Cell maturity gradient determines light regulated accumulation of proteins in pearl millet leaves

Developing pearl millet leaves possess cells of increasing maturity from the leaf base to the tip with oldest cells at the leaf tip. This natural developmental gradient was used to analyze the photoregulation of enzymes located in cytosolic, peroxisomal, and plastidic compartments of the leaf in relation to the cell age. In dark-grown leaves, the level of plastidic protochlorophyllide oxidoreductase A (PORA) (EC 1.3.1.33) protein increased from the leaf base to tip. Exposure to light reduced the level of PORA protein and stiumulated accmulation of PORB protein with increasing level from the base to the leaf tip. Light induced formation of cytosolic PEP carboxylase (EC 4.1.1.31) in the leaf with induction being maximal in the leaf tip. The levels of peroxisomal protein catalase (EC 1.11.1.6) showed gradual increase from the base to leaf tip in dark and light-grown leaves. By contrast, the distribution profile of cytosolic enzyme peroxidase (EC 1.11.1.7) followed reverse pattern in the dark-and light-grown leaves. In dark-grown leaves, peroxidase level showed increase towards the leaf tip, whereas light exposure lowered peroxidase level near the leaf tip and stimulated near the leaf base. These results indicate that the cell maturity gradient in cooperation with plastids modulates in a dual fashion the magnitude and pattern of photoregulation of protein levels in pearl millet leaves. It promotes the level of proteins functionally related to plastids towards the leaf tip and at the same time suppresses the level of cytosolic proteins, restricting them to the leaf base.