The manganese transporter MTP8 determines tolerance to Fe deficiency-induced chlorosis in Arabidopsis

The Cation Diffusion Facilitator MTP8 transports manganese into root vacuoles of iron-deficient plants and thereby prevents inhibition of iron deficiency-induced ferric reductase ABSTRACT Iron (Fe) deficiency is a widespread nutritional disorder on calcareous soils. To identify 54 genes involved in the Fe deficiency response, Arabidopsis T-DNA insertion lines were 55 screened on a high-pH medium with low Fe availability. This approach identified MTP8, a 56 member of the Cation Diffusion Facilitator family, as critical determinant for the tolerance 57 to Fe deficiency-induced chlorosis, also on soil substrate. Subcellular localization to the 58 tonoplast, complementation of a Mn-sensitive yeast strain, and Mn sensitivity of mtp8 59 knockout mutants characterized the protein as a vacuolar Mn transporter suitable to 60 prevent plant cells from Mn toxicity. MTP8 expression was strongly induced on low-Fe as 61 well as high-Mn media, which was both strictly dependent on the transcription factor FIT, 62 indicating that high-Mn stress induces Fe deficiency. mtp8 mutants were only 63 hypersensitive to Fe deficiency when Mn was present in the medium, which further 64 suggested a Mn-specific role of MTP8 during Fe limitation. Under those conditions, mtp8 65 mutants did not only translocate more Mn to the shoot than did wild-type plants, but 66 suffered in particular from critically low Fe concentrations, and hence Fe chlorosis, 67 although the transcriptional Fe deficiency response was upregulated more strongly in 68 mtp8 . The diminished uptake of Fe from Mn-containing low-Fe media by mtp8 mutants 69 was caused by an impaired ability to boost the ferric chelate reductase activity, which is 70 an essential process in Fe acquisition. These findings provide a mechanistic explanation 71 for the long-known interference of Mn in Fe nutrition and define the molecular processes 72 by which plants alleviate this antagonism. 73 situation in calcareous soil. In this screen, we identified two lines that showed severe chlorosis. In both lines, the MTP8 gene, a member of the CDF family, was knocked out. In line with previously characterized MTP8 orthologs, subcellular localization and yeast complementation characterized the protein as a vacuolar Mn transporter suitable to prevent plant cells from Mn toxicity. However, our investigations revealed a crucial role of this protein in the Fe acquisition machinery, thereby safeguarding an efficient acquisition and translocation of Fe to aboveground plant parts. We further identified ferric chelate reduction as the Mn-sensitive process in the Fe deficiency response. These findings provide a mechanistic insight into the long-known interference of Mn in Fe acquisition and define the molecular process by which plants alleviate this antagonism. The general relevance of this mechanism is underlined by the hypersensitivity of mtp8 mutants to Fe deficiency chlorosis also on calcareous soil. Mutations in Arabidopsis Yellow Stripe-Like1 and Yellow Stripe-Like3 reveal their roles in metal ion homeostasis and loading of metal ions in seeds.