Functional diversity of plant ABC transporters : from iron mining to volatile emission

ABC transporters mediate ATP-dependent translocation of a wide variety of structurally and functionally unrelated compounds across biological membranes. In plants, they were shown to be involved in many different physiological processes, such as pathogen resistance, heavy metal detoxification or hormone transport. This work aimed at the functional characterization of two plant ABC transporters, NtPDR3 and PhABCG1. NtPDR3 was identified in our laboratory as an ABC transporter expressed at the plasma membrane of Nicotiana tabacum root epidermal cells under conditions of iron deficiency. The characterization of transgenic plants silenced for NtPDR3 expression revealed that these plants were less tolerant to iron-deficient conditions than wild-type plants, displaying stronger chlorosis and growth retardation when not supplied with iron. However, identification of NtPDR3 substrate(s) is required to fully understand its physiological role. For this purpose, a metabolomic analysis of roots and root exudates was undertaken. This approach identified several O-methylated coumarins as putative substrates of the transporter. Interestingly, these metabolites are known to be abundant in iron-deficient exudates of other plant species, and some of them have the ability to mobilize iron in vitro. Flavins were also found to be secreted by N. tabacum roots upon iron depletion, but NtPDR3 does not seem to be involved in their export from the roots. To confirm O-methylated coumarins are indeed substrates of NtPDR3, over-expression of the transporter in N. tabacum BY-2 suspension cells was achieved and direct transport tests were performed using intact cells as well as microsomal vesicles. No transport of coumarins was observed. However, transport assays with cells indicated that glycosylated coumarins might be substrates of NtPDR3. On the other hand, PhABCG1 was suggested to be involved in the active emis-sion of volatile organic compounds (VOCs) from Petunia hybrida flowers. Heterolo-gous expression of the protein in BY-2 cells and direct transport assays validated the hypothesis that VOCs are indeed translocated by PhABCG1. Altogether, the data obtained in this work indicate that NtPDR3 mediates secre-tion of O-methylated coumarins to the rhizosphere under low iron supply, whereas PhABCG1 actively transports VOCs across biological membranes, providing the first direct proof of a biologically mediated mechanism for volatile emission.(AGRO - Sciences agronomiques et ingénierie biologique) -- UCL, 201