Israel D. K. Agorsor

: Background: Root exudation is an important trait that enables plants to shape their interactions with soil-borne organisms. Amino acids present in root exudates play important roles in bacterial chemotaxis, bacterial metabolism, and root colonization, contributing to plant nutrition and health. Notwithstanding the importance of amino acids in shaping the rhizosphere microbiome, the identities of the plant amino acid transporters that mediate their root exudation have remained elusive. Results: Here, we report that the Arabidopsis UMAMIT30 transporter, robustly expressed in root and shoot tissues, significantly contributes to amino acid root exudation. umamit30 loss-of-function mutants were compromised for amino acid root exudation as shown by the low concentration of amino acids, particularly glutamine, recovered from root exudates compared to wild-type plants. Amino acid quantification, as well as uptake and secretion assessments using radiolabelled glutamine, revealed that the shoots of umamit30 accumulate amino acids and have a reduced capacity to secrete glutamine, impacting root exudation. Conclusions: Our results identify UMAMIT30 as a broadly specific amino acid exporter strongly expressed in Arabidopsis vasculature. Loss-of-function mutants displayed reduced amino acid levels in root exudates, with significant drops in glutamine and asparagine among others, yet exhibited no detectable growth defects. UMAMIT30 disruption led to elevated shoot amino acid content and reduced glutamine efflux from shoots, suggesting a role in phloem uploading rather than root exudation alone. Despite decreased levels of root exuded amino acids, beneficial Pseudomonas interactions and plant-growth-promotion remained unaffected.

Plant immunity relies on the perception of microbe-associated molecular patterns (MAMPs) from invading microbes to induce defense responses that suppress attempted infections. It has been proposed that MAMP-triggered immunity (MTI) suppresses bacterial infections by suppressing the onset of bacterial virulence. However, the mechanisms by which plants exert this action are poorly understood. Here, we showed that MAMP perception in Arabidopsis (Arabidopsis thaliana) induces the accumulation of free amino acids in a salicylic acid (SA)-dependent manner. When co-infiltrated with Glutamine and Serine, two of the MAMP-induced highly accumulating amino acids, Pseudomonas syringae pv. tomato DC3000 expressed low levels of virulence genes and failed to produce robust infections in otherwise susceptible plants. When applied exogenously, Glutamine and Serine directly suppressed bacterial virulence and growth, bypassing MAMP perception and SA signaling. In addition, an increased level of endogenous Glutamine in the leaf apoplast of a gain-of-function mutant of Glutamine Dumper-1 rescued the partially compromised bacterial virulence- and growth-suppressing phenotype of the SA-induced deficient-2 (sid2) mutant. Our data suggest that MTI suppresses bacterial infections by delaying the onset of virulence with an excess of amino acids at the early stages of infection.

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 Arabidopsis thaliana roots, limits the growth of the plant-growth-promoting bacteria Pseudomonas simiae WCS417r (Ps WCS417r). The amino acid profiling of the lht1 mutant root exudates showed increased levels of glutamine, among other amino acids. Interestingly, lht1 exudates or Gln-supplemented wild-type exudates enhance Ps WCS417r growth. However, despite promoting bacterial growth and robust root colonization, lht1 exudates and Gln-supplemented wild-type exudates inhibited plant growth in a Ps 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 Ps WCS417r amino acids metabolism on auxin signaling. These data suggest that an excess of amino acids in the rhizosphere impacts Ps WCS417r metabolism, which, in turn, inhibits plant growth. Together, these results show that LHT1 regulates the amino-acid-mediated interaction between plants and Ps WCS417r and suggest a complex relationship between root-exuded amino acids, root colonization by beneficial bacteria, bacterial metabolism, and plant growth promotion.