Betaine lipids overproduced in seed plants are excluded from plastid membranes and promote endomembrane expansion

Plants and algae have to adapt to environmental changes and face various stresses that negatively affect their growth and development. One common stress is phosphate (Pi) deficiency, which is often present in the environment at limiting levels. In response to Pi deficiency, these organisms increase Pi uptake and remobilize intracellular Pi. Phospholipids are degraded to provide Pi and replaced by non-phosphorus lipids, such as glycolipids or betaine lipids. During evolution, seed plants lost the ability to synthesize betaine lipids. By expressing BTA1 genes, which are involved in the synthesis of diacylglyceryl-N,N,N-trimethyl-homoserine (DGTS), from different species, our work shows that DGTS can be produced in seed plants. In Arabidopsis, expression of BTA1 under a phosphate starvation-inducible promoter resulted in limited DGTS production without having any impact on plant growth or lipid remodeling. In transient expression systems in Nicotiana benthamiana, leaves were able to accumulate DGTS up to 30 % of their glycerolipid content at a slight expense of galactolipid and phospholipid production. At the subcellular level, we showed that DGTS is absent from the plastids and seems to be enriched in the endomembranes, driving an ER membrane proliferation. Finally, the DGTS synthesis pathway seems to compete with PC synthesis via the Kennedy pathway but does not seem to be derived from the PC diacylglycerol backbone and therefore does not interfere with the eukaryotic pathway involved in galactolipid synthesis.