Agriculture Reference
In-Depth Information
8.3.3 SnRK2S: A Central Node of ABA-Signaling Pathways
8.3.3.1 Studies in Crop Plants
Compelling evidence has been provided to reveal that members of the SnRK2 sub-
family play critical roles in ABA-signaling pathway and regulate plant responses
to environmental stresses. Earlier studies in crop plants identified two members of
the SnRK2 subfamily, PKABA1 from wheat ( Triticum aestivum ) and AAPK from
broad bean ( Vicia faba ), as the ABA-stimulated protein kinases and positive regu-
lators of ABA response (Anderberg and Walker-Simmons 1992 ; Li and Assmann
1996 ; Gomez-Cadenas et al. 1999 , 2001 ; Li et al. 2000 , 2002 ; Shen et al. 2001 ;
Johnson et al. 2002 ). Transcription of PKABA1 is induced by ABA and occurs
in embryos and seedlings, and PKABA1 functions to mediate ABA-suppressed
gene expression (Gomez-Cadenas et al. 1999 ), and phosphorylates and activates
the ABA-responsive basic-domain leu-zipper (bZIP) transcription factor TaABF, a
seed-specific homolog of ABI5 of Arabidopsis (Johnson et al. 2002 ), suggesting
that the TaABF may serve as a physiological substrate for PKABA1 which may
regulate ABA signaling through activating this transcription factor in seed devel-
opment and germination (Shen et al. 2001 ; Johnson et al. 2002 ). AAPK mediates
ABA-induced stomatal closure and anion channels (Li et al. 2000 ) and modulates
an RNA-binding protein AKIP1 by phosphorylating it and inducing its transloca-
tion into subnuclear speckles in guard cells (Li et al. 2002 ). In rice, expression
of three members of the rice SnRK2 subfamily, SAPK8, SAPK9, and SAPK10,
homologs of PKABA1/AAPK, is induced by osmotic stress and ABA (Kobayashi
et al. 2004 ), suggesting that these rice SnRK2s may be involved in ABA and stress
signaling.
8.3.3.2 SnRK2.2, SnRK2.3, and SnRK2.6 Constitute a Core Signaling
Point in Arabidopsis
The kinase activities of five Arabidopsis SnRK2 members, SnRK2.2, SnRK2.3,
SnRK2.6, SnRK2.7, and SnRK2.8, can be activated by ABA, and all of
the SnRKs except SnRK2.9 can be activated by osmotic stress (Boudsocq
et al. 2004 ; Furihata et al. 2006 ; Yoshida et al. 2006b ), suggesting that most
Arabidopsis SnRK2 members may be involved in ABA and stress signaling. The
OST1/SnRK2.6 is the first member of the Arabidopsis SnRK2 subfamily that
was identified by a forward genetic approach as a regulator of ABA signaling; the
snrk2.6 ( ost1 ) mutant cannot exhibit ABA-induced stomatal closure, revealing
that it positively regulates stomatal response to ABA (Mustilli et al. 2002 ; Yoshida
et al. 2002 ). However, the SnRK2.6 gene is mainly expressed in guard cells and
in the vascular system, and seed germination and postgermination growth were
not affected in snrk2.6 loss-of-function mutant (Mustilli et al. 2002 ; Yoshida et al.
2002 ), suggesting that other SnRK2 protein kinases may function in seed germi-
nation and seedling growth. The studies through a reverse genetic approach in
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