AtNHX5 and AtNHX6 are endosomal Na+ ,K+ /H+ antiporters that are critical for growth and development in Arabidopsis, but the mechanism behind their action remains unknown. Here, we report that AtNHX5 and AtNHX6, functioning as H+ leak, control auxin homeostasis and auxin-mediated development. We found that nhx5 nhx6 exhibited growth variations of auxin-related defects. We further showed that nhx5 nhx6 was affected in auxin homeostasis. Genetic analysis showed that AtNHX5 and AtNHX6 were required for the function of the endoplasmic reticulum (ER)-localized auxin transporter PIN5. Although AtNHX5 and AtNHX6 were colocalized with PIN5 at ER, they did not interact directly. Instead, the conserved acidic residues in AtNHX5 and AtNHX6, which are essential for exchange activity, were required for PIN5 function. AtNHX5 and AtNHX6 regulated the pH in ER. Overall, AtNHX5 and AtNHX6 may regulate auxin transport across the ER via the pH gradient created by their transport activity. H+ -leak pathway provides a fine-tuning mechanism that controls cellular auxin fluxes.

Institute of Science and Technology Austria Am Campus 1 3400 Klosterneuburg Austria

Laboratory of Growth Regulators Centre of the Region Haná for Biotechnological and Agricultural Research Institute of Experimental Botany AS CR and Palacký University Šlechtitelů 27 CZ 78371 Olomouc Czech Republic

Mendel Centre for Plant Genomics and Proteomics Central European Institute of Technology Masaryk University Kamenice 5 CZ 625 00 Brno Czech Republic

MOE Key Laboratory of Cell Activities and Stress Adaptations School of Life Sciences Lanzhou University Lanzhou Gansu 730000 China

School of Life Sciences Centre for Cell and Developmental Biology and State Key Laboratory of Agrobiotechnology The Chinese University of Hong Kong Shatin New Territories Hong Kong

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ER-Localized PIN Carriers: Regulators of Intracellular Auxin Homeostasis