No long-term costs of meristem allocation to flowering in stoloniferous Trifolium species

Clonal plants propagate by means of clonal growth and sexual reproduction. The commitment of meristems to branching and flowering govern the expression of these two mutually exclusive life-history functions. We used a modelling and an experimental approach to examine the consequences of a structural trade-off between flowering and clonal growth on future growth and fitness in stoloniferous species with a determinate module architecture. The model revealed negative effects of flowering on vegetative growth due to a structural trade-off at the meristem level. Total fecundity was maximized at intermediate flowering frequencies. In addition, optimal meristem commitment to flowering depended strongly on the time available for growth and reproduction. This indicates an interaction between optimal flowering frequency, the length of the growing period and the rate of ontogenetic development. The greenhouse study made use of 15 genotypes of two closely related, stoloniferous Trifolium species. Despite the existence of a structural trade-off at the meristem level, we found no evidence for costs of flowering on the whole-plant level. High allocation to flowering did not result in reduced plant performance (biomass and module production) and total fecundity, indicating that there were no demographic costs of meristem investment to different life-history functions. Flowering frequencies never exceeded the model prediction for optimal commitment of meristems to sexual reproduction, suggesting strong past selection to eliminate high levels of meristem allocation to flowering. Hence, clonal growth seems to have evolutionary priority over sexual reproduction in our species. This revised version was published online in November 2006 with corrections to the Cover Date.     

THE COST OF MERISTEM LIMITATION IN POLYGONUM ARENASTRUM: NEGATIVE GENETIC CORRELATIONS BETWEEN FECUNDITY AND GROWTH

Growth and reproduction in higher plants depend on meristems, which have three developmental fates. A meristem can become reproductive, but doing so terminates its activity, it can differentiate vegetatively, or it can remain quiescent for extended periods. The first two fates are mutually exclusive, and only the second leads to the production of additional meristems for subsequent growth and reproduction. In Polygonum arenastrum (frequently referred to as P. aviculare in North American Floras), an annual species lacking quiescent meristems, a quantitative genetic analysis of inbred full-sibling families revealed genetic variation in the developmental pattern of axillary meristem commitment to vegetative growth versus reproduction. Developmental variation resulted in family differences in the age of first reproduction, in age-specific fecundity and growth, and in final plant size and reproductive output. Furthermore, there were strong negative genetic correlations between age-specific growth and fecundity. Early commitment of meristems to reproduction favors high early fecundity, but reduces the number of meristems available for vegetative differentiation, and leads to lowered growth rates and fecundity later in life, when meristems are limiting. Conversely, meristem commitment to vegetative growth early in life results in low early fecundity but high late fecundity and growth. Meristem limitation, like resource limitation, is a proximate mechanism that generates trade-offs between life history traits. Differences between meristem limitation and resource limitation are discussed. Meristem limitation leads automatically to a senescent life history because of the determinate fate of reproductive meristems. Developmental characters were also found to be genetically correlated with metamer characters (leaf size, internode length) and seed size in this selfing species. The pattern of correlation is suggestive of selection for particular suites of life history and morphological characters.     

Plant clonality, mutation, diplontic selection and mutational meltdown

Apomixis is a very common characteristic in vascular plants. It occurs in two general forms: either subversion of the sexual system (agamospermous seeds or apogamous sporophytes in non-seed plants) or vegetative reproduction. In this communication, only the mutational consequences of vegetative reproduction are considered. Vegetative reproduction involves the replication of apical meristems, especially shoot apical meristems. Three general types of shoot apical meristems occur in the vascular plants: single tetrahedral apical initial, unstratified with impermanent initials and stratified with impermanent initials. Each meristem type has different consequences with regard to mutation, diplontic selection and the possibility of mutational meltdown.  © 2003 The Linnean Society of London. Biological Journal of the Linnean Society , 2003, 79 , 61–67.     

barren inflorescence2 regulates axillary meristem development in the maize inflorescence

