokinin production in the roots and its possible delivery to lateral buds. In turn, the increased delivery of cytokinins to the lateral buds promotes a strong increase in IAA production and export. Thus there is a strong mutual interaction between auxin production in the shoots and cytokinin production in the roots, which may be important in regulating the balance between root and shoot growth.     

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; )     

Involvement of auxin and CKs in boron deficiency induced changes in apical dominance of pea plants (Pisum sativum L.)

It has previously been shown that boron (B) deficiency inhibits growth of the plant apex, which consequently results in a relatively weak apical dominance, and a subsequent sprouting of lateral buds. Auxin and cytokinins (CKs) are the two most important phytohormones involved in the regulation of apical dominance. In this study, the possible involvement of these two hormones in B-deficiency-induced changes in apical dominance was investigated by applying B or the synthetic CK CPPU to the shoot apex of pea plants grown in nutrient solution without B supply. Export of IAA out of the shoot apex, as well as the level of IAA, Z/ZR and isopentenyl-adenine/isopentenyl-adenosine (i-Ade/i-Ado) in the shoot apex were assayed. In addition, polar IAA transport capacity was measured in two internodes of different ages using 3H-IAA. In B-deficient plants, both the level of auxin and CKs were reduced, and the export of auxin from the shoot apex was considerably decreased relative to plants well supplied with B. Application of B to the shoot apex restored the endogenous Z/ZR and IAA level to control levels and increased the export of IAA from the shoot apex, as well as the 3H-IAA transport capacity in the newly developed internodes. Further, B application to the shoot apex inhibited lateral bud growth and stimulated lateral root formation, presumably by stimulated polar IAA transport. Applying CPPU to the shoot apex, a treatment that stimulates IAA export under adequate B supply, considerably reduced the endogenous Z/ZR concentration in the shoot apex, but had no stimulatory effect on IAA concentration and transport in B-deficient plants. A similar situation appeared to exist in lateral buds of B-deficient plants as, in contrast to plants well supplied with B, application of CKs to these plants did not stimulate lateral bud growth. In contrast to the changes of Z/ZR levels in the shoot apex, which occurred after application of B or CPPU, the levels of i-Ade/i-Ado stayed more or less constant. These results suggest that there is a complex interaction between B supply and plant hormones, with a B-deficiency-induced inhibition of IAA export from the shoot apex as one of the earliest measurable events.     

Effect of auxins and cytokinins on plant regeneration from hypocotyls and cotyledons in niger (Guizotia abyssinica Cass.)

Tissue cultures were established from hypocotyl and cotyledonary leaf segments ofGuizotia abyssinica Cass. on MS medium supplemented with various concentrations of auxins (IAA, NAA, IBA or 2,4-D) and cytokinins (KN or BA). Expiants cultured on media with cytokinins or in combination with auxins produced shoot buds. Maximum number of shoot buds (20–25 per culture) were differentiated from cotyledonary leaf segments on medium with 2 mg 1^-1 each of KN and IBA. Rooting of regenerated shoot buds was acheived on medium with NAA. The obtained plantlets were successfully transferred to soil.     

Transport of exogenous auxin in two-branched dwarf pea seedlings (Pisum sativum L.)

Dwarf pea plants bearing two cotyledonary shoots were obtained by removing the epicotyl shortly after germination, and the patterns of distribution of ¹⁴C in these plants was investigated following the application of [¹⁴C]IAA to the apex of one shoot. Basipetal transport to the root system occurred, but in none of the experiments was ¹⁴C ever detected in the unlabelled shoot even after transport periods of up to 48 h. This was true both of plants with two equal growing shoots and of plants in which one shoot had become correlatively inhibited by the other, and in the latter case applied whether the dominant or subordinate shoot was labelled. In contrast, when [¹⁴C]IAA was applied to a mature foliage leaf of one shoot transfer of ¹⁴C to the other shoot took place, although the amount transported was always low. Transport of ¹⁴C from the apex of a subordinate shoot on plants bearing one growing and one inhibited shoot was severely restricted compared with the transport from the dominant shoot apex, and in some individual plants no transport at all was detected. Removal of the dominant shoot apex rapidly restored the capacity of the subordinate shoot to transport apically-applied [¹⁴C]IAA, and at the same time led to rapid cambial development and secondary vascular differentiation in the previously inhibited shoot. Applications of 1% unlabelled IAA in lanolin to the decapitated dominant shoot maintained the inhibition of cambial development in the subordinate shoot and its reduced capacity for auxin transport. These results are discussed in relation to the polarity of auxin transport in intact plants and the mechanism of correlative inhibition.Abbreviations IAA Indol-3-yl-acetic acid - TIBA 2,3,5-triiodobenzoic acid - 2,4D 2,4-dichlorophenoxyacetic acid - IAAsp Indol-3-yl-acetyl aspartic acid     

Abscisic acid, indole-3-acetic acid and cytokinin changes in buds of Pseudotsuga menziesii during bud quiescence release

