The Arabidopsis UMAMIT30 transporter contributes to amino acid root exudation

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 under our growth conditions. UMAMIT30 disruption led to elevated shoot amino acid content and reduced glutamine efflux from shoots, suggesting a role in phloem uploading as an upstream step necessary for root exudation. Despite decreased levels of root-exuded amino acids, the plant growth-promotion conferred by the soil-borne beneficial bacterium Pseudomonas simiae WCS417r remained unmodified in umamit30 mutants.