Session 6

Do plant cells behave like bubbles? Photoinhibition atlas of photosynthetic sea slugs Anyone who has ever isolated intact chloroplasts can tell that they are fickle organelles that do not last long after isolation. Sacoglossan sea slugs do not share this experience, as some of them can isolate chloroplasts from their prey algae and incorporate them intracellularly as new foreign organelles – kleptoplasts. Separated from the algal nucleus and in the face of light induced damage, kleptoplasts challenge the paradigms of photosynthesis by remaining active inside the slugs for months. There is currently no definitive explanation to this phenomenon known to exist in several species of sacoglossans that incorporate the chloroplasts from different algal species. We measured the kinetics of photoinhibition, the irreversible damage to photosystem II (PSII), under high light in multiple combinations of photosynthetic slugs and their prey algae to understand how much the resilience against photoinhibition contributes to the overall longevity of the kleptoplasts. Out of all the tested slugs, Elysia timida is the only species that eats only one alga, Acetabularia acetabulum, and it was also by far the best species in terms of protecting kleptoplasts from photoinhibition. The molecular mechanisms behind the remarkable photoprotection in E. timida are currently under investigation. Unveiling the role of cell wall mechanical properties in regulating cell division and differentiation How topological and geometrical cell properties instruct cells behavior is a fundamental question in developmental biology. Despite the intrinsic relationship between cell expansion and cell differentiation, how cell expansion induces cell differentiation remains unknown. Here, by using genetic and molecular approaches, and by taking advantage of high-throughput techniques, we demonstrated that changes of the cell wall mechanical properties are translated into variations of the cell state, thus determining if a cell will divide or differentiate. Stiffer cell wall promotes cell division while a more elastic cell wall induces cell differentiation. To understand how these different cell wall mechanical properties are translated in different cell activities, we performed a transcriptome analysis, where cell wall mechanical properties are altered in a time-controlled and zone-specific manner. These analyses established the foundation for identifying the genes that respond to these changes and subsequently influence cell activity. This represents one of the first evidences that cell wall mechanical properties can instruct cell fate. Where did the seed evolve from? An investigation of SPOROCYTLESS homologs in the fern Ceratopteris richardii Seeds are a recent evolutionary innovation that arose once from an ancestral spore-based reproductive mechanism that is still present in living seedless plant groups (e.g. mosses, liverworts and ferns). How this dramatic transition to seed-based reproduction occurred is currently unknown. SPOROCYTELESS/NOZZLE (SPL/NZZ) is a crucial regulator of seed development in Arabidopsis, but related genes (homologs) also exist in plants that do not make seeds. Identifying the functions of SPL/NZZ genes in seedless plants may therefore help to explain how seeds first evolved. Here we use a model seedless plant, the fern Ceratopteris richardii, to investigate the function of such SPL homologs. We have identified three SPL-like genes in Ceratopteris richardii. Phylogenetic analysis places two of these genes within the Arabidopsis SPL/NZZ clade, providing evidence that SPL function may predate the evolutionary origin of the seed. We further compare the expression of these three homologs in different reproductive and non-reproductive tissue stages. We demonstrate that these homologs have different patterns and levels of expression across tissue types, suggesting that there are functional differences between them. This work provides the foundation upon which to begin further investigation into a potential role for SPL-like genes during the evolution of the seed.