Is auxin the repressor signal of branch growth in apical control?

"Apical control" is the repression of branch growth by a higher dominating branch or shoot. There has been some confusion in the literature concerning the meaning and causal mechanisms of this correlative phenomenon with those of "apical dominance," which term is often used in a strict sense to connote the repression of the initiation of axillary bud outgrowth by an active shoot apex. Although the term "apical control" is most commonly employed with respect to woody species, this phenomenon also widely occurs in herbaceous plants. Because of the strong evidence for a role of auxin as a repressor signal in apical dominance and partly because of this lack of distinction in terminology, a similar role for auxin in apical control is often assumed in spite of the obvious acropetal auxin transport difficulty and the lack of direct evidence for the acropetal transport of any inhibitor influence. In the present study with the herbaceous Ipomoea nil, it has been clearly demonstrated that while exogenous auxin (1% NAA) strongly restores apical dominance in the Thimann-Skoog experiment, auxin treatments to decapitated dominant shoots do not, in any observable way, restore apical control in lower dominated branches. Hence, in this fast-growing species, the hypothesis for the role of auxin as a repressor signal for apical control is not supported.     

Effect of gravity on apical dominance in Pharbitis nil.

When the upper part of main shoot of morning glory (Pharbitis nil) is gently bent down, lateral bud on the bending region is released from apical dominance and starts to elongate. But, clinorotating the bending shoots prevents the release of the lateral bud from apical dominance. These results suggest that gravity affects apical dominance in morning glory. Here we verified the gravity-regulated apical dominance by using a weeping morning glory defective in gravitropic response due to abnormal differentiation of endodermis. That is, bending main shoot of the weeping morning glory hardly caused the lateral bud to elongate. In addition, decapitation of apical bud released the lateral bud from apical dominance, and exogenous auxin applied to the cut surface of the decapitated stem was inhibitory to the outgrowth of the lateral bud in the wild type. However, the effect of auxin was much less in the weeping morning glory. Thus, apical dominance of the weeping morning glory was weaker and less influenced by gravity than that of the wild type, which could occur due to abnormal differentiation of endodermis required for graviperception.     

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.     

The effects of 1-naphthaleneacetic acid and N6-benzyladenine on the growth of Cymbidium forrestii rhizomes in vitro

An Asiatic orchid, Cymbidium forrestii, was clonally propagated using seed-derived rhizomes as explants. The rhizomes were cultured and proliferated on Murashige and Skoog medium supplemented with various growth substances. Auxins stimulated rhizome growth by increasing branching and fresh weight of the explant, with 1-naphthaleneacetic acid (NAA) being the most effective auxin. All auxins tested suppressed normal shoot formation. The apical meristem of the rhizome reacted to exogenously applied auxin by reducing the cytoplasmic zone of the apical meristem and causing meristem derivatives to rapidly differentiate into vacuolated parenchyma cells. Leaf formation and development was retarded in the presence of auxin. Cytokinins generally reduced rhizome growth and the number of branches, but benzyladenine (BA) can induce shoot formation in vitro. BA induced the cytoplasmic zone of the apical meristem to enlarge and enhanced leaf development. A 5% (w/v) sucrose concentration was most effective in shoot induction when combined with 5 mg1^-1 BA. Activated charcoal promoted rhizome growth; however, shoot formation was inhibited.     

A preliminary investigation of the role of auxin and cytokinin in sylleptic branching of three hybrid poplar clones exhibiting contrasting degrees of sylleptic branching

Sylleptic branches grow out from lateral buds during the same growing season in which the buds are formed. This type of branching is present in poplar and in many tropical species. It results in the production of more branches, more leaves and expanded photosynthetic capacity and is thought to assist in increasing the overall growth and biomass of the tree at a young age. However, very little is known about the physiology of sylleptic branching in poplar, which is an extremely important source of fibre and fuel. In the present study of three hybrid poplar clones (11-11, 47-174 and 49-177) of Populus trichocarpa x P. deltoides exhibiting contrasting degrees of sylleptic branching, an analysis was carried out on parent shoot elongation and sylleptic branching, together with a preliminary comparison of the parent shoots' sensitivity to auxin (naphthaleneacetic acid) as a repressor of lateral bud outgrowth, and cytokinin (benzyladenine) as a promoter. Suggestive evidence was found for an inverse correlation between parent shoot sensitivity to auxin and the degree of sylleptic branching, as well as a partially positive correlation with respect to sensitivity to cytokinin. The present data are consistent with the hypothesis that auxin and cytokinin may play repressive and promotive roles, respectively, in the sylleptic branching of hybrid poplar.     

