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.     

Nauclea diderrichii: rooting of stem cuttings,clonal variation in shoot dominance,and branch plagiotropism

Summary Nauclea diderrichii (De Wild, and Th. Dur.) Merill (Rubiaceae), an indigenous hardwood of West Africa, is increasingly being grown commercially. This study investigates the potential for vegetative propagation and clonal selection, and raises some fundamental questions about the physiology of apical dominance and of plagiotropism. Rooting ability was high, with up to 100% rooting in 2–4 weeks, when different Indole-3-butyric acid (IBA) concentrations and leaf areas were tested. Auxin applications greatly increased the numbers of roots per cutting. The decapitation of unbranched plants revealed clonal variation in apical dominance and also in the establishment of outright dominance by the two shoots formed from the outgrowth of the axillary buds of the opposite leaves at the top node. Regression analysis of the Dominance Ratio (length of dominant: length of the sub-dominant shoot at the time of achieving dominance) against overall lateral bud activity (r = 0.82), showed that when the two top shoots co-dominate they provide a more powerful source of Correlative Inhibition than when one of the top shoots dominates the other. The imposition of plagiotropism in the axillary bud occurred over a period of a few days as the terminal and axillary buds emerged from the stipule. Growth of accessory buds on intact plants and debranched cuttings was orthotropic. These results are discussed with regard to the role of the leaf in root formation and the understanding of dominance relationships, branching and crown development in trees.     

Impact of a gall-inducing aphid on Pistacia atlantica Desf. trees

The impact of gall-inducing aphids on shoot development was analyzed in 900 shoots from 20 pistachio trees, Pistacia atlantica Desf. (Anacardiaceae): 600 in which the axillary—lateral buds were galled by Slavum wertheimae HRL during the previous growth season, and 300 ungalled shoots. Although P. atlantica is a compensating tree, and the aphids do not attack the apical buds, further development of shoots from the apical buds was stopped in 62% of the galled shoots, while only 8.7% of nongalled shoots stopped their growth. Further development was stopped more often on shoots carrying two or more galls than on shoots supporting only one gall. To assess the hypothesis that bud destruction by the aphids explains this pattern, a field experiment was conducted in 140 shoots, distributed across seven trees. One, two or three axillary buds from five shoots of each tree were removed for each treatment, and five other shoots were marked as controls. Only 14 shoots (10%) of the 140 did not develop. The growth of the other shoots was not very different among the treatments. The colonization of the apical shoots, which developed on previously treated shoots, by three other galling aphid species was monitored. Removing lateral buds considerably reduced the establishment of Geoica sp. galls (70% of them colonized control shoots), but weakly influenced Forda riccobonii (Stefani). It also contributed only 5% of the total variance of the distribution of Smynthurodes betae West. The different results of the survey and the experiment show that the impact of S. wertheimae galls on the future growth of shoots from apical buds is more complex than the simple physical destruction of the axillary buds. Handling editor: Graham Stone     

Branching in Pea (Action of Genes Rms3 and Rms4)

The nonallelic ramosus mutations rms3-2 and rms4 of pea (Pisum sativum L.) cause extensive release of vegetative axillary buds and lateral growth in comparison with wild-type (cv Torsdag) plants, in which axillary buds are not normally released under the conditions utilized. Grafting studies showed that the expression of the rms4 mutation in the shoot is independent of the genotype of the root-stock. In contrast, the length of the branches at certain nodes of rms3-2 plants was reduced by grafting to wild-type stocks, indicating that the wild-type Rms3 gene may control the level of a mobile substance produced in the root. This substance also appears to be produced in the shoot because Rms3 shoots did not branch when grafted to mutant rms3-2 rootstocks. However, the end product of the Rms3 gene appears to differ from that of the Rms2 gene (C.A. Beveridge, J.J. Ross, and I.C. Murfet [1994] Plant Physiol 104: 953-959) because reciprocal grafts between rms3-2 and rms2 seedlings produced mature shoots with apical dominance similar to that of rms3-2 and rms2 shoots grafted to wild-type stocks. Indole-3-acetic acid levels were not reduced in apical or nodal portions of rms4 plants and were actually elevated (up to 2-fold) in rms3-2 plants. It is suggested that further studies with these branching mutants may enable significant progress in understanding the normal control of apical dominance and the related communication between the root and shoot.     

