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National audience; Nitric oxide (NO) is an important cellular signaling molecule across kingdoms. During bioticor abiotic stresses, NO burst is detected in both plants and mammals although no sequenceencoding the well described mammalian NO synthase (NOS) is highlighted in plants. Comparedto terrestrial plants, some algae present transcripts encoding the NOS-like enzyme. Amongthem, Klebsormidium nitens the model alga to study the early transition step from aquatic algaeto land plants is found. As mechanisms governing NO synthesis and signaling in green lineageremain unclear, the study of NOS from K. nitens (KnNOS) through (i) the identification ofregulator proteins, (ii) the identification of target proteins, and (iii) their expression levels mightbe an important breakthrough knowledge. To achieve this goal, the identification of proteinpartners will be performed by pull down experiments followed by mass spectrometry analysis.Despite the KnNOS is a challenging protein to produce and purify, we are currently optimizinga promising protocol to obtain recombinant GST-tagged KnNOS protein in Escherichia coli.Since expression level analysis require suitable reference genes for reliable normalization, weselected 13 candidate reference genes based on literature, and performed RT-qPCR to studyexpression stability during 4 abiotic stresses commonly used. Final ranking list of most stablereference genes was obtain based on results from algorithms including, the ΔCt method,geNorm, NormFinder and BestKeeper. This finding opens the way for a deeper characterizationof KnNOS and its protein partners and will facilitate further investigation of gene expressionin K. nitens.

Studies performed in our Agroecology Department show that the immune response of plants is linked to their iron nutrition and is modulated by pyoverdine, a siderophore produced by the plant beneficial rhizobacteria Pseudomonas fluorescens C7R12. Accordingly, Arabidopsis thaliana plantlets exposed to iron deficiency and treated with pyoverdine in its iron non‐chelated structure (apo‐pyo) show an enhanced growth but a decreased immune response capacity. We hypothesize that nitric oxide (NO), a universal signaling molecule, is a key component of the regulation of the immune response in plants exposed to apo‐pyo and to the C7R12 strain. We checked by fluorescence microscopy that NO is actually produced in response to iron deficiency in presence or in absence of apo-pyo. To verify a role of NO in these processes, three tasks will be developed: ‐ identification of S‐nitrosylated proteins in response to apo‐pyo and P. fluorescens C7R12. S-nitrosylation is a major NO‐dependent posttranslational protein modification which targets cysteine residues (Cys). ‐ analysis of the impact of NO on the structure and function of the selected proteins. The influence of NO on the structure and function of the proteins will be investigated in vitro and the Cys residues regulated by NO will be identified by directed mutagenesis. ‐ study of the incidence of the S‐nitrosylation of the proteins of interest in planta. A. thaliana plants impaired in the expression of the proteins of interest or expressing them in a non‐nitrosylable structure will be generated and their interaction with pathogenic or beneficial micro‐organisms will be analyzed.

The existence of a tightly regulated balance between growth and immunity in plants has recently emerged. In this study, we challenged this concept thanks to the biological model pyoverdine-Arabidopsis thaliana. Pyoverdine is a siderophore produced by the plant growth promoting rhizobacteria Pseudomonas fluorescens C7R12. Under iron deficiency, P. fluorescens excretes the iron free form of pyoverdine (apo‐pyo) in the soil. Once chelated with iron (ferri‐pyo), the complex is internalized by the bacteria. We demonstrated that Arabidopsis thaliana plants treated by apo‐pyo in a medium containing or not iron internalize pyoverdine. Interestingly, apo‐pyo-treated plants did not show a typical growth reduction induced by iron deficiency. Accordingly, the expression of genes related to growth, development and iron uptake/transport was induced, these latter being involved in the growth improvement induced by apo‐pyo under iron deficiency. In contrast, a strong down-regulation of the expression of genes related to immunity was observed. Furthermore, the resistance to the fungal pathogen Botrytis cinerea conferred by iron deficiency was partially impaired following apo‐pyo treatment. The overexpression of the HBI1 transcription factor, known to be involved in the growth‐immunity tradeoff, was linked to the above observations. These apo‐pyo effects were not observed after treatment of plants under iron sufficient conditions, indicating that apo‐pyo effects are dependent on the plant iron status. To conclude, this work draws first elements of pyoverdine effects on plant physiology. In a larger view, this work supports the recent concept of the existence of a cross‐regulation between growth, immunity and iron homeostasis in plants.

Stilbenes, especially resveratrol and its derivatives, have become famous for their positive effects on a wide range of medical disorders, as indicated by a huge number of published studies. A less investigated area of research is their antimicrobial properties. A series of 13 trans-resveratrol analogues was synthesized via Wittig or Heck reactions, and their antimicrobial activity assessed on two different grapevine pathogens responsible for severe diseases in the vineyard. The entire series, together with resveratrol, was first evaluated on the zoospore mobility and sporulation level of Plasmopara viticola (the oomycete responsible for downy mildew). Stilbenes displayed a spectrum of activity ranging from low to high. Six of them, including the most active ones, were subsequently tested on the development of Botrytis cinerea (fungus responsible for grey mold). The results obtained allowed us to identify the most active stilbenes against both grapevine pathogens, to compare the antimicrobial activity of the evaluated series of stilbenes, and to discuss the relationship between their chemical structure (number and position of methoxy and hydroxy groups) and antimicrobial activity.

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