Localization of the C4 and C 3 pathways of photosynthesis in the leaves of Pennisetum purpureum and other C4 species. Insignificance of phenol oxidase

Abstract

Mesophyll protoplasts and bundle-sheath cells of Pennisetum purpureum Schum., a C4 plant with low phenol-oxidase activity, were enzymatically separated according to methods recently developed with sugarcane (Saccharum officinarum L.), maize (Zea mays L.), and sorghum (Sorghum bicolor L.). The phosphoenolpyruvate carboxylase and NADP-malic dehydrogenase of the C4 pathway were found to be localized in the mesophyll protoplasts while ribulose-1,5-diphosphate (RuDP) carboxylase, phosphoribulokinase and NADP-malic enzyme were localized in the bundle-sheath cells. The levels of these enzyme activities in the leaf extracts and in certain cellular preparations of P. purpureum are sufficient to account for the rate of photosynthesis in the leaf. These results on the activities and distribution of photosynthetic enzymes with P. purpureum preparations are consistent with our previous evidence for cellular separation of the C4 and the reductive pentose-phosphate pathways in C4 species.With chlorogenic acid as the substrate, P. purpureum, Setaria lutescens (Weigel) Hubb. and Panicum texanum Buckl. have relatively low phenol-oxidase activity, similar to that found in spinach (Spinacia oleracea L.); while sorghum, sugarcane, maize, Panicum capillare L. and P. miliaceum L. have relatively high phenoloxidase activity, similar to that in tobacco (Nicotiana tabacum L.). C4 species having high phenol-oxidase activity have substantial activity of the enzyme in both mesophyll and bundle-sheath extracts. Since phenol oxidase is found in both cell types it is not logical to expect preferential inhibition of RuDP carboxylase or other photosynthetic enzymes through phenol oxidation in mesophyll extracts, as has been previously suggested. When dithiothreitol and polyvinylpyrrolidone were included in the enzyme extraction medium, the activity of RuDP carboxylase increased 10% in P. purpureum and 59% in sugarcane leaf extracts.