Effects of endogenous hormones on variation of shoot branching in a variety of non-heading Chinese cabbage and related gene expression

Shoot branching (tillering) primarily determines plant shoot architecture and has been studied in many plants. Shoot branching is an important trait in non-heading Chinese cabbage (Brassica rapa ssp. chinensis Makino). The B. rapa ssp. chinensis var. multiceps exhibits unique and multiple shoot branching characteristics. Here, we analyzed the variation in shoot branching between ‘Maertou,’ with multiple shoot branching, and ‘Suzhouqing,’ a common variety. The levels of endogenous indole-3-acetic acid (IAA), zeatin riboside and active gibberellins in the shoot meristem tissues of the two cultivars were quantified by enzyme-linked immunosorbent assay during the vegetative growth stage. High levels of IAA maintained axillary bud dormancy and repressed axillary bud outgrowth allowing shoot branching to form in the vegetative stage in ‘Suzhouqing.’ In contrast, low levels of IAA did not inhibit axillary buds in ‘Maertou,’ while a high level of cytokinin promoted axillary bud growth and branch shoot development. Exogenous hormone (rac-GR24 and 6-benzylaminopurine) treatment showed that ‘Maertou’ was relatively sensitive to cytokinin, because the fold changes of cytokinin-responsive genes in ‘Maertou’ were significantly more frequent than those in ‘Suzhouqing’. Cytokinin was the direct regulator for axillary bud growth of ‘Maertou’. Compared with ‘Suzhouqing’, ‘Maertou’ was sensitive to cytokinin and this weakened the strigolactone–cytokinin branching pathway.     

Roles for Auxin, Cytokinin, and Strigolactone in Regulating Shoot Branching

Many processes have been described in the control of shoot branching. Apical dominance is defined as the control exerted by the shoot tip on the outgrowth of axillary buds, whereas correlative inhibition includes the suppression of growth by other growing buds or shoots. The level, signaling, and/or flow of the plant hormone auxin in stems and buds is thought to be involved in these processes. In addition, RAMOSUS (RMS) branching genes in pea (Pisum sativum) control the synthesis and perception of a long-distance inhibitory branching signal produced in the stem and roots, a strigolactone or product. Auxin treatment affects the expression of RMS genes, but it is unclear whether the RMS network can regulate branching independently of auxin. Here, we explore whether apical dominance and correlative inhibition show independent or additive effects in rms mutant plants. Bud outgrowth and branch lengths are enhanced in decapitated and stem-girdled rms mutants compared with intact control plants. This may relate to an RMS-independent induction of axillary bud outgrowth by these treatments. Correlative inhibition was also apparent in rms mutant plants, again indicating an RMS-independent component. Treatments giving reductions in RMS1 and RMS5 gene expression, auxin transport, and auxin level in the main stem were not always sufficient to promote bud outgrowth. We suggest that this may relate to a failure to induce the expression of cytokinin biosynthesis genes, which always correlated with bud outgrowth in our treatments. We present a new model that accounts for apical dominance, correlative inhibition, RMS gene action, and auxin and cytokinin and their interactions in controlling the progression of buds through different control points from dormancy to sustained growth.     

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.     

Effect of Night Temperature on the Activity of Sucrose Phosphate Synthase, Acid Invertase, and Sucrose Synthase in Source and Sink Tissues of Rosa hybrida cv Golden Times

Sucrose phosphate synthase and acid invertase activities in the mature leaves of roses (Rosa hybrida cv Golden Times) were greater in plants grown under a higher night temperature than under a lower temperature regime. In young shoots, the activity of acid invertase was promoted by the lower temperature while that of sucrose synthase was increased at the higher temperature. At both temperatures benzyladenine when applied to the axillary bud stimulated sucrose phosphate synthase activity and advancement of its peak of activity in the leaf subtending to the bud, and also stimulated sucrose synthase activity in the young shoot. At the lower temperature, application of benzyladenine to the axillary bud stimulated acid invertase activity in the young shoot but not in the leaves.     

