Growth Context and Fate of Axillary Meristems of Young Peach Trees. Influence of Parent Shoot Growth Characteristics and of Emergence Date

The relationship between several growth components of a shootand the fates of the axillary meristems (developing in the axilsof the leaves) borne by that shoot were studied, on first-ordershoots of young peach trees. A comprehensive picture of thoserelationships was obtained by a discriminant analysis. Shootgrowth at meristem emergence date was characterized by internodelength, leaf-production rate and leaf-unfolding duration. Allpossible fates of axillary meristems at the end of the growingseason (i.e. blind nodes, single vegetative or flower bud, budassociations, sylleptic or proleptic shoots) were considered.Shoot-elongation rate determined meristem fates quantitatively.The number of buds produced by a meristem increased when theshoot-elongation rate increased. Qualitatively, the fate of axillary meristems was related tothe balance between shoot-growth components. If the subtendingleaf unfolded slowly, sylleptic or proleptic shoots were morelikely to develop than bud associations, for high shoot-elongationrates; and flower buds were more frequent than vegetative buds,for low shoot-elongation rates. Compared to flower buds, blindnodes appeared for similar shoot-elongation rates but longerinternodes and lower leaf-production rates. The emergence dateslightly modified the relation between shoot growth and axillary-meristemfates, but the main features held true throughout the growingseason. The relationships between shoot growth and meristem fates mayresult from competitive interactions between the growing subtendingleaf and the developing axillary meristem. Growing conditionsmight also influence both shoot growth and meristem fates byfavouring either cell enlargement or cell division.Copyright1995, 1999 Academic Press Peach tree, Prunus persica (L.) Batsch, axillary meristem, meristem fate, branching, flowering, shoot growth, discriminant analysis, exploratory analysis     

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.     

Expression of CENTRORADIALIS (CEN) and CEN-like genes in tobacco reveals a conserved mechanism controlling phase change in diverse species.

Plant species exhibit two primary forms of flowering architecture, namely, indeterminate and determinate. Antirrhinum is an indeterminate species in which shoots grow indefinitely and only generate flowers from their periphery. Tobacco is a determinate species in which shoot meristems terminate by converting to a flower. We show that tobacco is responsive to the CENTRORADIALIS (CEN) gene, which is required for indeterminate growth of the shoot meristem in Antirrhinum. Tobacco plants overexpressing CEN have an extended vegetative phase, delaying the switch to flowering. Therefore, CEN defines a conserved system controlling shoot meristem identity and plant architecture in diverse species. To understand the underlying basis for differences between determinate and indeterminate architectures, we isolated CEN-like genes from tobacco (CET genes). In tobacco, the CET genes most similar to CEN are not expressed in the main shoot meristem; their expression is restricted to vegetative axillary meristems. As vegetative meristems develop into flowering shoots, CET genes are downregulated as floral meristem identity genes are upregulated. Our results suggest a general model for tobacco, Antirrhinum, and Arabidopsis, whereby the complementary expression patterns of CEN-like genes and floral meristem identity genes underlie different plant architectures.     

Herbivory could unlock mutations sequestered in stratified shoot apices of genetic mosaics

Many higher plants have shoot apical meristems that possess discrete cell layers, only one of which normally gives rise to gametes following the transition from vegetative meristem to floral meristem. Consequently, when mutations occur in the meristems of sexually reproducing plants, they may or may not have an evolutionary impact, depending on the apical layer in which they reside. In order to determine whether developmentally sequestered mutations could be released by herbivory (i.e., meristem destruction), a characterized genetic mosaic was subjected to simulated herbivory. Many plants develop two shoot meristems in the leaf axils of some nodes, here referred to as the primary and secondary axillary meristems. Destruction of the terminal and primary axillary meristems led to the outgrowth of secondary axillary meristems. Seed derived from secondary axillary meristems was not always descended from the second apical cell layer of the terminal shoot meristem as is expected for terminal and primary shoot meristems. Vegetative and reproductive analysis indicated that secondary meristems did not maintain the same order of cell layers present in the terminal shoot meristem. In secondary meristems reproductively sequestered cell layers possessing mutant cells can be repositioned into gamete-forming cell layers, thereby adding mutant genes into the gene pool. Herbivores feeding on shoot tips may influence plant evolution by causing the outgrowth of secondary axillary meristems.     

