lysine-63-linked ubiquitin chain-forming conjugase, UBC13, promotes the developmental responses to iron deficiency in Arabidopsis roots

Iron-deficiency responses comprise molecular, physiological and developmental adjustments, ultimately leading to an improved cellular Fe homeostasis. By using a proteomic approach, we identified the ubiquitin-conjugating enzyme UBC13 as being highly responsive to the Fe regime at the post-transcriptional level in the tips of cucumber (Cucumis sativus) roots. UBC13 has been shown to catalyze non-canonical Lys63-linked ubiquitin chains, playing important roles in signal transduction among eukaryotes. Ectopic expression of the cucumber UBC13 gene in Arabidopsis thaliana led to a more pronounced and Fe-responsive formation of branched root hairs, a key response of Arabidopsis roots to Fe deficiency. Plants carrying a mutation in the Arabidopsis ortholog UBC13A were unable to form branched root hairs upon Fe deficiency and showed a perturbed expression of Fe-regulated genes. Mutants defective in both Arabidopsis UBC13 genes, UBC13A and UBC13B, showed a marked reduction in root hair density. Mutations in the cognate E3 ligases RGLG2 and RGLG1 caused the constitutive formation of branched root hairs independent of the Fe supply, indicating the involvement of polyubiquitination in the altered differentiation of rhizodermal cells. It is concluded that UBC13, probably via the formation of Lys63-linked ubiquitin chains, has a critical function in epidermal cell differentiation and is crucial for the regulation of Fe-responsive genes and developmental responses to Fe deficiency.