Abstract
The chemical composition of root exudates strongly impacts the interactions of plants with microorganisms in the rhizosphere and the efficiency of nutrient acquisition. Exudation of metabolites is in part mediated by ATP-binding cassette (ABC) transporters. In order to assess the contribution of individual ABC transporters to root exudation, we performed an LC-MS based non-targeted metabolite profiling of semi-polar metabolites accumulating in root exudates of Arabidopsis thaliana plants and mutants deficient in the expression of ABCG36 (PDR8/PEN3), ABCG37 (PDR9) or both transporters. Comparison of the metabolite profiles indicated distinct roles for each ABC transporter in root exudation. Thymidine exudation could be attributed to ABCG36 function, whereas coumarin exudation was strongly reduced only in ABCG37 deficient plants. However, coumarin exudation was compromised in abcg37 mutants only with respect to certain metabolites of this substance class. The specificity of ABCG37 for individual coumarins was further verified by a targeted LC-MS based coumarin profiling method. The response to iron deficiency, which is known to strongly induce coumarin exudation, was also investigated. In either treatment, the distribution of individual coumarins between roots and exudates in the investigated genotypes suggested the involvement of ABCG37 in the exudation specifically of highly oxygenated rather than monohydroxylated coumarins.
Highlights
Functional promiscuity has been described for ABCG36/PDR8/PEN3, a transporter involved in resistance of Arabidopsis to nonadapted and/or host-adapted pathogens[20,21,22,23,24]
Abcg[37] mutants and feruloyl-CoA 6′ hydroxylase[1] (f6′h1) knockout lines that are incapable of coumarin biosynthesis show more severe Fe deficiency symptoms and accumulate less Fe when compared to wild type plants under Fe limiting conditions[7, 33, 34]
Plant genomes typically encode more than 120 ATP-binding cassette (ABC) transporters, which exceeds the number in animals by almost 3-fold, and it remains a challenging task to determine their transport properties and preferred substrates, which are often structurally and functionally unrelated[9]
Summary
Functional promiscuity has been described for ABCG36/PDR8/PEN3, a transporter involved in resistance of Arabidopsis to nonadapted and/or host-adapted pathogens[20,21,22,23,24]. Non-targeted metabolite profiling indicated that both ABC transporters mediate the exudation of specific as well as common metabolites based on the absence of m/z-retention time pairs (features) in root exudates of the mutants compared to the wild-type. The remaining features should not exhibit differential abundance between root exudates of wild-type and pdr[] or between pdr[] and the double mutant.
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