MAX1 and MAX2 control shoot lateral branching in Arabidopsis

Plant shoots elaborate their adult form by selective control over the growth of both their primary shoot apical meristem and their axillary shoot meristems. We describe recessive mutations at two loci in Arabidopsis, MAX1 and MAX2, that affect the selective repression of axillary shoots. All the first order (but not higher order) axillary shoots initiated by mutant plants remain active, resulting in bushier shoots than those of wild type. In vegetative plants where axillary shoots develop in a basal to apical sequence, the mutations do not clearly alter node distance, from the shoot apex, at which axillary shoot meristems initiate but shorten the distance at which the first axillary leaf primordium is produced by the axillary shoot meristem. A small number of mutant axillary shoot meristems is enlarged and, later in development, a low proportion of mutant lateral shoots is fasciated. Together, this suggests that MAX1 and MAX2 do not control the timing of axillary meristem initiation but repress primordia formation by the axillary meristem. In addition to shoot branching, mutations at both loci affect leaf shape. The mutations at MAX2 cause increased hypocotyl and petiole elongation in light-grown seedlings. Positional cloning identifies MAX2 as a member of the F-box leucine-rich repeat family of proteins. MAX2 is identical to ORE9, a proposed regulator of leaf senescence ( Woo, H. R., Chung, K. M., Park, J.-H., Oh, S. A., Ahn, T., Hong, S. H., Jang, S. K. and Nam, H. G. (2001) Plant Cell 13, 1779-1790). Our results suggest that selective repression of axillary shoots involves ubiquitin-mediated degradation of as yet unidentified proteins that activate axillary growth.     

FASCIATION AND DICHOTOMOUS BRANCHING IN ECHINOCEREUS (CACTACEAE)

In Echinocereus reichenbachii dichotomous branching and fasciation (cresting) are rare events. Both were found together in only a few of many populations investigated and are interpreted as variants of a single phenomenon. They may occur at any stage of shoot development, but crest meristems arise most commonly on young branches among clusters of normal shoots. Sometimes they appear on unbranched young plants or seedlings, very rarely on older shoots. Dichotomy results from the division of an apical meristem into equal parts each of which functions independently, producing a forked shoot. Fasciation involves the extension of a single meristem into an apical ridge. The product is a flabellate shoot that becomes undulate if growth along the summit continues. In longisection linear meristems appear similar to radial sections of normal shoots; in median sagittal section they have a much extended central mother cell zone within which the cell pattern resembles a rib meristem. Although crest meristems become sluggish or even inactive with age, localized renewed growth may occur spontaneously or be induced by injury. In this species the random production of normal shoots from crest meristems (defasciation) was not observed, but if much or all of such a meristem is removed, branches may arise from lateral areoles, and these are always normal. It seems, therefore, that whatever induces fasciation in E. reichenbachii originates in and is restricted to the apical meristem and its immediate vicinity.     

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.     

Morphogenesis in Selaginella. III. Meristem determination and cell differentiation.

Selaginella willdenovii Baker is a prostrate vascular cryptogam with a dorsiventral stem. At each major branching of the stem tip a dorsal and a ventral angle meristem are formed. The ventral meristem becomes determined as a root and the dorsal meristem as a shoot. Indoleacetic acid (IAA) is transported basipetally in the stem and has been found to be the regulatory agent for meristem determination both in vitro and in vivo.Growth measurements of intact plants indicated that the sequence of development for each stem unit is frond expansion, internodal elongation, ventral meristem growth as a root, and dorsal meristem growth as a shoot. The principal experimental findings of this study are as follows. Triiodobenzoic acid (TIBA), an inhibitor of auxin transport alters the normal pattern of development in intact plants, causing ventral meristems to develop as shoots and dorsal meristems to develop precociously. Dorsal meristems grown in sterile culture on an auxin-free medium develop as shoots, but in the presence of IAA develop as roots. Meristems transferred after excision from auxin-free to plus-auxin medium on successive days showed an increasing tendency to develop as shoots, with more than 50% doing so by day 5. The mitotic index is low at the time of excision of the meristem, rises to a peak on day 5 and then declines.     

A morphological and quantitative characterization of early floral development in apple (Malus x domestica Borkh.)

