How Sorghum Plants Modulate Root Citrate Exudation Under Aluminum Stress

Aluminum (Al) activated root organic acid exudation plays an important role in Al resistance in many plants.  However, organic acids are also an important carbon source for plant growth and development.  Thus, a finely controlled organic acid exudation system is essential not only for Al resistance but also for maintaining yields under Al stress.  We have previously identified the major sorghum Al resistance protein , SbMATE, that is a root citrate efflux transporter activated by Al. Here, we report on a metal binding protein (SbMBP) from sorghum root tips that interacts with SbMATE. If only SbMATE is expressed in heterologous systems (yeast, oocytes), we observed a large constitutive citrate efflux in the absence of Al.  Co-transformation of SbMATE and SbMBP in yeast restores Al-activated citrate exudation and confers a high level of Al resistance in yeast.  These findings suggest that SbMBP might function as an Al sensor and negative regulator of citrate flux through the SbMATE transporter.