Strigolactone may interact with gibberellin to control apical dominance in pea (Pisum sativum)

The role of strigolactones as plant growth regulators has been demonstrated through research on biosynthesis and signaling mutant plants and through the use of GR24, a synthetic analog of this class of molecules. Strigolactone mutants show a bushy phenotype and GR24 application inhibits the growth of axillary buds in these mutants, thus restoring the phenotype of a wild plant, which is characterized by a stronger apical dominance. In this work, we tested the effectiveness of this chemical on pea (Pisum sativum) plants following apex removal, which disrupts apical dominance and leads to axillary bud outgrowth. Moreover, we searched for relationships between the response to the strigolactone and gibberellin metabolism by applying GR24 to both climbing and dwarf peas, the latters being mutants for gibberellin biosynthesis. The results suggest that the endogenous level of the bioactive gibberellin GA₁ might modulate the response of decapitated pea plants to GR24, by changing bud sensitivity to the applied strigolactone.     

Apical Dominanace in Xanthium strumarium: A DISCUSSION IN RELATION TO CURRENT HYPOTHESES OF CORRELATIVE INHIBITION

The influence of the spectral distribution of illumination onthe gibberellin, cytokinin, auxin, and abscisic acid levelsand the correlation with the degree of branching in Xanthiumstrumarium is presented and discussed. Gibberellins do not appearto play a major role in apical dominance but may be importantfor bud extension following the initial release from dominance.The cytokinin level was much higher in inhibited buds than inreleased buds. It is suggested that the cytokinins present wereprobably not able to participate in bud growth because of anauxin-induced accumulation of abscisic acid in the buds themselves.The concentration of abscisic acid as measured by bioassay andgas-liquid chromatography was between 50 and 250 times thatoccurring in all other plants parts examined. This level felldramatically following release from apical dominance by decapitation.The results are discussed in relation to current hypothesesof apical dominance.     

Concepts and terminology of apical dominance

Apical dominance is the control exerted by the shoot apex over lateral bud outgrowth. The concepts and terminology associated with apical dominance as used by various plant scientists sometimes differ, which may lead to significant misconceptions. Apical dominance and its release may be divided into four developmental stages: (I) lateral bud formation, (II) imposition of inhibition on lateral bud growth, (III) release of apical dominance following decapitation, and (IV) branch shoot development. Particular emphasis is given to discriminating between Stage III, which is accompanied by initial bud outgrowth during the first few hours of release and may be promoted by cytokinin and inhibited by auxin, and Stage IV, which is accompanied by subsequent bud outgrowth occurring days or weeks after decapitation and which may be promoted by auxin and gibberellin. The importance of not interpreting data measured in Stage IV on the basis of conditions and processes occurring in Stage III is discussed as well as the correlation between degree of branching and endogenous auxin content, branching mutants, the quantification of apical dominance in various species (including Arabidopsis ), and apical control in trees.     

Dormancy and Flowering Are Regulated by the Reciprocal Interaction Between Cytokinin and Gibberellin in <Emphasis Type="Italic">Zantedeschia</Emphasis>

Floral productivity of Zantedeschia is dependent on the conversion rate of buds to shoots, which is controlled by varying intensities of para- (apical dominance), endo- (dormancy), and ecodormancy. We present evidence of cross-talk between cytokinin and gibberellin in their complementary roles to alleviate bud dormancy and enhance flowering in a perennial geophyte. We assessed the impact of cytokinin and gibberellin, applied alone and in sequential combinations, on bud fate during three phases along the ontogeny of growth, which coincide with the progressive transition of buds from apical dominance to dormancy. Given that cytokinin can stimulate branching and gibberellin can induce flowering in Zantedeschia, we measured these phenotypic responses as parameters of bud commitment. The efficacy of cytokinin alone to stimulate branching declined with the transition to dormancy (phase 1 = 3.8 ± 0.2 shoots; phase 3 = 1.0 ± 0.3 shoots). To sustain branching during this transition, a sequential application of gibberellin was necessary. Gibberellin alone failed to stimulate branching. The efficacy of gibberellin alone to stimulate flowering diminished with the transition to dormancy. Any flowering during this transition occurred only after the sequential application of cytokinin. Cytokinin alone failed to stimulate flowering. Alleviating bud dormancy and enhancing flowering in Zantedeschia, achieved by the reciprocal cross-talk between cytokinin and gibberellin, contributes to the pool of evidence drawing common mechanisms between dormancy and flowering and may have commercial implications.     

