Monogalactosyldiacylglycerol synthase isoforms play diverse roles inside and outside the diatom plastid

Abstract

Diatoms derive from a secondary endosymbiosis event, which occurred when a eukaryotic host cell engulfed a red alga. This led to the formation of a complex plastid enclosed by 4 membranes: 2 innermost membranes originating from the red alga chloroplast envelope, and 2 additional peri- and epiplastidial membranes (PPM and EpM). The EpM is linked to the endoplasmic reticulum (ER). The most abundant membrane lipid in diatoms is monogalactosyldiacylglycerol (MGDG), synthesized by galactosyltransferases called MGDG synthases (MGDs), conserved in photosynthetic eukaryotes and considered to be specific to chloroplast membranes. Similar to angiosperms, a multigenic family of MGDs has evolved in diatoms, but through an independent process. We characterized MGDα, MGDβ, and MGDγ in Phaeodactylum tricornutum, combining molecular analyses, heterologous expression in Saccharomyces cerevisiae, and studying overexpressing and CRISPR-Cas9-edited lines. MGDα localizes mainly to thylakoids, MGDβ to the PPM, and MGDγ to the ER and EpM. MGDs have distinct specificities for diacylglycerol, consistent with their localization. The results suggest that MGDα is required for thylakoid expansion under optimal conditions, while MGDβ and MGDγ play roles in plastid and nonplastid membranes and in response to environmental stress. Functional compensation among MGDs likely contributes to diatom resilience under adverse conditions and to their ecological success.