Insight into the role of sugars in bud burst under light in the rose

Bud burst is a decisive process in plant architecture that requires light in Rosa sp. This light effect was correlated with stimulation of sugar transport and metabolism in favor of bud outgrowth. We investigated whether sugars could act as signaling entities in the light-mediated regulation of vacuolar invertases and bud burst. Full-length cDNAs encoding two vacuolar invertases (RhVI1 and RhVI2) were isolated from buds. Unlike RhVI2, RhVI1 was preferentially expressed in bursting buds, and was up-regulated in buds of beheaded plants exposed to light. To assess the importance of sugars in this process, the expression of RhVI1 and RhVI2 and the total vacuolar invertase activity were further characterized in buds cultured in vitro on 100 mM sucrose or mannitol under light or in darkness for 48 h. Unlike mannitol, sucrose promoted the stimulatory effect of light on both RhVI1 expression and vacuolar invertase activity. This up-regulation of RhVI1 was rapid (after 6 h incubation) and was induced by as little as 10 mM sucrose or fructose. No effect of glucose was found. Interestingly, both 30 mM palatinose (a non-metabolizable sucrose analog) and 5 mM psicose (a non-metabolizable fructose analog) promoted the light-induced expression of RhVI1 and total vacuolar invertase activity. Sucrose, fructose, palatinose and psicose all promoted bursting of in vitro cultured buds under light. These findings indicate that soluble sugars contribute to the light effect on bud burst and vacuolar invertases, and can function as signaling entities.     

Computational Modeling and Molecular Physiology Experiments Reveal New Insights into Shoot Branching in Pea

Bud outgrowth is regulated by the interplay of multiple hormones, including auxin, cytokinin, strigolactones, and an unidentified long-distance feedback signal that moves from shoot to root. The model of bud outgrowth regulation in pea (Pisum sativum) includes these signals and a network of five RAMOSUS (RMS) genes that operate in a shoot-root-shoot loop to regulate the synthesis of, and response to, strigolactones. The number of components in this network renders the integration of new and existing hypotheses both complex and cumbersome. A hypothesis-driven computational model was therefore developed to help understand regulation of shoot branching. The model evolved in parallel with stepwise laboratory research, helping to define and test key hypotheses. The computational model was used to verify new mechanisms involved in the regulation of shoot branching by confirming that the new hypotheses captured all relevant biological data sets. Based on cytokinin and RMS1 expression analyses, this model is extended to include subtle but important differences in the function of RMS3 and RMS4 genes in the shoot and rootstock. Additionally, this research indicates that a branch-derived signal upregulates RMS1 expression independent of the other feedback signal. Furthermore, we propose xylem-sap cytokinin promotes sustained bud outgrowth, rather than acting at the earlier stage of bud release.     

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.     

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)     

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.     

Role of ethylene and cytokinins in the initiation of lateral shoot growth in bromeliads

Aechmea victoriana var discolor L. B. Foster and Aechmea dactylina Bal. are commercially propagated in vitro through lateral shoot growth. A modified Murashige and Skoog medium is used which contains both BA and IAA. These growth substances were shown in the present study to synergistically stimulate the production of ethylene by the cultured plants. The stimulation of ethylene production is correlated with the outgrowth of the lateral buds. The rise in ethylene production was concluded to induce lateral shoot growth, because: (a) outgrowth of the shoots was blocked by preventing an increase in ethylene production, (b) 1-aminocyclopropane-1-carboxylic acid (ACC), the natural precursor of ethylene biosynthesis, substituted for IAA in the promotion of ethylene production and lateral bud outgrowth. Although ACC could substitute for IAA, it could not substitute for BA; therefore, cytokinins are concluded to be essential for lateral bud outgrowth in vitro in Aechmea. These results suggest that cytokinins and ethylene both play roles in natural lateral bud initiation and that the cytokinin function involves two stages of the process.     

Molecular role of cytokinin in bud activation and outgrowth in apple branching based on transcriptomic analysis

Key message

Axillary bud activation and outgrowth were dependent on local cytokinin, and that bud activation preceded the activation of cell cycle and cell growth genes in apple branching.

Abstract

Cytokinin is often applied to apple trees to produce more shoot branches in apple seedlings. The molecular response of apple to the application of cytokinin, and the relationship between bud activation and cell cycle in apple branching, however, are poorly understood. In this study, RNA sequencing was used to characterize differential expression genes in axillary buds of 1-year grafted “Fuji” apple at 4 and 96 h after cytokinin application. And comparative gene expression analyses were performed in buds of decapitated shoots and buds of the treatment of biosynthetic inhibitor of cytokinin (Lovastatin) on decapitated shoots. Results indicated that decapitation and cytokinin increased ZR content in buds and internodes at 4–8 h, and induced bud elongation at 96 h after treatment, relative to buds in shoots receiving the Lovastatin treatment. RNA-seq analysis indicated that differential expression genes in auxin and cytokinin signal transduction were significantly enriched at 4 h, and DNA replication was enriched at 96 h. Cytokinin-responsive type-A response regulator, auxin polar transport, and axillary meristem-related genes were up-regulated at 4 h in the cytokinin and decapitation treatments, while qRT-PCR analysis showed that cell cycle and cell growth genes were up-regulated after 8 h. Collectively, the data indicated that bud activation and outgrowth might be dependent on local cytokinin synthesis in axillary buds or stems, and that bud activation preceded the activation of cell cycle genes during the outgrowth of ABs in apple shoots.

