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The aim of this study is to determine the combined effect of copper and hydrodynamic conditions on the response of certain biomarkers of an aquatic macrophyte, namely Myriophyllum alterniflorum . Watermilfoil biomarkers are monitored in a synthetic medium enriched or not with copper (100 μg.L −1 ) for 21 days in aquarium systems (150 L), under three hydrodynamic conditions: laminar, turbulent, and calm. The studied biomarkers are: respiratory and photosynthetic activities; concentrations of chlorophyll a , b and carotenoids; osmotic potential; hydrogen peroxide content; and growth. In addition, Cu contents in water and in Myriophyllum alterniflorum (roots and shoots) are investigated. The hydrodynamic conditions only affect watermilfoil morphology. Copper accumulates less in turbulent zones; moreover, it is more likely to accumulate in shoots than in roots, except within the calm zone. Cu leads to: i) a significant increase in H 2 O 2 content, ii) a decrease in root growth, pigment content, osmotic potential, photosynthesis and respiration rates, and iii) an inhibition of shoot branching. Differential effects are also observed between younger and older parts, thus indicating the benefit of considering these two plant parts separately in water quality biomonitoring.

International audience; Citrus seedlings (Citrus sinensis L. Osbeck x Poncirus trifoliata Blanco) were used to describe the effects of different N treatments on the NH4+ influx mediated by high- and low-affinity transport systems (HATS and LATS, respectively) and CitAMT1 gene expression. Results show that Citrus plants favor NH4+ over NO3- influx mediated by HATS and LATS when both N sources are present in the nutrient solution and Citrus plants display a much higher capacity to take up NH4+ than NO3-. Furthermore, NH4+ exerts a regulatory effect on NH4+ HATS activity and CitAMT1 expression, both are down-regulated by high N status of the plant, but specifically stimulated by NH4+ and the balance between these two opposite effects depends on the prior nutrition regime of the plant. On the other hand, supply of NO3- inhibits CitAMT1 expression but doesn't affect NH4+ HATS activity on the roots. To explain this discrepancy, it is possible that other CitAMT1 transporters, up-regulated by N limitation, but not repressed by NO3- could be involved in the stimulation of NH4+ HATS activity under pure NO3- nutrition or CitAMT1 transporter could be regulated at the post-transcriptional level.

The organogenesis of nitrogen-fixing nodules in legume plants is initiated in specific root cortical cells and regulated by long-distance signaling and carbon allocation. Here, we explore cell-to-cell communication processes that occur during nodule initiation in Medicago species and their functional relevance using a combination of fluorescent tracers, electron microscopy, and transgenic plants. Nodule initiation induced symplasmic continuity between the phloem and nodule initials. Macromolecules such as green fluorescent protein could traffic across short or long distances from the phloem into these primordial cells. The created symplasmic field was regulated throughout nodule development. Furthermore, Medicago truncatula transgenic plants expressing a viral movement protein showed increased nodulation. Hence, the establishment of this symplasmic field may be a critical element for the control of nodule organogenesis.

International audience; Salt stress limits crop yield and sustainable agriculture in most arid and semiarid regions of the world. Arbuscular mycorrhizal fungi (AMF) are considered bio-ameliorators of soil salinity tolerance in plants. In evaluating AMF as significant predictors of mycorrhizal ecology, precise quantifiable changes in plant biomass and nutrient uptake under salt stress are crucial factors. Therefore, the objective of the present study was to analyze the magnitude of the effects of AMF inoculation on growth and nutrient uptake of plants under salt stress through meta-analyses. For this, data were compared in the context of mycorrhizal host plant species, plant family and functional group, herbaceous vs. woody plants, annual vs. perennial plants, and the level of salinity across 43 studies. Results indicate that, under saline conditions, AMF inoculation significantly increased total, shoot, and root biomass as well as phosphorous (P), nitrogen (N), and potassium (K) uptake. Activities of the antioxidant enzymes superoxide dismutase, catalase, peroxidase, and ascorbate peroxidase also increased significantly in mycorrhizal compared to nonmycorrhizal plants growing under salt stress. In addition, sodium (Na) uptake decreased significantly in mycorrhizal plants, while changes in proline accumulation were not significant. Across most subsets of the data analysis, identities of AMF (Glomus fasciculatum) and host plants (Acacia nilotica, herbs, woody and perennial) were found to be essential in understanding plant responses to salinity stress. For the analyzed dataset, it is concluded that under salt stress, mycorrhizal plants have extensive root traits and mycorrhizal morphological traits which help the uptake of more P and K, together with the enhanced production of antioxidant enzymes resulting in salt stress alleviation and increased plant biomass.

