Thesis presented October, 2024

Abstract:
During evolution, organisms have developed different strategies to adapt to environmental stresses. Nutrient scarcity is a significant stress that can lead to substantial metabolic changes. Phosphorus is an essential nutrient for growth, it is assimilated as inorganic phosphate (Pi) and is often found to be limiting in ecosystems. In plants, phosphate deficiency greatly affects growth and induces massive membrane lipid remodeling to mobilize intracellular Pi reserves. A significant portion of intracellular Pi is stored in a particular class of glycerolipids called phospholipids (PL), which are the major components of extraplastidial membranes. Under deficiency conditions, PLs are degraded to provide new Pi sources to the cell. The degraded PLs are then replaced by non-phosphorus lipids to maintain membrane structure and integrity. However, this lipid remodeling is limited and only recycles a portion of the membrane PLs. Nutrient deficiency adaptation mechanisms are also present in microalgae, a highly diversified group of unicellular photosynthetic organisms. In these organisms, the response to Pi deficiency involves another class of non-phosphorus glycerolipids called betaine lipids (BL). BLs can completely replace the degraded PLs during Pi deficiency in certain species, such as the microalga Phaeodactylum tricornutum, representing a particularly efficient adaptive process. Throughout evolution, BL synthesis has gradually diminished and was definitively lost in seed plants, raising questions about the reasons for their disappearance. This project focuses on the study and comparison of lipid remodeling during Pi deficiency in two photosynthetic organisms: microalgae and terrestrial plants. The first objective was to generate BL-producing plants to study the impact of their production on their phenotype and their tolerance to Pi deficiency. Stable transformations of Arabidopsis thaliana were performed with the BTA1 gene encoding the enzyme responsible for DGTS synthesis, the best-known and studied BL species. The BTA1 gene from the microalga Microchloropsis gaditana was used, resulting in low DGTS production in the transformed A. thaliana plants. No major phenotypic impact was observed, but when the enzyme was transiently expressed in the leaves of the plant Nicotiana benthamiana, DGTS production was massive, representing approximately 20% of total glycerolipids. In this model, DGTS appears to accumulate in a proliferation of endoplasmic reticulum membranes, suggesting that it fulfills the same structural role as PLs. The second objective of this project was to study the impact of the absence of another BL species, DGTA, in P. tricornutum. Knockout mutants for the BTA1 gene were generated and analyzed. The results provided several interesting insights into the DGTA synthesis pathway and its importance in the algal response to Pi deficiency. Although DGTS is not detected in P. tricornutum, it acts as an intermediate in DGTA synthesis. DGTA was also found to be essential for maintaining the growth of P. tricornutum under Pi deficiency conditions and in the process of PLs degradation under this condition. Further research is needed to expand the study of BLs to other organisms.

Keywords:
betaine lipids, phosphate deficiency, lipid metabolism