C4-type gene expression is not directly dependent on Kranz anatomy in an amphibious sedge Eleocharis vivipara Link

Abstract

Eleocharis vivipara Link alters its photosynthetic mode depending on the growth environment. It utilizes C4 photosynthesis when grown under terrestrial conditions (terrestrial form) and C3 photosynthesis when grown under submerged conditions (submerged form). The photosynthetic organ (the mature internodal region of the culm) of the terrestrial form shows typical Kranz anatomy with well-developed bundle sheath cells, while the bundle sheath cells of the submerged form are not developed. In the mature internodal region of the terrestrial form, expression of the genes encoding two carboxylases, the small subunit of ribulose 1,5-bisphosphate carboxylase (RbcS) and phosphoenolpyruvate carboxylase (Ppc), occurred mainly in bundle sheath cells and in mesophyll cells, respectively, as seen in a typical C4 leaf. In the submerged form, RbcS was expressed in both bundle Sheath cells and mesophyll cells, and no expression of Ppc was observed. In the immature internodal region with undeveloped bundle sheath cells, both life forms showed the same expression pattern as in C3 plants: RbcS expression was localized in mesophyll cells and no Ppc expression was observed. The C4-type expression pattern was established concomitantly with the development of bundle sheath cells during tissue maturation in the terrestrial internode. In contrast to the terrestrial form, the submerged form maintains C3-type gene expression during tissue maturation. When the terrestrial culm was submerged, a region of transition from the terrestrial form to the submerged form was established in newly sprouting culms. In this transitional region, C4-type expression of the two carboxylase genes was still maintained even though the development of bundle sheath cells was repressed. This observation suggests that the C4-type cell-specific gene expression pattern does not depend on the formation of Kranz anatomy.