Effects of lead contamination on the clonal propagative ability of Phragmites australis (common reed) grown in wet and dry environments

Clonal propagation is important for the survival and maintenance of the common reed Phragmites australis. Pot culture experiments were conducted to investigate the effects of lead (Pb) concentration (0, 500, 1500, 3000, 4500 mg·kg^?1) and water stress on the clonal reproductive ability of this species. The Pb concentration found in plant organs, in decreasing order, was roots >shoots >rhizomes. There was a negative relationship between the growth of clonal propagative modules (excluding axillary shoot buds) and Pb concentrations, which caused a decrease in biomass, rhizome growth and number of axillary and apical rhizome buds. Daughter axillary shoots exhibited a tolerance strategy, with no significant change in their number; the axillary and apical rhizome buds, daughter apical rhizome shoots and rhizomes exhibited compensatory growth during the late stage of Pb (excluding 4500 mg·kg^?1) treatment in a wet environment. Pb applications above 500 mg·kg^?1 reduced these parameters significantly in the drought treatment, except for the number of axillary shoot buds, which did not change. Our results indicate that clonal propagative resistance to Pb contamination can occur via tolerance strategies, compensatory growth and a Pb allocation strategy, enabling these reeds to maintain population stability in wet environments. However, clonal modular growth and reproductive ability were inhibited significantly by the interaction between drought and Pb, which would cause a decline in P. australis populations in a dry environment. Lead concentrations of 4500 and 500 mg·kg^?1 in soils might meet or exceed the Pb tolerance threshold of clonally propagated reeds in wet and dry environments, respectively.     

The gravity-regulated growth of axillary buds is mediated by a mechanism different from decapitation-induced release

When the upper part of the main shoot of the Japanese morning glory (Pharbitis nil or Ipomoea nil) is bent down, the axillary bud situated on the uppermost node of the bending region is released from apical dominance and elongates. Here, we demonstrate that this release of axillary buds from apical dominance is gravity regulated. We utilized two agravitropic mutants of morning glory defective in gravisensing cell differentiation, weeping (we) and weeping2 (we2). Bending the main shoots of either we or we2 plants resulted in minimal elongation of their axillary buds. This aberration was genetically linked to the agravitropism phenotype of the mutants, which implied that shoot bending-induced release from apical dominance required gravisensing cells. Previous studies have shown that basipetal translocation of auxin from the apical bud inhibits axillary bud growth, whereas cytokinin promotes axillary bud outgrowth. We therefore compared the roles of auxin and cytokinin in bending- or decapitation-induced axillary bud growth. In the wild-type and we plants, decapitation increased cytokinin levels and reduced auxin response. In contrast, shoot bending did not cause significant changes in either cytokinin level or auxin response, suggesting that the mechanisms underlying gravity- and decapitation-regulated release from apical dominance are distinct and unique.     

Changes after Decapitation in Concentrations of Indole-3-Acetic Acid and Abscisic Acid in the Larger Axillary Bud of Phaseolus vulgaris L. cv Tender Green

Early changes in the concentrations of indole-3-acetic acid (IAA) and abscisic acid (ABA) were investigated in the larger axillary bud of 2-week-old Phaseolus vulgaris L. cv Tender Green seedlings after removal of the dominant apical bud. Concentrations of these two hormones were measured at 4, 6, 8, 12 and 24 hours following decapitation of the apical bud and its subtending shoot. Quantitations were accomplished using either gas chromatography-mass spectrometry-selected ion monitoring (GS-MS-SIM) with [¹³C₆]-IAA or [²H₆]-ABA as quantitative internal standards, or by an indirect enzyme-linked immunosorbent assay, validated by GC-MS-SIM. Within 4 hours after decapitation the IAA concentration in the axillary bud had increased fivefold, remaining relatively constant thereafter. The concentration of ABA in axillary buds of decapitated plants was 30 to 70% lower than for buds of intact plants from 4 to 24 hours following decapitation. Fresh weight of buds on decapitated plants had increased by 8 hours after decapitation and this increase was even more prominent by 24 hours. Anatomical assessment of the larger axillary buds at 0, 8, and 24 hours following decapitation showed that most of the growth was due to cell expansion, especially in the intermodal region. Thus, IAA concentration in the axillary bud increases appreciably within a very few hours of decapitation. Coincidental with the rise in IAA concentration is a modest, but significant reduction in ABA concentration in these axillary buds after decapitation.     

