Sulfoquinovosyldiacylglycerol (SQDG) in chloroplast-poor tissues: What can lipidomics tell us?

Sulfoquinovosyldiacylglycerol (SQDG) is an anionic sulfolipid of plastid membranes, classically associated with chloroplasts and thylakoids. Its detection by lipidomics in chloroplast-poor tissues raises a central problem: how can a reported SQDG signal be validated, and what evidence is required to determine its plastid context and biological meaning? In this critical narrative review, we reassess evidence beyond chloroplast-dominated tissues and evaluate both the strength of evidence for SQDG occurrence or function and the steps required to test lipidomics-derived hypotheses. The literature supports context-dependent rather than universal interpretations. Direct organelle-level lipid evidence from isolated non-green plastids remains limited; etioplast genetics supports an SQDG-related role in non-photosynthetic plastid membrane development; phosphate-deprived roots support phosphorus-economy remodeling; and sulfur-starvation studies establish regulated SQDG turnover and sulfur remobilization. By contrast, chromoplast and woody-tissue interpretations remain unresolved. These findings show that SQDG abundance alone is hypothesis-generating rather than diagnostic of function. We therefore propose a decision-tree framework that moves from chemical confirmation of the lipid signal to spatial localization, SQD1/SQD2-associated biosynthetic capacity, nutrient context, tissue-specific perturbation, and phenotype testing. This framework provides criteria for moving from lipidomic detection to supported biological interpretation in chloroplast-poor tissues.