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)     

The Presentation Time for Shoot Inversion Release of Apical Dominance in Pharbitis nil

The presentation time for shoot inversion release of apicaldominance in Pharbitis nil is between 1 and 1.5 d. Five to 6d of shoot inversion are required for persistent outgrowth ofthe highest lateral bud. Pharbitis nil, apical dominance, shoot inversion, lateral bud growth, presentation time     

Gravistimulus Direction, Ethylene Production and Shoot Elongation in the Release of Apical Dominance in Pharbitis nil

Prasad, T. K. and Cline, M. G. 1985. Gravistimulus direction,ethylene production and shoot elongation in the release of apicaldominance in Pharbitis nil.—. exp. Bot. 36: 1969–1975.Release of apical dominance can be induced in Pharbitis nilby the inversion of the upper shoot. This promotion of outgrowthof the highest lateral bud adjacent to the bend of the stemappears to be mediated by ethylene inhibition of growth of theinverted main shoot. In the present investigation the existenceof a direct correlation between ethylene evolution and the directionof gravistimulus is demonstrated as well as an inverse correlationbetween ethylene production by the inverted upper shoot andits elongation. An inverse correlation also exists between elongationof the inverted upper shoot and the outgrowth of the highestlateral bud if the lower portion of the shoot (below the bend)is oriented in an upright position. The latent period for shoot–inversioninduction of ethylene production is about 2 h. These resultssupport the hypothesis of indirect ethylene control of apicaldominance release by retardation of elongation of the invertedshoot. Key words: Shoot inversion, gravistimulus, ethylene, latent period, bud outgrowth, pharbitis nil     

The Growth of the Shoot Apex and the Apical Dome of Barley During Ear Initiation

The growth of the floral main shoot apex of spring barley wasstudied during the period of ear initiation (that is, from initiationof the collar primordium until maximum primordium number wasattained). While floral primordia were being initiated the relativelength growth rate of the shoot apex was low. After maximumprimordium number there was about a twofold increase in relativelength growth rate. Estimates of the volume, fresh and dry weightof the floral apex indicated that the relative weight growthrate was also low at first and increased after maximum primordiumnumber. The rates of growth and the size at initiation of thefloral primordia was affected by their position on the floralshoot apex. The relative volume growth rate increased acropetallyfrom the first initiated (collar) primordium. The collar wasthe smallest and each subsequently-initiated primordium increasedin length. The diameter of the newly-initiated primordium alsoincreased until more than half the primordia had been initiatedand then it declined. The apical dome increased in both lengthand diameter and both were at a maximum at the time of the double-ridgestage and then both measurements declined. Length and diameterwere at a minimum at maximum primordium number. Subsequentlythere was an increase in the length of the dome, after whichboth the dome and some of the last formed, distal primordiadied. The period of spikelet initiation therefore is a stage duringwhich the relative growth rate of the floral shoot apex is low,there are changes in the size of the dome and the primordiashow a progression of increasing relative growth rates acropetallyalong the shoot apex. These changes produce the embryo ear inwhich the most advanced spikelets are in the lower mid-partof the ear. Changes in size of the apical dome prior to maximumprimordium number may be related to the subsequent death ofspikelet primordia and therefore also to grain number in themature ear.     

The Role of Gravity in Apical Dominance : Effects of Clinostating on Shoot Inversion-Induced Ethylene Production, Shoot Elongation, and Lateral Bud Growth

Shoot inversion-induced release of apical dominance in Pharbitis nil is inhibited by rotating the plant at 0.42 revolutions per minute in a vertical plane perpendicular to the axis of rotation of a horizontal clinostat. Clinostating prevented lateral bud outgrowth, apparently by negating the restriction of the shoot elongation via reduction of ethylene production in the inverted shoot. Radial stem expansion was also decreased. Data from experiments with intact tissue and isolated segments indicated that shoot-inversion stimulates ethylene production by increasing the activity of 1-aminocyclopropane-1-carboxylic acid synthase. The results support the hypothesis that shoot inversion-induced release of apical dominance in Pharbitis nil is due to gravity stress and is mediated by ethylene-induced retardation of the elongation of the inverted shoot.     

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.     

植物茎端分生组织中的茎干细胞调控机制

介绍了高等植物茎端分生组织茎干细胞维持自我更新和产生分化细胞之问平衡的分子机制研究进展.     

The Mechanism of Apical Dominance in Coleus

The development of lateral buds in isolated stems of Coleus blumei is inhibited by low concentrations of indoleacetic acid or other auxins, just as in other plants. The inhibition can be fully reversed by kinetin, about 3 times as much kinetin as IAA being needed. However, the outgrowth of the same lateral buds on intact Coleus plants is sensitive to environmental conditions, well-nourished plants in full daylight often showing little inhibition by applied auxin. It is shown that (a) the solvent used for IAA, (b) the light intensity and (c) the nitrogen and phosphorus nutrition, all control the sensitivity of the buds to auxin inhibition. Using water instead of lanolin, lowering the light intensity or decreasing the supply of either nitrogen or phosphorus all increase the degree of apical dominance.     

