Differential Expression of C4 Pathway Genes in Mesophyll and Bundle Sheath Cells of Greening Maize Leaves

Pyruvate orthophosphate dikinase, phosphoenolpyruvate carboxylase, and NADP-malate dehydrogenase function in a series of reactions for fixing CO2 in mesophyll cells and NADP-malic enzyme (ME) catalyzes the production of CO2 and NADPH in bundle sheath cells of maize which is a NADP-ME type C4 plant. Northern blot analyses with cDNA clones for pyruvate orthophosphate dikinase and phosphoenolpyruvate carboxylase and in vitro translation-immunoprecipitation experiments with antiserum to NADP-malate dehydrogenase showed that pools of transcripts of these three genes grow and shrink coordinately in mesophyll cells but not in bundle sheath cells upon illumination of dark-grown maize seedlings. Western blot analyses indicated that the protein levels of phosphoenolpyruvate carboxylase and pyruvate orthophosphate dikinase are low in dark-grown maize seedlings and increase progressively following light-induced transient accumulation of their mRNAs in mesophyll cells. These proteins continue to accumulate and plateau in late-greening and green leaves in spite of a rapid drop in the sizes of their mRNA pools. Surprisingly, relatively large amounts of NADP-malate dehydrogenase are present in mesophyll cells of etiolated leaves despite the low level of the corresponding mRNA. No phosphoenolpyruvate carboxylase or NADP-malate dehydrogenase were detected in bundle sheath cells. On the other hand, the ME gene responds to light induction at both the transcriptional and translational levels only in bundle sheath cells. Moreover, the steady-state level of ME mRNA stays high in late-greening and green leaves in contrast to the rapid decline of mRNA levels of three other C4 pathway genes in mesophyll cells. In addition, low levels of both the mRNA and protein encoded by the PPDK gene were detected in bundle sheath cells. These levels were not influenced by light as distinguished from the patterns observed in mesophyll cells.