Immunogold localization of primary carboxylases in leaves of aquatic and A C 3-C 4 intermediate species

Abstract

Using protein A-gold labeled antibodies, attempts were made to determine the cellular location of ribulose bisphosphate carboxylase/oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPC) in two aquatic macrophytes and a terrestrial C 3-C 4 intermediate plant. Hydrilla verticillata and Myriophyllum spicatum are submersed angiosperms which exhibit variable photorespiratory states. Leaf sections indicated that neither possessed Kranz anatomy. In Myriophyllum, the chloroplasts were confined to a single epidermal layer. The Hydrilla leaf was composed of two morphologically distinct chloroplastic cell layers. Immunocytochemical studies showed that Rubisco was distributed in the chloroplasts of all the cells of both layers, and that no difference in Rubisco labeling between cells was apparent, even though low-photorespiration Hydrilla has many biochemical and physiological characteristics of C 4-photosynthesis. In Hydrilla the location of PEPC was largely cytosolic and was in all leaf cells. These data support the postulate that an intracellular separation of C 4 and C 3 fixation events may account for the low photorespiration state in Hydrilla. PEPC labeling was undectectable in Myriophyllum, which correlates with its lack of C 4-like photosynthetic biochemistry. The terrestrial C 3-C 4 intermediate species, Moricandia arvensis, has leaves with photosynthetic cells differentiated into a chloroplast-containing bundle sheath layer surrounded by mesophyll tissue. Rubisco labeling was found in the chloroplasts of both cell types. Thus, the reduced apparent photorespiration in Moricandia is not achieved through a separation of CO 2 fixation events into different cells, as occurs in C 4 plants with Kranz anatomy, but is more likely due to efficient refixation of photorespiratory CO 2 by Rubisco.