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Ecole Doctorale COMPLEXITE DU VIVANT – Fiche Projet CONCOURS
Fiche à nommer selon le format Nom_Prénom_Projet17, à enregistrer en format PDF et à renvoyer à l'adresse:
[email protected].
Nom et prénom du directeur de thèse (et si besoin du co-directeur) : CHARRIER Bénédicte
Le directeur de thèse et le co-directeur doivent impérativement avoir l'HDR ou équivalent
Coordonnées
Tel :02-98-29-56-53
e-mail :[email protected]
Nom et prénom du co-encadrant (non HdR ) (s’il y a lieu) :
Coordonnées Tel e-mail
Y-a-t-il un candidat déjà identifié pour le projet:
OUI
Nom et prénom du responsable de l’équipe : Bénédicte Charrier
Intitulé de l'équipe : Morphogenèse des Macro Algues
Nombre de chercheurs et enseignants-chercheurs statutaires de l’équipe titulaires d’une HDR (ou équivalent) : 1
Nom et prénom du responsable d'unité ou de département: Catherine Boyen
Intitulé et N° d’unité ou de département: UMR8227
Signature du directeur d'unité ou de département (vaut avis favorable pour le dépôt du projet) :
Avis favorable
Titre du projet de thèse : Mechanisms of rhizoid growth in the green alga Ulva mutabilis
Spécialité : Biologie cellulaire, Evolution
Résumé du projet de thèse (1 page maximum, en anglais)
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Ecole Doctorale COMPLEXITE DU VIVANT – Fiche Projet CONCOURS
Rhizoid growth in the green alga Ulva
Ulva is a marine green seaweed (responsible for the green tides). Phylogenetically, it belongs to the Chlorophyta lineage,
which is a sister group of the Streptophyta comprising the land plants. Ulva mutabilis (Wichard, Front Plant Sci 2015) is
currently developed as a model for the Ulvophyceae class. Its genome sequence will soon be published (first one for a
multicellular chlorophyte) and genetic transformation has been developed (Oertel et al., J Phycol 2014).
U. mutabilis morphology is simple: the thallus is divided into the blade, made of 2 cell layers, stem cells at the basis of the
blade, and rhizoids. The latter are thin row of cells. When the rhizoid is 4 cell long, the terminal cell of the rhizoid undergoes
an asymmetrical division, while the other 3 ones stop dividing. This cell ensures the further growth of the overall rhizoid.
Recent data suggests that the rhizoid growth in this species relies on a different molecular toolkit than that described in the
other organisms (not published). The present PhD project will characterize rhizoid growth in U. mutabilis. This will enlarge
the range of organisms for evo-devo studies.
Objectives
1- Acquire knowledge about the mechanisms of rhizoid growth in the multicellular green alga Ulva mutabilis.
2- Compare with the other tip-growing organisms from the green lineage (filaments of the moss Physcomitrella, land plant
pollen tubes and root hairs, the green filamentous alga Chara), and from other multicellular algae (among which the brown
algae Ectocarpus filaments).
3- Develop supported scenarii about the evolutionary history of this cellular process.
Approaches
1) Modeling of Ulva rhizoid growth; this part will rely on protocols recently developed by the host team. It includes first the
acquisition of biological data and secondly a relevant model. The frame of the model needs to be defined beforehand by
answering basic questions:

Is Ulva rhizoid growing by tip-growth? This will be carried out by pulse-chase experiments using e.g. calcofluor as
a stain for cell wall component (cellulose and callose). Time-lapse observations will allow to define the growth area
in the rhizoid and its growth rate in different conditions (mainly osmotic conditions).

How does the cell wall expand during growth? Cell wall can reach different conformation during growth (expansion
or compression, rotation). This pattern reflects the mechanical stretches of the cell wall during growth and is
informative regarding the underlying mechanisms. This will be achieved by time-lapse experiments monitoring
fluorescent markers oaded on the cell wall (Rabillé & Charrier, in prep).

What turgor in rhizoid cells? In plant cells, turgor provides the basic force promoting cell growth. It will be measured
either by the technique of Limit plasmolysis or by using a pressure probe.

What is the detailed cell shape of rhizoid apical cell? Curvatures modulates the tensile stress undergone by the cell
wall. Cell curvatures in apical cells will be inferred from the drawing of the contour of these cells. This work will be
carried out with B. Billoud, bioinformatician in the team.

What are the mechanical properties of the cell wall? Both physiological experiments (cell wall yielding by osmotic
chocks) and biophysics measurements (atomic force microscopy; collaboration Plateform BiBS, INRA Nantes) will
allow to infer the x,y elasticity and plasticity of the cell wall.