Effects of Gibberellin on Shoot Development in the dgt Mutant of Tomato

The morphological effects of gibberellin A₃ (GA₃) on the dgtmutant of tomato were investigated. The mutant effectively showedthe normal range of responses, including a promotion of stemlength due to an increased number of longer internodes, a dramaticincrease in apical dominance, and effects on leaf shape andcolour. In the case of stem elongation, the quantitative responseof the mutant was greater than normal. The morphological abnormalitiescharacteristic of the dgt mutant, such as horizontal growth,a thin stem and hyponastic leaves, were not normalized by GA₃. It is concluded that the demonstrated lack of response to auxinof the dgt mutant does not impair its gibberellin responses. Tomato, gibberellin, auxin, mutant, shoot development     

Hormone physiology of pea mutants prevented from flowering by mutations gi or veg1

The veg1 ( vegetative ) mutant in pea ( Pisum sativum L.) does not flower under any circumstances and gi ( gigas ) mutants remain vegetative under certain conditions. gi plants are deficient in production of floral stimulus, whereas veg1 plants lack a response to floral stimulus. During long days in particular, these non-flowering mutant plants eventually enter a stable compact phase characterised by a large reduction in internode length, small leaves and growth of lateral shoots from the upper-stem (aerial) nodes. The first-order laterals in turn produce second-order laterals and so on in a reiterative pattern. The apical bud is reduced in size but continues active growth. Endogenous hormone measurements and gibberellin application studies with gi-1 , gi-2 and veg1 plants indicate that a reduction in gibberellin and perhaps indole-3-acetic acid level may account, at least partially, for the compact aerial shoot phenotype. In the gi-1 mutant, the compact phenotype is rescued by transfer from a 24- to an 8-h photoperiod. We propose that in plants where flowering is prevented by a lack of floral stimulus or an inability to respond, the large reduction in photoperiod gene activity during long days may lead to a reduction in apical sink strength that is manifest in an altered hormone profile and weak apical dominance.     

Flowering in tobacco needs gibberellins but is not promoted by the levels of active GA1 and GA4 in the apical shoot

Flowering of Nicotiana tabacum cv Xhanti depends on gibberellins because gibberellin-deficient plants, due to overexpression of a gibberellin 2-oxidase gene (35S:NoGA2ox3) or to treatment with the gibberellin biosynthesis inhibitor paclobutrazol, flowered later than wild type. These plants also showed inhibition of the expression of molecular markers related to floral transition (NtMADS-4 and NtMADS-11). To investigate further the role of gibberellin in flowering, we quantified its content in tobacco plants during development. We found a progressive reduction in the levels of GA1 and GA4 in the apical shoot during vegetative growth, reaching very low levels at floral transition and beyond. This excludes these two gibberellins as flowering-promoting factors in the apex. The evolution of active gibberellin content in apical shoots agrees with the expression patterns of gibberellin metabolism genes: two encoding gibberellin 20-oxidases (NtGA20ox1 = Ntc12, NtGA20ox2 = Ntc16), one encoding a gibberellin 3-oxidase (NtGA3ox1 = Nty) and one encoding a gibberellin 2-oxidase (NtGA2ox1), suggesting that active gibberellins are locally synthesized. In young apical leaves, GA1 and GA4 content and the expression of gibberellin metabolism genes were rather constant. Our results support that floral transition in tobacco, in contrast to that in Arabidopsis, is not regulated by the levels of GA1 and GA4 in apical shoots, although reaching a threshold in gibberellin levels may be necessary to allow meristem competence for flowering.     

