The promoters of two carboxylases in a C 4 plant (maize) direct cell‐specific, light‐regulated expression in a C 3 plant (rice)

Abstract

C 4 plants have two carboxylases which function in photosynthesis. One, phosphoenolpyruvate carboxylase (PEPC) is localized in mesophyll cells, and the other, ribulose bisphosphate carboxylase (RuBPC) is found in bundle sheath cells. In contrast, C 3 plants have only one photosynthetic carboxylase, RuBPC, which is localized in mesophyll cells. The expression of PEPC in C 3 mesophyll cells is quite low relative to PEPC expression in C 4 mesophyll cells. Two chimeric genes have been constructed consisting of the structural gene encoding β‐glucuronidase (GUS) controlled by two promoters from C 4 (maize) photosynthetic genes: (i) the PEPC gene ( pepc ) and (ii) the small subunit of RuBPC ( rbcS ). These constructs were introduced into a C 3 cereal, rice. Both chimeric genes were expressed almost exclusively in mesophyll cells in the leaf blades and leaf sheaths at high levels, and no or very little activity was observed in other cells. The expression of both genes was also regulated by light. These observations indicate that the regulation systems which direct cell‐specific and light‐inducible expression of pepc and rbcS in C 4 plants are also present in C 3 plants. Nevertheless, expression of endogenous pepc in C 3 plants is very low in C 3 mesophyll cells, and the cell specificity of rbcS expression in C 3 plants differs from that in C 4 plants. Rice nuclear extracts were assayed for DNA‐binding protein(s) which interact with a cis ‐regulatory element in the pepc promoter. Gel‐retardation assays indicate that a nuclear protein with similar DNA‐binding specificity to a maize nuclear protein is present in rice. The possibility that differences in pepc expression in a C 3 plant (rice) and C 4 plant (maize) may be the result of changes in cis ‐acting elements between pepc in rice and maize is discussed. It also appears that differences in the cellular localization of rbcS expression are probably due to changes in a trans ‐acting factor(s) required for rbcS expression.