Auxin–cytokinin interactions in the control of shoot branching

In many plant species, the intact main shoot apex grows predominantly and axillary bud outgrowth is inhibited. This phenomenon is called apical dominance, and has been analyzed for over 70 years. Decapitation of the shoot apex releases the axillary buds from their dormancy and they begin to grow out. Auxin derived from an intact shoot apex suppresses axillary bud outgrowth, whereas cytokinin induced by decapitation of the shoot apex stimulates axillary bud outgrowth. Here we describe the molecular mechanisms of the interactions between auxin and cytokinin in the control of shoot branching.     

The shoot apex of Podostemaceae: De novo structure or reduction of the conventional type?

The paper reviews and discusses various interpretations of the shoot apex of Podostemaceae with special reference to subfamily Podostemoideae. Main questions concern (1) the proposed absence of a shoot apical meristem (SAM) in apical “meristemless” shoot tips of Podostemoideae and, as the consequence, the endogenous inception of leaf-borne leaves and branches and (2) the predicted stem bifurcation below a “terminal” dithecous (double-sheathed) leaf positioned instead of a shoot apex, as it is reported for subfamily Podostemoideae. Does the “meristemless” shoot apex represent a true evolutionary novelty? Does the view of stem bifurcation represent a new ramification pattern with the consequence that the “classical root–shoot model” of angiosperms is not valid for Podostemaceae? Both interpretations do not conform to previous studies that are complemented here by new data on the SAM of Zeylanidium olivaceum and Thelethylax minutiflora (Podostemoideae). Although a SAM is difficult to observe in the vegetative shoots of many Podostemoideae, it becomes well visible when the shoot passes into the flowering stage approaching the conspicuous shoot apex of floriferous shoots. The arguments of the absence of a SAM in vegetative shoots are not convincing and the endogenous origin of “leaf-borne leaves” appears questionable. Consequently, the “meristemless” shoot apex cannot be considered as a structure having evolved de novo. In the less advanced subfamilies Tristichoideae and Weddellinoideae, the leaf primordia develop only from a few apical cells of the outer shoot layer. This allows the conclusion that the surface layer of the apex in these subfamilies corresponds to the horizontally spread single-layered apical meristem of subfamily Podostemoideae. Similarly, the view of shoot bifurcation does not conform to the diachsial–sympodial branching pattern occurring in the cymose inflorescences of many Podostemoideae. This fact contradicts the presence of a terminal leaf.     

Regulation of extent of vegetative development of the maize shoot meristem

In maize plants ( Zea mays L.), the extent of vegetative development in the shoot is precisely regulated such that the apical meristem produces a predictable number of leaves before converting to tassel development. In previous experiments using shoot apex culture, we showed that the developmental program that limits vegetative development in maize is not intrinsic to the shoot apical meristem. Rather, the meristem receives information from elsewhere in the plant and responds by either continuing leaf initiation or becoming determined for determinate growth and forming an inflorescence, the tassel. Here we examine leaf primordia as potential sources for that information using shoot apex culture. Our results show that the presence of the four to six youngest leaf primordia on the shoot apex is sufficient to provide such information. The ability to reset shoot development by meristem culture also allows us to examine the basis for expression of a specific phenotype at a particular developmental stage. We found that the mutation hcf106 , which is typically expressed only during seedling stages, is not re-expressed when the shoot morphogically has regained a juvenile phase.     

Signal peptide peptidases are expressed in the shoot apex of rice,localized to the endoplasmic reticulum

Signal peptide peptidase (SPP) is a multi-transmembrane aspartic proteinase involved in regulated intramembrane proteolysis, which is implicated in fundamental life processes such as immunological response, cell signaling, tissue differentiation, and embryogenesis. In this study, we identified two rice SPPs: OsSPP1 and OsSPP2. Green fluorescent protein-fused OsSPP1 and OsSPP2 were localized to the ER in cultured plant cells. In situ hybridization showed that OsSPPs were strongly expressed in vegetative shoot apex, young panicle, developing panicle, and the early developing florets. Undifferentiated cells, which have the potential to differentiate into all of the aerial parts of the plant are presented in the shoot apex. OsSPPs are located in both the undifferentiated cells, and the early differentiated cells at the shoot apex. These results suggest that rice SPPs have an important function in differentiation and development at the shoot apex. The expression of the shoot apex and ER localization is equal to dicot Arabidopsis thaliana, and will have common crucial roles in plant.     

