C 3- und C 4-Photosynthese in Mais (Zea mays): das Hüllblatt der weiblichen Infloreszenz als Modellsystem zur Analyse photosynthetischer Genexpression

C4 plants like maize adopt physiological and anatomical modifications for the establishment and maintenance of their type of photosynthesis. The regulation of these processes is mostly unknown. Within this study the ear of the female inflorescence of maize was established as a system for analysing C4-specific gene expression. This leaf like organ shows a clearly increased number of mesophyll cells in between the vascular bundles compared to foliar leaves. Only mesophyll cells in direct contact with bundle sheath cells fix CO2 by the C4 mechanism, whereas mesophyll cells more distant from the vein seemingly use the C3 metabolism. Within this study, the C3 mesophyll was isolated and characterised. The isolation of C3 and C4 tissues enabled us to work out gene expression analyses comparing both types of photosynthetic regulation in a single plant. The photosynthetic mechanism of the isolated tissues was verified by the detection of typical transcripts via quantitative Real-Time-RT-PCR. Furthermore the two known isogenes encoding the small subunit of Rubisco were shown to be expressed differentially in C3 and C4 tissues with the subunit RbcS1 showing a stronger expression in C3 mesophyll. For the transcription factors DOF1 und DOF2 an important participation in light induced gene expression has been shown before. However, the gene expression patterns of C3 and C4 tissues generated in this study, exclude a controlling function of these factors in the establishment of a particular photosynthetic subtype. Thus, light induction and tissue specifity seem to be regulated independently. The isolation of the C3 mesophyll also facilitated the subtractive hybridisation of C3 and C4 tissues. Six new bundle sheath specific transcripts were identified applying this technique. The function of the deduced proteins is still unknown but at least one transcript is seemingly expressed exclusively in C4 plants. In the discussion the results are implicated with the already described regulating mechanisms of C4-photosynthesis and the methods for investigating differential gene expression used within this study are considered critically. By this, the husk leaf is compared with other model systems for the analysis of the C4 syndrome.