Potassium Stimulation of IAA Transport Mediated by the Arabidopsis Importer AUX1 Investigated in a Heterologous Yeast System

Auxin regulates diverse processes involved in plant growth and development. AUX1 is the first identified and most widely investigated auxin importer, and plays an important role in root gravitropism and the development of lateral root and root hair. However, the regulation of auxin transport by AUX1 is still not well understood. In this study, we examined the effect of metal ions on AUX1 transport function and found that the activity could be specifically stimulated four times by K + . Further experiments revealed the preference of KF on the enhancement of transport activity of AUX1 over KCl, KBr, and KI. In addition, the interaction between K + and AUX1 confers AUX1 more resistant to thermal stress but more vulnerable to proteolysis. Conventional chemical modification indicated that the extracellular acidic amino acids of AUX1 play a key role in the K + stimulation. Site-specific mutagenesis showed that the replacement of Asp 166 , Asp 293 , and Asp 312 of AUX1 to alanine deteriorated the K + -stimulated auxin transport. By contrast, when these residues were mutated to glutamate, lysine, or asparagine, only the D312E variant restored the IAA transport activity to the wild-type level. It is thus convinced that D312 is presumably the most promising residue for the K + stimulation on AUX1.