A light-controlled propagation system for Undaria pinnatifida based on protoplast-derived aposporous filaments

Propagation systems based on somatic tissues or cells of elite material is a powerful approach in plant cultivation; however, comparable effective and controllable systems remain limited for kelps, an economically important group of marine macroalgae. In this study, we present a method for the controlled and sustained propagation of the commercial brown alga Undaria pinnatifida using protoplast-derived aposporous filaments (PDAFs). We found that green LED light promotes the formation and maintenance of PDAFs, while sporophyte regeneration is successfully triggered under dichromatic (red plus blue) light. PDAFs produced under green light yielded significantly more sporophytes than those from white or red-light conditions. Optimal and sustained sporophyte production was achieved using 500–750 filaments/mL, light intensities of 40–80 µmol photons m−² s− 1, and subcultures younger than four months with at least six weeks of regrowth. Comparative nuclear DNA content analysis using haploid and diploid reference materials indicated that both PDAFs and regenerated sporophytes exhibited relative DNA content consistent with diploid levels. Furthermore, cryopreservation using proline-based cryoprotectants resulted in high post-thaw viability (73–76%) without impairing sporophytic regeneration capacity. Our findings demonstrate the potential of PDAFs as a controllable, non-spore-based propagation stage for U. pinnatifida, providing an alternative method for sporophyte production and germplasm preservation in kelps. This study demonstrates the potential of protoplast-derived aposporous filaments for controllable propagation in Undaria pinnatifida via light quality, culture conditions, and cryopreservation, advancing brown seaweed in vitro methods.