A reappraisal of the role of abscisic acid and its interaction with auxin in apical dominance

BACKGROUND AND AIMS: Evidence from pea rms1, Arabidopsis max4 and petunia dad1 mutant studies suggest an unidentified carotenoid-derived/plastid-produced branching inhibitor which moves acropetally from the roots to the shoots and interacts with auxin in the control of apical dominance. Since the plant hormone, abscisic acid (ABA), known to inhibit some growth processes, is also carotenoid derived/plastid produced, and because there has been indirect evidence for its involvement with branching, a re-examination of the role of ABA in apical dominance is timely. Even though it has been determined that ABA probably is not the second messenger for auxin in apical dominance and is not the above-mentioned unidentified branching inhibitor, the similarity of their derivation suggests possible relationships and/or interactions. METHODS: The classic Thimann-Skoog auxin replacement test for apical dominance with auxin [0.5 % naphthalene acetic acid (NAA)] applied both apically and basally was combined in similar treatments with 1 % ABA in Ipomoea nil (Japanese Morning Glory), Solanum lycopersicum (Better Boy tomato) and Helianthus annuus (Mammoth Grey-striped Sunflower). KEY RESULTS: Auxin, apically applied to the cut stem surface of decapitated shoots, strongly restored apical dominance in all three species, whereas the similar treatment with ABA did not. However, when ABA was applied basally, i.e. below the lateral bud of interest, there was a significant moderate repression of its outgrowth in Ipomoea and Solanum. There was also some additive repression when apical auxin and basal ABA treatments were combined in Ipomoea. CONCLUSION: The finding that basally applied ABA is able partially to restore apical dominance via acropetal transport up the shoot suggests possible interactions between ABA, auxin and the unidentified carotenoid-derived branching inhibitor that justify further investigation.     

The role of AUX1 gene and auxin content to the branching phenotype of Kenaf (Hibiscus cannabinus L.)

The objectives of this research were to identify auxin gene, AUX1, and to determine the plant auxin content and their role in conferring branching on Kenaf. PCR analysis using AUX1 primer capable to amplify the DNA of non branching (KR11) and branching kenaf mutant, resulting in 800 bp PCR product. The sequence of the PCR product showed high degree of homology with the sequence of AUX1 gene of other plants in the NCBI GenBank database, confirming kenaf possession of the gene AUX1. However, some variation on the DNA sequence was found between branching and non branching phenotype indicated allele differences of the same gene which were responsible for the variation in the type of branching. Identification of auxin content in the roots, apical shoot, and axillary branches using spectrophotometry method showed that the branching plant has higher auxin content in the apical shoot compared to the content in the branches. This indicate that AUX1 controls the formation of branches by controlling either the content of auxin in the apical shoot and branches, or the ratio of auxin content in the shoot and branches.     

CONTROL OF SHOOT-RHIZOME DIMORPHISM IN THE WOODY MONOCOTYLEDON,CORDYLINE (AGAVACEAE)

In Cordyline terminalis negatively geotropic leafy shoots and positively geotropic rhizomes develop from single axillary buds on either shoots or rhizomes. All axillary buds have similar morphogenetic potential when released from apical dominance. Experiments in which the orientation of the apex is changed, organs removed, or growth regulators applied indicate that after a rhizome is initiated, it is maintained as a rhizome by auxin originating in the leafy shoot. When auxin levels are lowered by changes in the orientation of the axis or shoot removal, the rhizome apex becomes a shoot apex, which appears to be the stable state of the actively growing apex. Benzyl adenine when applied exogenously to the apex or lateral buds has the same effect as lowering the auxin level. Gibberellic acid has no effect on the apex or lateral buds. High levels of exogenous naphthaleneacetic acid cause bud release and development of rhizomes from previously inhibited axillary buds of the shoot. However, it was not possible to convert a shoot apex into a rhizome apex by auxin treatment. It is suggested that the release of buds on the lower side of horizontal branches and of buds directly above a stem girdle is caused by high auxin levels on the lower side or distal to the girdle. The experimental results are discussed in relation to naturally occurring shoot-rhizome dimorphism.     

In vitro phyllomorphic regeneration of shoot buds and shoots in Picea abies

Needles (10–15 mm) of frost-hardened 20–22-week-old (physiological age equivalent to 1 year) plants of Picea abies L. excised just after flushing, were induced to form adventitious shoot buds and shoots on media supplemented with BAP (6-benzylaminopurine) and NAA (1-napthaleneacetic acid). The addition of nanomolar concentrations (0.5–50) of NAA combined with 1–10 μM BAP considerably stimulated formation of pseudobulbils on the basal to mid-part of the needle axis, as well as their subsequent development into shoot buds and shoots. On a medium containing 10 μM BAP, pseudobulbils that formed at the needle base did not develop further, but became necrotic and died with the omission of NAA. With 5 μM BAP + 50 nM NAA the initial phase of development was slow, but later showed good response and up to 22% of the needles produced shoot buds. Two to three shoots per needle could be excised and subcultured individually onto fresh media. It is concluded that the level of endogenous auxin decreases progressively from the needle's base to its tip, so that that concentration of exogenous auxin (50 nM NAA) which promotes pseudobulbil and shoot-bud formation part-way along the needle axis, simultaneously inhibits their induction at the needle base.     

