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Doctoral thesis, 2015

New insights into plant amino acid transport and its contribution to nitrogen nutrition

Ahmad, Iftikhar


Nitrogen (N) is a crucial element for plant growth and development, as it is a key constituent of numerous plant metabolites and structural components. However, most plant N is in the form of amino acids, which are building blocks of proteins, precursors of many secondary metabolites and the “currency” of N within plants (i.e. the form in which N is transported and transferred). Recent studies have shown that amino acids are prevalent in soils and may also serve as N sources for plants. This thesis (and four appended papers) focuses on these aspects of amino acids, particularly the molecular mechanisms of amino acid transport in plants and their potential contribution to plants N nutrition. In Arabidopsis thaliana (Arabidopsis) at least 67 genes are annotated as putative amino acid transporters. In this thesis, results are presented suggesting that two amino acid transporters, LHT1 and AAP5, in Arabidopsis play important roles in amino acid uptake from soil at field-relevant concentrations (Paper I). This conclusion is based on the uptake characteristics of mutant plants (lht1, aap5 and lht1xaap5), displaying very little residual root uptake of amino acids. Furthermore, these two transporters have complementary affinity spectra: LHT1 for acidic and neutral amino acids; and AAP5 for basic amino acids. To probe the disputed contribution of organic N to plant N nutrition, mutants with both suppressed and enhanced expression of LHT1 were grown in agricultural soil. The results indicate that plants can take up significant amounts of amino N, despite microbial competition, suggesting that uptake of amino acids contributes to plant N nutrition (Paper II). Moreover it was shown that disrupted N cycling in leaf mesophyll cell affects leaves of different developmental status differently with respect to N and C status. Thus amino acid transport is critical for the maintenance of C and N status and leaf metabolism and LHT1 plays a key role in the process (Paper III). To broaden the studies from herbaceous plants to trees, a Populus orthologue of LHT1 (PtrLHT1.2) was identified. The PtrLHT1.2 gene from Populus tremula L. x tremuloides Michx. was cloned and expressed in a LHT1 uptake mutant of Arabidopsis. Results of its heterologous expression in Arabidopsis suggest that PtrLHT1.2 is a functional orthologue of AtLHT1 (Paper IV), indicating that PtrLHT1.2 could be involved in root amino acid uptake in Poplar tree.


amino acids; organic N transport; amino acid cycling

Published in

Acta Universitatis Agriculturae Sueciae
2015, number: 2015:33
ISBN: 978-91-576-8264-2, eISBN: 978-91-576-8265-9
Publisher: Department of Forest Genetics and Plant Physiology, Swedish University of Agricultural Sciences

Authors' information

Swedish University of Agricultural Sciences, Department of Forest Genetics and Plant Physiology

UKÄ Subject classification

Plant Biotechnology

URI (permanent link to this page)