The results show clearin vitrointeraction between proteins studied (Fig. 3A). environmental conditions. SNF1-related protein kinases 2 (SnRK2s) are plant specific enzymes involved in ABA-dependent plant development and in responses to environmental stresses, mainly drought and salinity (for review see refs. 1-5and references herein). All SnRK2s, except Arabidopsis SnRK2. 9, are activated rapidly and transiently in response to osmotic stress (water deficit, salt stress) and some of them are also activated by ABA treatment. Based on phylogenetic SBI-477 analysis SnRK2s are classified into 3 groups. The classification correlates with their response to exogenously applied ABA; group 1 consists of kinases non-activated by ABA, group 2 – kinases non-activated or weakly activated by ABA (depending on the plant species), and group 3 – strongly activated by this hormone. 6, 7The activity of SnRK2s is dependent on phosphorylation of specific Ser/Thr residues within their activation loops. 7-11It is well established that phosphoprotein phosphatases type-2C (PP2C) belonging to clade A are negative regulators of ABA-activated SnRK2s. 12, 13In control conditions, when ABA level in plant cells is low, SBI-477 the phosphatases interact with ABA-activated SnRK2s blocking their activity. With the rise of the level of ABA (e. g., during seed maturation or in response to stress) ABA receptors – RCAR/PYR/PYL (RCAR, Regulatory Component of ABA Receptor/PYR1, Pyrabactin Resistance 1/PYL, PYR1-like) bind the hormone, which results in interaction with clade A PP2Cs and their inhibition, allowing ABA-dependent SnRK2 activation. 14-19This mechanism of PP2Cs inhibition is strongly dependent on ABA. It was recently shown that also Type One Protein Phosphatase 1 (TOPP1) regulates the activity of ABA-activated SnRK2s, 20and it was proposed that regulation of TOPP1 activity and subsequently that of SnRK2s was dependent on PYL11 and ABA. On the other hand, the SnRK2 OST1 interacts with regulatory PP2AA- and PP2AB-subunits andpp2aaregulatory subunit double mutants show reduced ABA sensitivity in stomatal closure, althoughpp2acatalytic subunit double mutants show enhanced ABA-dependent SnRK2 activation. 21Much less is known on enzymes controlling the activity of ABA-non-activated SnRK2, from group 1 . Our recently published results showed that at least one ABA-non-activated SnRK2, namely SnRK2. 4 is inhibited by ABI1, a phosphatase from clade A of the PP2C family. 22Moreover, we showed that ABI1 and SnRK2. 4 regulate primary DC42 root growth in response to salinity; the phenotype ofABI1knockout mutant (abi1td) exposed to salt stress was opposite to that of thesnrk2. 4mutant. Now, in order to check whether the differences in the phenotypes of thesnrk2. 4andabi1mutants correlate with differences in SnRK2 activity, we monitored the SnRK2. 4/SnR2. 10 activity in WT andabi1tdmutant seedlings exposed to NaCl. The SnRK2. 4/SnRK2. 10 activity was measured by immunocomplex in gel-kinase activity assay using MBP as substrate. We measured simultaneously the activity of SnRK2. 4 and SnRK2. 10, due to the specificity of antibodies used. Unfortunately, we did not observe any significant differences in the kinase activity between WT andabi1tdmutant. Since most of clade A phosphatases are redundant in respect to inhibition of ABA-activated SnRK2s, we considered that it might also be the case for ABA-non-activated SnRK2s. Therefore , we decided to analyze the salt stress-induced SnRK2. 4/SnRK2. 10 SBI-477 activity in seedlings of doubleabi12 pp2ca-1mutant plants, with insertions in bothABI1andPP2CAgenes, and additionally in thepp2ca-1single mutant. We have chosenpp2ca-1mutants for our studies since our results showed thatin vitrorecombinant PP2CA phosphatase dephosphorylates and inhibits the activity of SnRK2. 4 similarly to ABI1 (Fig. 1A). Measurement of the kinase activity in seedlings exposed to salt stress showed that the activity of SnRK2. 4/SnRK2. 10 was higher by about 50% after 60 min of salt treatment in theabi12 pp2ca-1mutant in comparison to wild type Arabidopsis (Fig. 1B), whereas we did not observe higher SnRK2. 4/SnRK2. 10 activity in the singlepp2ca-1mutant, as it was in the case ofabi1td. The enhanced activity of the ABA-non-activated SnRK2s in the doubleabi12 pp2ca-1mutant indicates that not only ABI1, but also PP2CA, might be a physiological regulator of SnRK2. 4. Moreover, their function is at least SBI-477 to some extent redundant. == Figure 1 . == GST-PP2CA dephosphorylates and inhibits SnRK2. 4in vitro(A). Recombinant SnRK2. 4 (1 g) and in parallel SnRK2. 6 (1 g) and SnRK2. 8 (1 SBI-477 g) as controls were incubated with increasing amounts of recombinant GST-PP2CA for 30 min at 30C and kinase activity was analyzed.