Protein Kinases and Phosphatases Involved in ABA Signaling - Abscisic Acid: Metabolism, Transport and Signaling - page 149

ERF7 that negatively regulates ABA response, suggesting that CIPK15 may reg-
ulate ABA signaling in cooperation with these two ABA-signaling components.
Additionally, SnRK3.11/CIPK24/SOS2, a key regulator of plant response to salt
stress, interacts with ABI2, which may constitute a point of ABA action on plant
salt tolerance. CIPK6/SnRK3.14 and CIPK20/PKS8/SnRK3.6 are positive regula-
tors of ABA signaling in Arabidopsis , which may possibly antagonize CIPK1/3/15
(Fig. 8.2 ). Multigene family-encoded CBLs and CIPKs provide a high level of
diversity, complexity, and flexibility in the function of the CBL-CIPK network in
ABA signaling.
8.4 Mitogen-Activated Protein Kinases (MAPKs) Involved
in ABA Signaling
Mitogen-activated protein kinase (MAPK) cascades are universal signal transduc-
tion modules in eukaryotes, including yeasts, animals, and plants. These protein
phosphorylation cascades link extracellular stimuli to a wide range of cell sign-
aling pathways (Widmann et al. 1999 ; Chen and Thorner 2007 ; Colcombet and
Hirt 2008 ). MAPKs are serine/threonine kinases that phosphorylate a variety of
substrates, such as transcription factors, protein kinases, and cytoskeletal proteins.
MAPKKs (or MEKs) are dual-specificity kinases that phosphorylate MAPKs
on their threonine and tyrosine residues. MAPKKKs (or MEKKs) are serine/
threonine kinases and phosphorylate MAPKKs on two serine/threonine resi-
dues. MAPKs form the terminal components of the MAPK sequential phospho-
rylation cascades and are activated by MAPKKs, which are in turn activated by
MAPKKKs (Widmann et al. 1999 ; Ichimura et al. 2002 ; Chen and Thorner 2007 ;
Colcombet and Hirt 2008 ). The Arabidopsis genome encodes 20 MAPKs, 10
MAPKs, and 80 MAPKs (Colcombet and Hirt 2008 ), which provides a potential
for functional redundancy and diversity in cell signaling. MAPK cascades have
been shown to be involved in responses to various environmental stresses and
developmental processes (Ichimura et al. 2002 ; Colcombet and Hirt 2008 ).
Earlier studies revealed that some members of the MAPK family could be acti-
vated by ABA and potentially involved in ABA signaling. For example, several
barley ( Hordeum vulgare ) MAPK isoforms that are activated by ABA (Knetsch
et al. 1996 ), an ABA-inducible MAPK in Vicia guard cell protoplasts (Mori and
Muto 1997 ), a Pisum sativum epidermal MAPK that may correlate with stomatal
responses to ABA (Burnett et al. 2000 ), a rice ( Oryza sativa ) MAPK, OsMAPK5,
which is involved in disease resistance and abiotic stress tolerance (Xiong and
Yang 2003 ), and a maize ( Zea mays ) MAPK that may be involved in ABA-
induced antioxidant defense (Zhang et al. 2006 ).
The studies in Arabidopsis help to understand the mechanisms of MAPK
signaling in response to ABA. ABA treatment activates AtMPK3, and overex-
pression of AtMPK3 increases ABA sensitivity in the postgermination arrest of
seedling growth (Lu et al. 2002 ), revealing a role of this MAPK in response of the