Bud quiescence release, considered as the ultimate dormancy breaking phase, was achieved in Pseudotsuga menziesii (Mirb.) Franco by a 9-week cold (5°C) treatment, under short daylength (9 h) followed by a transfer to mild temperature (22°C) under long daylength (16 h). Indole-3-acetic acid (IAA), abscisic acid (ABA), zeatin-type (Z) and isopentenyladenine-type (iPA) cytokinin (CK) levels were measured by means of an ELISA technique performed on HPLC-fractionated extracts of terminal and axillary buds. During the cold period, all hormones except IP-type CK levels decreased, whereas the opposite observation was made after transfer to mild temperature and long daylength, when buds started to grow. Some other immunoreactive compounds were also detected and quantified. The ABA-glucosyl ester (ABA-GE) level pattern was similar to that of ABA, but no accumulation occurred at mild temperatures. A putative IAA conjugate, more polar than IAA, was also detected. Its level increased transiently like IAA in terminal buds and, to a lesser extent, in axillary buds during the 10th week of the experiment. In terminal buds, isopentenyladenosine ([9R]-iP) was released by alkaline hydrolysis of a polar immunoreactive compound detected with anti-[9R]iP antibodies. This compound accumulated during the cold period and quickly dropped at 22°C. Relationships between environmental conditions and endogenous hormones are discussed.     

Polarity of the stem and ontogenetic changes in sunflow (Helianthus annuus L.) leaves in relation to endogenou cytokinins and ethylene

The content of endogenous cytokinin-like substances and the release of ethylene were determined in leaves of different insertion of sunflower plants during their ontogeny. The content of cytokinin-like substances was highest in the leaves on the middle part of the stem (that is in leaves just before full expansion), with a decrease occurring both towards the base and the apex of the stem, when followed at four growth phases (vegetative plants, plants with inflorescence diameter up to 0.5 cm, plants with inflorescence diameter up to 3 cm, and plants in flower). Changes in the content of cytokinin-like substances during the ontogeny of the leaf also corresponded to this pattern. Data obtained with the leaf at the third node from the basis of the stem showed that the level of cytokinin-like substances first sharply increased, and then after reaching maximal value (at the time when leaf blade area reached approximately 70 % of the final value) slowly and continuously decreased. The highest amount of ethylene released from the leaves was recorded in basal leaves and then also in apical leaves, whereas the leaves with the largest blade area situated at the central part of the stem released the lowest amount of ethylene. This pattern was repeatedly found at all four selected growth phases of sunflower plants.     

Hierarchy establishment among potentially similar buds

A plant has a great excess of buds each with the potential of developing an entire shoot system. The general question tackled was to what extent shoot size and time of bud development are important for bud hierarchy. Pea seedlings with two shoots, which were either equal or unequal in size, were obtained by the early removal of the seminal shoot. When these two shoots were also removed, one of the two cotyledon buds next to the bases of the shoots developed into the new shoot system. The determination of which of the buds became dominant was studied as a function of the relative sizes of the two primary cotyledonary shoots, of differences in the timing of the removal of these shoots and of the size of the buds. The bud that became dominant was not necessarily the larger one, nor did it always emerge from the axil of the larger shoot. Instead, it was usually the bud that was inhibited for a shorter period by the shoot next to it. It is suggested that the fate of a bud is predominantly determined by developmental parameters, for example lime of release, which are correlated with its developmental status and not necessarily with its physical size or with the past development of its shoot.     

Role of mineral nutrients and growth regulators in the apical dominance in Solanum sisymbrifolium

Summary The lower axillary buds of intact plants of Solanum sisymbrifolium are released from complete inhibition by supplying high doses of potassium to the soil while complete apical dominance is shown by plants grown at low, but not deficiency levels of K. Nitrate and phosphate supplied alone or together are ineffective but when either of them is supplied with K, the effect of the latter on the growth of axillary buds is somewhat enhanced. The buds thus released from inhibition elongate further only when supplied with gibberellin A₃ (GA₃). Indoleacetic acid (IAA) alone has a considerably weaker effect but when supplied with GA₃ the stimulatory effect is greater than is caused by the latter alone. The completely inhibited buds of low-K plants can be released from inhibition by supplying kinetin. In such buds IAA promotes further extension but GA₃ does not. The partially inhibited buds of the high-K plants, on the other hand, are not affected by exogenous kinetin.Dedicated to Professor H. D. Wulff on the occasion of his 60th birthday.     