Dormancy regulation by morphactin in aerial tubers of Begonia evansiana

Summary The sprouting of aerial tubers of Begonia evansiana was promoted by treatment with morphactin. As with cytokinins, the promotion of sprouting occurred in both the immature and mature tubers. Unlike cytokinins, however, morphactin did not stimulate tuber enlargement. The sprout-inhibiting action of applied gibberellin (GA) was overcome by morphactin. The possible mechanism of the inhibitory action of GA is diseussed in relation to apical dominance.     

Die Bedeutung der Attraktion von Gibberellin durch Indolylessigsäure bei der apikalen Dominanz

Zusammenfassung Aus den Versuchen mit 20 cm langen Segmenten von etiolierten Erbsenepikotylen, die mit 0,5% Gibberellinpaste und außerdem entweder in der Nähe des apikalen oder des basalen Pols mit 0,5% Indolylessigsäurepaste ringförmig angestrichen wurden, ergab sich nach 24 Std in allen Fällen ein bedeutend stärkerer Gibberellintransport zu der mit IES-Paste behandelten Region der Epikotylsegmente, gleichgültig ob es sich um den apikalen oder den basalen Pol handelte. Da sich die Wurzel an der Gibberellinbiosynthese wesentlich beteiligt, hängt der akropetale Gibberellintransport in der intakten Pflanze auch wesentlich damit zusammen, daß dieser Regulator durch das im Apikalmeristem aktivierte Auxin angezogen wird. Auf diese Weise mag dann sowohl das stärkere Wachstum des Apikalteiles der Pflanze als auch die apikale Dominanz zustande kommen.

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Attraction of gibberellin by indoleacetic acid as a factor in apical dominance

Summary Experiments with 20 cm long segments of etiolated pea epicotyls treated in the middle with 0,5% gibberellin paste (GA₃) in the form of a ring and also with a 0,5% indoleacetic acid (IAA) paste either near the apical or near the basal pole showed that in all cases after 24 hours there was a stronger transport of GA₃ to the IAA treated pole without regard to the polarity. Because the root participates significantly in the biosynthesis of gibberellin, the acropetal transport of gibberellin from this organ in an intact plant is probably connected with the attractive force of auxin activated in the shoot apex. This factor may be responsible for the more intensive growth of the apical part of shoot and therewith also for the apical dominance.

     

Relation of leaf deformities induced by 2,3,5-triiodobenzoic acid to growth correlations inBryophyllum rosei

Deformities of leaves induced by 2,3,5-triiodobenzoic acid (TIBA) may be helpful in elucidating certain plant growth correlations. The different behaviour of long and short shoots as regards apical dominance may be tested with this substance. Shoots growing out through ring fasciations first form leaves lacking the teeth and marginal primordia corresponding to bud scales. Are fasciations facilitate correlative studies on anisophylly. Sylleptic branching is cleared by additional fasciations provoked by the cytokinin supply. The effect of gibberellin which decreases these deformities corresponds to the correlative influence of roots. The TIBA induced modification of aerial roots on internodes treated with naphthalenacetic acid reveals the importance of polarity and periodicity in the shoot development.     

The New Rga Locus Encodes a Negative Regulator of Gibberellin Response in Arabidopsis Thaliana

We have identified a new locus involved in gibberellin (GA) signal transduction by screening for suppressors of the Arabidopsis thaliana GA biosynthetic mutant ga1-3. The locus is named RGA for repressor of ga1-3. Based on the recessive phenotype of the digenic rga/ga1-3 mutant, the wild-type gene product of RGA is probably a negative regulator of GA responses. Our screen for suppressors of ga1-3 identified 17 mutant alleles of RGA as well as 10 new mutant alleles at the previously identified SPY locus. The digenic (double homozygous) rga/ga1-3 mutants are able to partially repress several defects of ga1-3 including stem growth, leaf abaxial trichome initiation, flowering time, and apical dominance. The phenotype of the trigenic mutant (triple homozygous) rga/spy/ga1-3 shows that rga and spy have additive effects regulating flowering time, abaxial leaf trichome initiation and apical dominance. This trigenic mutant is similar to wild type with respect to each of these developmental events. Because rga/spy/ga1-3 is almost insensitive to GA for hypocotyl growth and its bolting stem is taller than the wild-type plant, the combined effects of the rga and spy mutations appear to allow GA-independent stem growth. Our studies indicate that RGA lies on a separate branch of the GA signal transduction pathway from SPY, which leads us to propose a modified model of the GA response pathway.     

