Ppo Location In Plant

The vascular elements of many fruits and vegetables darken rapidly when they are cut and exposed to air. The relative PPO activity of different parts of apricots, apples, or potato plants vary. PPO is considered to be an intracellular enzyme in plants except in a few cases where it is found in the cell wall fraction. The enzyme is located in a variety of cell fractions, both in organelles where it may be tightly bound to membranes and in the soluble fraction (17,18). Histochemical studies using the electron microscope and the density gradient centrifugation support the idea that PPO is bound within chloroplast lamellae and grana and in mitochondria (34). Recent study suggested more specifically that PPO is located in the grana lamellae loosely associated with photosynthetic photo system II (35). Many studies indicated that PPO is membrane-bound in plastides of nonsenescing tissues (3,5). It was suggested that PPO is located exclusively in plastids and is only released to the cytosol upon wounding, senescence, or deterioration of the organelle (36). So far, no PPO genes encoding a nonplastidic enzyme have been isolated (8). Conversion of particulate forms of the enzyme to soluble forms occurs in fruits when they are exposed to stress conditions or during ripening and storage. Browning reactions that take place after the disruption of tissues rich in PPO may cause binding of soluble PPO to a particulate fraction. The microbial extracellular laccase has been studied extensively but the location of laccase in higher plants is not well known. Although it is believed that the enzyme is found in a cytoplasmic soluble form, more information is needed on its location and distribution in plant tissues.

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