Ethylene and apical dominance

Ethylene is involved in at least two discrete mechanisms in the control of apical dominance: the release of lateral buds from inhibition and their subsequent growth and development. Generally, high levels of freely diffusible ethylene in the apical region of the shoot are conducive to lateral-bud-outgrowth, while high ethylene levels in the region of the lateral buds themselves tend to be inhibitory. Threshold ethylene levels concerned with the release of buds from inhibition and with the growth that follows may differ between species. Thus, in some species (e.g. Gossypium ) lateral-bud growth proceeds in the continuing presence of ethylene supplied to the whole plant, whereas in others (e.g. Petunia ) the growth of the released lateral buds occurs only when the ethylene is remaved.
When ethylene production in Pisum nodal sections is enhanced by exomgenous auxin, growth of the attached buds is suppressed. In the intact plant system, unequivocal evidence has not been established for a role of endogenous ethylene acting directly on lateral buds to effect their inhibition. Apical dominance is not affected by the apptication of ethylene antagonists to the lateral buds of intact plants. Results from different studies have been inconsistent regarding the changes in endogenous ethylene levels in the node/lateral-bud tissue when the plant is decapitated or when auxin is applied to the stump of the decapitated plant to maintain lateral bud inhibition.
While exogenous ethylene supplied to the lateral bud generally increases inhibition, the availability of ethylene, regulated endogenously, is essential to the released bud on the decapitated plant in order to sustain its subsequent development into a lateral shoot. There is evidence that, in certain instances, endogenous ethylene may also be essential in the initial stages of bud development, e.g. in thee early growth that is promoted bmy auxin in Phaseolus or by kinetin in Avena .     

Embryonal growth-correlative effects in pea seedlings as reflected in the level of endogenous gibberellin-like and cytokinin-like substances

The level of endogenous gibberellin-like and cytokinin-like substances was estimated in the plumule and the root of 2-to 3-day-old intact pea seedlings, in the plumule of seedlings from which the radicle was cut off, and in the root of seedlings from which the plumule was removed. An increase in the level of gibberellin-like substances and a decrease in the level of cytokininlike substances in the plumule as a consequence of radicle amputation were observed within 48 h. Plumule amputation resulted in a decreased level of both gibberellin-like and cytokinin-like substances in the root in the same period.     

Autoinhibition of indoleacetic acid transport in the shoots of two-branched pea (Pisum sativum) plants and its relationship to correlative dominance

Two-branched pea plants ( Pisum sativum L. cv. Lisa ZS) with different dominance degrees, obtained by removing the epicotyl shortly after germination, were used to study the interaction between the polar transport of indoleacetic acid (IAA) in both branches of the plants and its relationship to correlative dominance. The dominant shoot had higher transport capacity for 3H-IAA, exported more IAA out of its apex and possessed more endogenous IAA in apex and the first internode than the dominated one. Decapitation of the dominant shoot resulted in a rapid resumption of growth in the dominated shoot, accompanied by a considerable increase in its capacity to export endogenous IAA and to transport 3H-IAA. Parallel experiments with intact two-branched plants and Y-formed explants showed that the 3H-IAA transport on one side was inhibited by the other branch apex or by pre-application of 12C-IAA to the cut stump of the decapitated side. The higher the concentration of 12C-IAA applied to the cut stump of one side of the Y-form explant was used, the stronger the 3H-IAA transport was inhibited and the more the transported IAA was conjugated above the junction on the other side. The results of these experiments support the autoinhibition hypothesis at junctions. The relationship between elongation growth and IAA export/transport in the two-branch pea plants is considered.     

Apical Dominance in the Tomato: Some Further Observations on Isogenic Lines Showing Varying Degrees of Side-shoot Development

A study has been made of the distribution of substances resemblingindol-3-ylacetic acid (IAA), abscisic acid (ABA) and cytokinin-likesubstances in the stem tissue of Craigella tomato plants ascompared with that found in two isogenic lines of this variety,Craigella Blind (blbl) and Craigella Lateral Suppressor (lsls),in both of which side shoot growth is suppressed to varyingdegrees. There was no evidence to suggest that the distributionof these hormones in the stem had any association with the differentpatterns of side shoot development of the three types, thoughsome of the lateral suppressor plants which exhibited only partialbud inhibition did show a relation between high auxin and abscisicacid levels and lack of side shoot development from the centralnodes of the shoot. Decapitation led to a stimulation of bud outgrowth from allnodes of the Craigella plants but the lateral suppressor plantsremained inhibited. The blind plants were found to initiatebud primordia at the cotyledonary nodes only when the severedapex was replaced by exogenous IAA. The results are discussed in relation to our knowledge of themechanisms controlling apical dominance in the tomato. Lycopersicon esculentumL, tomato, apical dominance, growth regulation, indol-3-ylacetic acid, abscisic acid, cytokinins     

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.     

Stimulatory effect of cytokinins and interaction with IAA on the release of lateral buds of pea plants from apical dominance

Lateral buds of pea plants can be released from apical dominance and even be transformed into dominant shoots when repeatedly treated with synthetic exogenous cytokinins (CKs). The mechanism of the effect of CKs, however, is not clear. The results in this work showed that the stimulatory effects of CKs on the growth of lateral buds and the increase in their fresh weights in pea plants depended on the structure and concentration of the CKs used. The effect of N-(2-chloro-4-pyridyl)-N'-phenylurea (CPPU) was stronger than that of 6-benzylaminopurine (6-BA). Indoleacetic acid (IAA) concentration in shoot, IAA export out of the treated apex and basipetal transport in stems were markedly increased after the application of CPPU or 6-BA to the apex or the second node of pea plant. This increase was positively correlated with the increased concentration of the applied CKs. These results suggest that the increased IAA synthesis and export induced by CKs application might be responsible for the growth of lateral shoots in intact pea plants.