Organogenesis in plants is controlled by meristems. Shoot apical meristems form at the apex of the plant and produce leaf primordia on their flanks. Axillary meristems, which form in the axils of leaf primordia, give rise to branches and flowers and therefore play a critical role in plant architecture and reproduction. To understand how axillary meristems are initiated and maintained, we characterized the barren inflorescence2 mutant, which affects axillary meristems in the maize inflorescence. Scanning electron microscopy, histology and RNA in situ hybridization using knotted1 as a marker for meristematic tissue show that barren inflorescence2 mutants make fewer branches owing to a defect in branch meristem initiation. The construction of the double mutant between barren inflorescence2 and tasselsheath reveals that the function of barren inflorescence2 is specific to the formation of branch meristems rather than bract leaf primordia. Normal maize inflorescences sequentially produce three types of axillary meristem: branch meristem, spikelet meristem and floral meristem. Introgression of the barren inflorescence2 mutant into genetic backgrounds in which the phenotype was weaker illustrates additional roles of barren inflorescence2 in these axillary meristems. Branch, spikelet and floral meristems that form in these lines are defective, resulting in the production of fewer floral structures. Because the defects involve the number of organs produced at each stage of development, we conclude that barren inflorescence2 is required for maintenance of all types of axillary meristem in the inflorescence. This defect allows us to infer the sequence of events that takes place during maize inflorescence development. Furthermore, the defect in branch meristem formation provides insight into the role of knotted1 and barren inflorescence2 in axillary meristem initiation.     

Significance of the simultaneous growth of vegetative and reproductive organs in the prostrate annual Chamaesyce maculata (L.) Small (Euphorbiaceae)

To elucidate the significance of the simultaneous growth of vegetative and reproductive organs in the prostrate annual Chamaesyce maculata (L.) Small (Euphorbiaceae) from the standpoint of meristem allocation, we investigated plant architecture, meristem allocation, and the spatial and temporal patterns in vegetative growth and reproduction in the reproductive stage. The numbers of secondary and tertiary shoots successively increased by branching in the reproductive stage, and the sum of shoot length was greater in secondary shoots than in primary shoots. The specific shoot length (shoot length per shoot biomass) was greater in lateral shoots than in primary shoots, indicating efficient lateral shoot elongation. The internode length was shorter in secondary shoots than in primary shoots, increasing the number of nodes per shoot length in secondary shoots. Many nodes on a shoot generated two meristems, one of which committed to a flower and one to a lateral shoot. The number of reproductive meristems was greatest in tertiary shoots, and 96% of total reproductive meristems on shoots were generated in lateral shoots. On almost all nodes, the reproductive meristem developed into a flower, and 95–98% of the flowers produced a fruit. Therefore, vegetative growth by branching in the reproductive stage contributed to the increase in reproductive outputs. From the standpoint of meristem allocation, the simultaneous growth of vegetative and reproductive organs in prostrate plant species might be important for increasing the number of growth and reproductive meristems, resulting in the increase in reproductive outputs.     

barren inflorescence2 Encodes a co-ortholog of the PINOID serine/threonine kinase and is required for organogenesis during inflorescence and vegetative development in maize

Organogenesis in plants is controlled by meristems. Axillary meristems, which give rise to branches and flowers, play a critical role in plant architecture and reproduction. Maize (Zea mays) and rice (Oryza sativa) have additional types of axillary meristems in the inflorescence compared to Arabidopsis (Arabidopsis thaliana) and thus provide an excellent model system to study axillary meristem initiation. Previously, we characterized the barren inflorescence2 (bif2) mutant in maize and showed that bif2 plays a key role in axillary meristem and lateral primordia initiation in the inflorescence. In this article, we cloned bif2 by transposon tagging. Isolation of bif2-like genes from seven other grasses, along with phylogenetic analysis, showed that bif2 is a co-ortholog of PINOID (PID), which regulates auxin transport in Arabidopsis. Expression analysis showed that bif2 is expressed in all axillary meristems and lateral primordia during inflorescence and vegetative development in maize and rice. Further phenotypic analysis of bif2 mutants in maize illustrates additional roles of bif2 during vegetative development. We propose that bif2/PID sequence and expression are conserved between grasses and Arabidopsis, attesting to the important role they play in development. We provide further support that bif2, and by analogy PID, is required for initiation of both axillary meristems and lateral primordia.     

