A Phosphorus-Limitation Induced, Functionally Conserved DUF506 Protein is a Repressor of Root Hair Elongation in Arabidopsis thaliana

ABSTRACT Root hairs (RHs) function in nutrient and water acquisition, root metabolite exudation, soil anchorage and plant-microbe interactions. Longer or more abundant RHs are potential breeding traits for developing crops that are more resource-use efficient and can improve soil health. RH elongation is controlled by both environmental and endogenous factors. While many genes are known to promote RH elongation, relatively little is known about genes and mechanisms that constrain RH growth. Here we demonstrate that a DOMAIN OF UNKNOWN FUNCTION 506 (DUF506) protein, AT3G25240, negatively regulates Arabidopsis thaliana RH growth. The AT3G25240 gene is strongly and specifically induced during P-limitation. Mutants of this gene, which we call R EPRESSOR OF E X CESSIVE R OOT HAIR ELONGATION 1 (RXR1) , have much longer RHs, while over-expression of the gene results in much shorter RHs. RXR1 physically interacts with a Rab-GTPase (RXR2), and an rxr2 mutant phenocopies the rxr1 mutant. Overexpression of a Brachypodium distachyon RXR1 homolog resulted in repression of RH elongation in Brachypodium. Taken together, our results reveal a DUF506-GTPase module with a prominent role in repression of RH elongation that is conserved in monocots and dicots.