One mutant line eto with salt tolerance was screened from a T-DNA insertion mutant collection of Arabidopsis thaliana. In addition to a reduced rate of seed germination, NaCl and ABA also inhibited the growth and the greening of cotyledons of wild-type seedlings, but not the eto mutant. TAIL-PCR analysis showed that T-DNA tag insertion in the eto was located at nucleotide 27,502 in BAC F3M18, upstream (at position -487 relative to the translation initiation codon) of gene At1g77740 (encoding a putative phosphatidylinositol-4-phosphate 5-kinase, AtPIP5K2). This inserted mutation cosegregated closely with the eto phenotype. Another analysis not only indicated that AtPIP5 K2 transcript is expressed predominantly in roots and rosette leaves, but also showed the T-DNA insertion resulted higher accumulation of the AtPIP5K2 in eto mutant plants and did not influenced the expression of the upstream At1g77730 gene. This change may play an essential role in the tolerance of eto mutant plant to the osmotic stress.