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.     

Shoot branching and leaf dissection in tomato are regulated by homologous gene modules

Aerial plant architecture is predominantly determined by shoot branching and leaf morphology, which are governed by apparently unrelated developmental processes, axillary meristem formation, and leaf dissection. Here, we show that in tomato (Solanum lycopersicum), these processes share essential functions in boundary establishment. Potato leaf (C), a key regulator of leaf dissection, was identified to be the closest paralog of the shoot branching regulator Blind (Bl). Comparative genomics revealed that these two R2R3 MYB genes are orthologs of the Arabidopsis thaliana branching regulator REGULATOR OF AXILLARY MERISTEMS1 (RAX1). Expression studies and complementation analyses indicate that these genes have undergone sub- or neofunctionalization due to promoter differentiation. C acts in a pathway independent of other identified leaf dissection regulators. Furthermore, the known leaf complexity regulator Goblet (Gob) is crucial for axillary meristem initiation and acts in parallel to C and Bl. Finally, RNA in situ hybridization revealed that the branching regulator Lateral suppressor (Ls) is also expressed in leaves. All four boundary genes, C, Bl, Gob, and Ls, may act by suppressing growth, as indicated by gain-of-function plants. Thus, leaf architecture and shoot architecture rely on a conserved mechanism of boundary formation preceding the initiation of leaflets and axillary meristems.     

LAX PANICLE2 of rice encodes a novel nuclear protein and regulates the formation of axillary meristems

Aerial architecture in higher plants is dependent on the activity of the shoot apical meristem (SAM) and axillary meristems (AMs). The SAM produces a main shoot and leaf primordia, while AMs are generated at the axils of leaf primordia and give rise to branches and flowers. Therefore, the formation of AMs is a critical step in the construction of plant architecture. Here, we characterized the rice (Oryza sativa) lax panicle2 (lax2) mutant, which has altered AM formation. LAX2 regulates the branching of the aboveground parts of a rice plant throughout plant development, except for the primary branch in the panicle. The lax2 mutant is similar to lax panicle1 (lax1) in that it lacks an AM in most of the lateral branching of the panicle and has a reduced number of AMs at the vegetative stage. The lax1 lax2 double mutant synergistically enhances the reduced-branching phenotype, indicating the presence of multiple pathways for branching. LAX2 encodes a nuclear protein that contains a plant-specific conserved domain and physically interacts with LAX1. We propose that LAX2 is a novel factor that acts together with LAX1 in rice to regulate the process of AM formation.     

The Arabidopsis MAX pathway controls shoot branching by regulating auxin transport

BACKGROUND: Plants achieve remarkable plasticity in shoot system architecture by regulating the activity of secondary shoot meristems, laid down in the axil of each leaf. Axillary meristem activity, and hence shoot branching, is regulated by a network of interacting hormonal signals that move through the plant. Among these, auxin, moving down the plant in the main stem, indirectly inhibits axillary bud outgrowth, and an as yet undefined hormone, the synthesis of which in Arabidopsis requires MAX1, MAX3, and MAX4, moves up the plant and also inhibits shoot branching. Since the axillary buds of max4 mutants are resistant to the inhibitory effects of apically supplied auxin, auxin and the MAX-dependent hormone must interact to inhibit branching. RESULTS: Here we show that the resistance of max mutant buds to apically supplied auxin is largely independent of the known, AXR1-mediated, auxin signal transduction pathway. Instead, it is caused by increased capacity for auxin transport in max primary stems, which show increased expression of PIN auxin efflux facilitators. The max phenotype is dependent on PIN1 activity, but it is independent of flavonoids, which are known regulators of PIN-dependent auxin transport. CONCLUSIONS: The MAX-dependent hormone is a novel regulator of auxin transport. Modulation of auxin transport in the stem is sufficient to regulate bud outgrowth, independent of AXR1-mediated auxin signaling. We therefore propose an additional mechanism for long-range signaling by auxin in which bud growth is regulated by competition between auxin sources for auxin transport capacity in the primary stem.     

Auxin, cytokinin and the control of shoot branching

BACKGROUND: It has been known for many decades that auxin inhibits the activation of axillary buds, and hence shoot branching, while cytokinin has the opposite effect. However, the modes of action of these two hormones in branching control is still a matter of debate, and their mechanisms of interaction are equally unresolved. SCOPE: Here we review the evidence for various hypotheses that have been put forward to explain how auxin and cytokinin influence axillary bud activity. In particular we discuss the roles of auxin and cytokinin in regulating each other's synthesis, the cell cycle, meristem function and auxin transport, each of which could affect branching. These different mechanisms have implications for the main site of hormone action, ranging from systemic action throughout the plant, to local action at the node or in the bud meristem or leaves. The alternative models have specific predictions, and our increasing understanding of the molecular basis for hormone transport and signalling, cell cycle control and meristem biology is providing new tools to enable these predictions to be tested.     

