ABA Regulation of the Cold Stress Response in Plants - Abscisic Acid: Metabolism, Transport and Signaling - page 334

transgenic Arabidopsis (Dai et al. 2007 ). Recent studies suggest that some NAC-
domain family members are involved in stress responses (Olsen et al. 2005 ). For
example, expression of a rice NAC gene, OsNAC5 , is induced by osmotic stress
and ABA, and overexpression of OsNAC5 increases stress-induced proline and
soluble-sugar levels and enhances tolerance to cold, salt, and drought stresses
(Takasaki et al. 2010 ; Song et al. 2011 ). Thus, it can be assumed that cold induc-
tion of some ABA-responsive genes is regulated by different kinds of transcription
factors.
17.5 Second Messengers Integrate Cold
and ABA Signalling
Both cold stress and ABA treatment induce the production and accumulation of
IP3, Ca 2 + , and ROS inside the cell. These molecules act as second messengers
in signalling networks to amplify and transduce signals through activating pro-
tein kinases or transcription-factor cascades. Increased inositol 1,4,5-triphosphate
(IP 3 ) levels, which are catalysed by phospholipase C (PLC) under cold stress, are
known to release Ca 2 + from vacuoles into the cytosol (Allen and Sanders 1995 ;
Munnik et al. 1998 ). In Arabidopsis , FRY1 encodes an inositol polyphosphate 1—
phosphatase that is involved in IP 3 metabolism. fry1 mutant plants show signifi-
cantly higher IP 3 levels when treated with ABA, and they display defects in cold
acclimation and hypersensitivity to osmotic stress, with lowered expression of cer-
tain stress-responsive genes (Xiong et al. 2001b ).
In plants, Ca 2 + is a widely used and important signalling molecule during
early responses to abiotic stresses, as it functions by activating protein kinase
cascades, which in turn can activate transcription factors and stress-respon-
sive genes. More specifically, a transient influx of cytoplasmic Ca 2 + occurs in
response to cold shock, and it has been suggested that low-temperature-induced
changes in membrane fluidity are the primary thermosensor signal that activates
the Ca 2 + influx in plants (Knight et al. 2004 ). Furthermore, calcium is required
for the full expression of the COR genes in Arabidopsis . CAX1 , which encodes a
vacuolar membrane-located Ca 2 + /H + antiporter, participates in the regulation of
cold acclimation. Loss-of-function mutations in CAX1 increase CBF expression
and enhance freezing tolerance following acclimation in Arabidopsis (Catala
et al. 2003 ). One calmodulin proteins in Arabidopsis , AtCML9, has recently
been demonstrated to be responsive to ABA, cold, and salt stress through an
ABA-dependent pathway. The cml9 mutant exhibits hypersensitivity to ABA and
enhanced tolerance to stress and drought (Magnan et al. 2008 ). CaM methyla-
tion is also involved in the early growth of Arabidopsis and responds to ABA,
cold, and heat stress during germination (Banerjee et al. 2013 ). As mentioned
earlier, the calmodulin-binding transcriptional activator CAMTA3 is involved
in the regulation of freezing tolerance (Doherty et al. 2009 ). More importantly,
a recent study suggested that calcium channels may involve in temperature