Protein Kinases and Phosphatases Involved in ABA Signaling - Abscisic Acid: Metabolism, Transport and Signaling - page 155

(substitution of an aspartic acid residue for a glycine residue) at an equivalent
position close to the Mg 2 + -coordinating center (G180D for abi1 - 1 mutation and
G168D for abi2 - 1 mutation) in the catalytic domain (Leung et al. 1994 , 1997 ;
Meyer et al. 1994 ), but it remained puzzling, for a long time, how this mutation
results in strong ABA insensitivity (Schweighofer et al. 2004 ; Cutler et al. 2010 ).
Currently, it has been known that the ABA-activated PYR/PYL/RCAR receptors
interact with and antagonize the clade-A PP2Cs that dephosphorylate and inacti-
vate their substrates SnRK2s and SnRK1s (Ma et al. 2009 ; Park et al. 2009 ; Fujii
et al. 2009 ; Cutler et al. 2010 ; Rodrigues et al. 2013 ). It has been uncovered that
the abi1 - 1 and abi2 - 1 mutations disrupt the interactions between the PP2Cs and
the ABA receptors PYR/PYL/RCAR, but do not affect the interactions between
the PP2Cs and their downstream regulatory components SnRK2s (Yoshida et al.
2006b ; Ma et al. 2009 ; Park et al. 2009 ; Umezawa et al. 2009 ; Vlad et al. 2009 ).
These mutations reduce PP2C activity of both ABI1 and ABI2 in vitro (Leung
et al. 1997 ; Leube et al. 1998 ; Gosti et al. 1999 ), but appear to enhance consti-
tutively the dephosphorylation activity of the PP2Cs for their natural substrates
SnRK2s in vivo (Umezawa et al. 2009 ), which explains the dominant mutation of
strong ABA-insensitive phenotypes of the abi1 - 1 and abi2 - 1 mutants (Leung et al.
1994 , 1997 ; Meyer et al. 1994 ; Gosti et al. 1999 ), and is consistent with their cen-
tral, upstream roles in ABA signaling.
In addition to the SnRK2s, the Arabidopsis clade-A PP2Cs ABI1 and/or ABI2
may use CDPK, SnRK1, and SnRK3/CIPK as substrates, such as CPK21, CPK23,
SnRK1.1, SnRK3.1/CIPK15, and SnRK3.11/CIPK24/SOS2 (see earlier descrip-
tion in the CDPKs and SnRKs sections). The multiple substrates may be likely to
allow the clade-A PP2Cs to execute specific and/or redundant functions in ABA
signaling (Fig. 8.2 ).
It is noteworthy that two homologous members of clade B PP2Cs, PP2C5 and
AP2C1, were also reported to regulate ABA signaling (Brock et al. 2010 ), sug-
gesting that PP2Cs of other groups than A group may also be involved in ABA
signaling. Further studies will be needed to identify additional members of PP2Cs
involved in ABA signaling and to elucidate their functional mechanisms in the
complicated ABA-signaling network.
8.6.3 PP2As and PP2A-Like Proteins: Positive or Negative
Regulators in ABA Signaling?
As mentioned above, the PPP family proteins are composed of PP1, PP2A, and
PP2B proteins, which are ubiquitous enzymes in all eukaryotes except for PP2B
that is absent in plants, while their functions are largely unknown in higher plants.
In the Arabidopsis genome, twenty-six catalytic subunits C-encoding genes of the
PPP family have been found, which are related to PP1, PP2A, and a novel class
of phosphatases PP4, PP5, PP6, and PP7, of which PP4 and PP6 are PP2A-like
proteins according to their sequence similarities (Farkas et al. 2007 ). Protein