International audience; In plants, the shoot apical meristem (SAM) serves as a reservoir of pluripotent stem cells from which all above ground organs originate. To sustain proper growth, the SAM must maintain homeostasis between the self-renewal of pluripotent stem cells and cell recruitment for lateral organ formation. At the core of the network that regulates this homeostasis in Arabidopsis are the WUSCHEL (WUS) transcription factor specifying stem cell fate and the CLAVATA (CLV) ligand-receptor system limiting WUS expression. In this study, we identified the ERECTA (ER) pathway as a second receptor kinase signaling pathway that regulates WUS expression, and therefore shoot apical and floral meristem size, independently of the CLV pathway. We demonstrate that reduction in class III HD-ZIP and ER function together leads to a significant increase in WUS expression, resulting in extremely enlarged shoot meristems and a switch from spiral to whorled vegetative phyllotaxy. We further show that strong upregulation of WUS in the inflorescence meristem leads to ectopic expression of the AGAMOUS homeotic gene to a level that switches cell fate from floral meristem founder cell to carpel founder cell, suggesting an indirect role for ER in regulating floral meristem identity. This work illustrates the delicate balance between stem cell specification and differentiation in the meristem and shows that a shift in this balance leads to abnormal phyllotaxy and to altered reproductive cell fate.

International audience; Plant-beneficial effects of bacteria are often underestimated, especially for well-studied strains associated with pathogenicity or originating from other environments. We assessed the impact of seed inoculation with the emblematic bacterial models Agrobacterium tumefaciens C58 (plasmid-cured) or Escherichia coli K-12 on maize seedlings in nonsterile soil. Compared with the noninoculated control, root biomass (with A. tumefaciens or E. coli) and shoot biomass (with A. tumefaciens) were enhanced at 10 days for 'PR37Y15' but not 'DK315', as found with the phytostimulator Azospirillum brasilense UAP-154 (positive control). In roots as well as in shoots, Agrobacterium tumefaciens and E. coli triggered similar (in PR37Y15) or different (in DK315) changes in the high-performance liquid chromatography profiles of secondary metabolites (especially benzoxazinoids), distinct from those of Azospirillum brasilense UAP-154. Genome sequence analysis revealed homologs of nitrite reductase genes nirK and nirBD and siderophore synthesis genes for Agrobacterium tumefaciens, as well as homologs of nitrite reductase genes nirBD and phosphatase genes phoA and appA in E. coli, whose contribution to phytostimulation will require experimental assessment. In conclusion, the two emblematic bacterial models had a systemic impact on maize secondary metabolism and resulted in unexpected phytostimulation of seedlings in the Azospirillum sp.-responsive cultivar.

International audience; Studies related to phytoremediation by conifers are still at their beginning. Thus, we investigated the ability of a hybrid larch (Larix x eurolepis) to accumulate cadmium (Cd). One-month-old clonal plantlets grown in vitro were exposed for 1 week to a high Cd concentration (1.5 mM). No significant effect was observed on root and shoot biomass, root length, and needle number as a result of Cd treatment. Leaf photosynthetic pigment content and total soluble protein concentration in roots and shoots remained unchanged compared to control plantlets. Taken together, these results suggested that hybrid larch tolerated Cd in our conditions. The high Cd concentration in shoots (200 μg Cd gram(-1) dry weight) showed the good capacity of larch to translocate Cd and thus a potential use of this species for phytoremediation. Furthermore, under our conditions, phytochelatin biosynthesis pathway was slightly stimulated, suggesting that this pathway did not reach the threshold and/or another mechanism of Cd storage may be involved to explain larch tolerance to Cd.

International audience; Practically all human populations are environmentally exposed to cadmium (Cd), mostly through plant-derived food. A growing body of epidemiological evidence suggests that there is no margin of safety between current Cd exposure levels and the threshold for adverse health effects and, hence, there is an urgent need to lower human Cd intake. Here we review recent studies on rice (Oryza sativa) and Cd-hyperaccumulating plants that have led to important insights into the processes controlling the passage of Cd from the soil to edible plant organs. The emerging molecular understanding of Cd uptake, root retention, root-to-shoot translocation and grain loading will enable the development of low Cd-accumulating crops.

