Note the low confidence level of the alignment with CLC-e and CLC-f members. (B) Boxshade alignment of the region involved in NO − 3/Cl − selectivity with an arrow on the critical amino acid position. (A) Phylogenetic tree (Phylip) of the CLC family from Arabidopsis and rice. 2008) and eukaryotic CLC (Zifarelli and Pusch 2009).įeatures of CLC genes. Furthermore, mutations at an identical position affected the anion/proton coupling ratio in a prokaryotic (Jayaram et al. The alignment with these latter CLCs must therefore be done with caution ( Fig. CLC-c, -d and -g have a serine at this position, while CLC-e and -f differ in that region in their protein sequence. Mutant analysis has recently shown that eukaryotic CLCs with a proline at the residue corresponding to 159 in CLC-b had an increased conductance for nitrate, compared with those with a serine (Bergsdorf et al. The proteins encoded by these three CLC genes differ from the other CLCs in a site that determines the anion selectivity of CLCs (Bergsdorf et al. Among these, CLC-a and CLC-b cluster together with a single ortholog in rice. 1A), individual subgroups are identified, which may suggest a conserved function associated with individual sub-branches. When compared with CLC genes from the monocot rice ( Fig. Seven CLC sequences have been identified in the Arabidopsis genome. Furthermore, a loss-of-function mutant of the plastid-localized CLC-e revealed a common phenotype with clca: lower nitrate, but higher nitrite content and reduced nitrate root influx (Monachello et al. The morphology of clca and clcc mutants was similar to that of the wild type. The loss of a related gene, CLC-c, affected the overall anion homeostasis (Harada et al. 2006) and is essential for normal nitrate levels (Geelen et al. 1997).ĬLC-a encodes a vacuolar nitrate transporter in the model plant Arabidopsis (De Angeli et al. Since nitrate reduction occurs primarily during the light period, the ratio between metabolized and stored nitrate can vary greatly during the daily cycle (Scheible et al. Following the uptake into roots, nitrate is allocated to the leaves, transiently stored in vacuoles and remobilized, when nitrogen supply is insufficient to meet demand. 2007, Amtmann and Armengaud 2009, Gojon et al. The primary genes and proteins involved in NO − 3 acquisition have been identified in several plants (Miller et al. NO − 3 is thought to be a signaling molecule itself, activating its uptake, reduction and compensatory organic acid metabolism (Miller et al. In aerated soils, the main nitrogen source for most plants is nitrate (NO − 3), which is taken up by regulated plasma membrane transporters (Miller et al. Nitrogen availability affects the plant at all levels, such as root morphology, growth and development (Amtmann and Armengaud 2009, Gojon et al. Nitrogen is the most important plant nutrient acquired from the soil by mass. The total chloride and nitrate content was identical in clcb lines and the wild type, potentially suggesting that mutants were able to compensate the loss of CLC-b. Both lines grew as the wild type in various conditions. The physiological role of CLC-b was analyzed using two independent knock-out alleles. CLC-b expression was strongest in young roots, hypocotyl and cotyledons. CLC-b was localized at the tonoplast, as was CLC-c, when tagged with the green fluorescent protein. Fluorescence ratio changes of oocytes loaded with a pH-dependent fluorescent dye suggested that NO − 3 transport is associated with H + exchange. ![]() CLC-b conducted strongly outwardly rectifying anionic currents that were largest in the presence of nitrate. Here, CLC-b, a close relative of CLC-a, was functionally expressed in oocytes and analyzed. ![]() CLC-a, a member of the CLC family of anion transporters, is critically involved in this nitrate storage in the vacuole, while other CLC family members apparently have different roles in diverse cell organelles. In the absence of light, nitrate can transiently accumulate in the vacuolar lumen via tonoplast transporters. Nitrate is frequently the major nitrogen source for plants and is generally assimilated during the day.
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