Abstract
Plant immune responses to invading bacteria are initiated by the recognition of Pathogen-Associated Molecular-Patterns (PAMPs) such as bacterial flagellin in the leaf apoplast. Pathogenic bacteria have evolved virulence mechanisms that suppress plant immunity, a necessary condition to produce infections. Pseudomonas syringae, an important bacterial pathogen that infects leaves of several crops as well as the model plant Arabidopsis (Arabidopsis thaliana), suppresses plant immunity in part through the secretion of the small molecule coronatine (COR). It is well documented that bacterial COR mimics the plant-made intracellular signaling molecule jasmonic acid, whose activation suppresses plant defense signaling initiated by PAMPs and by the plant-made intracellular signaling molecule salicylic acid. In this study, I investigated the mechanisms by which COR promotes bacterial growth in Arabidopsis beyond its known function as a suppressor of salicylic acid- and PAMP-induced signaling. Since P. syringae colonizes the leaf apoplast, changes in extracellular concentrations of amino acids have a profound impact on bacterial infections (Zhang et al 2023). While the elicitation of PAMP-triggered immunity (PTI) induces an early increase and a late decrease in extracellular concentrations of several amino acids, including Gln and Ser, in Arabidopsis seedlings (Zhang et al 2022), I found that COR counters the effect of flagellin by promoting the late increase of extracellular concentrations of Glu and Asp, among other amino acids.
Furthermore, I unveiled that an intact jasmonic acid signaling pathway is required for COR to drive these changes, and that salicylic acid signaling suppression is not required. Gene expression analyses of Arabidopsis amino acid transporters (AATs) in seedlings revealed that COR induces the expression Usually Multiple Acids Move In and out Transporters (UMAMITs) family of AATs, suggesting that some of these transporters would contribute to the late increase in extracellular amino acids concentrations observed in COR-treated Arabidopsis seedlings. In a reverse genetic screening of Arabidopsis loss-of-function mutants for COR-responsive AATs, I found that three COR-induced AATs, UMAMIT18, UMAMIT21, and UMAMIT42, are necessary for the late increase in extracellular concentrations of amino acids in response to COR in Arabidopsis seedlings. These data suggest that, while AATs that facilitate the late export of amino acids would contribute to producing robust bacterial infections, AATs that contribute to the late uptake of amino acids would suppress bacterial infections. Bacterial infection in planta showed that umamit18, umamit21, and umamit42 single mutants were as susceptible to P. syringae infections as wild-type plants, suggesting that functional redundancy may compensate for the loss of function of any one of these genes individually. On the other hand, the loss-of-function for the COR-suppressed amino acid importers AAP6 (Amino Acid Permease 6) and LHT7 (Lysine Histidine Transporter 7), produced enhanced susceptibility against P. syringae infections suggesting that these genes play a role in suppressing bacterial infections by lowering the concentration of amino acids in the leaf apoplast. Moreover, aap6 failed to load amino acids into the phloem and was compromised to transport amino acids away from the infected leaves to distal tissues, a response associated with the restriction of P. syringae growth in wild-type plants. Further analysis revealed that the expression of other AAP genes was downregulated in aap6, suggesting that the aap6 phenotypes result from the concerted activity of several members of the AAP family of importers with partially overlapping functions. These studies demonstrate that COR plays a major role in extracting amino acids from the host, an important function at late stages of infections, when leaf apoplast colonizing bacteria have already suppressed plant immunity and require a readily usable source of carbon and nitrogen to support fast growth.
