Protein Kinases and Phosphatases Involved in ABA Signaling - Abscisic Acid: Metabolism, Transport and Signaling - page 148

of which 3 (SnRK1.1 to SnRK1.3) are SnRK1s, 10 (SnRK2.1 to SnRK2.10) are
SnRK2s, and 25 are (SnRK3.1 to SnRK3.25) SnRK3s. The SnRK1 group is the
most closely related to SNF1 from yeast and to AMP-activated protein kinases
(AMPK) from animals, whereas the SnRK2 and SnRK3 groups appear to be plant-
specific classes of kinases. The SnRK3s are the CBL-interacting protein kinases
CIPKs.
SnRK1s likely function as a convergence point of ABA and sugar signaling
pathways. The Arabidopsis SnRK1.1, a major form of the AtSnRK1s, positively
regulates sugar- and ABA signaling in seedling growth, possibly by phosphoryl-
ating ABF transcription factors. The SnRK1s of wheat were also suggested to
be involved in ABA signaling. Two members of the clade-A PP2Cs, ABI1 and
PP2CA, inhibit SnRK1.1 activity by dephosphorylating it to repress SnRK1-
mediated sugar signaling, while ABA promotes SnRK1-mediated sugar signaling
by inhibiting the PP2Cs, which allows the coordination of ABA and energy sign-
aling, strengthening the stress response through the cooperation of two key and
complementary pathways (Fig. 8.2 ).
SnRK2s play critical roles in ABA-signaling pathway and regulate plant
responses to environmental stresses. A wheat SnRK2, PKABA1, may mediate
ABA signaling in seed development and germination, and a broad bean SnRK2,
AAPK, regulates guard cell signaling in response to ABA. In Arabidopsis ,
OST1/SnRK2.6 is a key player in ABA regulation of stomatal movement.
SnRK2.2 and SnRK2.3 function redundantly in ABA-induced inhibition of seed
germination and postgermination growth. However, these functionally seg-
regated kinases SnRK2.2, SnRK2.3, and SnRK2.6 still work redundantly in
the three major ABA responses, acting together as a core signaling point in
ABA-signaling pathways. Other members of the Arabidopsis SnRK2 subfam-
ily, such as SnRK2.7/2.8, are likely to play roles in ABA and/or stress signal-
ing. An Arabidopsis decuple mutant carrying mutations in all 10 members of the
SnRK2 subfamily grows poorly under hyperosmotic stress conditions, reveal-
ing critical functions of the SnRK2s in osmotic stress signaling and tolerance.
SnRK2s phosphorylate and activate ABA-responsive bZIP transcription factor
ABFs and regulate ABA-responsive genes to mediate ABA signaling. In addi-
tion, guard cell anion channel SLAC1 and inward K + channel are substrates of
the SnRK2.6/OST1. Whereas Ca 2 + -dependent activation of such ion channels is
directly associated with the CDPKs, the Ca 2 + -independent activation of the ion
channels are mainly mediated by SnRK2.6. Phosphoproteomic approaches iden-
tified a set of new potential substrates of SnRK2 protein kinase in Arabidopsis ,
suggesting a highly complicated SnRK2-mediated phosphorylation network
(Fig. 8.2 ).
SnRK3s/CIPKs, together with corresponding CBLs, have been suggested to
regulate Ca 2 + -dependent ABA signaling in concert with CDPKs. In Arabidopsis ,
the interactions of CIPK15/SnRK3.1 with CBL1, or CIPK3/SnRK3.17 with
CBL9, or CIPK1/SnRK3.16 with CBL9, negatively mediate ABA signaling,
thought it still remains controversial whether CBL1 regulates ABA responses.
CIPK15 interacts with ABI1 and ABI2 and targets an AP2 transcription factor