Plant immunity relies on the perception of Microbe-Associate 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.

Plant immunity relies on the perception of Microbe-Associate 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 uncover that MAMP-perception in Arabidopsis induces the accumulation of free amino acids (AA) 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 suppressed bacterial virulence and bacterial growth directly without the need for MAMP-perception and SA-signaling, bypassing MAMP-elicited defense. 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 the direct suppressing effect of accumulating amino acids on the onset of bacterial virulence.

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.

The ability of multicellular organisms to detect and eliminate pathogens is key to their survival. Toll‐like receptors (TLRs) are one of the key components of the immune response in invertebrates and vertebrates. Although the general families of TLRs are conserved between species in the detection of microbe‐associated molecular patterns, their specific role may be altered in these distinct organisms. Beyond the vast role of TLRs in immunity, they are also involved in development, metabolic homoeostasis and pain sensation. Key Concepts The role of TLRs in facilitating the innate immune response is well established, but they are now known to be involved in development, metabolic homoeostasis and nociception. TLRs are functionally conserved between species but can play different roles depending on the tissue in which they are expressed as well as the nature of the activating ligand and downstream effectors. TLRs use two main adaptor proteins to facilitate their downstream signalling pathways: Myd88 dependent and TRIF. The ligands recognised by TLRs can be described as microbe‐associated molecular patterns (MAMPs) and damage‐associated molecular patterns (DAMPs). TLRs have been targeted for therapeutic applications, but a balance must be struck between their inhibition and activation to maintain homoeostasis.