| The distribution of amino acids (AAs) in plants is a complex process that involves 67 amino acid transporters (AATs) in Arabidopsis, many of which have yet to be characterized. The L-type Amino acid Transporter (LAT) family, which in Arabidopsis contains five members, remained uncharacterized in plants. This thesis presents the functional characterization of three members (LAT1, LAT4, and LAT5) of the Arabidopsis LAT family. In planta studies of knockout and over-expressing lines of these transporters showed increased toxicity to exogenous AAs and variation in AA-mobilization at cellular and whole-plant level compared to WT. Thus, their role in the amino-N distribution in Arabidopsis was investigated in this research. LATs were characterized through in silico studies, subcellular localization, organ- and tissue-specific expression, heterologous expression and in planta studies. Single knockout mutant or over-expressing lines were used in phenotypic growth studies, organ specific free AA analysis, analysis of nitrogen (N) and carbon (C) content in the seeds, and radio-labeled AA uptake and export studies.;LAT4 is a widely expressed AAT in the whole plant with high levels of expression in the photosynthetic tissues. In planta studies of a mutant lat4-1, together with the tissue-specific expression, provided evidence that LAT4 mobilizes AAs from leaf mesophyll cells and green carpel cells under low C conditions. LAT5 is also a widely expressed AAT in the whole plant including expression in the phloem. In planta mutant analysis suggested a role for LAT5 in delivering AAs to the leaves under high C conditions. In the lat5-1 mutant, decreased leaf size and increased accumulation of AAs in the stem was associated with an increased AA-translocation to the siliques and total N content in the seeds. The mutant lat5-1 showed increased salt and drought stress tolerance and altered free AA content in the seedlings under salt and osmotic stresses indicating that LAT5 plays a role in AA homeostasis under abiotic stresses. In the absence of a knockout mutant, a LAT1 over-expressing line was analyzed. In planta studies suggested a role for LAT1 in mobilizing AAs from leaves. |
