The PAP/SAL1 retrograde signaling pathway is involved in iron homeostasis

There is a link between PAP/SAL retrograde pathway, ethylene signaling and Fe metabolism in Arabidopsis. Nuclear gene expression is regulated by a diversity of retrograde signals that travel from organelles to the nucleus in a lineal or classical model. One such signal molecule is 3'-phosphoadenisine-5'-phosphate (PAP) and it's in vivo levels are regulated by SAL1/FRY1, a phosphatase enzyme located in chloroplast and mitochondria. This metabolite inhibits the action of a group of exorribonucleases which participate in post-transcriptional gene expression regulation. Transcriptome analysis of Arabidopsis thaliana mutant plants in PAP-SAL1 pathway revealed that the ferritin genes AtFER1, AtFER3, and AtFER4 are up-regulated. In this work we studied Fe metabolism in three different mutants of the PAP/SAL1 retrograde pathway. Mutant plants showed increased Fe accumulation in roots, shoots and seeds when grown in Fe-sufficient condition, and a constitutive activation of the Strategy I Fe uptake genes. As a consequence, they grew more vigorously than wild type plants in Fe-deficient medium. However, when mutant plants grown in Fe-deficient conditions were sprayed with Fe in their leaves, they were unable to deactivate root Fe uptake. Ethylene synthesis inhibition revert the constitutive Fe uptake phenotype. We propose that there is a link between PAP/SAL pathway, ethylene signaling and Fe metabolism.