International audience; Legume roots develop two types of lateral organs, lateral roots and nodules. Nodules develop as a result of a symbiotic interaction with rhizobia and provide a niche for the bacteria to fix atmospheric nitrogen for the plant. * The Arabidopsis NAC1 transcription factor is involved in lateral root formation, and is regulated post-transcriptionally by miRNA164 and by SINAT5-dependent ubiquitination. We analyzed in Medicago truncatula the role of the closest NAC1 homolog in lateral root formation and in nodulation. * MtNAC1 shows a different expression pattern in response to auxin than its Arabidopsis homolog and no changes in lateral root number or nodulation were observed in plants affected in MtNAC1 expression. In addition, no interaction was found with SINA E3 ligases, suggesting that post-translational regulation of MtNAC1 does not occur in M. truncatula. Similar to what was found in Arabidopsis, a conserved miR164 target site was retrieved in MtNAC1, which reduced protein accumulation of a GFP-miR164 sensor. Furthermore, miR164 and MtNAC1 show an overlapping expression pattern in symbiotic nodules, and overexpression of this miRNA led to a reduction in nodule number. * This work suggests that regulatory pathways controlling a conserved transcription factor are complex and divergent between M. truncatula and Arabidopsis.

International audience; The possibility of eradicating the pelargonium flower break virus (PFBV) and pelargonium line pattern virus (PLPV) by cryotherapy of axillary shoot apices was investigated using five Pelargonium cultivars. Viruses were detected by DAS-ELISA and their location was determined by immunolocalization. Apex culture did not permit elimination of PFBV and only 15% regenerated plants of 'Stellar Artie' cultivar were ELISA PLPV-negative. Plants regenerated from cryotherapy-treated apices were tested by DAS-ELISA after a 3-month in vitro culture period. Viruses were not detected in 25% and 50% of the plants tested for PFBV and PLPV, respectively. However, immunolocalization carried out on apices originating from cryopreserved shoot tips sampled from DAS-ELISA negative plants showed that they were still virus-infected. Using immunolocalization, PFBV and PLPV could be detected in Pelargoniuni apices, even in the meristematic dome. However, viral particles were more numerous in basal zone cells than in meristematic cells. Our results demonstrate that PFBV and PLPV are present within meristematic cells and that cryopreservation can partly reduce the quantity of these viruses in Pelargonium plants but not eliminate them totally. Additional knowledge on localization and behaviour of viruses during cryopreservation is essential to optimize cryotherapy and plant genetic resource management.

Equipe AFEF ‘Architecture et Fonctionnement des Espèces fruitières’ ‘Team AFFS Architecture and Functioning of Fruit Species’ Contact: tjchiou@gate.sinica.edu.tw, manuel.echeverria@univ-perp.fr; International audience; Here we report on the characterization of rice osa-miR827 and its two target genes, OsSPX-MFS1 and OsSPX-MFS2, which encode SPX-MFS proteins predicted to be implicated in phosphate (Pi) sensing or transport. We first show by Northern blot analysis that osa-miR827 is strongly induced by Pi starvation in both shoots and roots. Hybridization of osa-miR827 in situ confirms its strong induction by Pi starvation, with signals concentrated in mesophyll, epidermis and ground tissues of roots. In parallel, we analyzed the responses of the two OsSPX-MFS1 and OsSPX-MFS2 gene targets to Pi starvation. OsSPX-MFS1 mRNA is mainly expressed in shoots under sufficient Pi supply while its expression is reduced on Pi starvation, revealing a direct relationship between induction of osa-miR827 and down-regulation of OsSPX-MFS1. In contrast, OsSPX-MFS2 responds in a diametrically opposed manner to Pi starvation. The accumulation of OsSPX-MFS2 mRNA is dramatically enhanced under Pi starvation, suggesting the involvement of complex regulation of osa-miR827 and its two target genes. We further produced transgenic rice lines overexpressing osa-miR827 and T-DNA knockout mutant lines in which the expression of osa-miR827 is abolished. Compared with wild-type controls, both target mRNAs exhibit similar changes, their expression being reduced and increased in overexpressing and knockout lines, respectively. This suggests that OsSPX-MFS1 and OsSPX-MFS2 are both negatively regulated by osa-miR827 abundance although they respond differently to external Pi conditions. We propose that this is a complex mechanism comprising fine tuning of spatial or temporal regulation of both targets by osa-miR827.

International audience; The shoot apical meristem of Arabidopsis thaliana contains a reservoir of pluripotent stem cells that functions as a continuous source of new cells for organ formation during development. The SAM forms during embryogenesis, when it becomes stratified into specific cell layers and zones that can be delineated based on morphological and molecular criteria. The primary SAM produces all the aerial structures of the adult plant, and alterations in SAM organization or function can have profound effects on vegetative and reproductive plant morphology. Such SAM-specific defects can be identified, evaluated, and quantified using specialized microscopic and histological techniques.