Inhibition of Auxin Movement from the Shoot into the Root Inhibits Lateral Root Development in Arabidopsis

In roots two distinct polar movements of auxin have been reported that may control different developmental and growth events. To test the hypothesis that auxin derived from the shoot and transported toward the root controls lateral root development, the two polarities of auxin transport were uncoupled in Arabidopsis. Local application of the auxin-transport inhibitor naphthylphthalamic acid (NPA) at the root-shoot junction decreased the number and density of lateral roots and reduced the free indoleacetic acid (IAA) levels in the root and [³H]IAA transport into the root. Application of NPA to the basal half of or at several positions along the root only reduced lateral root density in regions that were in contact with NPA or in regions apical to the site of application. Lateral root development was restored by application of IAA apical to NPA application. Lateral root development in Arabidopsis roots was also inhibited by excision of the shoot or dark growth and this inhibition was reversible by IAA. Together, these results are consistent with auxin transport from the shoot into the root controlling lateral root development.     

Overcompensation and adaptive plasticity of apical dominance in Erysimum strictum (Brassicaceae) in response to simulated browsing and resource availability

In the cases where overcompensation has been observed in monocarpic herbs, overcompensation is associated with an apically dominant shoot architecture of intact plants, increased lateral branching following herbivory, and increased reproductive success as a consequence of damage. The compensatory continuum hypothesis expects overcompensation to be more prevalent in resource rich environments compared to poor environments. This is paradoxical since in resource rich conditions the intact plants should branch most vigorously and hence any further increase in branch number should lead to lower seed yield. An explanation could be that apical dominance is rather insensitive to changes in resource availability, and that overcompensation is possible in conditions where plants experience meristem limitation (due to apical dominance) in relation to available resources. We explored the branching patterns and fitness responses of tall wormseed mustard (Erysimum strictum) to simulated browsing, soil nutrients, and competition in common garden. Competition increased apical dominance and reduced plant fitness whereas fertilization had the reverse effects. Simulated browsing increased lateral branching and had little impact on plant fitness. Fitness overcompensation was observed only among plants grown in competition and in the absence of fertilization – the most resource poor treatment combination in the experiment. The results contradict both with the compensation continuum and the assumption that apical dominance shows no or very little plasticity in relation to growing conditions. Because directional selection gradients on branch number were invariantly positive irrespective of growing conditions, we propose that, in spite of phenotypic plasticity of apical dominance, the plants appear to be meristem rather than resource limited, and that meristem limitation is strongest in conditions where intact plants produce fewest lateral branches. Our results deviate from the compensation continuum because resource availability affected compensation ability more strongly through phenotypic plasticity of shoot architecture rather than via changes in resource availability per se.     

Flux-based transport enhancement as a plausible unifying mechanism for auxin transport in meristem development

Plants continuously generate new organs through the activity of populations of stem cells called meristems. The shoot apical meristem initiates leaves, flowers, and lateral meristems in highly ordered, spiralled, or whorled patterns via a process called phyllotaxis. It is commonly accepted that the active transport of the plant hormone auxin plays a major role in this process. Current hypotheses propose that cellular hormone transporters of the PIN family would create local auxin maxima at precise positions, which in turn would lead to organ initiation. To explain how auxin transporters could create hormone fluxes to distinct regions within the plant, different concepts have been proposed. A major hypothesis, canalization, proposes that the auxin transporters act by amplifying and stabilizing existing fluxes, which could be initiated, for example, by local diffusion. This convincingly explains the organised auxin fluxes during vein formation, but for the shoot apical meristem a second hypothesis was proposed, where the hormone would be systematically transported towards the areas with the highest concentrations. This implies the coexistence of two radically different mechanisms for PIN allocation in the membrane, one based on flux sensing and the other on local concentration sensing. Because these patterning processes require the interaction of hundreds of cells, it is impossible to estimate on a purely intuitive basis if a particular scenario is plausible or not. Therefore, computational modelling provides a powerful means to test this type of complex hypothesis. Here, using a dedicated computer simulation tool, we show that a flux-based polarization hypothesis is able to explain auxin transport at the shoot meristem as well, thus providing a unifying concept for the control of auxin distribution in the plant. Further experiments are now required to distinguish between flux-based polarization and other hypotheses.     