AXR1 acts after lateral bud formation to inhibit lateral bud growth in Arabidopsis

The AXR1 gene of Arabidopsis is required for many auxin responses. The highly branched shoot phenotype of mature axr1 mutant plants has been taken as genetic evidence for a role of auxin in the control of shoot branching. We compared the development of lateral shoots in wild-type Columbia and axr1-12 plants. In the wild type, the pattern of lateral shoot development depends on the developmental stage of the plant. During prolonged vegetative growth, axillary shoots arise and develop in a basal-apical sequence. After floral transition, axillary shoots arise rapidly along the primary shoot axis and grow out to form lateral inflorescences in an apical-basal sequence. For both patterns, the axr1 mutation does not affect the timing of axillary meristem formation; however, subsequent lateral shoot development proceeds more rapidly in axr1 plants. The outgrowth of lateral inflorescences from excised cauline nodes of wild-type plants is inhibited by apical auxin. axr1-12 nodes are resistant to this inhibition. These results provide evidence for common control of axillary growth in both patterns, and suggest a role for auxin during the late stages of axillary shoot development following the formation of the axillary bud and several axillary leaf primordia.     

The interaction of two homeobox genes,BREVIPEDICELLUS and PENNYWISE,regulates internode patterning in the Arabidopsis inflorescence

Plant architecture results from the activity of the shoot apical meristem, which initiates leaves, internodes, and axillary meristems. KNOTTED1-like homeobox (KNOX) genes are expressed in specific patterns in the shoot apical meristem and play important roles in plant architecture. KNOX proteins interact with BEL1-like (BELL) homeodomain proteins and together bind a target sequence with high affinity. We have obtained a mutation in one of the Arabidopsis BELL genes, PENNYWISE (PNY), that appears phenotypically similar to the KNOX mutant brevipedicellus (bp). Both bp and pny have randomly shorter internodes and display a slight increase in the number of axillary branches. The double mutant shows a synergistic phenotype of extremely short internodes interspersed with long internodes and increased branching. PNY is expressed in inflorescence and floral meristems and overlaps with BP in a discrete domain of the inflorescence meristem where we propose the internode is patterned. The physical association of the PNY and BP proteins suggests that they participate in a complex that regulates early patterning events in the inflorescence meristem.     

Photoregulation of growth and branching of plum shoots: Physiological action of two photosystems

Summary Plum shoot proliferation was investigated in terms of two distinct processes: axillary bud differentiation and axillary shoot development. Results showed that light quality influenced bud differentiation and interacted with apical dominance in determining shoot outgrowth, resulting in a differentiated structure of shoot clusters and type of branching. Results suggested that blue light, acting through its photoreceptor, increased the number of axillary buds differentiated from apical meristem, but did not remove the apical dominance. Red light removed apical dominance, while reducing the formation of axillary buds; both events appeared to be dependent on the putative amount of phytochrome active form, and independent of light photon fluence rate. On the contrary, blue light action appeared to be dependent on photon fluence rate. In addition, apparent blue-red interactions related to photomorphogenic events fit an antagonistic model for branching regulated by light via cryptochrome and phytochrome photoreceptors. Our results show that the dynamics of shoot cluster development is the product of two events: the formation of new axillary buds and their release from apical dominance.     

Development and structure of trichotomous branching in Edgeworthia chrysantha (Thymelaeaceae)

We studied the development and structure of the unusual trichotomous branching of Edgeworthia chrysantha. Three "branch primordia" are formed sequentially on the shoot apex of a main axis and develop into trichotomous branching. The branch primordia are clearly distinguishable from the typical axillary buds of other angiosperms; they develop much more rapidly than axillary buds, and the borders between the branch primordia and shoot apex of the main axis are anatomically unclear. Furthermore, at a later stage, leaves subtending the branch primordia produce typical axillary buds. These results suggest that the trichotomous branching in this species involves the division of the shoot apical meristem. Expression analysis of genes involved in branching or maintenance of the shoot apical meristem in this species should clarify the control mechanism of this novel branching pattern in angiosperms. We also observed the phyllotactic patterns in trichotomous branching and have related these patterns to the shoot system as a whole.     

Early events of multiple bud formation and shoot development in soybean embryonic axes treated with the cytokinin, 6-benzylaminopurine

Early events of multiple bud formation and shoot development in germinating soybean embryonic axes treated for 24 hr with the cytokinin, 6-benzylaminopurine (BAP), were compared to the development of untreated control axes using four different techniques: photomicrography, scanning electron microscopy, histology, and autoradiography. Shoot apex development in BAP-treated embryonic axes was delayed by about 9 to 15 hr. A transient inhibition of DNA synthesis in the primary apical meristem and axillary buds was observed with subsequent changes in the timing of cell division patterns in these regions. Meristematic regions (supernumerary vegetative buds) were observed in BAP-treated axes around the perimeter of the apical dome at and above the level of the axillary buds. Cells elongated from some of the BAP-induced meristematic regions to form four to six shoots. In the absence of BAP, excision of the primary apical meristem and/or axillary buds did not result in multiple bud formation. These results suggest that transient exposure to BAP interrupted chromosomal DNA replication and reprogrammed the developmental fate of a large number of cells in the shoot apex. We postulate that interruption of DNA synthesis, either directly, by interfering with DNA replication, or indirectly, by preventing entry into S-phase, effected redetermination of the shoot apex cells.     