Normal and Abnormal Development in the Arabidopsis Vegetative Shoot Apex

Vegetative development in the Arabidopsis shoot apex follows both sequential and repetitive steps. Early in development, the young vegetative meristem is flat and has a rectangular shape with bilateral symmetry. The first pair of leaf primordia is radially symmetrical and is initiated on opposite sides of the meristem. As development proceeds, the meristem changes first to a bilaterally symmetrical trapezoid and then to a radially symmetrical dome. Vegetative development from the domed meristem continues as leaves are initiated in a repetitive manner. Abnormal development of the vegetative shoot apex is described for a number of mutants. The mutants we describe fall into at least three classes: (1) lesions in the shoot apex that do not show an apparent alteration in the shoot apical meristem, (2) lesions in the apical meristem that also (directly or indirectly) alter leaf primordia, and (3) lesions in the apical meristem that alter meristem size and leaf number but not leaf morphology. These mutations provide tools both to genetically analyze vegetative development of the shoot apex and to learn how vegetative development influences floral development.     

Some Aspects of Ontogeny of the Shoot Apices of Solanum melongena L. and Capsicum annuum L.

The ontogeny of the vegetative shoot apices of two importantvegetable plants, Solanum melongena L. and Capsicum annuum L.is described. Each shoot apex is studied at different stagesof seed germination. The relationship between time of germinationand (i) area of vacuoles in the cell, (ii) total area of thecell, (iii) area of the nucleus in the cell, and (iv) ratioof area of vacuoles in the cell to the total area of the cellin each apex is examined. The differentiation of cytohistologicalzonation in both apices is distinct only after one or two leavesare initiated and developed. At a certain stage heterogeneityin staining in the peripheral region of the shoot apex of S.melongena is found. The zonation in both plants differentiatesgradually in histological and cytological features. Vacuolationincreases or decreases in all the cells of the shoot apex orin the cells of a particular region of the shoot apex at differentstages of its ontogeny.     

A Catastrophe Model for the Switch from Vegetative to Reproductive Growth in the Shoot Apex

A mathematical model is constructed to describe the morphopneticswitch that occurs when a vegetative plant apex becomes reproductive.The cusp equation from catastrophe theory is modified, and isused to relate primordial size at initiation to apex size. Theresulting equation may be viewed as an equation of state definingthe allowed organizational modes of the shoot apex. The modelsimulates the growth of the apex from the vetative stage toearly reproductive growth, and makes reasonable predictionsabout apex size and growth rate, primordial sizes, and the lengthsof the plastochron. flowering, mathematical model, catastrophe theory, shoot apex     

Ontogeny of the reproductive shoot apex ofClethra barbinervis,especially on the superficial view

The structure and the ontogenetic process of the reproductive shoot apex forming a terminal inflorescence ofClethra barbinervis were examined, especially concerning the superficial view of the apex. The system of contact parastichies is 2+3 in phyllotaxis in the vegetative phase, changing to 5+8 for bract arrangement in the reproductive phase. At the same time the size of the apex is conspicuously enlarged. The size of the foliage leaf primordia in the vegetative phase is larger than that of the bract primordia in the reproductive phase. The radial cell files, which are clear in the vegetative shoot apex, are not recognizable at least in the early stage of the reproductive phase. The author proposes a close correlation between the appearance of the radial cell files, as well as the construction of the apical sectors, and the sizes of the shoot apex and leaf primordia. It may be proposed also that the construction of the apical sectors is closely correlated with the phyllotaxis.     

Long-day induction of flowering in Lolium temulentum involves sequential increases in specific gibberellins at the shoot apex

One challenge for plant biology has been to identify floral stimuli at the shoot apex. Using sensitive and specific gas chromatography-mass spectrometry techniques, we have followed changes in gibberellins (GAs) at the shoot apex during long day (LD)-regulated induction of flowering in the grass Lolium temulentum. Two separate roles of GAs in flowering are indicated. First, within 8 h of an inductive LD, i.e. at the time of floral evocation, the GA(5) content of the shoot apex doubled to about 120 ng g(-1) dry weight. The concentration of applied GA(5) required for floral induction of excised apices (R.W. King, C. Blundell, L.T. Evans [1993] Aust J Plant Physiol 20: 337-348) was similar to that in the shoot apex. Leaf-applied [(2)H(4)] GA(5) was transported intact from the leaf to the shoot apex, flowering being proportional to the amount of GA(5) imported. Thus, GA(5) could be part of the LD stimulus for floral evocation of L. temulentum or, alternatively, its increase at the shoot apex could follow import of a primary floral stimulus. Later, during inflorescence differentiation and especially after exposure to additional LD, a second GA action was apparent. The content of GA(1) and GA(4) in the apex increased greatly, whereas GA(5) decreased by up to 75%. GA(4) applied during inflorescence differentiation strongly promoted flowering and stem elongation, whereas it was ineffective for earlier floral evocation although it caused stem growth at all times of application. Thus, we conclude that GA(1) and GA(4) are secondary, late-acting LD stimuli for inflorescence differentiation in L. temulentum.     