     

Trehalose 6‐phosphate is involved in triggering axillary bud outgrowth in garden pea (Pisum sativum L.)

Trehalose 6‐phosphate (Tre6P) is a signal of sucrose availability in plants, and has been implicated in the regulation of shoot branching by the abnormal branching phenotypes of Arabidopsis (Arabidopsis thaliana) and maize (Zea mays) mutants with altered Tre6P metabolism. Decapitation of garden pea (Pisum sativum) plants has been proposed to release the dormancy of axillary buds lower down the stem due to changes in sucrose supply, and we hypothesized that this response is mediated by Tre6P. Decapitation led to a rapid and sustained rise in Tre6P levels in axillary buds, coinciding with the onset of bud outgrowth. This response was suppressed by simultaneous defoliation that restricts the supply of sucrose to axillary buds in decapitated plants. Decapitation also led to a rise in amino acid levels in buds, but a fall in phosphoenolpyruvate and 2‐oxoglutarate. Supplying sucrose to stem node explants in vitro triggered a concentration‐dependent increase in the Tre6P content of the buds that was highly correlated with their rate of outgrowth. These data show that changes in bud Tre6P levels are correlated with initiation of bud outgrowth following decapitation, suggesting that Tre6P is involved in the release of bud dormancy by sucrose. Tre6P might also be linked to a reconfiguration of carbon and nitrogen metabolism to support the subsequent growth of the bud into a new shoot.     

Auxin and cytokinin related gene expression during active shoot growth and latent bud paradormancy in Vitis riparia grapevine

Cultural practices for canopy management in grapevines rely on intensive manipulation of shoot architecture to maintain canopy light levels. In contrast to common model plant systems used to study regulation of branch outgrowth, the grapevine has a more complex architecture. The node contains first, second and third order axillary meristems. The prompt bud (N+1) develops into a summer lateral and a latent compound bud develops in the basal node of the summer lateral (N+2, N+3(1,2)). The outgrowth potential of latent buds was determined using common canopy management treatments (shoot tip decapitation and removal of summer laterals and leaves) and monitoring the rate of latent bud outgrowth. Two shoot node regions (apical and basal) with differential outgrowth potential were characterized and it was noted that the shoot tip, summer laterals and leaves in addition to node position contributed to the inhibition of latent bud outgrowth. To advance the understanding of the molecular regulation of bud outgrowth and paradormancy in the complex shoot architecture of grapevines, the expression of auxin and cytokinin genes involved in branching (amidase (VrAMI1), PINFORMED-3 (VrPIN3) and isopentenyl transferase (VrIPT)) were monitored in shoot tips and differentially aged buds of Vitis riparia grapevine shoots. In addition, Histone 3 (VrH3) and a hexose transporter (VrHT1) expression were monitored as a measure of tissue activity. The expression of VrAMI1 and VrPIN3 remained constant in actively growing shoot tips and decreased significantly with increasing bud maturation in paradormant buds. VrHT1 expression was greater in buds than in any other plant tissue tested. VrHT1 may have the potential to be used as an indicator of paradormancy status in grapevines. These characterizations in the complex architecture of the grapevine provide an excellent model system for molecular analysis of bud outgrowth and shoot architecture development.     

Hormonal control of shoot branching

Shoot branching is the process by which axillary buds, located on the axil of a leaf, develop and form new flowers or branches. The process by which a dormant bud activates and becomes an actively growing branch is complex and very finely tuned. Bud outgrowth is regulated by the interaction of environmental signals and endogenous ones, such as plant hormones. Thus these interacting factors have a major effect on shoot system architecture. Hormones known to have a major influence are auxin, cytokinin, and a novel, as yet chemically undefined, hormone. Auxin is actively transported basipetally in the shoot and inhibits bud outgrowth. By contrast, cytokinins travel acropetally and promote bud outgrowth. The novel hormone also moves acropetally but it inhibits bud outgrowth. The aim of this review is to integrate what is known about the hormonal control of shoot branching in Arabidopsis, focusing on these three hormones and their interactions.     

