ABA Regulation of the Cold Stress Response in Plants - Abscisic Acid: Metabolism, Transport and Signaling - page 338

found to negatively regulate the expression of ABA-related genes by directly
interacting with ABI3 (Kurup et al. 2000 ). Moreover, TOC1 can also bind to the
promoter of ABAR , a chloroplastic ABA binding protein, to control its circadian
expression, and the gated-induction of TOC1 by ABA is abolished in ABAR RNAi
plants. Furthermore, overexpression of TOC1 causes significant changes in plant
responses to ABA and drought stress, which are indicative of a feedback mecha-
nism linking the circadian clock to the ABA response (Legnaioli et al. 2009 ).
Collectively, these findings illustrate the mechanisms by which the circadian clock
regulates the cold and ABA responses.
17.8 Post-Transcriptional Modification Mediates Cold and
ABA Signalling
RNA splicing and transport can lead to tissue-specific differences in gene expres-
sion as well as affect mRNA stability and turnover via nonsense-mediated decay
(McGlincy and Smith 2008 ). By performing a genetic screen for deregulated
expression of RD29A promoter-driven luciferase reporter gene, several genes
encoding RNA processing proteins were identified. Loss of osmotic responsive-
ness 4 ( LOS4 ) encodes a putative DEAD-box RNA helicase that has been impli-
cated in nucleo-cytoplasmic mRNA transport. The recessive mutations los4 - 1
and los4 - 2 affect cold-induced CBF transcription and lead to changes in chilling
and freezing tolerance (Gong et al. 2002 , 2005 ), suggesting that mRNA trans-
location may also be a control point in cold acclimation. It was also found that
a los4 - 2 mutant was hypersensitive to ABA (Gong et al. 2005 ). A recent study
identified a regulator of CBF2 gene expression1 ( rcf1 - 1 ) mutant that was
hypersensitive to cold stress. RCF1 also encodes a cold-inducible DEAD-box
RNA helicase. However, unlike LOS4, RCF1 maintains the proper splicing of
COR pre-mRNAs under cold stress conditions (Guan et al. 2013 ). Another pre-
mRNA splicing factor Stabilized1 ( STA1 ) which is required for pre-mRNA splic-
ing and mRNA turnover is upregulated by cold stress, but not by ABA or NaCl
(Lee et al. 2006 ). A sta1 mutant displays a chilling-sensitive phenotype and
hypersensitivity to ABA-induced root growth, as well as defects in splicing of
the cold-induced COR15A gene (Lee et al. 2006 ). Arabidopsis FIERY2 ( FRY2 ),
which encodes an RNA polymerase II C-terminal domain (CTD) phosphatase,
may function in mRNA processing. FRY2 plays a negative role in regulating salt
stress and the ABA response during seed germination, and it has been shown
to be a repressor of the CBF transcription factors. However, a fry2 mutant was
shown to be hypersensitive to freezing stress (Xiong et al. 2002a ). Sensitive to
ABA and drought2 (SAD2) was found to be involved in nucleo-cytoplasmic traf-
ficking during ABA treatment, and sad2 mutant plants show a hypersensitive
response to ABA and enhanced expression of stress genes in response to ABA,
salt, and low temperatures (Verslues et al. 2006 ). Therefore, post-transcriptional