Identification and Characterization of Amino Acid Exporters in Arabidopsis thaliana
Abstract
Amino acids are the major form of nitrogen (N) transported within the plant body, and their transport between different plant tissues through the xylem and the phloem is indispensable for optimal growth. Such transport processes rely on critical import and export steps at the cellular membranes, mediated by transporters. Many amino acid importers and their roles in the plant body have previously been discovered, providing not only a better understanding of amino acid transport mechanisms but also tools to improve N use efficiency of crops. However, amino acid exporters are not well studied due to a lack of efficient techniques for their identification. The overall goal of this Ph.D. was to advance our understanding of amino acid export in plants by 1) developing methods in Saccharomyces cerevisiae that allow a faster identification of putative amino acid exporters and 2) investigating the roles of the identified amino acid exporters using genetic tools available in Arabidopsis thaliana.
The studies presented here revealed several novel amino acid exporter proteins, Usually Multiple Acids Move In and Out Transporters (UMAMIT) 14, UMAMIT24 and UMAMIT25, which mediated amino acid export when expressed in yeast, with little to no import activities. In plants, UMAMIT14 functions synergistically with previously characterized amino acid facilitator UMAMIT18 to mediate phloem unloading of shoot-derived amino acids. Knockout lines for these genes did not show any deleterious effects on yield; however the plants had reduced amino acid transfer from the shoot to the roots, as well as from roots to the growth medium. On the other hand, UMAMIT24 and UMAMIT25 are expressed in various seed tissues during embryogenesis, and are responsible for amino acid transport to the seed. An increased expression of UMAMIT24 and 25 using their own promoters increased the seed yields, whereas ectopic expression of UMAMIT transporters caused a stress response linked with an amino acid imbalance. The results suggest that optimizing key amino acid export steps in plants could benefit crop production, whereas excessive amino acid export can cause deleterious effects on plant growth.
Citation
Besnard, Julien Charles Claude (2018). Identification and Characterization of Amino Acid Exporters in Arabidopsis thaliana. Doctoral dissertation, Texas A & M University. Available electronically from https : / /hdl .handle .net /1969 .1 /173335.