Developmental analysis of the evolutionary origin of vegetative propagules in Mimulus gemmiparus (Scrophulariaceae)

Mimulus gemmiparus (Scrophulariaceae), a rare endemic of Colorado, has a novel life history that depends on an unusual method of vegetative reproduction. The plants are functionally annuals; however, reproduction is asexual via propagules that have been termed gemmae. The morphological identity and the evolutionary antecedent of these propagules are unclear. We approached this problem through comparative developmental analyses of M. gemmiparus and the presumed progenitor species, Mimulus guttatus. In M. gemmiparus there are two meristems initiated in the axil of each leaf primordium. The distal meristem has the potential to produce either a lateral branch or a flower, and the proximal meristem becomes a vegetative propagule (the gemma) that is ultimately surrounded by an expanded, ensheathing petiole. The first leaves of the propagules are thickened and are the site of nutrient storage. Consequently, these propagules can be characterized morphologically as brood bulbils. Mimulus guttatus also has two meristems in each leaf axil; however, the proximal meristem typically remains dormant and serves no function in the life history of this species. Based on architectural and developmental correspondence, we hypothesize that the propagule of M. gemmiparus is homologous to the proximal meristem of M. guttatus. Comparative analysis shows that evolution of the bulbil has involved both the incorporation of features present in shoots of M. guttatus and the acquisition of novel features.     

Effect of water stress on root meristems in woody and herbaceous plants during the first stage of development

We investigated the effect of water stress on the root system architecture of pine saplings and pea seedlings during the first stage of development. Attention was focused on meristematic tissue situated at the root tip because of the leading role played by the tissue in the planning of root system architecture. The data showed that both species are extremely sensitive and that plants arrest their growth immediately during water stress treatment. When stress treatment was not intense, both species recovered growth but presented modifications in the root system architecture. In pine saplings, the modification in root system architecture was the consequence of fine root meristems not recovering from water stress. The saplings survived by producing new lateral meristems from the cortical tannin zone above the fine root tip. In the case of pea seedlings, the meristematic tissues in the primary root arrested proliferation during water stress although they recovered when the event occurred during the first hours of germination. The response was different when water stress was enforced on older seedlings. In this case, root meristems never completely recovered their proliferation despite the increase in proline content observed in the cells. The modification of root system architecture in pea seedlings depended on the arrest of primary root elongation and the formation of new root laterals. As regards the primary roots, water stress treatment induced along the axis the formation of irregular ‘swellings’ in the cortical zone above the meristematic zone. Anatomical investigations suggested that such swellings may have derived from the changes in elongation direction of derivatives. The formation of new laterals was observed in hydroponic cultures when water stress treatment was enforced slowly and prolonged for a long time. The production of new lateral meristems may have been a similar response of woody and herbaceous plants to water stress conditions. It is not known whether these new meristems present characteristics of resistance to water stress. This revised version was published online in June 2006 with corrections to the Cover Date.     

Herbivore-mediated ecological costs of reproduction shape the life history of an iteroparous plant

Plant reproduction yields immediate fitness benefits but can be costly in terms of survival, growth, and future fecundity. Life-history theory posits that reproductive strategies are shaped by trade-offs between current and future fitness that result from these direct costs of reproduction. Plant reproduction may also incur indirect ecological costs if it increases susceptibility to herbivores. Yet ecological costs of reproduction have received little empirical attention and remain poorly integrated into life-history theory. Here, we provide evidence for herbivore-mediated ecological costs of reproduction, and we develop theory to examine how these costs influence plant life-history strategies. Field experiments with an iteroparous cactus (Opuntia imbricata) indicated that greater reproductive effort (proportion of meristems allocated to reproduction) led to greater attack by a cactus-feeding insect (Narnia pallidicornis) and that damage by this herbivore reduced reproductive success. A dynamic programming model predicted strongly divergent optimal reproductive strategies when ecological costs were included, compared with when these costs were ignored. Meristem allocation by cacti in the field matched the optimal strategy expected under ecological costs of reproduction. The results indicate that plant reproductive allocation can strongly influence the intensity of interactions with herbivores and that associated ecological costs can play an important selective role in the evolution of plant life histories.     

