Cloning of Cell-Type Specific Genes in Maize Leaves and Characterization of PEP Carboxykinase in Bundle-Sheathstrands

Abstract

C4 plants are adapted well to grow under high temperature and intense light conditions. Because they have a unique mechanism for CO2 concentration, their photorespiration is negligible. The C4 photosynthesis functions under the cooperation of mesophyll cells (MC) and bundle-sheath-cells (BSC). Primary CO2 fixation reaction by PEP carboxylase occurs in MC. The C4 acids, malate and aspartate, are transported into BSC and decarboxylated. There CO2 is refixed into the Calvin cycle via ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO) [1]. Most enzymes involved in the C4 metabolism have been investigated well about their cell-type localization, regulation of gene expression [2], post-translational regulation [3], and so on. However, many other proteins indispensable for C4 photosynthesis remain to be elucidated. To isolate cDNAs for the proteins functioning specifically in MC and BSC, we carried out differential screening and isolated a number of MC-specific- and bundle sheath strands (BSS)-specific clones. During the course of analysis of these clones, we found a phosphoenolpyruvate carboxykinase (PCK) [EC4.1.1.49] gene as a member of BSS-specific genes. In this report, we describe novel genes, which were expressed specifically in MC or in BSS, and characterization of maize PCK gene.