Which specific features for rhizoid apical cells?: Transmission electronic microscopy performed on longitudinal
section of U. mutabilis rhizoids will show the sub-cellular organisation and details about the cell components, with
a particular focus on the cell wall thickness (collaboration S. Le Panse, Platform Merimage, SB Roscoff).
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Ecole Doctorale COMPLEXITE DU VIVANT – Fiche Projet CONCOURS
These biological parameters will be used to “feed” biophysical models.
Modeling rhizoid growth

Ulva rhizoid growth will be modelled using elastic or viscoplastic models (depending on the results obtained above)
previously used for land plants apical cells or other algal cells (e.g. Chara, Streptophyta). Simulations will allow to
identify the master parameters in rhizoid growth in this species.
2) Identify genes involved in Ulva rhizoid growth; this part will be carried out in collaboration with Dr T. Wichard’s lab,
Univ Jena.
Transgenic lines of Ulva expressing promoter-trap constructs fused to GFP will be produced by applying the protocol
developed by the partner team (Oertel et al. J Phycol 2014). Lines expressing GFP in the rhizoids (which construct is inserted
downstream from a start codon) will be screened for and isolated, and the genes into which the construct has been inserted
will be identified e.g. by inverse PCR. The identity of the genes will be put in the context of the biophysical mechanism
characterised in part 1).
Conclusion:
Thanks to the biophysical simulation approach, the PhD project will provide a mechanistic and dynamic view of how growth
occurs in Ulva rhizoid, which might be further supported by the identification of molecular factors through the genetic
approach (promoter-trapping). The results will be compared to the mechanisms known in moss filament, pollen tube, root
hair, fungal and other algal rhizoid growth, and will supply insight about the evolution of this cellular process.
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Ecole Doctorale COMPLEXITE DU VIVANT – Fiche Projet CONCOURS
Thèses actuellement en cours dans l’équipe
Tous les encadrements doivent être indiqués (y compris les co-directions avec un autre HDR, et les encadrements dans
le cadre de programmes doctoraux tels qu'IPV, FDV...)
Nom et Prénom du doctorant
Hervé Rabillé
Directeur(s) de thèse
Bénédicte Charrier
Année de 1ère
inscription
2015
ED
515
Financement
Région Bretagne +
UPMC
Trois publications récentes du directeur de thèse (du co-directeur ou du co-encadrant s’il y a lieu). Mettre en gras le
nom du directeur de thèse.
Jia F, Ben Amar M, Billoud B, Charrier B. Morphoelasticity in the development of brown alga
Ectocarpus siliculosus: from cell rounding to branching. J. R. Soc. Interface 14 : 20160596.
http://dx.doi.org/10.1098/rsif.2016.0596, 2017
Charrier B, Coates J.C., Stavridou I. Surfing amongst oil-tankers: Developing under-represented fields
in the current landscape. Trends Plant Sci, 22(1) : 1-3, 2017
Wichard T, Charrier B, Mineur F, Bothwell JH, De Clerck O and Coates JC. The green seaweed Ulva:
A model system to study morphogenesis. Front. Plant Sci. 6:72. doi: 10.3389/fpls.2015.00072, 2015
Docteurs encadrés par le directeur de thèse ayant soutenu après septembre 2011 et publications relatives à leur sujet de thèse.
Mettre en gras le nom du directeur de thèse et celui du docteur.
Nom Prénom : Zofia Nehr
Date de soutenance : Nov 2013
Durée de thèse (en mois): 38
Ecole Doctorale : 515
Publications :
Billoud B, Jouanno É, Nehr Z, Carton B, Rolland É, Chenivesse S and Charrier B. Localisation of
causal locus in the genome of the brown macroalga Ectocarpus: NGS-based mapping and positional
cloning approaches. Front. Plant Sci. 6:68. doi: 10.3389/fpls.2015.00068, 2015
Nehr Z & Charrier B. “Algal Morpho-inducers”, in “Outstanding Marine Molecules”, J-M
Kornprobst & S. Labarre Eds. Wiley Blackwell, pp217-223, 2014. ISBN: 978-3-527-33465-0
Billoud B, Nehr Z, Le Bail A, Charrier B. Computational prediction and experimental validation
of microRNAs in the brown alga Ectocarpus siliculosus. Nucleic Acids Research, 42(1):417-29,
doi: 10.1093/nar/gkt856, 2013
Collen et al. (puis par ordre alphabétique ~ 30 auteurs, dont Charrier et Nehr), Genome structure
and metabolic features in the red seaweed Chondrus crispus shed light on evolution of the
Archaeplastida. Proc. Natl. Acad. Sci. USA, 110(13):5247-52, 2013
Nehr Z, Billoud B, Le Bail A, Charrier B. Space-time decoupling in the branching process in the
mutant etoile of the filamentous brown alga Ectocarpus siliculosus, Plant Signaling & Behavior, 6
(12): 1889 - 1892, 2011
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Ecole Doctorale COMPLEXITE DU VIVANT – Fiche Projet CONCOURS
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