Protein Ubiquitination and Sumoylation in ABA Signaling - Abscisic Acid: Metabolism, Transport and Signaling - page 179

and Hasegawa 2010 ). Up to now, several sumoylation components in sumoyla-
tion pathway were reported to regulate ABA signaling. AtSUMO1/2 overex-
pressing plants with increased level of sumoylation are less sensitive to ABA,
showing reduced seedling primary root growth inhibition (Lois et al. 2003 ).
AtSCE1a, the unique SUMO-conjugating enzyme in Arabidopsis, is colocalized
in nucleus with AtSUMO1/2. Transgenic plants with AtSCE1a co-suppression
showed ABA-mediated enhanced root growth inhibition compared to wide type
(Lois et al. 2003 ). Both known SUMO E3 ligases were also discovered to be
involved in ABA signaling. AtSIZ1, as a multiple biological functional SUMO
ligase E3, is confirmed to be related to ABA signaling, with the evidence that
the enhanced ABA response was observed in siz1 mutant (Miura and Hasegawa
2009 ). The other SUMO E3 ligase, HPY2/MMS21, firstly identified as a modula-
tor of cell cycle progression and meristem maintenance, was also demonstrated
to participate in ABA signaling. Similar to siz1 , enhanced ABA sensitivity was
also observed in mms21 (Zhang et al. 2013 ). Altogether, it seems that knock-
out/knockdown of components in sumoylation pathway all increased ABA
sensitivity.
How does sumoylation regulate plant ABA response? The effect of sumoyla-
tion on key components in ABA-signaling pathway has drawn the attention.
Genetic analysis showed mutant abi5 - 4 represses ABA sensitivity in siz1 - 2
mutant both in seed germination and primary root growth, which indicates the
genetic interaction between SIZ1 and ABI5. Biochemical evidence demon-
strated that ABI5 was sumoylated by SIZ1 and SIZ1-mediated ABI5 sumoyla-
tion protected the protein stability of ABI5 (Miura et al. 2009 ). Whereas AFP
(ABI 5-binding protein) and KEG (KEEP ON GOING, a RING finger ubiqui-
tin E3 ligase) facilitate ABI5 degradation through 26S proteasomes system, it
seems that SUMO E3 ligase SIZ1 protects ABI5 protein by sumoylation of ABI5
to against the ubiquitin-mediated ABI5 degradation (Lopez-Molina et al. 2003 ;
Miura et al. 2009 ; Liu and Stone 2010 ). The additive ABA-hypersensitive phe-
notype caused by siz1 - 2afp - 1 double mutation compared to single mutant sug-
gested that sumoylation and AFP facilitated ABI5 degradation may function in
different ways, perhaps other types of protein modifications involved (Miura
et al. 2009 ). Moreover, the recent study found phenotype of double mutants of
abi5 - 8 myb30 - 2 under ABA treatment is comparable to wild-type plants dur-
ing seed germination, so do the expression level of a group of stress-responsive
genes, including COR413 , COR15B , LOX3, and BGL2 . It is further found that
MYB30 can be sumoylated at residue K283 dependent on SIZ1. Due to the dif-
ferent sets of genes regulated by MYB30 and ABI5 in response to ABA, MYB30
and ABI5 were considered to balance different sets of downstream genes expres-
sion upon ABA treatment (Zheng et al. 2012 ). Additionally, ABI5 is phospho-
rylated by SnRK2.2 and SnRK2.3 in response to ABA, and then ABI5 was
activated as a transcription factor. Up to know, at least three different protein
modifications have been found on ABI5 protein, how ABI5 is precisely regulated
by these three post-translational modifications, and whether there is some kind
of balance needs further investigation.