Background: SNF1-related protein kinases 2 (SnRK2s) are key regulators of the plant response to osmotic stress. They are transiently activated in response to drought and salinity. Based on a phylogenetic analysis SnRK2s are divided into three groups. The classification correlates with their response to abscisic acid (ABA); group 1 consists SnRK2s non-activated in response to ABA, group 2, kinases non-activated or weakly activated (depending on the plant species) by ABA treatment, and group 3, ABA-activated kinases. The activity of all SnRK2s is regulated by phosphorylation. It is well established that clade A phosphoprotein phosphatases 2C (PP2Cs) are negative regulators of ABA-activated SnRK2s, whereas regulators of SnRK2s from group 1 remain unidentified. Results: Here, we show that ABI1, a PP2C clade A phosphatase, interacts with SnRK2.4, member of group 1 of the SnRK2 family, dephosphorylates Ser158, whose phosphorylation is needed for the kinase activity, and inhibits the kinase, both in vitro and in vivo. Our data indicate that ABI1 and the kinase regulate primary root growth in response to salinity; the phenotype of ABI1 knockout mutant (abi1td) exposed to salt stress is opposite to that of the snrk2.4 mutant. Moreover, we show that the activity of SnRK2s from group 1 is additionally regulated by okadaic acid-sensitive phosphatase(s) from the phosphoprotein phosphatase (PPP) family. Conclusions: Phosphatase ABI1 and okadaic acid-sensitive phosphatases of the PPP family are negative regulators of salt stress-activated SnRK2.4. The results show that ABI1 inhibits not only the ABA-activated SnRK2s but also at least one ABA-non-activated SnRK2, suggesting that the phosphatase is involved in the cross talk between ABA-dependent and ABA-independent stress signaling pathways in plants. This work was supported by the Ministry of Science and Higher Education, Poland (grant 500/N-COST/2009/0 to GD) and National Science Centre (grant 2011/03/B/NZ3/00297 to GD). Funding in the laboratory of Pedro L. Rodriguez was provided by grant BIO2014-52537-R. Financial support from the EU (COST FA0605) is gratefully acknowledged. Krzywinska, E.; Bucholc, M.; Kulik, A.; Ciesielski, A.; Lichocka, M.; Debski, J.; Ludwikow, A.... (2016). Phosphatase ABI1 and okadaic acid-sensitive phosphoprotein phosphatases inhibit salt stress-activated SnRK2.4 kinase. BMC Plant Biology. 16(136):1-12. doi:10.1186/s12870-016-0817-1 Senia 1 12 16 136