C3 Photosynthetic Traits in the C2 Species Sedobassia sedoides under Salinity

C2 photosynthesis is considered an “evolutionary bridge” between C3 and C4 photosynthesis and is characterized by significant plasticity, making it a unique model for studying evolution and a valuable resource for genetic engineering. In this study, plants of the C2 species Sedobassia sedoides were exposed to moderate salinity that did not suppress growth. Changes in the tissue and cellular localization of the main photosynthetic enzyme, Rubisco, as well as in the size of chloroplasts in bundle sheath (BS) cells, are important indicators of C2 photosynthesis formation. Under non-saline conditions, S. sedoides plants exhibited typical features of C2 photosynthesis: large BS chloroplasts, with Rubisco and starch grains predominantly localized within them. However, salinity led to a sharp decrease in Rubisco content in BS chloroplasts due to a decrease in rbcL expression. Moreover, salinity significantly decreased BS chloroplasts size and in their starch grain content, while increasing starch accumulation in the mesophyll. In addition, salinity downregulated expression of the PnsB5 gene, which encodes a component of the NDH-dependent of cyclic electron transport pathway around photosystem I (CET PSI) involved in the functioning of the C2 carbon-concentrating mechanism. Thus, salinity induces a restructuring of the tissue and subcellular localization of Rubisco in the C2 species S. sedoides, accompanied by a decrease in BS chloroplasts size and redistribution of starch, as well as a possible decrease in the role of NDH-mediated CET PSI, which may lead to weakening of C2 photosynthesis.