Immunocytochemical localization of phosphoenolpyruvate carboxylase and photosynthetic gas-exchange characteristics in ears of Triticum durum Desf.

The presence and distribution of phosphoenolpyruvate carboxylase (PEPCase) in the glumes and immature grains of durum wheat (Triticum durum Desf.) were studied by electron-microscopical immunolabeling of PEPCase with polyclonal antibodies followed by protein A-gold. Plants were grown under mediterranean field conditions and samples were obtained two weeks after anthesis. In the kernels, high gold label was associated with the unstained areas of the protein bodies of aleurone cells, whereas labeling in the cytoplasm and chloroplasts of the pericarp was slight, although significantly above the background. In the glumes, high gold label was only located in cytoplasmic granules (vesicles) of the mesophyll cells, although labeling in the cytoplasm and chloroplasts was also significantly above the background. These observations in immature kernels and glumes are in accordance with the anaplerotic role of this enzyme, as evidenced in C₃ plants. Measurements of apparent photosynthesis and its O₂ dependence and CO₂ compensation concentration were made on ears and flag leaves of durum wheat. In addition, an analog of phosphoenolpyruvate, 3,3-dichloro-2-dihydroxy-phosphinoylmethyl-2-propenoate, was used to inhibit PEPCase and, thereby, to assess the contribution of the PEPCase to photosynthesis in detached ears. There was no effect of the inhibitor on the apparent photosynthesis of ears. Whereas inhibition of apparent photosynthesis by 210 mL · L^–1 O₂ in flag leaves was typical of C₃ species, inhibition in ears was even greater. The CO₂ compensation concentrations in different ear parts were similar to or higher than in flag leaves and the O₂ dependence was also comparable (about 70%). Therefore, gas-exchange data give further support to the assumption that a C₄ cycle is absent or limited to very low rates in ears of durum wheat.Abbreviations and Symbol AP apparent photosynthesis - BSA bovine serum albumin - DCDP 3,3-dichloro-2-dihydroxyphosphinoylmethyl-2-propenoate - PBS phosphate-buffered saline - PEP phosphoenolpyruvate - PEPCase PEP carboxylase - CO₂ compensation concentration We thank Dr. Sam Sun (University of Hawaii) for kindly providing the PEPCase antibodies and Drs. C.L.D. Jenkins and M.D. Hatch (Division of Plant Industry, CSIRO, Canberra, Australia) for supplying DCDP for these experiments. We are also grateful to Dr. Robert Bargalló and Mrs. Ana Ribero (Servei de Microscopia Electronica, Universitat de Barcelona) for technical assistance in electron-microscopy studies. Participation of Drs. J.L. Araus and J. Bort in this work was supported by the Research Project of CICYT AGF92-0301, Spain.