Expression of Linear and Cyclic Electron Flow Components in the Bundle Sheath Chloroplasts Along the Developmental Leaf Gradient of Maize (Zea mays)
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During plant photosynthesis, ATP can be produced in the chloroplast by linear electron flow (LEF), as well as cyclic electron flow (CEF). As a C4 plant, maize (Zea mays) utilizes two distinct cell types, known as bundle sheath (BS) and mesophyll (M) cells to facilitate the photosynthetic activity. BS cells function to fix the carbon dioxide in the form of reduced carbohydrates and carry out mostly CEF for ATP production. In contrast M cells act as a carbon pump and provide carbon dioxide in the form of the C4 acid malate to BS cells and use LEF for ATP production. BS and M chloroplasts develop and differentiate from base to tip. In this honors thesis, I tested the hypothesis that with progressive leaf development, levels of Photosystem II proteins (OEC23, OEC33, D1, CP47) involved in LEF decrease in BS chloroplasts, while CEF proteins (NDH-L and PGR5) in the BS chloroplasts increase. This serves to switch from linear to cyclic electron flow in the BS chloroplasts along the leaf developmental gradient. Additionally, I investigated relationships between mRNA and protein levels of the selected CEF and LEF components to understand transcriptional and translational controls. Proteins and RNA were extracted from BS strands isolated from four different leaf sections of 9 day old plants and analyzed by western blot and reverse transcript - polymerase chain reactions (RT-PCR). My analysis showed increased levels of mRNA and proteins of all measured components from base to the middle of the leaf, followed by additional increase or decrease towards the leaf tip. These results suggest both transcriptional and post-transcriptional control mechanisms in maize leaf development. My data also suggest that BS cells isolated near the tip were not fully differentiated at the sampling stage.
photosystem; NDH; CEF; LEF; maize
dissertation or thesis