Protein Kinases and Phosphatases Involved in ABA Signaling - Abscisic Acid: Metabolism, Transport and Signaling - page 142

The SnRK1 group is the most closely related to SNF1 from yeast and to AMP-
activated protein kinases (AMPK) from animals. Plant SnRK1s are likely to form
heterotrimeric complexes (consisting of ʱ , ʲ , and ʳ subunits) and may regulate
carbon metabolism and stress signaling like their yeast and animal counterparts
(Halford et al. 2000 , 2003 ; Ferrando et al. 2001 ; Hardie 2000 ; Hrabak et al. 2003 ;
Polge and Thomas 2006 ; Halford and Hey 2009 ; Coello et al. 2011 ; Robaglia et al.
2012 ). However, the SnRK2 and SnRK3 groups appear to be plant-specific classes
of kinases, which diverge from AMPK and SNF1 than do SnRK1s, and may likely
emerge as a result of duplication and then evolve rapidly, taking on new roles to
enable plants to link metabolic and stress signaling (Halford et al. 2000 ; Hrabak
et al. 2003 ; Kolukisaoglu et al. 2004 ; Halford and Hey 2009 ; Coello et al. 2011 ).
The SnRK3s are the CBL-interacting protein kinases (CIPKs), which interact
with calcium-binding proteins related to animal neuronal calcium sensors and to
the regulatory B subunit of the protein phosphatase calcineurin (Kudla et al. 1999 ;
Hrabak et al. 2003 ; Luan 2009 ; Stefan and Kudla 2009 ). Subsequent analyses have
identified a complement of 10 CBLs and 25 CIPKs and 10 CBLs and 30 CIPKs in
the genomes of Arabidopsis and rice ( Oryza sativa ), respectively (Kolukisaoglu
et al. 2004 ). The SnRK members of the three groups have been shown to be
involved in plant response to ABA, drought, salt, and nutritional stresses, and dis-
ease (Kudla et al. 1999 ; Hrabak et al. 2003 ; Kolukisaoglu et al. 2004 ; Polge and
Thomas 2006 ; Halford and Hey 2009 ; Luan 2009 ; Stefan and Kudla 2009 ; Coello
et al. 2011 ).
8.3.2 SnRK1S: A Convergence Point of ABA and Sugar
Signaling Pathways
It has been demonstrated that ABA and sugar signaling pathways may share sign-
aling components (Cheng et al. 2002a ; Radchuk et al. 2006 ). Earlier studies indi-
cated that SnRK1 could be implicated in these interactions (Nemeth et al. 1998 ;
Bradford et al. 2003 ; Thelander et al. 2004 ; Radchuk et al. 2006 ; Lu et al. 2007 ).
Further, a report showed that overexpression of an Arabidopsis SnRK1.1, a major
form of the AtSnRK1s, confers sugar- and ABA-hypersensitive phenotype in seed-
ling growth (Jossier et al. 2009 ), and it is suggested that the AtSnRK1s function
by phosphorylating ABF (AREB) transcription factors (Zhang et al. 2008 ). The
SnRK1s of wheat ( Triticum aestivum ) were also suggested to be involved in ABA
signaling (Coello et al. 2012 ). Most recently, Rodrigues et al. ( 2013 ) revealed that
two members of the clade-A PP2Cs acting as major, negative regulators in ABA
signaling (reviewed in Cutler et al. 2010 , and see the section as follows), ABI1
and PP2CA, inhibit SnRK1.1 activity by dephosphorylating it to repress SnRK1-
mediated sugar signaling, and ABA promotes SnRK1-mediated sugar signaling by
inhibiting the PP2Cs (Fig. 8.2 ), which allows the coordination of ABA and energy
signaling, strengthening the stress response through the cooperation of two key
and complementary pathways.