Non-proteolytic protein ubiquitination is crucial for iron deficiency signaling.

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.