Normal and Abnormal Development in the Arabidopsis Vegetative Shoot Apex

Vegetative development in the Arabidopsis shoot apex follows both sequential and repetitive steps. Early in development, the young vegetative meristem is flat and has a rectangular shape with bilateral symmetry. The first pair of leaf primordia is radially symmetrical and is initiated on opposite sides of the meristem. As development proceeds, the meristem changes first to a bilaterally symmetrical trapezoid and then to a radially symmetrical dome. Vegetative development from the domed meristem continues as leaves are initiated in a repetitive manner. Abnormal development of the vegetative shoot apex is described for a number of mutants. The mutants we describe fall into at least three classes: (1) lesions in the shoot apex that do not show an apparent alteration in the shoot apical meristem, (2) lesions in the apical meristem that also (directly or indirectly) alter leaf primordia, and (3) lesions in the apical meristem that alter meristem size and leaf number but not leaf morphology. These mutations provide tools both to genetically analyze vegetative development of the shoot apex and to learn how vegetative development influences floral development.     

Early inflorescence and floral development in Zea mays land race Chapalote (Poaceae)

Tassel and ear primordia were collected from greenhouse-grown specimens of the Mexican maize landrace Chapalote and prepared for scanning electron microscopic (SEM) examination. Measurements of inflorescence apices and spikelet pair primordia (spp) were made from SEM micrographs. Correlation of inflorescence apex diameter with number of spikelet ranks showed no significant difference between tassels and ears, except at the two-rank level where the ear apical meristem had a significantly smaller diameter than corresponding two-ranked tassels. Within individual inflorescences, spp in different ranks enlarged at comparable rates, although the rates from one ear to the next along the stem differed. In both tassels and ears, spp divide to form paired sessile and pedicellate spikelet primordia when the spp is 150 μm wide; ear axes are significantly thicker than tassel axes at the time of bifurcation. The similarities in growth between ear and tassel primordia lend further support to the hypothesis that both the maize tassel and ear are derived from a common inflorescence pattern, a pattern shared with teosinte. Inflorescence primordial growth also suggests that a key character difference between teosinte and maize, distichous vs. polystichous arrangement of spikelets, may be related to size of the apical dome and/or rate of primordium production by the apical meristem. There appears to be more than a single morphological event in the shift from vegetative to reproductive growth. The evocation of axillary buds (ears) is independent of, and temporally separated from, the transition to flowering at the primary shoot apex (tassel).     

The liguleless2 gene of maize functions during the transition from the vegetative to the reproductive shoot apex

During a maize plant's (Zea mays) development, the shoot apical meristem (SAM) generates an apex that proceeds through different phases: juvenile vegetative, adult vegetative and reproductive. During each phase the structures produced are distinguishable from structures produced during the other phases. In this paper, we demonstrate that the LIGULELESS2 (LG2) function is required for an accurate vegetative to reproductive phase transition. The maize gene liguleless2 (lg2) has been shown to encode a basic-leucine zipper (bZIP) protein and to function in narrowing the region from which the ligule and auricle develop in a typical maize leaf. Here we show that lg2 mutant plants can have reduced long tassel branches, extra vegetative leaves and extra husk leaves when compared to wild-type siblings. This indicates a role for the lg2 gene in the vegetative to reproductive phase transition of the shoot apex. We also discuss a potential role for the lg2 gene in general phase transition processes.     

