Auxin Influx Carrier AUX1 Confers Acid Resistance for Arabidopsis Root Elongation Through the Regulation of Plasma Membrane H+-ATPase.

The plant plasma membrane (PM) H⁺-ATPase regulates pH homeostasis and cell elongation in roots through the formation of an electrochemical H⁺ gradient across the PM and a decrease in apoplastic pH; however, the detailed signaling for the regulation of PM H⁺-ATPases remains unclear. Here, we show that an auxin influx carrier, AUXIN RESISTANT1 (AUX1), is required for the maintenance of PM H⁺-ATPase activity and proper root elongation. We isolated a low pH-hypersensitive 1 (loph1) mutant by a genetic screen of Arabidopsis thaliana on low pH agar plates. The loph1 mutant is a loss-of-function mutant of the AUX1 gene and exhibits a root growth retardation restricted to the low pH condition. The ATP hydrolysis and H⁺ extrusion activities of the PM H⁺-ATPase were reduced in loph1 roots. Furthermore, the phosphorylation of the penultimate threonine of the PM H⁺-ATPase was reduced in loph1 roots under both normal and low pH conditions without reduction of the amount of PM H⁺-ATPase. Expression of the DR5:GUS reporter gene and auxin-responsive genes suggested that endogenous auxin levels were lower in loph1 roots than in the wild type. The aux1-7 mutant roots also exhibited root growth retardation in the low pH condition like the loph1 roots. These results indicate that AUX1 positively regulates the PM H⁺-ATPase activity through maintenance of the auxin accumulation in root tips, and this process may serve to maintain root elongation especially under low pH conditions.

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