Role of Plant Amino Acid Transporters in Shaping Root-Beneficial Microbe Associations

Crop agriculture fueled by reliance on chemical fertilizers has environmentally-damaging consequences. Proposed alternative approaches to enhancing plant growth include the use of plant growth-promoting bacteria (PGPB). Large-scale field adoption of PGPB has been hindered by the low rhizosphere competence of PGPB, among others. Plant-derived amino acids in the rhizosphere promote bacterial growth, facilitate bacterial chemotaxis towards the roots, and enable microbial biosynthesis of plant growth-promoting compounds such as auxins. In this dissertation, I set out to identify and characterize Arabidopsis amino acid (AA) transporters that shape rhizosphere AA concentrations, and to determine whether these transporters modulate root―PGPB interactions and PGPB-mediated plant fitness. I found that the loss of Arabidopsis AA transporter LHT1 enhanced AA accumulation in the rhizosphere, indicating that LHT1 contributes to the retrieval of root-secreted amino acids. Consequently, both root exudates and root tissues of lht1 plants promoted the growth of the PGPB Pseudomonas simiae WCS417r better than did wildtype root exudates and root tissues. Additionally, lht1 root exudates enhanced biofilm formation by Ps WCS417r cells. Thus, engineering the rhizosphere to enhance AA levels by modifying amino acid transporter function would appear an important strategy to boost plant―PGPB interactions. Surprisingly, Ps WCS417r-mediated plant growth was attenuated in lht1 plants, suggesting that rhizosphere AA homeostasis may be important for Ps WCS417r-mediated plant growth. In support of this hypothesis, I found plant growth was inhibited in in vitro experiments where wildtype roots were co-treated with glutamine and Ps WCS417r. Additional experiments are required to understand the mechanisms underlying how elevated concentrations of specific amino acids in the rhizosphere may impair rhizobacteria-mediated plant fitness. Furthermore, I found that Arabidopsis AA transporter UMAMIT30 contributes to the availability of AA in the rhizosphere. Loss of UMAMIT30 function depleted umamit30 root exudates of AAs. However, the low AA content of root exudates from umamit30, as well as from umamit14 (previously characterized for its defective root AA secretion), did not impair Ps WCS417r growth. Importantly, Ps WCS417r-mediated plant growth remained intact in both umamit30 and umamit14 plants. These suggest that plant-derived AA metabolism by Ps WCS417r was not affected under these conditions of low AA concentrations so as to impair Ps WCS417r-mediated plant growth.