ABA Regulation of the Cold Stress Response in Plants - Abscisic Acid: Metabolism, Transport and Signaling - page 339

regulation also serves as a major mechanism in the global control of the ABA
and cold responses.
17.9 Epigenetic Modifications in Cold and ABA Signalling
DNA and histones are epigenetically modified through acetylation and meth-
ylation following exposure to stress (Kim et al. 2010 ; Feng and Jacobsen 2011 ).
HOS15 encodes a protein similar to the human WD-40 repeat protein TBL1 (trans-
ducin-like protein-1), which is involved in histone deacetylation. Both ABA and
cold induce the expression of HOS15 , and hos15 - 1 mutants exhibit hypersensitiv-
ity to ABA-induced inhibition of germination and freezing stress. Furthermore,
HOS15 interacts with histone H4, and levels of acetylated histone H4 are higher
in hos15 mutants than in the wild type (Zhu et al. 2008 ). Therefore, HOS15 is
involved in the regulation of ABA and the cold stress response through H4 deacet-
ylation-dependent chromatin remodelling in Arabidopsis . Consistently, increased
levels of acetylated histone H4 are consistently associated with increased expres-
sion of RD29A in a hos15 mutant compared with the wild type under cold stress.
On the other hand, histone deacetylation results in a non-permissive chromatin
conformation that represses transcription. In Arabidopsis , histone deacetylase 6
(HDA6) has been reported to be involved in the ABA response and low-temper-
ature-mediated flowering (Luo et al. 2012 ; Chen and Wu 2010 ). In Arabidopsis ,
HDA6 encodes an RPD3-type histone deacetylase, and a loss-of-function muta-
tion in HDA6 , axe1 - 5 , displays hypersensitivity to ABA and salt stress (Luo et al.
2012 ). Interestingly, HOS1 was discovered as a chromatin remodelling factor for
cold-induced transcriptional silencing of the FLC gene through an interaction with
HDA6, thereby antagonising the actions of FVE and resulting in the inhibition of
flowering in these plants (He and Amasino 2005 ; Jung et al. 2013 ). These findings
suggest that plant flowering or stress response processes might be epigenetically
controlled through similar mechanisms.
17.10 Cross Talk Between Phytohormones and the Cold
Response
Emerging evidence indicates that cold acclimation is influenced by changes in the
homeostasis of various hormones and that hormone signalling is crucial for regula-
tion of the CBF pathway. Genetic analyses of Arabidopsis mutants with compro-
mised ABA biosynthesis or signalling have identified a complex interplay between
ABA and other phytohormone-signalling pathways. Generally, when plants are
challenged by stress conditions, the stress hormones ABA and jasmonate acid (JA)
inhibit growth by modulating the actions of auxin, GA, and cytokinin (Achard