Heterotrimeric G Protein-Regulated Ca2+ Influx and PIN2 Asymmetric Distribution Are Involved in Arabidopsis thaliana Roots' Avoidance Response to Extracellular ATP

Extracellular ATP (eATP) has been reported to be involved in plant growth as a primary messenger in the apoplast. Here, roots of Arabidopsis thaliana seedlings growing in jointed medium bent upon contact with ATP-containing medium to keep away from eATP, showing a marked avoidance response. Roots responded similarly to ADP and bz-ATP but did not respond to AMP and GTP. The eATP avoidance response was reduced in loss-of-function mutants of heterotrimeric G protein alpha subunit (G alpha) (gpa1-1 and gpa1-2) and enhanced in G alpha-over-expression (OE) lines (wG alpha and cG alpha). Ethylenebis(oxyethylenenitrilo) tetraacetic acid (EGTA) and Gd3+ remarkably suppressed eATP-induced root bending. ATP-stimulated Ca2+ influx was impaired in G alpha null mutants and increased in its OE lines. DR5-GFP and PIN2 were asymmetrically distributed in ATP-stimulated root tips, this effect was strongly suppressed by EGTA and diminished in G alpha null mutants. In addition, some eATP-induced genes' expression was also impaired in G alpha null mutants. Based on these results, we propose that heterotrimeric G alpha-regulated Ca2+ influx and PIN2 distribution may be key signaling events in eATP sensing and avoidance response in Arabidopsis thaliana roots.