The Arabidopsis LHT1 Amino Acid Transporter Contributes to -Mediated Plant Growth Promotion by Modulating Bacterial Metabolism in the Rhizosphere

The root microbiome structure ensures optimal plant host health and fitness, and it is, at least in part, defined by the plant genotype. It is well documented that root-secreted amino acids promote microbial chemotaxis and growth in the rhizosphere. However, whether the plant-mediated re-uptake of amino acids contributes to maintaining optimal levels of amino acids in the root exudates, and, in turn, microbial growth and metabolism, remains to be established. Here, we show that Lysine-Histidine Transporter-1 (LHT1), an amino acid inward transporter expressed in roots, limits the growth of the plant-growth-promoting bacteria WCS417r ( WCS417r). The amino acid profiling of the mutant root exudates showed increased levels of glutamine, among other amino acids. Interestingly, exudates or Gln-supplemented wild-type exudates enhance WCS417r growth. However, despite promoting bacterial growth and robust root colonization, exudates and Gln-supplemented wild-type exudates inhibited plant growth in a WCS417r-dependent manner. The transcriptional analysis of defense and growth marker genes revealed that plant growth inhibition was not linked to the elicitation of plant defense but likely to the impact of WCS417r amino acids metabolism on auxin signaling. These data suggest that an excess of amino acids in the rhizosphere impacts WCS417r metabolism, which, in turn, inhibits plant growth. Together, these results show that LHT1 regulates the amino-acid-mediated interaction between plants and WCS417r and suggest a complex relationship between root-exuded amino acids, root colonization by beneficial bacteria, bacterial metabolism, and plant growth promotion.