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.     

Developmental anatomy of the three-dimensional structure of the vegetative shoot apex

There have been many studies of the vegetative shoot apex, but the majority of such studies have been based on median longitudinal sections. Anatomical studies based on surface views of the apex are relatively few in number. Unfortunately, three-dimensional structures, based on surface and transverse-sectional views as well as longitudinal views, have scarcely been analyzed. However, Hara attempted such analyses (1961, 1962, 1971a, b, c, 1977). He identified radial files of cells in the peripheral meristem and recognized four or five apical sectors in the three-dimensional structure of the shoot apex. The size of the central zone and the sizes and relative positions of the sectors change rhythmically in each plastochron. Further studies on the three-dimensional structure of the shoot apex are needed if we are fully to understand the morphogenesis and histogenesis of vascular plants.     

The rate of cell division in the shoot apical meristem during photoperiodic induction and transition to flowering

Cell division contributing to longitudinal growth of the shoot apex was investigated inChenopodium rubrum in segments marked by the axils of leaf primordia. Plants treated with two short days (16h of darkness and 8h of light) were compared with two non-induced controls (cultivated in continuous light or treated by alternations of 8 h of darkness and 4 h of light for two days). During the short-day treatments the rate of cell division contributing to the longitudinal growth decreases in all segments of the shoot apex irrespective of whether the darkness was given in inductive or non-inductive photoperiods. The rate of cell division contributing to longitudinal growth increases in the upper internodes of the shoot apex after the termination of the photoperiodic treatment and transfer of the plants to continuous light. However, cell division remains inhibited in the lowest segment of the shoot apex. This inhibition in the differentiating parts of the shoot apical meristem is a direct consequence of photoperiodic induction. It is supposed that this inhibition is related to evocation similarly as the well-known phenomenon of stimulation of cell division in the apical dome.     

Development of the shoot apex ofChenopodium rubrum L. after photoperiodic induction in the cotyledon stage

Development of the shoot apex up to floral differentiation was investigated in the short-day plantChenopodium rubrum. The changes occurring in the apex from energence until full opening of the cotyledons (Figs 1–4), development during photoperiodic induction (Figs. 5–8), as well as the resulting floral differentiation (Figs. 9–10) are described. It was aimed at excluding the influence of plastochron changes on the interpretation of ontogeny of the shoot apex. For that reason two planes of longitudinal sections and two plastochron stages were compared. In young plants zonation does not become fully evident prior to floral differentiation. The anatomical structure of the shoot apex does not change substantially during the first two inductive cycles which proved to be obligatory under the given experimental conditions. The changes occurring during two further inductive cycles correspond to the total activation of the meristems as manifested by the growth and branching of the apex preceeding floral differentiation proper.     

Growth and Development of the Banana Plant: II. The Transition from the Vegetative to the Floral Shoot in Musa acuminata cv. Gros Michel