STRUCTURE AND DEVELOPMENT OF THE DIMORPHIC BRANCH SYSTEM OF THEOBROMA CACAO

The vegetative morphology of Theobroma cacao, the cacao tree, was studied in order to provide a foundation for further investigations on the morphogenesis of the cacao dimorphic shoot system. The seedling of cacao has a determinate orthotropic shoot with a (2+3) phyllotaxis. Branch dimorphism is initiated after 1 to 2 years of growth at which time the apical meristem of the orthotropic shoot aborts and a pseudowhorl of plagiotropic branches is initiated from axillary positions in the shoot tip. The plagiotropic branches are characterized by a distichous phyllotaxis and indeterminate growth. Subsequently an axillary bud below the pseudowhorl develops into a new orthotropic shoot. The apical meristem of this shoot eventually aborts and another pseudowhorl is formed. The apical anatomy of the two types of shoots is similar. The developmental potentiality of the orthotropic shoot axillary buds to form one or the other type of shoot was investigated. The phyllotaxis of the axillary buds of the orthotropic shoot is spiral and that of the axillary buds of the plagiotropic branch is distichous. Pruning and apical puncture experiments showed that the axillary buds of a plagiotropic branch, and of an orthotropic seedling shoot which has not yet formed a pseudowhorl, always give rise to the parent type of shoot. However, the axillary buds of an orthotropic shoot which already bears a pseudowhorl give rise to either type of shoot for several nodes below the point of origin of the pseudowhorl. The type of shoot has no influence on the form of branch which develops from an axillary bud grafted to it. This evidence supports the hypothesis that the axillary buds are initiated as one or the other type of shoot, i.e., once initiated they are predestined.     

The fall and rise of apical dominance

The plant hormone auxin, synthesised in the shoot apex, moves down the stem and inhibits lateral branching. Auxin does not travel upward into the branches, so it must act indirectly; for example, through a second messenger. However, recent work on auxin transport suggests a possible additional mechanism whereby auxin transport in the stem prevents the establishment of auxin transport out of the branches, inhibiting their growth.     

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)     

Seasonal variations in the polar-transport pathways and retention sites of [3H]indole-3-acetic acid in young branches ofFagus sylvatica L.

Branches were cut from young beeches (Fagus sylvatica L.) at various stages of the annual cycle and [³H]indole-3-acetic acid (0.35 nmol) was applied to the whole surface of the apical section of each branch, just below the apical bud. The labelled pulse (moving auxin) and the following weakly radioactive zone (auxin and metabolites retained by the tissues) were localized by counting: microautoradiographss were made using cross sections from these two regions. During the second fortnight of April, auxin was transported by nearly all the cells of the young primary shoot, but the label was more concentrated in the vascular bundles. Auxin transport became the more localized: the cortical parenchyma appeared to lose its ability to transport the hormone (end of April), followed in turn by the pith parenchyma (May). Polar auxin movement at that time was limited to the outer part of the bundle (cambial zone and phloem) and to the inner part (protoxylem parenchyma). Later protoxylem parenchyma ceased to carry auxin. During the whole period of cambial activity, auxin was transported and retained mainly by the cambial zone and its recent derivatives. In September, before the onset of dormancy, and in February, at the end of the resting period, the transport pathways and retention sites for auxin were mainly in the phloem, where sieve tubes often completely lacked radiolabel. When cambial reactivation occurred in the one-year shoot, auxin was mainly carried and retained again in the cambial zone and differentiating derivatives.Abbreviation IAA indole-3-acetic acid     

高表达水稻WRKY72基因影响拟南芥生长素信号传导

植物转录调控因子WRKY基因家族是一个拥有众多成员的超家族,功能涵盖了植物生长发育的控制与抗病耐逆的调节。我们主要分析了OsWRKY72基因在外源植物拟南芥中的生物学功能。通过转基因拟南芥(Arabidopsis thaliana)的遗传学研究发现外源高表达该基因不单明显地抑制转基因植株的顶端优势,增强植株侧枝的生长,还改变了转基因植株叶片和角果的发育。进一步分析证实,高表达OsWRKY72基因所导致转基因拟南芥植株的表型和其它生理现象都与生长素信号通路改变所导致的表型和生理变化极其相近。这些结果说明OsWRKY72基因在外源植物拟南芥体内高表达后很可能改变了其正常的生长素信号通路。     

Micropropagation of Sesbania aculeata (Pers.) by adventitious organogenesis

Multiple shoots differentiated from hypocotyl explants of Sesbania aculeata (Pers.) syn S. cannabina (Retz.) Pers., a leguminous woody shrub, when cultured on Murashige and Skoog's basal medium supplemented with auxin (IBA, NAA) or auxin and cytokinin (IBA + BAP, NAA + BAP). Shoot budding occurred directly from the explant as well as from callus. Differentiation of shoot and root occurred in one step in the same concentration of auxin or auxin and cytokinin. Elongation of shoots occurred in the shoot induction medium.     

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.