Correlative Inhibition in the Shoot of Agropyron repens ( L.) Beauv

Correlative inhibition was investigated in plants of Agropyronrepens at two temperatures. Reciprocal inhibition ocrurred betweenthe main shoot apex and the outgrowing axillary shoots, withthe balance of inhibition varying with temperature. Apical dominancewas stronger at 10 °C than at 20 °C , but even at 10°C release of apical dominance by decapitation had onlyminor effects on the timing of outgrowth, growth pattern andrate of dry weight aocumulation of the axillary shoots. Dominanceof the main shoot apex by the axillary shoots was stronger at20 °C than at 10 °C. Removal of axillary buds preventeddecline in size and activity of the main shoot apex ard resultedin increased rates of primordium initiation, leaf emergenceand dry weight accumulation in the main shoot. It is suggestedthat a system of reciprocal dominance provides a mechanism formaintaining the characteristic habit of the grass plant andlimits growth in height of vegetative shoots. Agropyron repens (L.) Beauv, couch grass, correlative inhibition, apical dominance, shoot, apex     

Influences of ascorbic acid and gibberellic acid in alleviating effects of salinity in Petunia under in vitro

Salinity is one of the abiotic stresses that limits the growth and productivity of many crops. A possible survival strategy for plant under saline conditions is to use compounds that could minimize the harmful effects of salt stress on the plant development. The objective of the presented study was to investigate the effect of exogenous ascorbic acid (ASA) with or without gibberellic acid (GA3) on key growth and biochemical parameters in two petunia cultivars ‘Prism Rose’ and ‘Prism White’ under saline (150 mM NaCl) and non-saline in vitro condition. Nodal cutting with an axillary buds were used as explants. Application of 1 mM ascorbic acid with or without 0.05 mM gibberellic acid into the MS medium stimulated the length of shoots and the number of new shoots of ‘Prism Rose’; whereas, it decreased the root length and the number of roots of both ‘Prism Rose’ and ‘Prism White’ under non-saline condition. The addition of ascorbic acid with or without gibberellic acid into the MS medium under saline condition, increased the length of plants and the number of new shoots, but did not affect their root number and length. NaCl treatments increased the proline content and lipid peroxidation which was indicated by the accumulation of malondialdehyde (MDA). The study revealed a correlation between chlorophylls a and b content and the leaf pigmentation intensity – parameter a*. Addition of 1 mM ascorbic acid with 0.05 mM gibberellic acid into the MS medium plays a protective role in salinity tolerance by improving the shoot growth and the development as well as increasing the activities of the antioxidant enzymes and other antioxidant substances.     

伊乐藻和黑藻断枝根和芽的发生及生长研究

为了解外来种伊乐藻的无性繁殖力、评价其生态安全性,采用插植方式比较研究了伊乐藻(Elodea nuttallii)和本土种黑藻(Hydrilla verticillata)两种沉水植物不同节数(1至4节)和不同节位断枝的不定根和新芽的发生及生长情况。通过室内4周的3次平行实验,结果表明:两者顶芽段均有形成不定根继而形成新植株的能力,而顶芽以下茎段只有本身具有腋芽的断枝才有形成新芽和不定根的能力。两者具相同节数的断枝形成不定根的百分率及根、芽长度,以具顶芽断枝的均明显高于不具顶芽断枝的,具顶芽四节断枝的不定根生成率最高达到90%以上。不具顶芽断枝形成新芽和不定根的百分率及长度随着断枝节数的增加均呈显著递增趋势,每类断枝的发芽率显著大于其生根率;伊乐藻和黑藻枝条一般分别每7节和5节具有一个腋芽,只有具腋芽断枝才能存活,因此,对不具顶芽断枝,7节和5节分别是其形成新苗所需的最短断枝长度。根和芽的长度随节位的下降大致呈递增的趋势。但是节数对形成根、芽的影响显著大于节位的影响。具顶芽断枝的顶芽的增长量和具顶芽4节断枝的生物量增量伊乐藻的高于黑藻,其余指标伊乐藻均显著低于黑藻。伊乐藻断枝的繁殖力总体上低于黑藻。       

Warm spring temperatures induce persistent season-long changes in shoot development in grapevines

Background and Aims

The influence of temperature on the timing of budbreak in woody perennials is well known, but its effect on subsequent shoot growth and architecture has received little attention because it is understood that growth is determined by current temperature. Seasonal shoot development of grapevines (Vitis vinifera) was evaluated following differences in temperature near budbreak while minimizing the effects of other microclimatic variables.