Induction of Zygotic Polyembryos in Wheat: Influence of Auxin Polar Transport

The effects of two auxin polar transport inhibitors, N-1-naphthylphthalamic acid (NPA) and 3,3[prime],4[prime],5,7-pentahydroxyflavone (quercetin), on attaining bilateral symmetry from radial symmetry during early wheat embryogenesis were investigated by using an in vitro culture system. Although NPA and quercetin belong to two different classes of auxin transport inhibitors, the phytotropins and the flavonoids, respectively, they induced the same specific abnormal phenotypes during embryo development. These abnormal embryos differentiated multiple meristems (i.e., multiple shoot and root meristems) and multiple organs (i.e., multiple coleoptiles and scutella). Multiple shoot apical meristem phenotypes were characterized by partly multiplied embryonic axes and supernumerary scutella. The differentiation of multiple primary roots in addition to multiple shoot meristems and multiple scutella led to the formation of polyembryos. The occurrence of multiple shoot meristem phenotypes depended on the concentration of the inhibitor and the developmental stage of the isolated embryo. Embryos treated with NPA or quercetin developed multiple radicle phenotypes less frequently than they developed multiple shoot meristem phenotypes. Our observations suggest that the root meristem differentiates later than the shoot meristem. Our data support the hypothesis that polar transport of auxin has a determining influence on the differentiation of the embryonic axis and the scutellum.     

Tolerance of Gentianella campestris in relation to damage intensity: an interplay between apical dominance and herbivory

Meristem allocation models suggest that the patterns of compensatory regrowth responses following grazing vary, depending on (i) the number of latent meristems that escape from being damaged, and (ii) the activation sensitivity of the meristems in relation to the degree of damage. We examined the shape of compensatory responses in two late-flowering populations (59°20′N and 65°45′N) of the field gentian. Plants of equal initial sizes were randomly assigned to four treatment groups with 0, 10, 50 and 75% removal of the main stalk. The plants were clipped before flowering, and their performance was studied at the end of the growing season. The northern population showed a linear decrease in shoot biomass and fecundity with increasing biomass removal, while the response in the southern population was quadratic with maximum performance at the damage level of 50% clipping. This nonlinear shape depended upon the activation sensitivity of dormant meristems in relation to their position along the main stem. The highest plant performance was achieved by inflicting intermediate damage which induced regrowth from basally located meristems. In contrast, the topmost branches took over the dominance role of the main stem after minor apical damage (10% clipping). Consequently, the breakage of apical dominance is a necessary precondition of vigorous regrowth in this species. However, compensation in the field gentian is unlikely to be a mere incidental by-product of apical dominance. The ability to regrow from basally located meristems that escape from being damaged by grazing may well be a sign of adaptation to moderate levels of shoot damage. This revised version was published online in July 2006 with corrections to the Cover Date.     

Density and the commitment of apical meristems to clonal growth and reproduction in Hieracium pilosella

Summary We examined responses to population density in the commitment of apical meristems to reproduction and clonal growth in a rosette-forming, stoloniferous herb (Hieracium pilosella). Despite close physiological coupling between the evocation of the terminal inflorescence bud and the development of one or more axillary buds into stolons, the allocation of meristems was extremely plastic.Genets at the higher sowing densities showed density-dependent mortality consistent with self-thinning along a-3/2 trajectory. The probability of inflorescence evocation and associated stolon development was negatively dependent on surviving density. The proportinal distribution of primary stolons amongst genets became strikingly more unequal (expressed as the Gini coefficient) with increasing density. Clonal growth was resolved into the number of primary stolons per stoloniferous genet and the extent of stolon branching (i.e. number of apices per primary stolon); both showed strongly negative density-dependence. Reproduction, expressed as the mean number of flowering capitula per stoloniferous genet, declined 15-fold with increasing density; although theoretically expected to be unity, greater values resulted from capitulum production by attached secondary rosettes and lower values reflected the increasing abortion rate of inflorescence buds with increasing density.Both the total number of apices produced per unit area and the corresponding number of reproductive apices were maximal at intermediate surviving densities (700–1,000 m^-2). The balance between reproductive and clonal growth may be expressed as the probability of an apical meristem producing a capitulum, that also peaked sharply at intermediate density. This finding does not conform with linear models that predict a shift from vegetative growth to sexual reproduction with increasing population density.     

