Pierre Lejeune

Physiological Signals That Induce Flowering.

The timing of the transition from vegetative growth to flowering is of paramount importance in agriculture, horticulture, and plant breeding because flowering is the first step of sexual reproduction. Studies to understand how this transition is controlled have occupied countless physiologists during the past half century and have produced an almost unmanageably large amount of information (Bernier et al., 1981a; Halevy, 1985-1989; Bernier, 1988; Kinet, 1993). A majority of plants use environmental cues to regulate the transition to flowering because all individuals of a species must flower synchronously for successful outcrossing and because all species must complete their sexual reproduction under favorable externa1 conditions. Any environmental variables exhibiting regular seasonal changes are potential factors that control the transition to flowering. The major factors are photoperiod, temperature, and water availability. Plants that do not require a particular photoperiod or temperature to flower, i.e., the so-called “autonomous-flowering” plants, are usually sensitive to irradiance. The environmental factors are perceived by different parts of the plant. Photoperiod and irradiance are perceived mainly by mature leaves in intact plants. Temperature is perceived by all plant parts, although low temperature (vernalization) is often perceived mainly by the shoot apex. Water availability is perceived by the root system. There are strong interactions between these different factors, so that each factor can change the threshold value for the effectiveness of the others. Plants, as opportunists, will thus make use of a different critical factor in different environments. Melilotus officinalis, for example, is a biennial with a vernalization requirement in temperate zones and an annual long-day (LD) plant with no cold requirement in arctic regions. In photoperiodic species, such as the short-day (SD) plant Pharbitis nil and the LD plant Silene armeria, flowering in unfavorable photoperiods can be caused by changing temperature, irradiance, or nutrition or by removing the roots. Similarly, in some late-flowering mutants of Arabidopsis, vernalization and an increase in the proportion of far-red light in the light source can substitute for one another in promoting the transition to flowering (Martinez-Zapater and Somerville, 1990; Bagnall, 1992). Clearly, there are alternate pathways to flowering in most, if

The role of carbohydrates in the induction of flowering in Arabidopsis thaliana : comparison between the wild type and a starchless mutant

Abstract. In order to test whether an increased export of carbohydrates by leaves and starch mobilization are critical for floral transition in Arabidopsis thaliana, the Columbia ecotype as well as its starchless mutant pgm and starch-in-excess mutant sex1 were investigated. Induction of flowering was achieved by exposure of plants to either one long day (LD) or one displaced short day (DSD). The following conclusions were drawn: (i) Both the pgm and sex1 mutants have a late-flowering phenotype in days shorter than 16 h. (ii) When inductive treatments cause a large percentage of induced plants, there is always a large, early and transient increase in carbohydrate export from leaves. By contrast, when an inductive treatment results in only a low percentage of induced plants (pgm plants exposed to one DSD), the export of carbohydrates from leaves is not increased, supporting the idea that phloem carbohydrates have a critical function in floral transition. (iii) Starch mobilization is not required to obtain an increased carbohydrate export when induction is by one LD (extended period of photosynthesis), but is absolutely essential when induction is by one DSD (period of photosynthesis unaffected). (iv) Floral induction apparently increases the capability of the leaf phloem-loading system.

Sucrose increase during floral induction in the phloem sap collected at the apical part of the shoot of the long-day plant Sinapis alba L.

Sinapis alba L., a long-day plant, has been induced to flower either by a single 22-h-long photoperiod or by an 8-h short photoperiod displaced by 10 h in a 24 h cycle. The ehtylenediametetraacetate method previously used for leaf exudation was modified to collect phloem sap at the apical part of the shoot. Carbohydrates in the phloem sap have been analysed comparatively in vegetative and induced plants, using high-performance liquid chromatography and refractometry. Sucrose was the major sugar detected. A dramatic increase of its flux in the apical sap occurred early and transiently during the floral transition in plants induced by both long days and displaced short days. These results indicate a message-like role for sucrose since they fit nicely with previous observations indicating that an early event in the floral transition in S. alba is the accumulation of sucrose in the meristem.

