Abstract
XBAT32, a member of the RING domain-containing ankyrin repeat subfamily of E3 ligases, was previously identified as a positive regulator of lateral root development. Arabidopsis (Arabidopsis thaliana) plants harboring a mutation in XBAT32 produce fewer lateral roots that wild-type plants. We found that xbat32 mutants produce significantly more ethylene than wild-type plants and that inhibition of ethylene biosynthesis or perception significantly increased xbat32 lateral root production. XBAT32 interacts with the ethylene biosynthesis enzymes AMINOCYCLOPROPANE-1-CARBOXYLIC ACID SYNTHASE4 (ACS4) and ACS7 in yeast-two-hybrid assays. XBAT32 is capable of catalyzing the attachment of ubiquitin to both ACS4 and ACS7 in in vitro ubiquitination assays. These results suggest that XBAT32 negatively regulates ethylene biosynthesis by modulating the abundance of ACS proteins. Loss of XBAT32 may promote the stabilization of ACSs and lead to increased ethylene synthesis and suppression of lateral root formation. XBAT32 may also contribute to the broader hormonal cross talk that influences lateral root development. While auxin treatments only partially rescue the lateral root defect of xbat32, they completely restore wild-type levels of xbat32 lateral root production when coupled with ethylene inhibition. Abscisic acid, an antagonist of ethylene synthesis/signaling, was also found to stimulate rather than inhibit xbat32 lateral root formation, and abscisic acid acts synergistically with auxin to promote xbat32 lateral root production.
Highlights
XBAT32, a member of the Really Interesting New Gene (RING) domain-containing ankyrin repeat subfamily of E3 ligases, was previously identified as a positive regulator of lateral root development
The higher Mr forms of His-Flag-ACID SYNTHASE4 (ACS4) or -ACS7 were not observed when ubiquitin was omitted from the assay (Fig. 5B). These results suggest that XBAT32 negatively regulates ethylene biosynthesis via ubiquitin-dependent degradation of ACS enzymes, in particular ACS4 and ACS7
We show that XBAT32, a RING domain-containing ankyrin repeat E3 ligase, is a negative regulator of ethylene biosynthesis and that through this activity XBAT32 modulates lateral root development
Summary
XBAT32, a member of the RING domain-containing ankyrin repeat subfamily of E3 ligases, was previously identified as a positive regulator of lateral root development. Members of several subgroups have been linked to important physiological and developmental processes, including XBAT32 (XB3 ortholog 2 in Arabidopsis), which belongs to the RING domain-containing ankyrin repeat subgroup (Nodzon et al, 2004; Stone et al, 2005) Members of this subgroup are referred to as XBAT due to their structural similarity to the rice (Oryza sativa) XB3 protein (Nodzon et al, 2004). Only certain pericycle cells in the primary root that are in contact with the xylem poles acquire the ability to undergo a series of asymmetric cell divisions This process, referred to as the lateral root initiation stage, leads to the formation of lateral root primordium (Malamy and Benfey, 1997; Dubrovsky et al, 2000; De Smet et al, 2006a). Other ethylene mutants, such as ethylene overproducer (eto1) and the constitutive ethylene signaling mutant constitutive triple response (ctr1), display a reduced lateral root phenotype (Negi et al, 2008)
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