Molecular Cloning and Function Analyses of Two Ammonium Transporter Protein Genes from Pyrus calleryana Cloning and Functional Analysis of a New IspF Gene from Ginkgo biloba Molecular Cloning and Function Analyses of Two Ammonium Transporter Protein Genes from Pyrus calleryana Functional Complement Between General Nitrogen Regulator GlnG in Escherichia coli and General Nitrogen Regulator NtrC in Pseudomonas Identification and functional characterization of the AGO1 ortholog in maize