Establishing research strategies, methodologies and technologies to link genomics and proteomics to seagrass productivity, community metabolism, and ecosystem carbon fluxes

A complete understanding of the mechanistic basis of marine ecosystem functioning is only possible through integrative and interdisciplinary research. This enables the prediction of change and possibly the mitigation of the consequences of anthropogenic impacts. One major aim of the European Cooperation in Science and Technology (COST) Action ES0609 “Seagrasses productivity. From genes to ecosystem management,” is the calibration and synthesis of various methods and the development of innovative techniques and protocols for studying seagrass ecosystems. During 10 days, 20 researchers representing a range of disciplines (molecular biology, physiology, botany, ecology, oceanography, and underwater acoustics) gathered at The Station de Recherches Sous-marines et Océanographiques (STARESO, Corsica) to study together the nearby Posidonia oceanica meadow. STARESO is located in an oligotrophic area classified as “pristine site” where environmental disturbances caused by anthropogenic pressure are exceptionally low. The healthy P. oceanica meadow, which grows in front of the research station, colonizes the sea bottom from the surface to 37 m depth. During the study, genomic and proteomic approaches were integrated with ecophysiological and physical approaches with the aim of understanding changes in seagrass productivity and metabolism at different depths and along daily cycles. In this paper we report details on the approaches utilized and we forecast the potential of the data that will come from this synergistic approach not only for P. oceanica but for seagrasses in general.

Shoot-root interactions during floral transition: A possible role for cytokinins

This review discusses the role of roots in the control of floral transition. Classical physiological understanding indicates that flowering and root initiation and/or elongation are usually antagonistic processes. In several species, root removal promotes flowering. High or low temperatures applied to the root system also influence flower initiation in some plants. These observations clearly implicate the root system as possessing at least partial control of flowering. In various species, the inhibitory effect of the root can be mimicked by cytokinins which are known to be produced by roots. A few studies revealed, however, that the effect of exogenous cytokinins is strongly dependent on other factors such as the applied concentration, the environmental conditions, and the time and site of application, and that promotion or inhibition can be observed. These findings indicate that there is a permissive range of cytokinin concentrations and that the endogenous status of the plant affects cytokinin action, which is not comparable in all plant organs. Studies with the long-day plant Sinapis alba and the short-day plant Xanthium strumarium suggested the existence of a shoot-to-root signal, which is under photoperiodic control and affects cytokinin synthesis in and/or release from the roots. As a result, cytokinin levels are altered in the plant; these changes, triggered by the inductive treatment, may be transient and are not necessarily similar in different plant organs. Remarkably, in all species investigated, increases in cytokinin levels, most usually in buds or phloem sap, have been detected during the floral transition, suggesting that cytokinins could be required. A role as a mitotic stimulus is possible, as indicated by the work with Sinapis.

C, N, P concentrations and requirements of flowering Posidonia oceanica shoots

The carbon, nitrogen and phosphorus contents in flowering and nonflowering shoots were compared after an important flowering event occurred in the Posidonia meadow of the Bay of Calvi. The flower formation caused a significant increase of C and a significant decrease of N concentrations in intermediate and adult leaves. Minimum daily requirements in mgshoot-1day-1 of 3.4 and 4.8 of C, 0.09 and 0.09 N, 0.01 and 0.02 of P respectively for nonflowering and flowering shoots were calculated. It shows that additional quantities of C and P are required for the inflorescence elaboration. The unchanged quantity of N required by the shoot for the inflorescence elaboration and the significant modification of N concentration in intermediate and adult leaves suggests that N is limited in the environment and that an efficient resorption of N occurs from leaves to ensure the inflorescence formation.

Contrasting Patterns of Phytoplankton Assemblages in Two Coastal Ecosystems in Relation to Environmental Factors (Corsica, NW Mediterranean Sea)

Corsica Island is a sub-basin of the Northwestern Mediterranean Sea, with hydrological features typical of both oligotrophic systems and eutrophic coastal zones. Phytoplankton assemblages in two coastal ecosystems of Corsica (the deep Bay of Calvi and the shallow littoral of Bastia) show contrasting patterns over a one-year cycle. In order to determine what drives these variations, seasonal changes in littoral phytoplankton are considered together with environmental parameters. Our methodology combined a survey of the physico-chemical structure of the subsurface water with a characterization of the phytoplankton community structure. Sampling provided a detailed record of the seasonal changes and successions that occur in these two areas. Results showed that the two sampled stations presented different phytoplankton abundance and distribution patterns, notably during the winter–spring bloom period. Successions in pico-, nano-, and microphytoplankton communities appeared mainly driven by differences in the ability to acquire nutrients, and in community-specific growth rates. Phytoplankton structure and dynamics are discussed in relation to available data on the Northwestern Mediterranean OPEN ACCESS Diversity 2014, 6 297 Sea. These results confirm that integrated monitoring of coastal areas is a requisite for gaining a proper understanding of marine ecosystems.

