Root phloem-specific expression of the plasma membrane amino acid proton co-transporter AAP3

Abstract

Amino acids are regarded as the nitrogen 'currency' of plants. Amino acids can be taken up from the soil directly or synthesized from inorganic nitrogen, and then circulated in the plant via phloem and xylem. AtAAP3, a member of the Amino Acid Permease (AAP) family, is mainly expressed in root tissue, suggesting a potential role in the uptake and distribution of amino acids. To determine the spatial expression pattern of AAP3, promoter-reporter gene fusions were introduced into Arabidopsis. Histochemical analysis of AAP3 promoter-GUS expressing plants revealed that AAP3 is preferentially expressed in root phloem. Expression was also detected in stamens, in cotyledons, and in major veins of some mature leaves. GFP-AAP3 fusions and epitope-tagged AAP3 were used to confirm the tissue specificity and to determine the subcellular localization of AtAAP3. When overexpressed in yeast or plant protoplasts, the functional GFP-AAP3 fusion was localized in subcellular organelle-like structures, nuclear membrane, and plasma membrane. Epitope-tagged AAP3 confirmed its localization to the plasma membrane and nuclear membrane of the phloem, consistent with the promoter-GUS study. In addition, epitope-tagged AAP3 protein was localized in endodermal cells in root tips. The intracellular localization suggests trafficking or cycling of the transporter, similar to many metabolite transporters in yeast or mammals, for example, yeast amino acid permease GAP1. Despite the specific expression pattern, knock-out mutants did not show altered phenotypes under various conditions including N-starvation. Microarray analyses revealed that the expression profile of genes involved in amino acid metabolism did not change drastically, indicating potential compensation by other amino acid transporters.

Global Characterization of Cell-Specific Gene Expression through Fluorescence-Activated Sorting of Nuclei

“…The entries also include At1g22710, the locus of a Suc/proton symporter (AtSUC2), consistent with a previous report that its protein product is specifically located within PCC plasma membranes (Stadler and Sauer, 1996), the acidic amino acid transporters encoded by the loci At5g49630 (AAP6) and At5g09220 (AAP2; Kwart et al, 1993;Okumoto et al, 2002), and a basic amino acid permease encoded by At1g77380 (AAP3), which has been reported previously to be specifically expressed in root phloem and may function in the distribution of amino acids absorbed by the roots (Okumoto et al, 2004). Finally, the gene list also contains At1g08200, which encodes a UDP-D-apiose/UDP-D-Xyl synthase and is expressed at enhanced levels in the vasculature as visualized by promoter-reporter fusions (Mølhøj et al, 2003).…”

Transcriptome Profiling of Tiller Buds Provides New Insights into PhyB Regulation of Tillering and Indeterminate Growth in Sorghum

“…S6). AAP3 amino-acid permeases are located in the plasmalemma and import amino acids from the apoplast into the phloem (Okumoto et al, 2004), but the exact role and localization of the bud-expressed member of the AAP3 gene family is unknown. Genes encoding a haloacid dehalogenaselike hydrolase and UDP-glucosyl transferase 85A2 were both expressed at higher levels in buds of phyB-1.…”

A Plasma Membrane-Anchored Fluorescent Protein Fusion Illuminates Sieve Element Plasma Membranes in Arabidopsis and Tobacco

“…The target of the SE-specific antibody RS6, raised against the isolated SEs of Streptanthus tortuosus (Brassicaceae), was recently found in Arabidopsis to be an ENOD-like integral protein (Khan et al, 2007). Other proteins have been immunolocalized to SEs, such as the Suc-H 1 symporter SUT1 in potato (Solanum tuberosum; Kü hn et al, 1997), a putative monosaccharide transporter and a protonATPase in developing apple (Malus domestica) fruit (Zhang et al, 2004), a proton-amino acid transporter in Arabidopsis roots (Okumoto et al, 2004), a calcium channel-like protein in tobacco and Pistia stratiotes leaves (Volk and Franceschi, 2000), and a plasma membrane aquaporin in spinach (Spinacia oleracea; Fraysse et al, 2005).…”