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

Abstract

Nitrogen assimilation in crabgrass Digitaria 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 NO(2) (-) coupled to the photochemical production of O(2) but are inactive with NO(3) (-). The NO(2) (-) -dependent O(2) evolution is light-dependent; inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea; stimulated by photophosphorylation uncouplers; and exhibits a stoichiometry of O(2) evolved to NO(2) (-) reduced of 1.45 and 0.67 in coupled and uncoupled experiments, respectively. Isolated bundle sheath strands are inactive in O(2) evolution with NO(3) (-) or NO(2) (-).Based on these results, plus literature data, two schemes for crabgrass leaf nitrogen assimilation are presented, depending on whether the plant is using ammonium or nitrate as its nitrogen source. It is proposed that the increased nitrogen use efficiency in crabgrass and other C(4) plants is due partially to a "division of labor" between mesophyll and bundle sheath cells, where NO(3) (-) and NO(2) (-) reductase in mesophyll cells act as nitrogen reduction traps in an analogous fashion to phosphoenolpyruvate carboxylase acting as a CO(2) trap during C(4) photosynthesis.