BRUTUS-LIKE proteins moderate the transcriptional response to iron deficiency in roots
Iron is an essential micronutrient but in excess is toxic inside cells. Under iron deficiency, the expression of iron uptake genes is increased, but it is not known how the transcriptional response is controlled to avoid uptake of too much iron. The hemerythrin E3 ligases BRUTUS (BTS) and BTS-LIKE (BTSL) have previously been identified as negative regulators of the iron deficiency response. Our phylogenetic analysis indicated that BTSL proteins are present in dicotyledonous plants only and form a separate clade from BTS homologs. BTSL1 and BTSL2 in Arabidopsis thaliana are in a network with nearly all iron uptake genes, whereas BTS is in a shoot-specific network. BTSL1 and BTSL2 are expressed predominantly in the root epidermis and cortex, separate from BTS in the root stele, shoot and embryos. Mutant analysis identified BTSL2 as the dominant paralog of the otherwise redundant BTSL genes. The btsl double mutant had increased protein levels of FIT, the FER-like Iron deficiency-induced Transcription factor, and failed to switch off the transcriptional response upon iron resupply, leading to dramatic iron accumulation in roots and shoots. Protein interaction between the C-terminus of BTSL proteins and FIT indicate that FIT is a direct target for degradation. Taken together, our studies show that BTSL1 and BTSL2 control iron uptake in the epidermis and cortex, upstream of BTS in the vasculature and leaves.