Effects of Developing Leaves on Stelar Pattern Development in the Shoot Apex of Matteuccia struthiopteris

A developmental study of the normal shoot apex of Matteucciastruthiopteris suggested that patterned stelar differentiationis initiated immediately beneath the single layer of promeristemand occurs prior to the initiation of the youngest leaf primordium.A developmental study in which all leaf primordia were suppressed,with or without lateral isolation of the terminal meristem byvertical incisions, has confirmed this interpretation of stelardifferentiation. Experimentally-induced changes in the tissueimmediately below the promeristem were reflected in the resultingmature structure of the stele. Failure of leaf gap initialsto differentiate, if all leaf primordia were suppressed at theincipient stage, resulted in a mature stele without leaf gaps.Similarly the disappearance of pith mother cells after severalweeks of leaf removal was associated with the formation of astele without pith. Leaf influence was further assessed by allowingone primordium to develop while all others were suppressed.The developing leaf had a small promoting effect on caulinevascular tissue differentiation but its major impact on theexpansion of the parenchymatous tissues of the stele. Characteristicprotoxylem and protophloem failed to differentiate when allleaves were suppressed and, when leaf was allowed to develop,formed only in relation to the leaf.Copyright 1995, 1999 AcademicPress Leaf influence, vascular pattern formation, experimental surgery, shoot apex development, protoxylem, protophloem, Matteuccia struthiopteris     

Shoot Inversion Inhibition of Stem Elongation in Pharbitis nil: A Possible Role for Ethylene-Induced Glycoprotein and Lignin

Inversion of the upper shoot of Pharbitis nil results in the inhibition of elongation in the inverted stem. The objective of the present study was to determine how shoot inversion-induced gravity stress inhibited elongation and to elucidate the possible role of ethylene-induced glycoprotein and lignin in this process. Determinations of hydroxyproline, peroxidase, phenylalanine ammonia-lyase (PAL), phenol, and lignin content/activity were carried out by appropriate spectrophotometric methods. It was found that inversion and Ethrel treatments of upright shoots caused significant increases in hydroxyproline content, peroxidase, and PAL activity in 12 hours and in phenol and lignin contents in 24 hours. All of these increases except for that of cytoplasmic peroxidase activity were partially reversed by AgNO₃, the ethylene action inhibitor. It is concluded that possible cross-linking associated with the accumulation of ethylene-induced hydroxyproline-rich glycoprotein and lignin may be responsible for the later stages of cessation of elongation in the inverted Pharbitis shoot.     

The Shoot Apex and its Dichotomous Branching in the Nypa Palm

Seedlings of the palm Nypa fruticans van Wurmb are viviparous,the plumule becoming exserted before the fruit is ripe and possiblyassisting in fruit detachment. Established seedlings have horizontalaxes, this growth orientation being maintained throughout thelife of the palm which may be described as ‘rhizomatous’.Inflorescences are axillary in adult plants, but their distributionis irregular. The shoot apex is small and very asymmetricalsince it is more or less displaced into a lateral position byprogressive enlargement of each leaf primordium during thisleaf's first plastochrone. The plastochrone interval is apparentlya long one so that leaves have a wide developmental gap betweenthem. This results in a leaf base which is more or less cylindricalbut with a groove to accommodate the next youngest leaf. All the available evidence suggests that vegetative branchingis by equal dichotomy of the shoot apex at wide intervals. Thedichotomy is marked superficially by a leaf, enclosing the twonew shoots, which has two grooves. The twin shoots are insertedin the lateral, not the dorsiventral plane of this enclosingleaf. The daughter shoots are always at precisely the same stageof development and they always have mirror-image symmetry withrelation to each other, the phyllotaxis of the parent shootbeing maintained without obvious interruption. There is no anatomicalevidence for abortion of the original apex and its replacementby two new ones. The vascular system is divided equally betweeneach daughter shoot without interruption, suggesting stronglythat there is continuity of growth from the undivided to thedivided condition.     

