Chloroplast differentiation in epidermal cells: an environmental response supporting submerged photosynthesis in <i>Rorippa aquatica</i>

While the epidermal cells of terrestrial plants typically lack chloroplasts, this is not the case for aquatic plants, a trait that likely evolved to enhance photosynthetic efficiency underwater. Amphibious plants display various response strategies to survive under both terrestrial and aquatic environments. In this study, we described an environmental response in the amphibious plant Rorippa aquatica that involves chloroplasts differentiating into epidermal cells upon submergence. This phenomenon has not been previously documented and was named environmentally responsive epidermal chloroplast differentiation (ECD). To elucidate the mechanisms underlying ECD, we conducted RNA-sequencing analysis. The results revealed ethylene signaling, hypoxia responses, and light-regulated chloroplast development as key pathways. Physiological experiments showed that ethylene is a central trigger; exogenous ethylene or 1-aminocyclopropane-1-carboxylic acid promote ECD, whereas silver nitrate inhibits it. Submergence alone was insufficient to fully induce ECD, with underwater light exposure accelerating chloroplast maturation. Comparative analysis of phylogenetically distant amphibious species revealed that ECD is not unique to R. aquatica, implying ECD occurs in other species as well, with its extent varying among them. These findings provide new insights on plant acclimation to aquatic environments and highlight the intricate interplay between submergence, ethylene, and light in regulating chloroplast differentiation.