Hypertonic stress increased extracellular ATP levels and the expression of stress-responsive genes in Arabidopsis thaliana seedlings.

Arabidopsis seedlings increased extracellular ATP (exATP) concentrations transiently in extracellular matrix (ECM) under hypertonic stresses. The increased transcription levels of two apyrase genes, AtAPY1 and AtAPY2, in accordance with exATP accumulation, suggests active regulation of exATP concentration. Arabidopsis seedlings subjected to hypertonic stresses survived after incubation with beta,gamma-methyleneadenosine-5'-triphosphate, which usually causes cell death through competitive exclusion of ATP. This confirms that the enhanced viability was due to accumulated exATP. The increased concentration of hydrogen peroxide through NADPH oxidase expression suggests the possible importance of exATP in stress response under hypertonic stresses. The mRNA levels of exATP inducible genes (AtMAPK3, AtACS6, and AtERF4) and the reactive oxygen species inducible gene (AtPAL1) were increased by hypertonic stresses. We suggest that exATP accumulation plays a role as a regulatory mechanism in the hypertonic stress response in Arabidopsis seedlings.