Protein Ubiquitination and Sumoylation in ABA Signaling - Abscisic Acid: Metabolism, Transport and Signaling - page 178

plant sensitivity of ABA (Irigoyen et al. 2014 ). So, DDA1 negatively regulates
ABA response by targeting the degradation of ABA receptor, which is very
important when ABA levels in plant decrease.
Other two F-box proteins are involved in ABA-signaling pathway. DOR,
a putative F-box protein, belongs to the S-locus F-box-like family related to
AhSLF-S2 and can interact with ASK14 and CUL1. dor mutant plants are hyper-
sensitive to ABA-induced stomatal closure and tolerant to drought stress. DOR -
overexpressing plants were more sensitive to drought stress (Zhang et al. 2008 ).
Based on the physiological analysis about the overexpresser lines and knockdown
mutant plants, the EID1-like protein 3 (EDL3) is another F-box protein who acts
as a positive regulator in ABA-signaling pathway (Koops et al. 2011 ). But up to
now, no substrate was discovered for both of F-box proteins.
9.3.2.4 E2 Involved in ABA Signaling
Besides ubiquitin E3 ligases control the substrate specificity, ubiquitin conju-
gases were also discovered to be important in substrate recognition and thus
may also works in the specific physiological regulation. The Mungbean VrUBC1
( Vignaradiata UBC1 ) encodes a functional UBC E2. Overexpression of VrUBC1
plants enhanced ABA-induced stomatal closing and germination inhibition. A
number of key ABA-signaling genes, such as ABI5 , ABF2 , ABF3, and ABF4 , were
increased in transgenic plants compared to the wild-type plants. VrUBC1 can
interact with AtVBP1, a C3HC4-type RING E3 ligase, indicating that VrUBC1
is involved in ABA response possibly through interaction with a RING E3 ligase
(Chung et al. 2013 ). Arabidopsis UBC32 was identified as a stress-induced trans-
membrane-contained E2, similar to its counterpart in yeast and mammals, it was
discovered to be involved in the ER-associated protein degradation of unfolding
protein and participate in salt and ABA response in Arabidopsis. The ubc32 lines
were more insensitive than wild-type plants to ABA in both the germination and
post-germination stages, while the 35S - UBC32 lines were more sensitive to ABA,
indicating that UBC32 is also a positive regulator in ABA-signaling pathway (Cui
et al. 2012 ).
9.4 Protein Sumoylation in ABA Signaling
Many biochemical and genetic evidences have revealed that SUMO post-trans-
lation modification participates in numerous crucial biological processes in
plants, including plant vegetative growth and development, flowering regula-
tion, embryogenesis, embryogenesis, hormonal response, nitrogen assimilation,
pathogen resistance, especially the abiotic stress response, including phos-
phate-deficiency, cold, drought, salt, heat shock, and copper tolerance (Miura