Plastic morphological response to density in Spergularia marina (L.) Griseb.: Leaf demography and meristem activity

In order to explain, in terms of a morphologically based plastic response, the absence of density-dependent mortality, and the regulation of reproduction in the halophilous annual Spergularia marina , leaf demography and the use of meristems were studied in populations of three different densities. Leaf birth and death rates were almost constant at the higher densities, whereas at the lowest density both increased throughout most of the observation period. This increase was caused by the addition of new vegetative shoots, each one resembling the individuals growing at higher densities in having an almost constant leaf birth and death rate. The capacity to adjust leaf death rate to birth rate with density was high. There was no difference in the proportions of meristems used for flowering as the density increased. After the first flower had been produced, each following node produced a flower. The proportions of meristems used for vegetative shoots were strongly density-dependent; more shoots were produced at lower densities. The regulation of flower production was almost entirely caused by the addition of vegetative shoots.     

Developmental disaster1: A novel mutation causing defects during vegetative and inflorescence development in maize (Zea mays, Poaceae)

Axillary meristems, which give rise to branches and flowers, play a critical role in plant architecture and reproduction. To understand how axillary meristems initiate, we have screened for mutants with defects in axillary meristem initiation to uncover the genes controlling this process. These mutants, called the barren class of mutants in maize (Zea mays), have defects in axillary meristem initiation during both vegetative and reproductive development. Here, we identify and characterize a new member of the barren class of mutants named Developmental disaster1 (Dvd1), due to the pleiotropic effects of the mutation. Similar to the barren mutants, Dvd1 mutants have fewer branches, spikelets, florets, and floral organs in the inflorescence due to defects in the initiation of axillary meristems. Furthermore, double mutant analysis with teosinte branched1 shows that dvd1 also functions in axillary meristems during vegetative development. However, unlike the barren mutants, Dvd1 mutants are semidwarf due to the production of shorter internodes, and they produce leaves in the inflorescence due to the outgrowth of bract leaf primordia. The suite of defects seen in Dvd1 mutants, together with the genetic interaction of Dvd1 with barren inflorescence2, suggests that dvd1 is a novel regulator of axillary meristem and internode development.     

Factors Controlling the Planktonic Community in the Shallow-Lake of Créteil,France

A two-year study on the seasonal pattern of plankton was performed in a shallow, polymictic, eutrophic lake. Autogenic successions typical of stratified lakes did not occur. Only small edible algae developed throughout the two years and Cyanophyceae rarely appeared, and did not bloom. Algal sequences depended on wind-induced periodic mixing of water. The seasonal pattern of Rotifers varied greatly from year to year and some species depended on sporadic increases of algae. In contrast, the seasonal pattern of microcrustacea was more regular. Resting stages played an important role in zooplankton development in this variable environment. Successful fish reproduction depended a great deal on climatic conditions. The changes from year to year in predation pressure from young fish appeared to be an important factor controlling zoo-plankton structure.     

Cytogenetic peculiarities of cell genesis in apical meristems under gametophytic apomixis (using autonomous apomicts of the Asteraceae as an example)

Cytogenetic peculiarities of cell genesis in apical meristems of apomicts has been analyzed using a series of the Asteraceae species as an example. The extent to which aneu- and mixoploids are spread among plants in the investigated populations of the Asteraceae species is so high (up to 30-60% of the studied plants and their offspring), that it seems reasonable to suppose that their rise is a natural phenomenon. It has been shown that in the aposporous facultative apomict Pilosella officinarum microgametophyte is a relatively stable element of the seed reproduction system from the point of view of caryotypical variation.     

Axillary shoot proliferation and in vitro flowering in an adult giant bamboo, Dendrocalamus giganteus Wall. Ex Munro

Summary Continuous axillary shoot proliferation and in vitro flowering were achieved using single node explants from a mature (over 70-yr-old) field clump of Dendrocalamus giganteus (giant bamboo). The shoots proliferated in a basal Murashige and Skoog medium with 6 mgl⁻¹ (26.6 μM) N⁶-benzyladenine (BA) and 2% sucrose. The rate of shoot proliferation gradually increased to over three-fold before in vitro flowering took place. In vitro flowering was not the expression of a species-specific mechanism believed to occur during gregarious flowering, as the mother clump did not flower. The rate of shoot proliferation decreased at flowering, accompanied by reversion of flowering. The development of axillary meristems into vegetative or generative shoots depended on the level of BA. The possible role of BA, changes in the rate of shoot proliferation decreased at flowering, accompanied by reversion of flowering. The development of axillary meristems into vegetative or generative shoots depended on the level of BA. The possible role of BA, changes in the rate of shoot proliferation leading to build up, and release of stress in relation to flowering and its reversion are discussed.     