A possible role of cytokinin in mediating long-distance nitrogen signaling in the promotion of sylleptic branching in hybrid poplar

Nitrogen fertilization of roots enhances shoot growth in plants and cytokinins are known to initiate bud outgrowth in shoots. Is it possible that root-derived cytokinins may play a role in long-distance signaling for nitrogen availability in the promotion of sylleptic branching in hybrid poplar? Nitrogen fertilization in the form of 5 mM NH4NO3, KNO3 or NH4Cl was applied to roots of three hybrid poplar clones exhibiting contrasting degrees of sylleptic branching. Cytokinin (0.1-1 mM benzyladenine, BA) was applied directly to lateral buds of shoots. Glutamate, asparagine and glutamine were also applied as drops to buds or as foliar sprays. NH4NO3, KNO3 and NH4Cl all usually enhanced sylleptic branching within a week in the high sylleptic clone (11-11) but in four out of five trials there was no effect in the low sylleptic clone (47-174). NH4NO3 added directly to buds had no effect. Also, glutamate, asparagine and glutamine had no effect. However, 1 mM BA promoted lateral bud outgrowth in all three clones. These results are consistent with the long-distance nitrogen signaling hypothesis of Forde and Sakakibara wherein nitrogen is transduced to cytokinin via enhanced ipt activity in the roots and is translocated up the shoot with the subsequent promotion of leaf/bud outgrowth.     

植物茎分枝的分子调控

植物茎分枝结构决定了不同植物的不同形态结构.本文从腋生分生组织的发生、腋芽的生长两个方面综述了近年来植物分枝发生发育相关的分子机理研究及其进展.发现在不同植物中腋分生组织形成的基本机制是相似的,LS（lateral suppressor）及其同源基因在不同植物中都参与腋生分生组织的形成,而BL(blind)及其同源基因也参与调控腋生分生组织的形成.腋生分生组织的形成可能也是受激素调控的.目前,对腋芽生长的分子调控机制的认识主要集中于生长素通过二级信使的作用调控腋芽的生长.而生长素调控腋芽生长的机制已经较为清楚的有两条途径：一是生长素通过抑制细胞分裂素合成来调控腋芽的生长;另一途径是一种类胡萝卜素衍生的信号物质参与生长素的运输调控(MAX途径)来调控腋芽的生长.最新研究表明,TB1的拟南芥同源基因在MAX途径的下游负调控腋芽的生长.此外,增强表达OsNAC2也促进腋芽的生长.     

Low temperature induction of invertase activity in grapefruit flavedo tissue

Invertase activity increased in the flavedo tissue of ‘Marsh’ grapefruit (Citrus paradisi Macf.) when trees were exposed to cold hardening temperatures and decreased at dehardening temperatures. Invertase activity also increased in the flavedo of detached fruit stored at 5δ. Reducing sugar levels paralleled invertase activity while sucrose levels were inversely related to invertase levels. The mechanism by which low temperatures induce invertase activity in grapefruit flavedo tissue was not determined. However, results indicated that a proteinaceous inhibitor, similar to the one found in potato tubers, is not involved in the regulation of invertase activity in flavedo tissue of grapefruit.     

Carbohydrates Modulate the In Vitro Growth of Olive Microshoots. I. The Analysis of Shoot Growth and Branching Patterns

The rate of microshoot proliferation during the micropropagation of olive (Olea europaea L.) plants is limited by the low rates of both bud sprouting and growth of secondary shoots following subculturing. The aim of this study was to determine (1) the effects of sucrose and mannitol on shoot growth, (2) whether either of these sugars modifies the pattern of shoot development of the explants, and (3) the influence of apical dominance on explant development. Working with single-node microcuttings of Olea europaea L. cv. Maurino with two opposite axillary buds, we added 17, 34, or 68 g L^?1 of sucrose or mannitol to the medium as the primary carbon source. Shoot development was classified as either (a) an outgrowth of the first bud on an explant (shoot-type A), (b) an outgrowth of the second bud (shoot-type B), or (c) an outgrowth of an axillary bud on either an A- or B-type shoot (shoot-type C). Explant survival, fresh-mass production, and patterns of shoot development were influenced by the type and concentration of sugar used. Mannitol promoted the sprouting and growth of A-, B-, and C-type shoots more than sucrose. The developmental responses observed indicate that the growth of axillary meristems of in vitro olive explants is not regulated by apical dominance. The results demonstrate that the sugar alcohol plays an important role in the developmental regulation of olive explants. Mannitol may also protect against detrimental effects associated with in vitro growth conditions.