Impact of Iron Deficiency on the <scp><i>Arabidopsis thaliana</i></scp> Phloem Sap Proteome, a Key Role for <scp>bHLH121</scp>

Iron (Fe) is an essential micronutrient for plant growth and development whose homeostasis must be tightly regulated to avoid deficiency or excess that could be detrimental to the cells. In Arabidopsis thaliana, this mechanism is regulated by a series of transcription factors that act in an intricate regulatory network among which URI/bHLH121 (UPSTREAM REGULATOR OF IRT1) plays a predominant role. Tremendous efforts were deployed to decipher the molecular mechanisms that regulate iron homeostasis in plants. Nonetheless, the nature of the long-distance signal that conveys, via the phloem sap, information on the iron status of aerial tissues to the roots in order to coordinate iron uptake with the plant needs for iron is still to be determined. With the aim to identify potential actors involved in this process, we set up a proteomic analysis of the phloem sap of wild type Arabidopsis plants and bhlh121 loss-of-function mutants grown in iron-replete and iron-deficient conditions. We found that modifications in iron availability or the loss of URI activity have a profound impact on the phloem sap protein composition. We also found that some proteins whose translocation through the phloem sap is inhibited in response to iron deficiency are also affected in bhlh121 mutants. Interestingly, we discovered that some of the genes encoding such proteins are direct targets of URI, which suggests that the encoded proteins might act as potential signaling factors to regulate root iron uptake and/or root growth.