Auxin enhances aluminium-induced citrate exudation through upregulation of GmMATE and activation of the plasma membrane H+-ATPase in soybean roots.

Background and Aims Aluminium (Al) toxicity is a limiting factor for plant growth and crop production in acidic soils. Citrate exudation and activation of the plasma membrane H+-ATPase are involved in soybean responses to Al stress. Auxin has crucial functions in plant growth and stress responses. However, little is known about possible interactions between auxin and citrate exudation under Al stress. In this study, we elucidated the regulatory roles of IAA in Al-induced citrate exudation in soybean roots. Methods We measured IAA content, Al concentration, citrate exudation, plasma membrane H+-ATPase activity, expression of the relevant genes and phosphorylation of the plasma membrane H+-ATPase by integrating physiological characterization and molecular analysis using hydroponically grown soybean. Key Results The concentration of IAA was increased by 25 and 50 μm Al, but decreased to the control level at 200 μm Al. External addition of 50 μm IAA to the root medium containing 25, 50 or 200 μm Al decreased root Al concentration and stimulated Al-induced citrate exudation and the plasma membrane H+-ATPase activity. Reverse transcription-PCR analysis showed that exogenous IAA enhanced the expression of citrate exudation transporter (GmMATE) but not the plasma membrane H+-ATPase gene. The western blot results suggested that IAA enhanced phosphorylation of the plasma membrane H+-ATPase under Al stress. Conclusions Auxin enhanced Al-induced citrate exudation through upregulation of GmMATE and an increase in phosphorylation of the plasma membrane H+-ATPase in soybean roots.