Protein Kinases and Phosphatases Involved in ABA Signaling - Abscisic Acid: Metabolism, Transport and Signaling - page 133

by modulating the conformation, activity, localization, and stability of substrate
proteins (Hunter 1995 ; Cohen 1997 ; Smith and Walker 1996 ; Ichimura et al.
2002 ; Luan et al. 2002 ; Hrabak et al. 2003 ; Luan 2003 ; Schweighofer et al. 2004 ;
McCormack et al. 2005 ; Morillo and Tax 2006 ; Farkas et al. 2007 ; Colcombet
and Hirt 2008 ; De Smet et al. 2009 ; Halford and Hey 2009 ; Luan 2009 ; Weinl
and Kudla 2009 ; Gish and Clark 2011 ; Perochon et al. 2011 ; Boudsocq and
Sheen 2013 ; Schulz et al. 2013 ; Hamel et al. 2014 ). Both protein kinases and
phosphatases were originally discovered in animal systems in which reversible
protein phosphorylation plays important roles in diverse cellular processes such
as signal transduction, glycogen metabolism, cell cycle progression, ion trans-
port, and developmental regulation (Hunter 1995 ). In plants, as in animals, it has
been recognized that nearly all aspects of cell function involve reversible phos-
phorylation, and significant progress has been made in the identification of plant
protein kinases and phosphatases and elucidation of their functional mechanisms
by which plants respond to developmental and environmental cues (Smith and
Walker 1996 ; Ichimura et al. 2002 ; Luan et al. 2002 ; Hrabak et al. 2003 ; Luan
2003 ; Schweighofer et al. 2004 ; McCormack et al. 2005 ; Morillo and Tax 2006 ;
Farkas et al. 2007 ; Colcombet and Hirt 2008 ; De Smet et al. 2009 ; Halford and
Hey 2009 ; Luan 2009 ; Weinl and Kudla 2009 ; Gish and Clark 2011 ; Perochon
et al. 2011 ; Boudsocq and Sheen 2013 ; Schulz et al. 2013 ; Hamel et al. 2014 ). In
the field of ABA signal transduction, the researches of the identification of protein
kinases and phosphatases involved in ABA signaling have attracted much attention
in the last decades, and an increasing number of protein kinases and phosphatases
have been reported to be important components of ABA signaling, covering a
wide range of protein kinase and phosphatases, such as Ca 2 + -dependent protein
kinases (CDPK), SNF1-related protein kinases (SnRKs), mitogen-activated pro-
tein kinases (MAPKs), receptor-like kinases (RLKs), clade-A protein phosphatase
2C (PP2C), and protein phosphatase 2A (PP2A). Some of these protein kinases
and phosphatases play pivotal roles in ABA signaling, forming a reversible protein
phosphorylation cascade, which deepens our understanding of highly complicated
ABA-signaling pathway. In this chapter, we examine these advances in the plant
protein kinases and phosphatases as well as the insights of these discoveries into
the mechanisms of ABA-signaling network.

8.2 Ca 2 + -Dependent Protein Kinases (CDPK) Involved in
ABA Signaling

8.2.1 CDPKs: Key Ca 2 + Sensors in Plant Cells

Ca 2 + has long been recognized as a major, conserved second messenger in eukary-
otic cells in responses to developmental and environmental stimuli. Ca 2 + signaling
is believed to be modulated by specific Ca 2 + signatures, i.e., the specific patterns
of variations in the amplitude, duration, location, and frequency of cytosolic free
Ca 2 + -spikes in response to different stimuli. It has been accepted that the specific