Premise of the study: Chloroplast development and structure are highly conserved in vascular plants, but the bizonoplast of Selaginella is a notable exception. In the shade plant S. erythropus , each dorsal epidermal cell contains one bizonoplast, while other cells have normal chloroplasts. Our quest was to (1) determine the origin of bizonoplasts, (2) explore developmental plasticity, and (3) correlate developmental changes with photosynthetic activity to provide insights unavailable in other green plants with more constrained development. Methods: Bizonoplast development was studied in juvenile prostrate and older erect shoots of S. erythropus. Plastid plasticity was studied in plants cultivated under different light conditions. Chlorophyll fl uorescence was measured and correlated with photosynthetic activity. Key results: The bizonoplast originates from a proplastid, forming a distinctive upper zone rapidly after exposure to low light. In the prostrate shoots, the proplastid develops through early stages only. When the shoot becomes erect, the proplastid soon develops into a mature bizonoplast. Erect shoots have signifi cantly higher photosynthetic effi ciency than prostrate shoots. No bizonoplasts were found in the plants growing in high light, where 2–4 spheroidal chloroplasts formed, or with light from below. Conclusions: The upper zone develops above a normal-looking chloroplast structure to produce a bizonoplast. Bizonoplast developmental plasticity suggests that regular lamellar structure and monoplastidy are adaptations to deep shade environments. Such novel variation in S. erythropus is in stark contrast to known plastid development in other vascular plants, possibly refl ecting retention of developmental fl exibility in the basal clade, Lycophyta, to which it belongs.