The changes that occur in the shoot apex of the banana, as itpasses from the vegetative to the flowering stage, are described.The crucial events occur well before floral primordia are evident,and they require a redistribution of activity in the variousgrowing regions. The vegetative shoot apex is in a central depressionin the rhizome; there is virtually no internodal growth in theaxis, the most active growth is in the leaf bases; vegetativebuds do not form in the leaf axils but only appear adventitiouslyfar from the tip of the shoot. With the onset of flowering thisis changed; growth in the axis itself, previously suppressed,occurs and flower buds arise as primordia in the axils of subtendingbracts. The bracts do not show the market growth in their baseswhich is so characteristic of leaves. Thus, the shoot apex risesto the level of the rhizome and then above it; as it does so,its tip changes in shape from a broad flattened some to a pointedcone. At the transitional stage, more activity occurs in thecells of the mantle, or tunica, which now consists of 3 to 4layers over the central dome. Below, in the central or mothercell zone of the corpus, which was quiescent in the vegetativeshoot, the cells spring into greater activity, becoming moreprotoplasmic and stain more deeply. Directly below this regionin the rib meristem, cells show transverse divisions. Bractprimordia occur high on the flanks of the apex, and, thoughthey originate in the manner of leaves, their subsequent growthis different. Flower primordia occur even in the axils of bractsclose to the shoot tip. Thus, the problem now is to designatethe source, nature, and mode of action of the stimuli whichinitiate and control this quite different distribution of growthin the floral, as contrasted with the vegetative, shoot. Thesignificance of the previously more quiescent central, or mothercell zone, of the apex as the source of such stimuli, is stressed.Thus, flowering first requires that the limiting controls whichapply to the vegetative shoot be released, and, secondly, thatthe apex of the shoot, rather than the leaf base, becomes themain centre of growth and development.     

Rates of cell division in the young prefloral shoot apex ofChrysanthemum segetum L.

Summary The rate of cell division was determined by the colchicine induced metaphase-accumulation technique in the young prefloral shoot apex of the quantitative long-day plantChrysanthemum segetum L. growing under conditions favourable to flowering (16-hour photoperiod; 124Em^–2s^–1; 22 °C). Cell cycle duration was evaluated in relation to the location of the cells in the intact apex. The cell cycle durations were 53.5 hours, 47.4 hours, and 97.7 hours in the axial, lateral and subapical central cells respectively. Compared with previous results, these data give evidence of the major role played by the early increase in cell division rate of axial cells in the new pattern of the prefloral shoot apex at its initial stage of development. By comparison with the vegetative shoot apex, the cell cycle duration was preferentially shortened in the axial zone; it was only slightly altered in the lateral zone while it was lengthened in the vacuolating subapical central cells. In the three zones within the prefloral shoot apex, the duration of mitosis was constant (3.2 to 3.3 hours) and the same as in the vegetative shoot apex.     

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     

Hormone-mediated regulative action of the sunflower shoot apex on growth and cation level in the cotyledons: an additional manifestation of apical control

Decapitation of the shoot apex of seedlings of Helianthus annuus Lin. above the cotyledonary node brought about promotion of growth in the cotyledons. Potassium level in the cotyledons of decapitated plants was higher, and that of sodium lower, than in those of intact plants. IAA applied to the cut stem surface imitated the effects of the apex. Application of kinetin to the cotyledons antagonized the apex or the auxin in their influence on growth and cation level. Labeled IAA applied to the cut stem surface penetrated into the cotyledons in significant amounts. It was concluded that growth and monovalent cation level in the cotyledons are regulated by auxin released from the shoot apex and that at least part of the auxin effect is exerted directly in the cotyledons. A function of the apex as a sink for cytokinins may also be involved in the control mechanism.     

Interrelationships of Cotyledonary Shoots in Pisum sativum

Unequal two shoot systems of tall and dwarf varieties of Pisum sativum L. ev, Improved Pilot and Meteor, respectively, were used in experiments where manipulative treatments involving the removal of one or other or both of the shoot apices were carried out. These were combined with GA treatments. Twenty four hours after treatment, ¹⁴CO₂ was applied to the lowest expanded leaf on the larger shoot, and the distribution of radioactivity was determined after a further 24 hours. Removal ot the apex of the larger shoot reduced the level of translocation from the treated leaf, the effect being enhanced by the additional removal of the other apex. This (Heel was more marked in the tall than in the dwarf variety. In untreated control plants of both varieties there was very little transfer ot labelled material from the larger (dominant) shoot to the smaller (weaker) shoot. This pattern was not affected by the removal of the apex from the weaker shoot. In Improved Pilot, removal of the apex from the stronger shoot led to considerable transfer of radioactive material to the weaker shoot, GA treatment having little effect. However, in Meteor, transfer of radioactivity from the stronger to the weaker shoot. after removal of the apex from the stronger shoot, only occurred after the application of GA. Removal of both shoot apices again resulted in low levels of transfer of labelled material to the weaker shoot in both varieties. These results are consistent with the hypothesis that there is competition between the two shoots for photosynthates produced by the leaves and that treatments reducing the competitive ability of one shoot, reduce the level of nutrients received by that shoot and can result in transfer of materials from it to the other shoot. Continuation of such a situation will result in increased inequality and perhaps ultimately in the death of the weaker shoot.