Protein Ubiquitination and Sumoylation in ABA Signaling - Abscisic Acid: Metabolism, Transport and Signaling - page 174

9.3 Protein Ubiquitination in ABA Signaling
Being sessile organisms, plants are continuously exposed in complex and vola-
tile environment, including abiotic stress, such as drought, salt, and cold, and
biotic stress, for examples, pathogens and insects. At the same time, plants have
evolved rapid and efficient mechanisms to adapt changeable conditions. ABA is
a key hormone for many plant physiological processes, including senescence,
seed dormancy and germination, and adapting to stress conditions. Recently, the
understanding of ABA signaling, from the perception to downstream regulation,
involves many intermediates and is very complicated (Ma et al. 2009 ; Park et al.
2009 ; Cutler et al. 2010 ). Furthermore, many aspects of ABA biology are regu-
lated by protein ubiquitination.
9.3.1 ABA Biosynthesis Impacted by Protein Ubiquitination
Protein ubiquitination is known to control a broad of protein stability, including
signaling molecules and metabolic enzymes. SENESCENCE-ASSOCIATED E3
UBIQUITIN LIGASE1/PLANT U-BOX E3 LIGASE 44 (SAUL1/AtPUB44) has
been shown to be a functional E3 ubiquitin ligase and directly target ABSCISIC
ALDEHYDE OXIDASES 3 (AAO3), an enzyme to convert abscisic aldehyde to
ABA, for ubiquitin-dependent degradation via the 26S proteasome. saul1 mutants
exhibit enhanced ABA level and show premature senescence (Raab et al. 2009 ).
XERICO , encoding a small protein with RING-H2 RING finger motif, has been
shown to increase ABA levels and drought tolerance of plants through upregula-
tion of AtNECD3 , which is a key ABA-biosynthesis gene. Furthermore, yeast two-
hybrid assay indicates that XERICO interacts with an E2 ubiquitin-conjugating
enzyme, AtUBC8 and also AtTLP9, which is an ASK1-interacting F-box protein
involved in ABA-signaling pathway (Ko et al. 2006 ).
9.3.2 ABA Signaling Regulated by Protein Ubiquitination
Recent progress in interpreting the ABA signaling shows a core ABA signal
pathway, in which ABA receptor proteins (PYR/PRL/RCAR) bind to the type
2C protein phosphatases (PP2Cs) and inhibit the phosphatase activity in the
presence of ABA, subsequently releasing the SnRK2s kinases and the accumu-
lated phosphorylated SnRK2s kinases phosphorylating the ABA-responsive
element binding factors (ABFs) (Ma et al. 2009 ; Park et al. 2009 ; Cutler et al.
2010 ). Besides a few of ubiquitin conjugases, the increased numbers of E3
ligases, belonging to different E3 subfamilies, playing important roles in ABA
signaling have been identified.