Doctorate thesis

Thursday, October 18, 2018
at 2:00 pm, amphi 206

Role of plant growth promoting rhizobacteria on the root development and the mineral nutrition of the model plant Arabidopsis thaliana

Sulaiman Mashkoor
Thesis Director: Guilhem Desbrosses
BPMP, “Electrophysiology of plant mineral nutrition and root symbioses” team

Jury :

• Guilhem Desbrosses, PR2, Biochimie et Physiologie Moléculaire des Plantes, Directeur de thèse
• Mathilde Fagard, Directeur de Recherche, Institut Jean-Pierre Bourgin, Rapporteur
• Claire Prigent-Combaret, Directeur de Recherche, Ecologie Microbienne Lyon (UMR CNRS 5557 / UMR INRA 1418), Rapporteur
• Pierre Berthomieu, Professeur des Universités, Biochimie et Physiologie Moléculaire des Plantes, Examinateur

The plant rhizosphere is the region of soil whose chemical and biological composition is modified by the physiological activity of the roots. Many bacteria (collectively called rhizobacteria) that have various effects on plants are present in the rhizosphere. Many of them appear to be able to stimulate plant growth directly and indirectly. These bacteria are collectively called PGPR (Plant Growth Promoting Rhizobacteria). Beside growth promotion, PGPR stimulate root hairs elongation, and modify root growth (inhibition and sometimes stimulation). It is not known whether this property is related to growth stimulation. In addition, similar changes in the root system are observed in response to nutritional deficiency such as the one of phosphate (Pi). Maybe the signalings involved in the response to Pi deficiency and in the response to a PGPR are partially redundant. The aim of the PhD was to understand how PGPRs modify root development and to assess the impact that modifications in root development may have on plant growth. To achieve this goal, we inoculated Arabidopsis thaliana seedlings grown in vitro in the presence of variable concentrations of Pi, with the symbiotic nitrogen-fixing rhizobium strain Mesorhizobium loti MAFF303099 (M. loti). We show that under our growth conditions, plant growth is dependent on the number and length of lateral roots. As for the signaling pathway, we confirm that Pi deficiency triggers the synthesis and deposition of callose in the stem cells niche of root meristem. We show that M. loti inoculation also triggers callose production in the roots. However, unlike for Pi deficiency, callose is deposited around the elongation zone of the roots tip. Finally, we show that these deposits are dependent on the auxin pathway. The root developmental responses that are shared between biotic and abiotic signaling pathways are both depending on callose deposition but at different places. Because callose is a polysaccharide deposited when a phytopathogenic microorganism attacks a plant, our results suggest that a PGPR associated signaling pathway may little differs from the one of pathogenic bacteria. This suggests that the use of beneficial microorganisms in sustainable agricultural practices should be considered with caution.