AtPDR9 and NtPDR3, two homologous pleiotropic drug resistance transporters, are involved in the plant response to iron deficiency

Iron is an essential micronutrient for virtually all living organisms. This work deals with the functional characterization of AtPDR9 and NtPDR3, two homologous plant ABC (ATP-binding cassette) transporters involved in the response to iron deficiency, from Arabidopsis and Nicotiana tabacum, respectively. ABC transporters mediate the ATP-dependent translocation of a wide range of structurally and functionally unrelated compounds across biological membranes. AtPDR9 and NtPDR3 expression was found to be induced by iron deficiency and both proteins appeared to localize to the plasma membrane of the root epidermal cells. The characterization of Arabidopsis and N. tabacum mutant plants that do not express AtPDR9 or NtPDR3 revealed that the mutant plants were less tolerant to iron-deficient conditions than wild type plants, displaying a stronger chlorosis and growth retardation when not supplied with iron. Phenolic compounds are known to be abundant in the iron-deficient plant root exudate. Interestingly, exogenous application of certain phenolics was shown to rescue the mutant plants phenotype on iron-depleted medium. Moreover, phenolics were shown to accumulate in the roots of iron-deficient mutant plants. These data provide strong evidence supporting phenolic compounds as putative substrates of AtPDR9 and NtPDR3. Finally, the obtaining of tools aiming at demonstrating the transport properties of AtPDR9 and NtPDR3 was undertaken. NtPDR3 was successfully expressed in N. tabacum BY2 cells. Transport assays demonstrated that this material is appropriate for testing putative substrates of PDR proteins. Altogether, the data obtained in this work indicate that AtPDR9 and NtPDR3 transport phenolics to the rhizosphere under low iron supply, a mechanism that appears to be critical for plant tolerance to iron deficiency.(AGRO 3) -- UCL, 201

The Nicotiana tabacum ABC transporter NtPDR3 secretes O-methylated coumarins in response to iron deficiency

Although iron is present in large amounts in the soil, its poor solubility means that plants have to use various strategies to facilitate its uptake. In this study, we show that expression of NtPDR3/NtABCG3, a Nicotiana tabacum plasma-membrane ABC transporter in the pleiotropic drug resistance (PDR) subfamily, is strongly induced in the root epidermis under iron deficiency conditions. Prevention of NtPDR3 expression resulted in N. tabacum plants that were less tolerant to iron-deficient conditions, displaying stronger chlorosis and slower growth than those of the wildtype when not supplied with iron. Metabolic profiling of roots and root exudates revealed that, upon iron deficiency, secretion of catechol-bearing O-methylated coumarins such as fraxetin, hydroxyfraxetin, and methoxyfraxetin to the rhizosphere was compromised in NtPDR3-silenced plants. However, exudation of flavins such as riboflavin was not markedly affected by NtPDR3-silencing. Expression of NtPDR3 in N. tabacum Bright Yellow-2 (BY-2) cells resulted in altered intra- and extracellular coumarin pools, supporting coumarin transport by this transporter. The results demonstrate that N. tabacum secretes both coumarins and flavins in response to iron deficiency and that NtPDR3 plays an essential role in the plant response to iron deficiency by mediating secretion of O-methylated coumarins to the rhizosphere