Plasmodesmata formation in the early development of <i>Saccharina japonica</i> sporophytes

SUMMARY Brown algae, as multicellular organisms, possess plasmodesmata (PD) that connect adjacent cells and facilitate cell‐to‐cell interactions, similar to land plants. In brown algae, PD are tiny tubular cytoplasmic channels measuring 10–20 nm in diameter. The distribution of PD appears to depend on body structure: in uni‐ and multiseriate filaments, PD are scattered across cross walls, whereas in thalli undergoing cellular differentiation, PD are concentrated into localized regions known as pit fields (PF). However, it remains unclear how the developmental reorganization of PD contributes to the regulation of intercellular communication, as information on the size exclusion limit (SEL) in PF remains limited. In this study, we investigated early development from zygotes and protoplasts in sporophytes of Saccharina japonica , the brown alga with a highly differentiated thallus, to determine the timing of PD distribution and the properties of PF‐mediated intercellular transport. We found that PF were already present at the two‐celled stage in both the zygote‐ and protoplast‐derived developmental pathways, in contrast to observations in Scytosiphon lomentaria , Mutimo cylindricus , Silvetia babingtonii , and Fucus distichus . Quantitative measurements revealed that PF width increased during development, whereas the spacing between individual PD remained relatively constant. Furthermore, analysis of intercellular transport using gold nanoparticles conjugated with 3 kDa and 10 kDa FITC–dextran, introduced into the epidermal cells of differentiated sporophytes by particle bombardment, demonstrated size‐dependent movement through PF.