Apple is an important crop and a focus of research worldwide. However, some aspects of floral commitment and morphogenesis remain unclear. A detailed characterization of bourse shoot apex development was undertaken to provide a framework for future genetic, molecular and physiological studies. Eight morphologically distinct stages of shoot apex development, prior to winter dormancy, were defined. Based on measurements of meristem diameter, two stages of vegetative development were recognized. Vegetative meristems were flat, and either narrow (stage 0) or broad (stage 1). Pronounced doming of the apex marked stage 2. During stage 3, the domed meristem initiated four to six lateral floral meristems and subtending bracts before converting to a terminal floral meristem (stage 4). The terminal floral meristem proceeded directly with bractlet and sepal initiation, while lateral floral meristems initiated bractlets (stage 5). Sepal initiation began on the basal lateral flower (stage 6) and continued in an acropetal direction until all floral meristems had completed sepal initiation (stage 7). In this study, only stage 0 and stage 7 apices were observed in dormant buds, indicating that stages 1-6 are transient. The results suggest that broadening of the apex (stage 1) is the first morphological sign of commitment to flowering.     

棉花花芽分化及部分内源激素变化规律的研究

棉花（Ｇｏｓｓｙｐｉｕｍ ｈｉｒｓｕｔｕｍ）的腋芽原基,有的将来发育成叶枝;有的将来发育成果枝。这２种不同命运的腋芽,在其刚分化的初期就表现出了不同的解剖学特征。将来发育为叶枝的腋芽,其生长锥呈圆锥形或扁圆球形,体积较小,原套层数为１－２层;而将来发育为果枝的腋芽,其生长锥为圆柱形,顶端表面平坦,体积较大,原套层数为２－３层。从子叶展平后到肉眼可见花芽（现蕾）,连续测茎尖的内源ＡＢＡ及ＩＡＡ的含量     

Flowering and apical meristem growth dynamics

The shoot apical meristem generates stem, leaves, and lateralshoot meristems during the entire shoot ontogeny. Vegetativeleaves are generated by the meristem in the vegetative developmentalphase, while in the reproductive phase either bracts subtendinglateral flower primordia (or paraclades), or perianth and strictlyreproductive organs are formed. Meristem growth is fully characterizedby the principal growth rates, directions, volumetric, and arealgrowth rates. Growth modelling or sequential in vivo methodsof meristem observation complemented by growth quantificationallow the above growth variables to be estimated. Indirectly,growth is assessed by cell division rates and other cell cycleparameters. Temporal and spatial changes of growth and geometrytake place at the meristem during the transition from the vegetativeto the reproductive phase. During the vegetative phase, meristemgrowth is generally indeterminate. In the reproductive phaseit is almost always determinate, but the extent of determinacydepends on the inflorescence architecture. In the vegetativephase the central meristem zone is the slowest growing region.The transition from the vegetative to the reproductive phaseis accompanied by an increase in mitotic activity in this zone.The more determinate is the meristem growth, the stronger isthis mitotic activation. However, regardless of the extent ofthe activation, in angiosperms the tunica/corpus structure ofthe meristem is preserved and therefore the mitotic activityof germ line cells remains relatively low. In the case of thethoroughly studied model angiosperm plant Arabidopsis thaliana,it is important to recognize that the flower primordium developsin the axil of a rudimentary bract. Another important featureof growth of the inflorescence shoot apical meristem is theheterogeneity of the peripheral zone. Finally, the role of mechanicalfactors in growth and functioning of the meristem needs furtherinvestigation. Key words: Flower primordium, geometry, growth, inflorescence, shoot apical meristem, transition from vegetative to reproductive phase Received 4 October 2007; Revised 5 November 2007 Accepted 6 November 2007     

