A novel calmodulin‐interacting Domain of Unknown Function 506 protein represses root hair elongation in Arabidopsis

Domain of Unknown Function 506 proteins are ubiquitous in plants. The phosphorus (P) stress‐inducible REPRESSOR OF EXCESSIVE ROOT HAIR GROWTH1 (AtRXR1) gene encodes the first characterized DUF506. AtRXR1 inhibits root hair elongation by interacting with RabD2c GTPase. However, functions of other P‐responsive DUF506 genes are still missing. Here, we selected two additional P‐inducible DUF506 genes for further investigation. The expression of both genes was induced by auxin. Under P‐stress, At3g07350 gene expressed ubiquitously in seedlings, whereas At1g62420 (AtRXR3) expression was strongest in roots. AtRXR3 overexpressors and knockouts had shorter and longer root hairs, respectively. A functional AtRXR3‐green fluorescent protein fusion localized to root epidermal cells. Chromatin immunoprecipitation and quantitative reverse‐transcriptase‐polymerase chain reaction revealed that AtRXR3 was transcriptionally activated by RSL4. Bimolecular fluorescence complementation and calmodulin (CaM)‐binding assays showed that AtRXR3 interacted with CaM in the presence of Ca(2+). Moreover, cytosolic Ca(2+) ([Ca(2+)](cyt)) oscillations in root hairs of rxr3 mutants exhibited elevated frequencies and dampened amplitudes compared to those of wild type. Thus, AtRXR3 is another DUF506 protein that attenuates P‐limitation‐induced root hair growth through mechanisms that involve RSL4 and interaction with CaM to modulate tip‐focused [Ca(2+)](cyt) oscillations.