Nitrogen Assimilation Pathways in Leaf Mesophyll and Bundle Sheath Cells of C4 Photosynthesis Plants Formulated from Comparative Studies with Digitaria sanguinalis (L.) Scop.

Abstract

N assimilation in crabgrass D. sanguinalis (L.) Scop. was studied by comparing leaf extracts with isolated mesophyll cell and bundle sheath strand extracts. The results show that both nitrate and nitrate reductase are localized in mesophyll cells; glutamine synthetase is nearly equally distributed in the mesophyll and bundle sheath; approximately 67% of the glutamate synthase activity is in the bundle sheath and 33% is in the mesophyll; and 80% of the glutamate dehydrogenase activity is in the bundle sheath, with the NADH-dependent form exhibiting a 2.5-fold higher activity than the NADPH-dependent form. Isolated crabgrass mesophyll cells reduce NO2- coupled to the photochemical production of O2 but are inactive with NO3-. NO2--dependent O2 evolution is light-dependent; inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea; stimulated by photophosphorylation uncouplers; and exhibits a stoichiometry of O2 evoled to NO2- reduced of 1.45 and 0.67 in coupled and uncoupled experiments, respectively. Isolated bundle sheath strands are inactive in O2 evolution with NO3- or NO2-. Based on these results, plus literature data, 2 schemes for crabgrass leaf N-assimilation are presented, depending on whether the plant is using ammonium or nitrate as its N source. It is proposed that the increased N use efficiency in crabgrass and other C4 plants is due partially to a division of labor between mesophyll and bundle sheath cells, where NO3- and NO2- reductase in mesophyll cells act as N reduction traps in an analogous fashion to phosphoenolpyruvate carboxylase acting as a CO2 trap during C4 photosynthesis.