Hormonal control of ear initiation in a stress Maise (**Zea mays**) inbred line

A cold-sensitive maize (Zea mays L.) inbred was used as a model for investigating the interactions between growth regulators, reproductive development, and environmental stress. In this genotype, a chilling treatment given just before floral transition caused the topmost ear to abort and be replaced at maturity by a sterile, leaf-like, structure. Exogenous applications of the synthetic auxin 2,4-dichlorophenoxyacetic acid or of the cytokinin benzyladenine respectively mimicked or prevented the abortive response caused by chilling. Chilling also induced a moderate decrease in the content of endogenous indoleacetic acid (IAA) in the apical shoot tissues. By contrast, zeatin-type cytokinins decreased dramatically (5–8 fold), both in the apical shoot tissues and in the xylem exudate of chilled plants. Overall, the ratio of free-IAA to zeatin-cytokinins was increased in the apical shoot of chilled plants. Our results suggest that: (1) ear abortion induced by chilling might be related to an altered cytokinin content; (2) the number of developing ears may be limited by the endogenous levels of cytokinins just before floral transition; and (3) cytokinins may have a potential for increasing yield in maize.

Abundance and Diversity of Crypto- and Necto-Benthic Coastal Fish Are Higher in Marine Forests than in Structurally Less Complex Macroalgal Assemblages

In Mediterranean subtidal rocky reefs, Cystoseira spp. (Phaeophyceae) form dense canopies up to 1 m high. Such habitats, called ‘Cystoseira forests’, are regressing across the entire Mediterranean Sea due to multiple anthropogenic stressors, as are other large brown algae forests worldwide. Cystoseira forests are being replaced by structurally less complex habitats, but little information is available regarding the potential difference in the structure and composition of fish assemblages between these habitats. To fill this void, we compared necto-benthic (NB) and crypto-benthic (CB) fish assemblage structures between Cystoseira forests and two habitats usually replacing the forests (turf and barren), in two sampling regions (Corsica and Menorca). We sampled NB fish using Underwater Visual Census (UVC) and CB fish using Enclosed Anaesthetic Station Vacuuming (EASV), since UVC is known to underestimate the diversity and density of the ‘hard to spot’ CB fish. We found that both taxonomic diversity and total density of NB and CB fish were highest in Cystoseira forests and lowest in barrens, while turfs, that could be sampled only at Menorca, showed intermediate values. Conversely, total biomass of NB and CB fish did not differ between habitats because the larger average size of fish in barrens (and turfs) compensated for their lower densities. The NB families Labridae and Serranidae, and the CB families Blenniidae, Cliniidae, Gobiidae, Trypterigiidae and Scorpaenidae, were more abundant in forests. The NB taxa Diplodus spp. and Thalassoma pavo were more abundant in barrens. Our study highlights the importance of using EASV for sampling CB fish, and shows that Cystoseira forests support rich and diversified fish assemblages. This evidence suggests that the ongoing loss of Cystoseira forests may impair coastal fish assemblages and related goods and services to humans, and stresses the need to implement strategies for the successful conservation and/or recovery of marine forests.

From mechanical to chemical impact of anchoring in seagrasses: The premises of anthropogenic patch generation in Posidonia oceanica meadows

Intensive anchoring of leisure boats in seagrass meadows leads to mechanical damages. This anthropogenic impact creates bare mat patches that are not easily recolonized by the plant. Several tools are used to study human impacts on the structure of seagrass meadows but they are not able to assess the indirect and long term implication of mechanical destruction. We chose to investigate the possible changes in the substrate chemistry given contrasted boat impacts. Our observations show that hydrogen sulfide concentrations remain high at 15 and 20m depth (42.6μM and 18.8μM) several months after the highest period of anchoring during the summer. Moreover, our multidisciplinary study reveals that anchoring impacts of large boats at 15 and 20m depth can potentially change the seascape structure. By taking into account both structural and chemical assessments, different managing strategies must be applied for coastal areas under anthropogenic pressures.