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vitro, suggesting that KEG can also modulate the degradation of ABF1 and ABF3.
Similarly, the loss of ABF1 , ABF3, or both of them cannot fully rescue the keg
phenotype (Chen et al. 2013 ), which is similar to the abi5 keg double mutant. All
above evidence indicates that other bZIP transcription factors, such as ABF2 and
ABF4, maybe function in the similar way (Jakoby et al. 2002 ). Calcineurin B-like
interacting protein kinase (CIPK26) was identified as another KEG-interacting
protein by using a yeast two-hybrid screen. CIPK26 and KEG can interact in
vitro and in vivo. The degradation of CIPK26 was increased along with the ele-
vated levels of KEG protein, CIPK26 -overexpressing plant was more sensitive to
ABA compared with wild-type plants. Furthermore, CIPK26 was able to phos-
phorylate ABI5 in vitro. So the data show that CIPK26 is involved in ABI5- and
KEG-mediated ABA-signaling network (Lyzenga et al. 2013 ). Since no evidence
showed the KEG can direct ubiquitinate and promote the degradation of CIPK26,
the triadic relation among CIPK26, KEG, and ABI5 still need to be addressed.
Through testing the germination of 100 different Arabidopsis RING finger gene
T-DNA insertion mutant lines in the presence of exogenous ABA, Kim's lab found
three RING E3 ubiquitin ligases, At Arabidopsis ABA-insensitive RING protein
1 (AIRP1), At AIRP2 and At AIRP3 were involved in ABA response. These three
E3 ubiquitin ligases are positive regulators in ABA-dependent response to drought
stress, among them AtAIRP1 and AtAIRP2 play combinatory roles in this process.
AtAIRP3 can target the Cys proteinase of the papain family, RESPONSIVE TO
DEHYDRATION21, for degradation in vitro and in vivo (Ryu et al. 2010 ; Cho
et al. 2011 ; Kim and Kim 2013 ).
In addition, a number of other RING-type E3 ligases have been shown to play
positive roles in ABA signaling in both seed germination and post-germination
growth, although their substrates remain to be identified. These include ATL43
(Serrano et al. 2006 ), SDIR1 (Zhang et al. 2007 ), RHA2a (Bu et al. 2009 ), and
RHA2b (Li et al. 2011 ).
9.3.2.2 U-Box-type E3 Ligases Involved in ABA Signaling
U-box family E3 ligase was classified based on their modified RING finger
domain, without the full complement of Zn 2 + -binding ligands, probable involved
in E2-dependent ubiquitination. At least four U-box-type E3 ligases have been
characterized in ABA signaling. The expression of AtPUB9 can be induced by
ABA treatment, accompanying the changes in AtPUB9 subcellular localization
from the nucleus to the plasma membrane in tobacco BY-2 cells. atpub9 mutant
plants were hypersensitive to ABA-induced seed germination inhibition, which
can be rescued by abi3 - 6 , indicating that AtPUB9 is involved in ABA response
(Bergler and Hoth 2011 ). AtPUB18 and AtPUB19 are two homologous U-box E3
ubiquitin ligases. Loss-of-function and overexpression assays reveal that AtPUB18
and AtPUB19 are negative regulators of ABA-mediated drought responses.
Moreover, the atpub18atpub19 double mutant displayed more sensitive to ABA
and enhanced drought tolerance than did each single mutant plant (Bergler and
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