Methods

Dormant buds and emerging shoots of field-grown grapevines were heated above or cooled below the temperature of ambient buds from before budbreak until individual flowers were visible on inflorescences, at which stage the shoots had four to eight unfolded leaves. Multiple treatments were imposed randomly on individual plants and replicated across plants. Shoot growth and development were monitored during two growing seasons.

Key Results

Higher bud temperatures advanced the date of budbreak and accelerated shoot growth and leaf area development. Differences were due to higher rates of shoot elongation, leaf appearance, leaf-area expansion and axillary-bud outgrowth. Although shoots arising from heated buds grew most vigorously, apical dominance in these shoots was reduced, as their axillary buds broke earlier and gave rise to more vigorous lateral shoots. In contrast, axillary-bud outgrowth was minimal on the slow-growing shoots emerging from buds cooled below ambient. Variation in shoot development persisted or increased during the growing season, well after temperature treatments were terminated and despite an imposed soil water deficit.

Conclusions

The data indicate that bud-level differences in budbreak temperature may lead to marked differences in shoot growth, shoot architecture and leaf-area development that are maintained or amplified during the growing season. Although growth rates commonly are understood to reflect current temperatures, these results demonstrate a persistent effect of early-season temperatures, which should be considered in future growth models.     

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.     

Endorhythmic development in Choisya ternata Kunth (Rutaceae), with a further elucidation of lammas shoots, as well as sylleptic and proleptic shoots

Most apical resting buds of Choisya tenata include inflorescence buds in the axils of their lower consecutive paired scales. These inflorescences develop as apical buds which burst in spring. The whole of the lateral inflorescence system on a shoot originating from an apical bud may be viewed as a single, proliferous inflorescence. After the spring flush there are usually two other flushes of the same shoot within the same season, each of which may be accompanied by the development of lateral inflorescences as in the spring flush. Each further flush produces an apical 'lammas shoot'. As an apical lammas shoot elongates, lateral lammas shoots may also develop from upper, previously resting, axillary buds on the underlying stem segment of the preceding flush. Lateral inflorescences on apical lammas shoots arise from axillary buds preformed within the briefly-dormant apical buds terminating the preceding flush. These inflorescences, as well as the spring ones, represent proleptic shoots. The production of resting apical buds between two intra-season flushes of a shoot may be fugacous, without the differentiation of perfect bud-scales, and with curtailmenl ol internode elongation. As no environmental influence seems to be responsible for intra-season rhythmicity in development, this is said to be endorhythmic. The interrelations of proleptic to sylleptic shoots are discussed.     

Participation of gibberellin in the control of apical dominance in soybean and redwood

Summary Loss of apical dominance in soybeans and redwood was increased when the plants were treated with the growth retardant AMO-1618. Simultaneous application of gibberellin reduced the number of elongating buds and promoted growth of the first or second uppermost axillary bud, thus restoring apical dominance. It is concluded that gibberellin participates in the expression of apical dominance.     

Changes in the level of endogenous gibberellins and auxins in apical buds ofChenopodium rubrum L. after application of growth substances reversing the effect of (2-chlorethyl)-trimethylammonium chloride (CCC) on flowering

The application of CCC at concentrations inhibiting flowering ofChenopodium rubrum reduces the level of endogenous gibberellins in the apical buds of the plants. The effect of CCC may be reversed by appropriate concentrations of gibberellin (GA-), indole acetic acid (IAA) or kinetin. Kinetin applied to the apical bud during floral induction reduced the level of endogenous gibberellins similarly as CCC and if both CCC and kinetin were applied simultaneously their action was additive. On the other hand IAA applied under the same conditions increased the level of endogenous gibberellins and after joint application of CCC and IAA their level was the same as in untreated control plants. After application of CCC during floral induction the level of endogenous auxins did not change markedly but an active substance “x” appeared on the chromatograms of indole compounds. This substance was found also after simultaneous application of GA- and CCC but not after joint application of CCC and kinetin. If follows from our results that the same morphological phenomenon (flowering) can take place in plants considerably differing as to their level of endogenous growth substances. The ratio of different growth substances is obviously more important than the actual level of the single substances.     