Isolation and identification of lateral bud growth inhibitor,indole-3-aldehyde,involved in apical dominance of pea seedlings

A lateral bud growth inhibitor was isolated from etiolated pea seedlings and identified as indole-3-aldehyde. The indole-3-aldehyde content was significantly higher in the diffusates from explants with apical bud and indole-3-acetic acid treated decapitated explants, in which apical dominance is maintained, than in those from decapitated ones releasing apical dominance. When the indole-3-aldehyde was applied to the cut surface of etiolated decapitated plants or directly to the lateral buds, it inhibited outgrowth of the latter. These results suggest that indole-3-aldehyde plays an important role as a lateral bud growth inhibitor in apical dominance of pea seedlings.     

Organs of gibberellin synthesis in light-grown sunflower plants

The sites of gibberellin (GA) synthesis in light grown sunflower plants were studied. The results of organ excision and the exogenous application of indole acetic acid and gibberellic acid indicated that gibberellin synthesis occurred in the young leaves of the apical bud. This was substantiated using a combination of diffusion and extraction techniques. Diffusion of sunflower apical buds on agar for 20 hours revealed a level of gibberellin greater than that obtained by solvent extraction of a similar number of apices, indicating that synthesis of gibberellin was occurring in those apices during the diffusion period. The gibberellin level of apices extracted following a 20 hour diffusion period was the same as that obtained from buds extracted immediately following excision from the plant, again suggesting that apical buds are sites of gibberellin synthesis. A similar experiment was conducted with young internodal sections, the results indicating that they were not sites of gibberellin synthesis.     

A Developmental Pattern of Flowering in Colored <Emphasis Type="Italic">Zantedeschia</Emphasis> spp.: Effects of Bud Position and Gibberellin

The restricted flowering of colored cultivars ofZantedeschia is a consequence of developmental constraints imposed by apical dominance of the primary bud on secondary buds in the tuber, and by the sympodial growth of individual shoots. GA₃ enhances flowering inZantedeschia by increasing the number of flowering shoots per tuber and inflorescences per shoot. The effects of gibberellin on the pattern of flowering and on the developmental fate of differentiated inflorescences along the tuber axis and individual shoot axes were studied in GA₃ and Uniconazole-treated tubers. Inflorescence primordia and fully developed (emerged) floral stems produced during tuber storage and the plant growth period were recorded. Days to flowering, percent of flowering shoots and floral stem length decreased basipetally along the shoot and tuber axes. GA₃ prolonged the flowering period and increased both the number of flowering shoots per tuber and the differentiated inflorescences per shoot. Activated buds were GA₃ responsive regardless of meristem size or age. Uniconazole did not inhibit inflorescence differentiation but inhibited floral stem elongation. The results suggest that GA₃ has a dual action in the flowering process: induction of inflorescence differentiation and promotion of floral stem elongation. The flowering pattern could be a result of a gradient in the distribution of endogenous factors involved in inflorescence differentialtion (possibly GAs) and in floral stem growth. This gradient along the tuber and shoot axes is probably controlled by apical dominance of the primary bud. Online publication: 7 April 2005     

Effects of Orientation and Root Position on Apical Dominance in a Tropical Woody Plant

Stem cuttings of cassava (Manihot esculenta Crantz), rootedat one or both ends, were grown at a range of orientations fromthe vertical. Basally rooted cuttings showed strong apical dominanceonly in upright or near-upright positions. Basal shoots generallydominated when the stem was horizontal, while completely invertedstems exhibited weak apical dominance or no dominance at all.Cuttings rooted at the apical end were little affected by changedorientation, apical dominance being present throughout. Effectsof each root system could be detected in cuttings rooted atboth ends. The results are discussed in relation to currentthinking on the mechanism of apical dominance, gravimorphiceffects in woody plants, and the role of the ‘root-factor’in the control of shoot growth.     

