Quantification of photosynthetic gene expression in maize C₃ and C₄ tissues by real-time PCR

Carbon assimilation in maize follows the C-4 mechanism. This requires the tissue-specific and light-induced expression of a set of different genes involved in CO2 fixation as well as adaptations in the leaf anatomy including a reduced distance between vascular bundles compared to C-3 plants. However, several maize tissues exist with larger bundle distances and there is significant evidence that CO2 fixation follows the C-3 mechanism in these tissues. We isolated maize C-3 and C-4 tissues and quantified the accumulation of mRNAs encoding PEPC, ME, the small subunit of Rubisco, and PPDK. For this, primer systems for the specific and sensitive detection by real-time PCR were established. The observed patterns show the expected distribution for foliar leaf tissues. Also in total husk leaves, all transcripts under investigation were detected, albeit at a lower level. When mesophyll cells which are located distant from bundles were isolated from husk leaves, only accumulation of RbcS was observed. Comparing the expression of two genes encoding for isoenzymes of the small subunit of RbcS in the different tissues differential patterns of relative transcript abundance were observed. Transcripts for the DOF1 transcription factor involved in the activation of photosynthetic genes in maize were found in leaf tissues performing both C-4 and C-3 photosynthesis with highest accumulation levels in C-4 mesophyll cells, whereas the homologous DOF2 gene was not expressed in any of the investigated samples. The results provide novel insights into the regulation of C-3 and C-4 carbon fixation pathways in maize.