CHANGE IN EPILOBIUM PHYLLOTAXY DURING REPRODUCTIVE TRANSITION

The ontogeny of Epilobium hirsutum grown under natural summer photoperiod in a glasshouse was divided into vegetative, early transitional, transitional, and floral stages. Bijugate phyllotaxy, common to both the vegetative and early transitional stages, is transformed into spiral phyllotaxy during the transitional stage by an initial change in the divergence angle of a single primordium inserted at a unique level on the shoot. Leaf primordia subsequently are inserted in a spiral arrangement in the indeterminate floral shoot apex. The early transitional shoot apical meristem is about 1.5 times the volume of the vegetative meristem but expands at about two-thirds the relative plastochron rate of volume increment of the vegetative meristem. There are progressive decreases in the plastochron and relative plastochron rates of radial and vertical shoot growth through ontogeny. Relative chronological rates of shoot growth, however, are not altered during ontogeny. Spiral transformation results from changes in the relative points of insertion of leaf primordia on the shoot meristem. These changes are accompanied by an increased rate of primordia initiation on a more circular shoot meristem. The change in phyllotaxy during ontogeny is similar to that which was artificially induced by chemical modification of auxin concentration gradients in the shoot apex, with the additional feature that there is an initial increase in the volume of the shoot meristem prior to the natural spiral transformation. Size of the shoot apical meristem, however, appears to have little influence on Epilobium phyllotaxy; but the geometric shape of the meristem is well correlated with bijugate to spiral transformations. This suggests that geometric parameters of the shoot meristem should be considered in theoretical models of phyllotaxy.     

Bi-directional inflorescence development inArabidopsis thaliana: Acropetal initiation of flowers and basipetal initiation of paraclades

In this study we investigated Arabidopsis thaliana (L.) Heynh. inflorescence development by characterizing morphological changes at the shoot apex during the transition to flowering. Sixteen-hour photoperiods were used to synchronously induce flowering in vegetative plants grown for 30 d in non-inductive 8-h photoperiods. During the first inductive cycle, the shoot apical meristem ceased producing leaf primordia and began to produce flower primordia. The differentiation of paraclades (axillary flowering shoots), however, did not occur until after the initiation of multiple flower primordia from the shoot apical meristem. Paraclades were produced by the basipetal activation of buds from the axils of leaf primordia which had been initiated prior to photoperiodic induction. Concurrent with the activation of paraclades was the partial suppression of paraclade-associated leaf primordia, which became bract leaves. The suppression of bract-leaf primordia and the abrupt initiation of flower primordia during the first inductive photoperiod is indicative of a single phase change during the transition to flowering in photoperiodically induced Arabidopsis. Morphogenetic changes characteristic of the transition to flowering in plants grown continuously in 16-h photoperiods were qualitatively equivalent to the changes observed in plants which were photoperiodically induced after 30 d. These results suggest that Arabidopsis has only two phases of development, a vegetative phase and a reproductive phase; and that the production of flower primordia, the differentiation of paraclades from the axils of pre-existing leaf primordia and the elongation of internodes all occur during the reproductive phase.     

TENDRILS OF PASSIFLORA FOETIDA: HISTOGENESIS AND MORPHOLOGY

Passiflora foetida bears an unbranched tendril, one or two laterally situated flowers, and one accessory vegetative bud in the axil of each leaf. The vegetative shoot apex has a single-layered tunica and an inner corpus. The degree of stratification in the peripheral meristem, the discreteness of the central meristem, and its centric and acentric position in the shoot apex are important plastochronic features. The procambium of the lateral leaf trace is close to the site of stipule initiation. The main axillary bud differentiates at the second node below the shoot apex. Adaxial to the bud 1–3 layers of cells form a shell-zone delimiting the bud meristem from the surrounding cells. A group of cells of the bud meristem adjacent to the axis later differentiates as an accessory bud. A second accessory bud also develops from the main bud opposite the previous one. A bud complex then consists of two laterally placed accessory bud primordia and a centrally-situated tendril bud primordium. The two accessory bud primordia differentiate into floral branches. During this development the initiation of a third vegetative accessory bud occurs on the axis just above the insertion of the tendril. This accessory bud develops into a vegetative branch and does not arise from the tissue of the tendril and adjacent two floral buds. The trace of the tendril bud consists of two procambial strands. There is a single strand for the floral branch trace. The tendril primordium grows by marked meristematic activity of its apical region and general intercalary growth.     