Non-axillary branching in the palms Eugeissona and Oncosperma (Arecaceae)

FISHER, J. B., GOH, C. J. & RAO, A. N., 1989. Non-axillary branching in the palms Eugeissona and Oncosperma (Arecaceae). The south-east Asian palms, Eugeissona (Calamoideae) and Oncosperma (Arecoideae) are multiple-stemmed. The morphology and development of branching in two species of each genus were examined in Singapore, Borneo, and the Malay Peninsula. Cultivated seedling and adult plants of 0. tigillarium were also observed in Florida. A new shoot arises most often from a longitudinal abaxial groove at the base of an enclosing leaf sheath. In some instances, especially in E. tristis , the enclosing leaf has two equal, adjacent grooves such that any distinction between an original mother shoot and a lateral daughter shoot is impossible. No axillary buds occur in Eugeissona which is hapaxanthic. In Oncosperma , which is pleonanthic, axillary buds are absent from young pre-flowering stems, but an inflorescence bud occurs in the axil of each leaf in older aerial stems. Early ontogenetic stages of vegetative branching, as seen in sectioned apices, indicate that a new vegetative shoot is present on the abaxial base of the first (youngest) leaf primordium. There is no ontogenetic evidence for the displacement of an originally axillary meristem as previously described for the palm Salacca (Calamoideae). Shoot development in Eugeissona is interpreted as a putative dichotomy of the apical meristem in which the meristem centres commonly develop unequally. In Oncosperma the smaller sucker bud meristem may be described as an abaxial leaf base bud, but ontogenetic variations indicate this form of branching is close to dichotomous branching. These new examples of non-axillary branching are compared to similar cases previously reported for palms and other monocotyledons.     

The bHLH protein ROX acts in concert with RAX1 and LAS to modulate axillary meristem formation in Arabidopsis

During post-embryonic shoot development, new meristems are initiated in the axils of leaves. They produce secondary axes of growth that determine morphological plasticity and reproductive efficiency in higher plants. In this study, we describe the role of the bHLH-protein-encoding Arabidopsis gene REGULATOR OF AXILLARY MERISTEM FORMATION (ROX), which is the ortholog of the branching regulators LAX PANICLE1 (LAX1) in rice and barren stalk1 (ba1) in maize. rox mutants display compromised axillary bud formation during vegetative shoot development, and combination of rox mutants with mutations in RAX1 and LAS, two key regulators of axillary meristem initiation, enhances their branching defects. In contrast to lax1 and ba1, flower development is unaffected in rox mutants. Over-expression of ROX leads to formation of accessory side shoots. ROX mRNA accumulates at the adaxial boundary of leaf and flower primordia. However, in the vegetative phase, axillary meristems initiate after ROX expression has terminated, suggesting an indirect role for ROX in meristem formation. During vegetative development, ROX expression is dependent on RAX1 and LAS activity, and all three genes act in concert to modulate axillary meristem formation.     

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.     

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.     

Bud Structure and Shoot Architecture of Canopy and Understorey Evergreen Broad-leaved Trees at their Northern Limit in East Asia

The morphology of winter buds, shoot growth and branching architecturewas studied in evergreen broad-leaved trees of subtropical/warm-temperaterain forests of southern and central Japan. Winter buds werecategorized into three types based on external morphology anddevelopmental processes: naked, hypsophyllary and scaled buds.Each shoot tip with intermittent growth was covered with a smallnumber of immature leaves or hypsophylls when growth ceased.Hypsophylls protect the apical meristem during its resting period,hence we termed them hypsophyllary buds. In trees with nakedbuds, immature leaves resumed their growth and developed tomature leaves the following spring; thus these trees had nospecial organs to cover shoot tips during winter. In trees withhypsophyllary buds, some hypsophylls covering the shoot tipsthrough the year were shed without further growth when new shootsstarted to grow in the spring. In trees with scaled buds, newlygrowing shoots had hypsophyllary buds at their tips in spring.After the completion of stem elongation, the buds were replacedby scaled buds (often covered with more than 30 scales) in summer.These scaled buds grew during autumn and winter until a newflush of growth the following spring. The three bud types correspondedto forest stratification in the northern-limit forest: the nakedbuds of Rubiaceae and Myrsinaceae in the ground layer; the hypsophyllarybuds of various families (e.g. Symplocaceae, Myrsinaceae) inthe understorey; and the scaled buds of Fagaceae and Lauraceaein the forest canopy. The position and activity of buds on abranch were reflected in the architectural patterns of the treesin different layers of the forest. The scaled-bud trees hadwell-protected, abundant axillary buds and are probably suitedto survive in the forest canopy (with frequent disturbances),whereas the single terminal bud of hypsophyllary-bud trees cansurvive in the less disturbed, resource-limited understoreyof the forest.Copyright 1998 Annals of Botany Company Bud structural type; bud formation; bud growth; shoot elongation; shoot-growth cycle; branching architecture; forest stratification.