Doctorate thesis

Saturday, December 17, 2016
at Faculté des sciences de Sfax, Tunisie

Physiological and molecular dissection of phytic acid role in response of plant to environmental stress

Nibras Belgaroui
BPMP, “Metal phytotoxicity” team

Jury :
M. Ali GARGOURI, Professeur (CBS) Président
M. Chedly ABDELLY, Professeur (CBBC) Rapporteur
M. Thierry DESNOS, Chargé de recherche, HDR (CEA) Rapporteur
M. Christophe ROBAGLIA, Professeur (CEA)
M. Moez HANIN, Professeur (ISBS) Examinateur Directeur de Thèse
M. Hatem ROUACHED, HDR (INRA Montpellier) Directeur de Thèse

Abstract :
The low availability of phosphorus (P) is a major constraint to growth and development of vegetable crops worldwide. In fact, up to 95% of total P in agricultural soils exists as organic P where phytic acid is the most dominant component, that is not available to plants unless hydrolyzed by specific enzymes called phytases. We are particularly interested to study a microbial phytase PHYC of Bacillus subtilis US417 strain (Farhat et al., 2008) to test its ability to enhance the mobilization of P from PA in Arabidopsis thaliana plants. Firstly, we have demonstrated in this work that overexpression of an intracellular form of phytase PHY-US417 in Arabidopsis resulted in reduction of PA levels in the grain.
Physiological analyzes showed that overexpression of this phytase improves plant growth after P deficiency, by increasing the intracellular level of Pi and sulfate. Remarkably, a stronger mobilization of iron was observed during germination in these transgenic lines. In addition, the perception of the outside Pi result of changes in the expression profiles of genes induced by phosphate deficiency. These results show that the AP is not only a storage form in seeds but also acts as a signaling molecule that regulates the interaction between the phosphorus and sulfate homeostasis.
These transgenic lines also led to conclude that the AP and its derivatives inositol phosphates may also be involved in the interconnection between the pathways regulating homeostasis of P and Zn. These components also appear to play a role as a positive regulator in the plant response to osmotic stress and this via stimulation of antioxidant activities.
In a second step, the effect of the secretion of the PHY-US417 phytase by Arabidopsis roots was studied. The results showed that the extracellular form ePHY-US417 was able to hydrolyze the PA used as the sole P source of Pi in the external environment. Therefore these lines over ePHY-US417 have a better acquisition of Pi and better growth.
Finally, this improved growth in Pi deficiency conditions has also affected other plants Arabidopsis or tobacco co-cultivated with plants secreting phytase. Such a step shows that the secreted phytase could have a significant upside potential through the development of new ways of “intercropping” that support sustainable agriculture by limiting the intensive use of phosphate fertilizers.

Full text (pdf – in french)