Adventitious shoot formation in decapitated dicotyledonous seedlings starts with regeneration of abnormal leaves from cells not located in a shoot apical meristem

Regeneration of new shoots in plant tissue culture is often associated with appearance of abnormally shaped leaves. We used the adventitious shoot regeneration response induced by decapitation (removal of all preformed shoot apical meristems, leaving a single cotyledon) of greenhouse-grown cotyledon-stage seedlings to test the hypothesis that such abnormal leaf formation is a normal regeneration progression following wounding and is not conditioned by tissue culture. To understand why shoot regeneration starts with defective organogenesis, the regeneration response was characterized by morphology and scanning electron and light microscopy in decapitated cotyledon-stage Cucurbita pepo seedlings. Several leaf primordia were observed to regenerate prior to differentiation of a de novo shoot apical meristem from dividing cells on the wound surface. Early regenerating primordia have a greatly distorted structure with dramatically altered dorsoventrality. Aberrant leaf morphogenesis in C. pepo gradually disappears as leaves eventually originate from a de novo adventitious shoot apical meristem, recovering normal phyllotaxis. Similarly, following comparable decapitation of seedlings from a number of families (Chenopodiaceae, Compositae, Convolvulaceae, Cucurbitaceae, Cruciferae, Fabaceae, Malvaceae, Papaveraceae, and Solanaceae) of several dicotyledonous clades (Ranunculales, Caryophyllales, Asterids, and Rosids), stems are regenerated bearing abnormal leaves; the normal leaf shape is gradually recovered. Some of the transient leaf developmental defects observed are similar to responses to mutations in leaf shape or shoot apical meristem function. Many species temporarily express this leaf development pathway, which is manifest in exceptional circumstances such as during recovery from excision of all preformed shoot meristems of a seedling.     

Microarray analysis of vegetative phase change in maize

Vegetative phase change is the developmental transition from the juvenile phase to the adult phase in which a plant becomes competent for sexual reproduction. The gain of ability to flower is often accompanied by changes in patterns of differentiation in newly forming vegetative organs. In maize, juvenile leaves differ from adult leaves in morphology, anatomy and cell wall composition. Whereas the normal sequence of juvenile followed by adult is repeated with every sexual generation, this sequence can be altered in maize by the isolation and culture of the shoot apex from an adult phase plant: an 'adult' meristem so treated reverts to forming juvenile vegetative organs. To begin to unravel the as-yet poorly understood molecular mechanisms underlying phase change in maize, we compared gene expression in two juvenile sample types, leaf 4 and culture-derived leaves 3 or 4, with an adult sample type (leaf 9) using cDNA microarrays. All samples were leaf primordia at plastochron 6. A gene was scored as 'phase induced' if it was up- or downregulated in both juvenile sample types, compared with the adult sample type, with at least a twofold change in gene expression at a P-value of < or =0.005. Some 221 expressed sequence tags (ESTs) were upregulated in juveniles, and 28 ESTs were upregulated in adults. The largest class of juvenile-induced genes was comprised of those involved in photosynthesis, suggesting that maize plants are primed for energy production early in vegetative growth by the developmental induction of photosynthetic genes.     

The shoot apex of Podostemaceae: De novo structure or reduction of the conventional type?

The paper reviews and discusses various interpretations of the shoot apex of Podostemaceae with special reference to subfamily Podostemoideae. Main questions concern (1) the proposed absence of a shoot apical meristem (SAM) in apical “meristemless” shoot tips of Podostemoideae and, as the consequence, the endogenous inception of leaf-borne leaves and branches and (2) the predicted stem bifurcation below a “terminal” dithecous (double-sheathed) leaf positioned instead of a shoot apex, as it is reported for subfamily Podostemoideae. Does the “meristemless” shoot apex represent a true evolutionary novelty? Does the view of stem bifurcation represent a new ramification pattern with the consequence that the “classical root–shoot model” of angiosperms is not valid for Podostemaceae? Both interpretations do not conform to previous studies that are complemented here by new data on the SAM of Zeylanidium olivaceum and Thelethylax minutiflora (Podostemoideae). Although a SAM is difficult to observe in the vegetative shoots of many Podostemoideae, it becomes well visible when the shoot passes into the flowering stage approaching the conspicuous shoot apex of floriferous shoots. The arguments of the absence of a SAM in vegetative shoots are not convincing and the endogenous origin of “leaf-borne leaves” appears questionable. Consequently, the “meristemless” shoot apex cannot be considered as a structure having evolved de novo. In the less advanced subfamilies Tristichoideae and Weddellinoideae, the leaf primordia develop only from a few apical cells of the outer shoot layer. This allows the conclusion that the surface layer of the apex in these subfamilies corresponds to the horizontally spread single-layered apical meristem of subfamily Podostemoideae. Similarly, the view of shoot bifurcation does not conform to the diachsial–sympodial branching pattern occurring in the cymose inflorescences of many Podostemoideae. This fact contradicts the presence of a terminal leaf.     

