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Boron (B) is essential in plants but often present at low concentrations in the environment. To investigate how plants survive under conditions of B limitation, we conducted a transcriptome analysis and identified NIP5;1, a member of the major intrinsic protein family, as a gene upregulated in B-deficient roots of Arabidopsis thaliana. Promoter-b-glucuronidase fusions indicated that NIP5;1 is strongly upregulated in the root elongation zone and the root hair zone under B limitation, and green fluorescent protein-tagged NIP5;1 proteins localized to the plasma membrane. Expression in Xenopus laevis oocytes demonstrated that NIP5;1 facilitated the transport of boric acid in addition to water. Importantly, two T-DNA insertion lines of NIP5;1 displayed lower boric acid uptake into roots, lower biomass production, and increased sensitivity of root and shoot development to B deficiency. These results identify NIP5;1 as a major plasma membrane boric acid channel crucial for the B uptake required for plant growth and development under B limitation.

The AMMONIUM TRANSPORTER (AMT) family comprises six isoforms in Arabidopsis thaliana. Here, we describe the complete functional organization of root-expressed AMTs for high-affinity ammonium uptake. High-affinity influx of 15 N-labeled ammonium in two transposon-tagged amt1;2 lines was reduced by 18 to 26% compared with wild-type plants. Enrichment of the AMT1;2 protein in the plasma membrane and localization of AMT1;2 promoter activity in the endodermis and root cortex indicated that AMT1;2 mediates the uptake of ammonium entering the root via the apoplasmic transport route. An amt1;1 amt1;2 amt1;3 amt2;1 quadruple mutant (qko) showed severe growth depression under ammonium supply and maintained only 5 to 10% of wild-type high-affinity ammonium uptake capacity. Transcriptional upregulation of AMT1;5 in nitrogen-deficient rhizodermal and root hair cells and the ability of AMT1;5 to transport ammonium in yeast suggested that AMT1;5 accounts for the remaining uptake capacity in qko. Triple and quadruple amt insertion lines revealed in vivo ammonium substrate affinities of 50, 234, 61, and 4.5 mM for AMT1;1, AMT1;2, AMT1;3, and AMT1;5, respectively, but no ammonium influx activity for AMT2;1. These data suggest that two principle means of achieving effective ammonium uptake in Arabidopsis roots are the spatial arrangement of AMT1-type ammonium transporters and the distribution of their transport capacities at different substrate affinities.

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-dependent oxidation of deoxyribose. This suggests that NA will have an important role in scavenging Fe and protecting the cell from oxidative damage. The pH dependence of metal ion chelation by NA and a typical phytosiderophore, 2 -deoxymugineic acid, indicated that although both have the ability to chelate Fe, when both are present, 2 -deoxymugineic acid dominates the chelation process at acidic pH values, whereas NA dominates at alkaline pH values. The consequences for the role of NA in the long-distance transport of metals in the xylem and phloem are discussed.

Boron (B) is essential for plants but toxic when present in excess.Arabidopsis thaliana BOR1 is a B exporter for xylem loading and is essential for efficient B translocation from roots to shoots under B limitation. B translocation to shoots was enhanced under B limitation in WT but not in bor1-1 mutant plants. The

Ammonium and nitrate are the prevalent nitrogen sources for growth and development of higher plants. 15 N-uptake studies demonstrated that ammonium is preferred up to 20-fold over nitrate by Arabidopsis plants. To study the regulation and complex kinetics of ammonium uptake, we isolated two new ammonium transporter ( AMT ) genes and showed that they functionally complemented an ammonium uptake-deficient yeast mutant. Uptake studies with 14 C-methylammonium and inhibition by ammonium yielded distinct substrate affinities between Յ 0.5 and 40 M. Correlation of gene expression with 15 NH 4 ؉ uptake into plant roots showed that nitrogen supply and time of day differentially regulated the individual carriers. Transcript levels of AtAMT1;1 , which possesses an affinity in the nanomolar range, steeply increased with ammonium uptake in roots when nitrogen nutrition became limiting, whereas those of AtAMT1;3 increased slightly, with AtAMT1;2 being more constitutively expressed. All three ammonium transporters showed diurnal variation in expression, but AtAMT1;3 transcript levels peaked with ammonium uptake at the end of the light period, suggesting that AtAMT1;3 provides a link between nitrogen assimilation and carbon provision in roots. Our results show that highaffinity ammonium uptake in roots is regulated in relation to the physiological status of the plant at the transcriptional level and by substrate affinities of individual members of the AMT1 gene family.

2. Auflage auch Messwerte zu Baumhöhen und Alter (Bonitä-ten) zur quantitativen Untermauerung der Wachstumsaussa-gen beitragen. Alle, die professionell in Waldböden forschen, diese kartie-ren, bewirtschaften oder beplanen, auch Studierende dieser Fachrichtungen, sollten den Bildatlas als Standardwerk auf-nehmen. Und allen denen, die mehr über den Aufbau, die Diversität und das Potenzial von Waldböden-ja Böden über-haupt-wissen wollen, wird der Bildatlas uneingeschränkt empfohlen.