N-Terminal Metal-Binding Domain of Arabidopsis IBR5 Is Important for Its in Planta Functions

Indole-3-acetic acid (IAA), the predominant natural auxin, is a plant hormone that regulates growth and development in response to various internal and external signals. <i>Arabidopsis thaliana</i> (Arabidopsis) indole-3-butyric acid response 5 (<i>AtIBR5</i>, <i>AT2G04550</i>) encodes a dual-specificity phosphatase in Arabidopsis. The <i>atibr5</i> mutant exhibits reduced sensitivity to indole-3-butyric acid (IBA), a precursor of IAA, but is also less responsive to another plant hormone, abscisic acid (ABA). We report that AtIBR5 contains a rubredoxin-like domain in its N-terminal region, in addition to the previously identified dual-specificity phosphatase domain. The rubredoxin-like domain of AtIBR5, when expressed in <i>Escherichia coli</i>, binds zinc through four cysteine residues in the rubredoxin-like domain and exhibits a characteristic absorption spectrum at 430 nm. The rubredoxin-like domain, more specifically the set of four cysteine residues, is essential for most in planta functions of AtIBR5 related to auxin and ABA. These functions include hypocotyl elongation, leaf serration, and germination phenotypes. However, this domain is dispensable for the inhibition of root elongation by ABA. All orthologs of AtIBR5 in the green plant lineage investigated encode the N-terminal rubredoxin-like domain, which features the specific arrangement of four cysteine residues. Our result provides a clue as to how various plant phenotypes can be subtly modulated by AtIBR5.