Effect of environment on the early steps of ear initiation in maize (Zea mays L.)

The effects of environmental conditions on ear-shoot initiation have been investigated in three inbred genotypes of Zea mays L. which are used for seed production. Scanning electron microscopy (SEM) and binocular examination during the vegetative phase showed that axillary meristems are initiated at the same rate as the leaf primordia on the apical meristem, but with a delay of 5.6–7.0 plastochrons, depending on the genotype. Furthermore, the topmost axillary meristem is initiated on the same day as the tassel, whatever the genotype. One of the inbreds (B22) used in this study has been reported to exhibit, in field conditions, a reproductive failure affecting car initiation, causing the topmost car to be replaced at maturity by a sterile, leaf-like, structure. Scanning electron microscopic study of the formation of the abnormal axillary buds indicated that ear failure resulted from the early collapse of the axillary meristem followed by elongation of the prophyll or of the meristem itself. Using controlled environments, ear abortion was mimicked by a chilling treatment (10°C), given just before tassel initiation. Other factors, such as high irradiance and flooding, enhanced the abortive response. The critical stage for the main car was just before the initiation of the topmost axillary meristem which also corresponded to tassel initiation. Chilling the plants before or after tassel initiation either induced an acclimation response or had no effect. The three inbreds showed differential responses to the stress treatment, indicating that a genetic factor is implicated as well. It is suggested that chilling causes a perturbation of apical dominance which, in the responsive genotypes, represses axillary meristem development. The use of a stress-sensitive inbred such as B22 as a model system could yield some interesting clues to the mechanism of endogenous control of ear initiation in maize.     

Developmental anatomy of the reproductive shoot in <Emphasis Type="Italic">Hydrobryum japonicum</Emphasis> (Podostemaceae)

Podostemaceae are unusual aquatic angiosperms adapting to extreme habitats, i.e., rapids and waterfalls, and have unique morphologies. We investigated the developmental anatomy of reproductive shoots scattered on crustose roots of Hydrobryum japonicum by scanning electron microscopy and using semi-thin serial sections. Two developmental patterns were observed: bracts arise either continuously from an area of meristematic cells that has produced leaves, or within differentiated root ground tissue beneath, and internal to, leaf base scars after an interruption. In both patterns, the bract primordia arise endogenously at the base of youngest bracts in the absence of shoot apical meristem, involving vacuolated-cell detachment to each bract separately. The different transition patterns of reproductive shoot development may be caused by different stages of parental vegetative shoots. The floral meristem arises between the two youngest bracts, and is similarly accompanied by cell degeneration. In contrast, the floral organs, including the spathella, arise exogenously from the meristem. Bract development, like vegetative leaf development, is unique to this podostemad, while floral-organ development is conserved.     