Elimination of five viruses from sugarcane using in vitro culture of axillary buds and apical meristems

Procedures were developed for the in vitro elimination of Sugarcane mosaic virus (SCMV), Sorghum mosaic virus (SrMV), Sugarcane streak mosaic virus (SCSMV), Sugarcane yellow leaf virus (SCYLV) and Fiji disease virus (FDV) from infected sugarcane. In vitro shoot regeneration, elongation and virus elimination through meristem tissue culture originating from both apical and axillary shoots were compared. The average rates of regeneration and elongation from apical meristem tissues were 91 and 66%, respectively, with the virus-free rate among elongated shoots ranging from 61–92%. Mature axillary buds were cultivated in vitro to produce axillary shoots, from which meristem tissues were excised and cultured. These meristem tissues regenerated (77–100%) and elongated (55–88%) in culture medium at approximately the same rate as the apical meristems. The average virus elimination rate was 90% among elongated shoots derived from mature axillary buds. All five viruses can be eliminated by meristem tissue culture from both apical and axillary shoots using a standardized procedure. The overall average efficiency of virus-free plant production was 45 and 58% from apical and axillary shoots, respectively. There were no significant differences for shoot induction or virus elimination when the meristems were harvested from either the apical or the axillary shoots. This is the first report of SrMV or SCSMV elimination from sugarcane, as well as elimination of any mixed virus infections. This new method of harvesting meristems from axillary buds greatly expands the amount of material available for therapeutic treatments and thereby increases the probability of eliminating viruses from infected sugarcane.     

Opuntia micropropagation by axillary proliferation

Cladode explants of Opuntia amyclaea were cultivated in Murashige and Skoog medium with different supplements. Benzyladenine was necessary for shoot development from pre-existing buds. Axillary proliferation was also stimulated in subsequent subcultures in the presence of benzyladenine and when the apical meristem was not present in the explant. The number of shoots and the total dry weight were maximum with 5% of sucrose in the medium. It was found that satisfactory rooting occurred when 5×10^-5 M indole butyric acid was added to the medium. Vascular contact between roots and shoots was clearly shown by histological observations. The micropropagation system developed here allows the production in 100 days of 25 000 rooted plantlets from a single cladode, by the stimulation of axillary proliferation in the absence of apical dominance.     

Hormonal control of the outgrowth of axillary buds in <Emphasis Type="Italic">Alstroemeria</Emphasis> cultured <Emphasis Type="Italic">in vitro</Emphasis>

We study apical dominance in Alstroemeria, a plant with an architecture very different from the model species used in research on apical dominance. The standard explant was a rhizome with a tip and two vertically growing shoots from which the larger part had been excised leaving ca. 1 cm stem. The axillary buds that resumed growth were located at this 1-cm stem just above the rhizome. They were released by removal of the rhizome tip and the shoot tips. Replacement of excised tips by lanolin with indole-3-butyric acid (IBA) restored apical dominance. The auxin transport inhibitors 2,3,5-triiodobenzoic acid (TIBA) and N-1-napthylphthalamic acid (NPA) reduced apical dominance. 6-Benzylaminopurine (BAP) enhanced axillary bud outgrowth but the highest concentrations (> 9 μM) caused fasciation. Thidiazuron (TDZ) did not show improvement relative to BAP. Even though the architecture of Alstroemeria and the model species are very different, their hormonal mechanisms in apical dominance are for the greater part very similar.     

AtMYB2 regulates whole plant senescence by inhibiting cytokinin-mediated branching at late stages of development in Arabidopsis

Whole plant senescence of monocarpic plants consists of three major processes: arrest of shoot apical meristem, organ senescence, and permanent suppression of axillary buds. At early stages of development, axillary buds are inhibited by shoot apex-produced auxin, a mechanism known as apical dominance. How the buds are suppressed as an essential part of whole plant senescence, especially when the shoot apexes are senescent, is not clear. Here, we report an AtMYB2-regulated post apical dominance mechanism by which Arabidopsis (Arabidopsis thaliana) inhibits the outgrowth of axillary buds as part of the whole plant senescence program. AtMYB2 is expressed in the compressed basal internode region of Arabidopsis at late stages of development to suppress the production of cytokinins, the group of hormones that are required for axillary bud outgrowth. atmyb2 T-DNA insertion lines have enhanced expression of cytokinin-synthesizing isopentenyltransferases genes, contain higher levels of cytokinins, and display a bushy phenotype at late stages of development. As a result of the continuous generation of new shoots, atmyb2 plants have a prolonged life span. The AtMYB2 promoter-directed cytokinin oxidase 1 gene in the T-DNA insertion lines reduces the endogenous cytokinin levels and restores the bushy phenotype to the wild type.