ARGININE DECARBOXYLASE IS LOCALIZED IN CHLOROPLASTS

Plants, unlike animals, can use either ornithine decarboxylase or argi nine decarboxylase (ADC) to produce the polyamine precursor putrescine . Lack of knowledge of the exact cellular and subcellular location of these enzymes has been one of the main obstacles to our understanding of the biological role of polyamines in plants. We have generated poly clonal antibodies to oat (Avena sativa L.) ADC to study the spatial di stribution and subcellular localization of ADC protein in different oa t tissues. By immunoblotting and immunocytochemistry, we show that ADC is organ specific. By cell fractionation and immunoblotting, we show that ADC is localized in chloroplasts associated with the thylakoid me mbrane. The results also show that increased levels of ADC protein are correlated with high levels of ADC activity and putrescine in osmotic ally stressed oat leaves. A model of compartmentalization for the argi nine pathway and putrescine biosynthesis in active photosynthetic tiss ues has been proposed. in the context of endosymbiote-driven metabolic evolution in plants, the location of ADC in the chloroplast compartme nt may have major evolutionary significance, since it explains (a) why plants can use two alternative pathways for putrescine biosynthesis a nd (b) why animals do not possess ADC.