DIFFERENTIATION OF LATERAL SHOOTS AS THORNS IN ULEX EUROPAEUS

Ulex europaeus is a much-branched shrub with small, narrow, spine-tipped leaves and axillary thorn shoots. The origin and development of axillary shoots was studied as a basis for understanding the changes that occur in the axillary shoot apex as it differentiates into a thorn. Axillary bud primordia are derived from detached portions of the apical meristem of the primary shoot. Bud primordia in the axils of juvenile leaves on seedlings develop as leafy shoots while those in the axils of adult leaves become thorns. A variable degree of vegetative development prior to thorn differentiation is exhibited among these secondary thorn shoots even on the same axis. Commonly the meristems of secondary axillary shoots initiate 3–9 bracteal leaves with tertiary axillary buds before differentiating as thorns. In other cases the meristems develop a greater number of leaves and tertiary buds as thorn differentiation is delayed. The initial stages in the differentiation of secondary shoot meristems as thorns are detected between plastochrons 10–20, depending on vigor of the parent shoot. A study of successive lateral buds on a shoot shows an abrupt conversion from vegetative development to thorn differentiation. The conversion involves the termination of meristematic activity of the apex and cessation of leaf initiation. Within the apex a vertical elongation of cells of the rib meristem initials and their immediate derivatives commences the attenuation of the apex which results in the pointed thorn. All cells of the apex elongate parallel to the axis and proceed to sclerify basipetally. Back of the apex some cortical cells in which cell division has persisted longer differentiate as chlorenchyma. Although no new leaves are initiated during the extension of the apex, provascular strands are present in the thorn tip. Fibrovascular bundles and bundles of cortical fibers not associated with vascular tissue differentiate in the thorn tip and are correlated in position with successive incipient leaves in the expected phyllotactic sequence, the more developed bundles being related to the first incipient leaves. Some secondary shoots displayed variable atypical patterns of meristem differentiation such as abrupt conversion of the apex resulting in sclerification with limited cell elongation and small, inhibited leaves. These observations raise questions concerning the nature of thorn induction and the commitment of meristems to thorns.     

SYMMETRY AND DEVELOPMENT OF LIMNOBIUM SPONGIA (HYDROCHARITACEAE)

Limnobium spongia produces upright vegetative axes and prostrate stolons. The upright axes bear new stolons, whereas the stolons bear new upright axes and fertile and sterile branching systems. Upright axes and fertile and sterile branching systems are all interpreted to have sympodial growth. However, it was not determined whether growth of stolons is monopodial or sympodial. Both stolons and upright axes branch in alternate plastochrons, and branching is achieved solely by the bifurcation of apical meristems. Each meristematic bifurcation is interpreted to represent the formation of a precocious lateral bud. The upright axes develop from presumed precocious lateral buds on stolons, whereas such buds on upright axes produce renewal shoots. Limnobium spongia exhibits a marked degree of mirror-image symmetry.     

Auxin Depletion from the Leaf Axil Conditions Competence for Axillary Meristem Formation in Arabidopsis and Tomato

The enormous variation in architecture of flowering plants is based to a large extent on their ability to form new axes of growth throughout their life span. Secondary growth is initiated from groups of pluripotent cells, called meristems, which are established in the axils of leaves. Such meristems form lateral organs and develop into a side shoot or a flower, depending on the developmental status of the plant and environmental conditions. The phytohormone auxin is well known to play an important role in inhibiting the outgrowth of axillary buds, a phenomenon known as apical dominance. However, the role of auxin in the process of axillary meristem formation is largely unknown. In this study, we show in the model species Arabidopsis thaliana and tomato (Solanum lycopersicum) that auxin is depleted from leaf axils during vegetative development. Disruption of polar auxin transport compromises auxin depletion from the leaf axil and axillary meristem initiation. Ectopic auxin biosynthesis in leaf axils interferes with axillary meristem formation, whereas repression of auxin signaling in polar auxin transport mutants can largely rescue their branching defects. These results strongly suggest that depletion of auxin from leaf axils is a prerequisite for axillary meristem formation during vegetative development.     

The Arabidopsis dual-affinity nitrate transporter gene AtNRT1.1 (CHL1) is activated and functions in nascent organ development during vegetative and reproductive growth

The AtNRT1.1 (CHL1) transporter provides a primary mechanism for nitrate uptake in Arabidopsis and is expected to localize to the epidermis and cortex of the mature root, where the bulk of nitrate uptake occurs. Using fusions to GFP/GUS marker genes, we found CHL1 expression concentrated in the tips of primary and lateral roots, with very low signals in the epidermis and cortex. A time-course study showed that CHL1 is activated in the primary root tip early in seedling development and at the earliest stages of lateral root formation. Strong CHL1 expression also was found in shoots, concentrated in young leaves and developing flower buds but not in the shoot meristem. These expression patterns were confirmed by immunolocalization and led us to examine CHL1 function specifically in the growth of developing organs. chl1 mutants showed a reduction in the growth of nascent roots, stems, leaves, and flower buds. The growth of nascent primary roots was inhibited in the mutants even in the absence of added nitrate, whereas elongation of lateral root primordia was inhibited specifically at low nitrate and acidic pH. Interestingly, chl1 mutants also displayed a late-flowering phenotype. These results indicate that CHL1 is activated and functions in the growth of nascent organs in both shoots and roots during vegetative and reproductive growth.