Light Alters Cytosolic and Plastidic Phosphorylase Distribution in Pearl Millet Leaves

In pearl millet (Pennisetum americanum) seedlings, although the cytosolic phosphorylase was present in all organs, the plastidic phosphorylase was restricted to the leaf. Intercellular fractionation of the leaf revealed that cytosolic and plastidic phosphorylase were localized in the mesophyll and bundle-sheath cells, respectively. In dark-grown leaves phosphorylase activity increased from the leaf base to the tip. The dark-grown leaves possessed both cytosolic and plastidic phosphorylase isoforms; however, their distribution followed different patterns along the length of the leaf. The plastidic phosphorylase level increased from the base to the tip of the leaf, and the cytosolic phosphorylase level was higher in the basal half of the leaf and declined toward the tip. In light-grown leaves phosphorylase activity was higher in a region near the leaf base and declined toward the leaf tip. Light stimulated cytosolic phosphorylase level and repressed plastidic phosphorylase level, resulting in an altered distribution of the respective phosphorylase isoforms along the length of the light-grown leaf. Since pearl millet leaf possesses a cell maturity and chloroplast development gradient from the leaf base to the tip, the inverse effect of light on cytosolic and plastidic phosphorylase levels might have been determined by its interaction with these gradients.