Anatomy and development of the fern sporophyte

Recent research on the developmental anatomy and morphology of the fern sporophyte is reviewed. Detailed histological and experimental studies of the organization of the fern shoot apical meristem have reconfirmed the recently controversial role of the shoot apical cell as the single apical initial of the meristem. The shoot apical meristem is nevertheless an anatomically and functionally complex structure with a strongly zoned cytohistological organization. Fern shoot apex organization can be compared with that of seed plants. The control of leaf initiation and phyllotaxy remains poorly understood. Studies differ as to whether leaf initiation in ferns involves one leaf mother cell or a multicellular region of the shoot apex. The concept of non-appendicular fronds is refuted for living ferns. The later developmental changes in the determinate leaf apical and marginal meristems of the leaf primordium form an area that is still largely unexplored but could be investigated by methods similar to those used to study shoot and root apices. Branching in ferns is morphologiclaly and developmentally diverse. There is apparently more than one developmental mode of dichotomous branching, and several modes of lateral bud formation have been described, including the phyllogenous initiation of branches at the base of leaf primordia. Developmental changes in bud meristems related to apical dominance, inhibition, and bud activation is another major area for continued study. The traditional concept of the role of the root apical cell has been reestablished by studies similar to those made of the shoot apex. Detailed ultrastructural investigations of the root ofAzolla have given a sophisticated new picture of developmental processes in that organ. Fern roots show remarkably precise patterns of histogenesis in relation to apical segmentation. The formation of secondary vascular tissue inBotrychium suggests that the Ophioglossales may be related to the seed plants. The causal relationship of leaf (and branch and root) formation and the initiation of vascular tissue in the shoot needs more study. Although still poorly understood, protoxylem systems in ferns are variable and may have morphological and systematic significance. Recent investigations of hydraulic conductance in fern stems have found possible correlations of conductance levels with growth forms. The anatomical diversity of ferns makes comparative functional anatomy a promising field for future study.     

Ontogeny of the reproductive shoot apex ofClethra barbinervis,especially on the superficial view

The structure and the ontogenetic process of the reproductive shoot apex forming a terminal inflorescence ofClethra barbinervis were examined, especially concerning the superficial view of the apex. The system of contact parastichies is 2+3 in phyllotaxis in the vegetative phase, changing to 5+8 for bract arrangement in the reproductive phase. At the same time the size of the apex is conspicuously enlarged. The size of the foliage leaf primordia in the vegetative phase is larger than that of the bract primordia in the reproductive phase. The radial cell files, which are clear in the vegetative shoot apex, are not recognizable at least in the early stage of the reproductive phase. The author proposes a close correlation between the appearance of the radial cell files, as well as the construction of the apical sectors, and the sizes of the shoot apex and leaf primordia. It may be proposed also that the construction of the apical sectors is closely correlated with the phyllotaxis.     

Inflorescence development in the “standard exotic” maize,Argentine popcorn (Poaceae)

Argentine popcorn is an exotic race considered by some to be similar to the earliest cultivated maize. We used scanning electron microscopy to examine inflorescence development in both the tassel and ear. In our material, and under our conditions, both two-ranked central tassel spikes and two-ranked ears were observed as well as more typical four-ranked structures. Subsequent development of spikelets and florets was similar to that observed in other varieties of maize and in their close relatives—the teosintes. We suggest that the switch from two-ranked to four-ranked inflorescences (a key trait difference between teosinte and maize) may be due to a change in developmental timing allowing an additional meristem bifurcation of axillary branch primordia prior to the initiation of spikelet pair primordia.     

Initiation of the Vascular System and the Transition to Flowering in Early Tassels of Zea mays Land Race Chapalote (Poaceae)

Serial transections of young tassels of (Zea mays land race) chapalote revealed relationships between the vascular system in its procambial state and the lateral primordia along the axis. A lateral tassel primordium usually consists of an indefinite rim with a prolongation that will become a tassel branch or spikelet pair. A lateral tassel primordium usually develops via modifications of the vegetative leaf primordium in which the leaf apex is enhanced but the leaf base and the bud it produces are suppressed. The clearest sign of the transition from the vegetative state to the tassel is the scale leaf, which is intermediate in form between a vegetative leaf and a lateral tassel primordium. Procambial traces differentiate in isolation in the tassel axis in response to the lateral tassel primordia. Adjacent procambial traces then link axially into sympodia to initiate the three-dimensional vascular system of the tassel axis. Older sympodia occur near the center of the axis interior to more recently initiated procambial traces. Procambial continuity does not occur between the tassel axis and the lateral primordia until isolated traces in the lateral primordia link with the sympodia in the tassel axis. The transition from distichy to polystichy by the lateral tassel primordia occurs as the narrowing of the leaf base makes space available on the tassel axis for lateral primordia out of the vegetative distichous plane.