Stem Cell Maintenance and Abiotic Stress Response in Shoot Apical Meristem for Developmental Plasticity

The shoot apical meristem (SAM) serves as a non-drying reservoir of pluripotent stem cells to supply new daughter cells forming above-ground tissues and organs such as leaves, stems, flowers and fruits throughout the life cycle of plants. Accordingly, the homeostasis control of stem cell division and differentiation must be an essential core mechanism for harmonic growth and development of plants as multicellular higher eukaryotes. Unlike animals, plants are sessile organisms and thus constantly face environmental factors, including abiotic stresses. Therefore, post-embryonic development derived from stem cells in the SAM likely interacts with surrounding abiotic stresses for plant adaptation and plastic development. For this reason, this review provides the most recent findings regarding comprehensive signaling networks involved in stem cell maintenance in the SAM, and then describes how stem cell signaling is related with abiotic stress response through involvement of phytohormones and reactive oxygen species in the SAM.     

Hormones and Apical Dominance in the Fern Davallia

Lateral buds of the fern Davallia trichomanoides are releasedfrom inhibition by the removal of the main shoot apex. However,auxin is not capable of substituting for the apex in decapitatedshoots nor can auxin in shoot tips be detected by bioassay orextraction and chromatography. Expanding leaves of this speciescontain auxin, but these organs are not responsible for inhibitionof lateral bud growth. The response of lateral buds to an exogenouslyapplied cytokinin does not result in initial bud break. It isconcluded that the hormonal factors known to govern apical dominancein seed plants are not responsible for the regulation of differentialbud expansion in this fern.     

The Structure of the Shoot Apex of Glyptostrobus pensilis Koch

1. A series of studies on the structure of the shoot apices in various genera of the Taxodiaceae, except Glyptostrobus was carried out by Cross (1939, 1941, 1942, 1943a, b). In the last two pieces of his above mentioned works, he repeatedly suggested that there was a basic structure pattern which characterized the family. Results obtained from the present investigation lend support to this view. 2. From a comparative point of view concerning the structure of the apical meristem, it may be said that the family Taxodiaceae is a very natural one, any attempt on subdividing it into several separate families, as Hayata (1932) did, seems to be unnecessary. 3. It is very interesting to note that the cellular pattern of the Glyptostrobus shows more affinity with that of the sequoias than with that of Taxodium. Whereas the structures of the apices of Taxoclium and Metasequoia possess more similarities. 4. Cross (1942) proposed that an entirely discrete surface layer might be evolved in the apical meristem of the Taxodiaceae as a result of a gradual diminution and ultimate loss of the periclinal divisions in the outer cells of the shoot apex. In his subsequent papers (1943a, b), more evidence were added in this respect. The present observation is in accord with Cross' findings.     

The Structure and Development of the Vegetative Shoot Apex in Glechoma hederacea L

The development of the vegetative shoot apex of Glechoma hederaceahas been followed through a double plastochron. During this period the apex grows from c. 20 to c. 260 µin height and c. 100 to c. 300 µ in width, whilst thepair of leaves inserted at the apex base increase from o toc. 600µ in height. The width of the apex and height ofthese leaves are directly related to apex height. Some variationoccurs in the average maximal dimensions of the apex with plastochronnumber but no regular increase or decrease in these dimensionsis apparent. Both a tunica-corpus organization and cytohistological zonationis visible within the apex throughout a double plastochron.The central initiation zone shows little change in size or appearanceduring this period but the rib and flank meristems grow considerablyand undergo some differentiation. The boundaries of these zonesare not sharply defined, but normally the rib meristem givesrise to the pith, and the flank meristem forms the epidermis,cortex, and provascular tissue. The provascular tissue differentiatesacropetally and in continuity with that in the axis below.     

Vascular Differentiation in the Shoot Apex of Matteuccia struthiopteris

Initial vascular differentiation is generally considered tooccur in procambium. In ferns, however, a provascular tissueimmediately subjacent to the promeristem has been suggestedas an initial stage within which the procambium is subsequentlyformed. In contrast to this interpretation, a zonation conceptapplied in ferns recognizes a promeristem consisting of severallayers of cells in which no differentiation takes place. Thisstudy demonstrates that the shoot apex of Matteuccia struthiopterishas one cell layer of promeristem. Immediately subjacent tothe promeristem is the provascular tissue surrounding a centralgroup of pith mother cells. The developmental continuity betweenthe provascular tissue and the mature vascular tissue, and betweenthe pith mother cells and the pith, through transitional stages,indicates that the initial differentiation of vascular tissueand pith takes place in this prestelar tissue. The continuityof vascular differentiation in the area confronting young leavesor incipient leaf positions is interrupted by the formationof leaf gap initials. Developing leaves thus begin to exertinfluence on the vascular system at the prestelar stage. Smallprotoxylem elements with helical cell wall thickening, and distinctiveprotophloem elements are present in the leaf traces, but endabruptly near the junction regions of leaf traces to the meristele.Copyright1994, 1999 Academic Press Initial vascular differentiation, provascular tissue, pith mother cells, shoot apex, Matteuccia struthiopteris