Geraniums (Pelargonium spp.) are among the most popular bedding and pot plants (25% of the French domestic market). On one hand, as vegetatively propagated plants, Pelargonium are submitted to pathogen pressure. On the other hand, innovation via interspecific hybridisation faces some difficulties. In this chapter, the two first protocols (from seeds and meristems) explain how in vitro plants free of virus could be obtained. The development of this technique is the long-term preservation of genetic resources via meristem cryopreservation. The third protocol describes propagation of Pelargonium with limited risks of variation. This technique also allows the constitution and the maintenance of a plant-stock from which explants can be taken for other studies. The two last protocols describe plant regenerations from leaf discs and mesophyll protoplasts, used for gene transfer and somatic hybridisation. These protocols were established mainly with Pelargonium x hortorum cultivars, but we propose possible solutions for the other species: P. x peltatum, P. x domesticum, P. capitatum and P. graveolens.

International audience; This study presents the isolation and characterization of a novel nonsymbiotic Hb gene from sessile oak (Quercus petraea) seedlings, herein designated QpHb1. The cellular and tissue expression of QpHb1 was analysed by Northern blotting and in situ hybridization. The encoded protein was predicted to consist of 161 amino acid residues, and shares 71 and 51% amino acid sequence identity with the Arabidopsis class 1 and 2 nonsymbiotic Hb, respectively. Northern blot analysis revealed that QpHb1 was strongly expressed in roots. Spatial expression analysis of QpHb1 in the root apical region of sessile oak by in situ hybridization indicated that transcripts were mostly abundant in protoxylem cell initials, some cortical cells and the protoderm. In addition, when comparing the expression profile of QpHb1 in sessile and pedunculate oak (Quercus robur), two species with contrasted hypoxia tolerance, the transcript level of QpHb1 rose early in the most flood-tolerant species, pedunculate oak, during root submergence. The spatial-temporal expression of QpHb1 suggests that this gene could participate in perception and signalling during hypoxia.

In all land plants, cellulose is synthesized from hexameric plasma membrane complexes. Indirect evidence suggests that in vascular plants the complexes involved in primary wall synthesis contain three distinct cellulose synthase catalytic subunits (CESAs). In this study, we show that CESA3 and CESA6 fused to GFP are expressed in the same cells and at the same time in the hypocotyl of etiolated seedlings and migrate with comparable velocities along linear trajectories at the cell surface. We also show that CESA3 and CESA6 can be coimmunoprecipitated from detergent-solubilized extracts, their protein levels decrease in mutants for either CESA3 , CESA6 , or CESA1 and CESA3, CESA6 and also CESA1 can physically interact in vivo as shown by bimolecular fluorescence complementation. We also demonstrate that CESA6-related CESA5 and CESA2 are partially, but not completely, redundant with CESA6 and most likely compete with CESA6 for the same position in the cellulose synthesis complex. Using promoter-β-glucuronidase fusions we show that CESA5 , CESA6 , and CESA2 have distinct overlapping expression patterns in hypocotyl and root corresponding to different stages of cellular development. Together, these data provide evidence for the existence of binding sites for three distinct CESA subunits in primary wall cellulose synthase complexes, with two positions being invariably occupied by CESA1 and CESA3, whereas at least three isoforms compete for the third position. Participation of the latter three isoforms might fine-tune the CESA complexes for the deposition of microfibrils at distinct cellular growth stages.

Although most of the components of the cell cycle machinery are conserved in all eukaryotes, plants differ strikingly from animals by the absence of a homolog of E-type cyclin, an important regulator involved in G1/S-checkpoint control in animals. By contrast, plants contain a complex range of A-type cyclins, with no fewer than 10 members in Arabidopsis. We previously identified the tobacco A-type cyclin Nicta;CYCA3;2 as an early G1/S-activated gene. Here, we show that antisense expression of Nicta;CYCA3;2 in tobacco plants induces defects in embryo formation and impairs callus formation from leaf explants. The green fluorescent protein (GFP)-Nicta;CYCA3;2 fusion protein was localized in the nucleoplasm. Transgenic tobacco plants overproducing GFP-Nicta;CYCA3;2 could not be regenerated from leaf disc transformation, whereas some transgenic Arabidopsis plants were obtained by the floral-dip transformation method. Arabidopsis plants that overproduce GFP-Nicta;CYCA3;2 showed reduced cell differentiation and endoreplication and a dramatically modified morphology. Calli regenerated from leaf explants of these transgenic Arabidopsis plants were defective in shoot and root regeneration. We propose that Nicta;CYCA3;2 has important functions, analogous to those of cyclin E in animals, in the control of plant cell division and differentiation.