Evolution of Thylakoid Structural Diversity

Examining interactions of animal cells with chloroplasts and their light-induced responses in in vitro cell culture systems

Chloroplasts are organelles that convert light energy into chemical energy in plants. The potential to integrate chloroplasts into animal cells presents an exciting frontier in synthetic biology, allowing for photo-controllable biochemical processes within these cells. However, the lack of well-established in vitro experimental systems to study chloroplast-animal cell interactions remains a significant challenge. This study investigates the behavior of human cervical cancer HeLa cells and mouse macrophage-like J774.1 cells, along with the light-induced responses of these cells, when introduced into culture media containing spinach-derived chloroplasts. Additionally, we examine isolated cells from Elysia marginata, a sacoglossan sea slug known for its unique ability to acquire and retain functional chloroplasts through a process known as kleptoplasty. Our results show that HeLa cells primarily adhere to chloroplasts with minimal intracellular uptake, while J774.1 cells actively engulf them. Co-incubation with chloroplasts increases the rate of cell death upon light irradiation. In contrast, naturally chloroplast-containing cells from E. marginata exhibit minimal light-induced damage. Excessive reactive oxygen species (ROS) production is observed in HeLa and J774.1 cells co-incubated with chloroplasts under light exposure, suggesting that photoinduced ROS generation contributes to cytotoxicity. These findings highlight three different patterns of interactions between animal cells and chloroplasts and underscore the importance of considering ROS generation induced by light exposure when analyzing chloroplast-animal cell interactions in vitro experimental systems.

Lipid polymorphism of plant thylakoid membranes. The dynamic exchange model - facts and hypotheses

The light reactions of oxygenic photosynthesis are performed by protein complexes embedded in the lipid bilayer of thylakoid membranes (TMs). Bilayers provide optimal conditions for the build-up of the proton motive force (pmf) and ATP synthesis. However, functional plant TMs, besides the bilayer, contain an inverted hexagonal (HII) phase and isotropic phases, a lipid polymorphism due to their major, non-bilayer lipid species, monogalactosyldiacylglycerol (MGDG). The lipid phase behavior of TMs is explained within the framework of the Dynamic Exchange Model (DEM), an extension of the fluid-mosaic model. DEM portrays the bilayer phase as inclusions between photosynthetic supercomplexes - characterized by compromised membrane impermeability and restricted sizes inflicted by the segregation propensity of lipid molecules, safe-guarding the high protein density of TMs. Isotropic phases mediate membrane fusions and are associated with the lumenal lipocalin-like enzyme, violaxanthin de-epoxidase. Stromal-side proteins surrounded by lipids give rise to the HII phase. These features instigate experimentally testable hypotheses: (i) non-bilayer phases mediate functional sub-compartmentalization of plant chloroplasts - a quasi-autonomous energization and ATP synthesis of each granum-stroma TM assembly; and (ii) the generation and utilization of pmf depend on hydrated protein networks and proton-conducting pathways along membrane surfaces - rather than on strict impermeability of the bilayer.

Lighting the way: Compelling open questions in photosynthesis research

Photosynthesis-the conversion of energy from sunlight into chemical energy-is essential for life on Earth. Yet there is much we do not understand about photosynthetic energy conversion on a fundamental level: how it evolved and the extent of its diversity, its dynamics, and all the components and connections involved in its regulation. In this commentary, researchers working on fundamental aspects of photosynthesis including the light-dependent reactions, photorespiration, and C4 photosynthetic metabolism pose and discuss what they view as the most compelling open questions in their areas of research.