Endogenous gibberellin transport and biosynthesis in relation to geotropic induction of excised sunflower shoot-tips

Summary Surgical experiments on Helianthus annuus and Phaseolus multiflorus seedlings involving the application of auxin and gibberellin to decapitated plants, suggested that internode extension growth occurs under the controlling influence of apically synthesised gibberellin rather than auxin. Studies were made of diffusible gibberellins from sunflower apical buds in relation to geotropic stimulation. Approximately ten times as much gibberellin was obtained from lower than from upper tissues of horizontal shoot-tips, whereas approximately equal quantities were obtained from the two halves of upright tips. Evidence was obtained suggesting both lateral transport of gibberellin in the young internode, and also enhanced gibberellin synthesis in buds maintained in a horizontal position during the collection of diffusible gibberellins into agar. The results are discussed in relation to current concepts of the role of auxin in geotropism.     

Phenotypic Suppression of the Gibberellin-Insensitive Mutant (gai) of Arabidopsis

The semidominant gibberellin-insensitive (gai) mutant of Arabidopsis thaliana shows impairment in multiple responses to the plant hormone gibberellin A3, which include effects on seed germination, stem elongation, apical dominance, and rapid flowering in short days. Results presented here show that the gai mutation also interferes with development of fertile flowers in continuous light. Mu-tagenesis of the gai mutant resulted in recovery of 17 independent mutants in which the gibberellin-insensitive phenotype is partially or completely suppressed. Sixteen of the suppressor mutations act semidominantly to restore gibberellin responsiveness. One representative of this class, the gar1 mutation, could not be genetically separated from the gai locus and is proposed to cause inactivation of the gai gene. The exceptional gar2 mutation partially suppresses the gai phenotype, is completely dominant, and is not linked to the gai locus. The gar2 mutation may define a new gene involved in gibberellin signaling. A recessive allele of the spindly (SPY) locus, spy-5, was also found to partially suppress the gai mutant phenotype.     

Some Observations on Factors Controlling Apical Dominance in the 'Rogue' Tomato

The rogue tomato differs from the normal plant in that it exhibitsa lesser degree of apical dominance. Grafting techniques andmeasurements of the endogenous levels of growth substances inthe two types have been used in order to establish whether thisdifference is due to an altered hormonal balance in the roguetype. The results suggest that root-produced cytokinins play no rolein the control of apical dominance in the tomato plant, andthat lateral bud out-growth is influenced by a balance betweenapically-produced auxin, abscisic acid produced at the sitesof bud development and cytokinins synthesized within the budsthemselves. Lycopersicon esculentum L., tomato, apical dominance, abscisic acid, auxins, cytokinins, growth regulation     

Effect of gibberellin and kinetin on the regeneration ability ofFucus vesiculosus L.

The influence of gibberellic acid (GA³) and kinetin (6-furfurylaminopurine) on the regeneration ability of the basal and apical thallus fragments ofFucus vesiculosus L. was examined. The naturally occurring gibberellin and kinetin-like substances in these thallus fragments were also studied. It was found that exogenously applied GA³ markedly increased the number of adventitious branches formed on the cut surface of the thallus fragments taken from the apical parts of plants. The concentration of 0.001 mg GA³ I-1 proved to be the most effective. The growth promoting effect of GA³ was increased by simultaneous action with kinetin. In experiments in which the fragments of the basal parts of the thallus were treated with GA³, as a rule a slight growth inhibition was observed. The growth responses of the investigated plant tissues to gibberellin and kinetin varied according to season. Usually their susceptibility to the applied plant hormones was greater in spring than is summer. The shifts in growth reaction were related to the seasonal changes in the content of endogenous gibberellin and kinetin-like substances in the investigated parts of the thallus. It is suggested that growth regulators of the gibberellin and cytokinin type are involved in the regeneration processes inFucus.