Gravi- and Chemotropisms of the Primary Maize Roots when Affected by Lead and Cadmium

Gravity-imposed growth orientation was studied in the roots of three-day-old maize seedlings treated for 3 h with 10^–5to 10^–2-M lead and cadmium nitrate solutions. Cubic agar blocks (1 mm³) containing lead and cadmium nitrate solutions were used to produce unilateral local chemostimulation of roots. Gravistimulation was induced when roots were in the horizontal position or slightly deviated from the initial vertical position at the beginning of chemotropic curvature response. Positive (towards the salt) and negative (away from the salt) chemotropic curvatures were observed most often when meristems of the initially vertical roots were chemostimulated. Negative curvatures were observed most often in response to medium salt concentrations, whereas high concentrations resulted in positive curvatures. Half of the roots with their meristems stimulated by salt solutions still continued growing vertically downward. Most roots exposed to simultaneous gravi- and chemostimulation and exposed to gravistimulation after salt treatment (except at the highest salt concentration) curved downward. It follows that the final growth orientation of these roots depended mostly on gravity. The author concludes that the primary roots of maize seedlings possess high gravitropic and low chemotropic sensitivity.     

Meristem differentiation in Riella helicophylla (Bory et Mont.) Mont. under the influence of auxin or antiauxin

During the development of the unistratose gemmae of Riella helicophylla, the single intercalary meristem of the very young gemmae is subdivided into two lateral meristems. The duration of the cell reproduction cycle increases from the margin to the median part of the gemmae. This polarization within the meristem disappears after addition of the antiauxin PCIB to the culture medium. PCIB leads to a retardation or blockage of the cell cycle during the light period of the culture. Under the influence of PCIB the amount of starch in the chloroplasts is strikingly increased, probably because of a reduction of starch degradation. Addition of sugars compensates the effect of PCIB on the cell cycle. The effects of PCIB are counteracted by auxin. The results are taken as evidence that auxin plays a role in directing the transport of substances needed for the continuation of the cell reproduction cycle between adjacent cells of the meristem.Abbreviations IAA indole-3-acetic acid - 2,4-D 2,4-dichlorophenoxyacetic acid - PCIB p-chlorophenoxyisobutyric acid - G-l-P glucose-l-phosphat - EDTA ethylenediaminetetraacetic acid     

Flowering pathway is regulated by bulb size in Lilium longiflorum (Easter lily)

Lilium longiflorum (Easter lily) vegetative propagation occurs through production of underground bulbs containing apical and axillary meristems. In addition, sexual reproduction is achieved by flowering of elongated shoots above the bulb. It is generally accepted that L. longiflorum has an obligatory requirement for vernalisation and that long day (LD) regime hastens flowering. However, the effect of bulb size and origin, with respect to axillary or apical meristems on flowering, as well as the interactions between these meristems are largely unknown. The aim of this study was to explore the effect of bulb size, vernalisation and photoperiod on L. longiflorum flowering. To this end, we applied vernalisation and photoperiod treatments to the different bulb sizes and used a system of constant ambient temperature of 25 °C, above vernalisation spectrum, to avoid cold‐dependent floral induction during plant growth. Vernalisation and LD hasten flowering in all bulbs. Large, non‐vernalised bulbs invariably remained at a vegetative stage. However, small non‐vernalised bulbs flowered under LD conditions. These results demonstrate for the first time that cold exposure is not an obligatory prerequisite for L. longiflorum flowering, and that an alternative flowering pathway can bypass vernalisation in small bulbs. We suggest that apical dominance interactions determine the distinct flowering pathways of the apical and axillary meristems. Similar floral induction is achieved in propagated bulblets from scaling. These innovative findings in the field of geophyte floral induction represent valuable applicative knowledge for lily production.