Ubiquitination generally targets proteins for recognition and degradation via the 26S proteasome. Activated ubiquitin (E1) is transferred to an ubiquitin conjugase (UBC, E2) which associates with a ubiquitin ligase (E3), and multiple ubiquitin molecules are attached via linkage of Lys 48. By contrast, Lys 63-linked ubiquitin chains modify proteins in a non-proteolytic manner. We recently reported that UBC13, the only known ubiquitin conjugase capable of catalyzing Lys 63-linked polyubiqitination, is responsive to the iron (Fe) regime at the post-transcriptional level and may play a crucial role for the morphological alterations triggered by Fe deficiency in cucumber and Arabidopsis roots. It is assumed that UBC13 participates, most likely via the non-proteolytic polyubiquitination of proteins, in the signal transduction cascade associated with the acclimation of plants to the prevailing availability of Fe. In this Addendum, we present a possible scenario that occurs downstream of UBC13, which ultimately leads to Fe deficiency-specific changes in post-embryonic development of Arabidopsis roots.