The BTB ubiquitin ligases ETO1, EOL1 and EOL2 act collectively to regulate ethylene biosynthesis in Arabidopsis by controlling type‐2 ACC synthase levels

Abstract

SummaryEthylene biosynthesis is directed by a family of 1-aminocyclopropane-1-carboxylic acid (ACC) synthases (ACS) that convert S-adenosyl-L-methionine to the immediate precursor ACC. Members of the type-2 ACS subfamily are strongly regulated by proteolysis with various signals stabilizing the proteins to increase ethylene production. In Arabidopsis, this turnover is mediated by the ubiquitin/26 S proteasome system, using a broad complex/tramtrack/bric-a-brac (BTB) E3 assembled with the ETHYLENE OVERPRODUCER 1 (ETO1) BTB protein for target recognition. Here, we show that two Arabidopsis BTB proteins closely related to ETO1, designated ETO1-like (EOL1) and EOL2, also negatively regulate ethylene synthesis via their ability to target ACSs for breakdown. Like ETO1, EOL1 interacts with type-2 ACSs (ACS4, ACS5 and ACS9), but not with type-1 or type-3 ACSs, or with type-2 ACS mutants that stabilize the corresponding proteins in planta. Whereas single and double mutants affecting EOL1 and EOL2 do not show an ethylene-related phenotype, they exaggerate the effects caused by inactivation of ETO1, and further increase ethylene production and the accumulation of ACS5 in eto1 plants. The triple eto1 eol1 eol2 mutant phenotype can be effectively rescued by the ACS inhibitor aminoethoxyvinylglycine, and by silver, which antagonizes ethylene perception. Together with hypocotyl growth assays showing that the sensitivity and response kinetics to ethylene are normal, it appears that ethylene synthesis, but not signaling, is compromised in the triple mutant. Collectively, the data indicate that the Arabidopsis BTB E3s assembled with ETO1, EOL1 and EOL2 work together to negatively regulate ethylene synthesis by directing the degradation of type-2 ACS proteins.

Arabidopsis RING E3 Ligase XBAT32 Regulates Lateral Root Production through Its Role in Ethylene Biosynthesis

“…The role of XBAT32 in regulating ethylene synthesis may also be responsible for the delayed growth phenotype of xbat32 plants. Similar to ethylene overproducers or constitutively active ethylene signaling mutants (Ogawara et al, 2003;Christians et al, 2009), xbat32 plants are shorter in stature than wild-type plants. However, unlike these ethylene mutants, the short stature phenotype of xbat32 is only observed early in development (Nodzon et al, 2004;this study).…”

“…XBAT32 mutant seedlings do not seem to have an exaggerated apical hook or shorter roots; however, the shorter hypocotyls compared with wildtype seedlings indicate that the level of ethylene overproduction in xbat32 does alter some aspects of growth. Blocking ethylene synthesis via cobalt or AVG application rescues the shortened hypocotyl phenotype and restores it to wild-type levels, as for ethyleneoverproducing mutants like eto1 (Guzman and Ecker, 1990) and the triple mutant eto1 eol1 eol2 (Christians et al, 2009). Similar to xbat32, the ethylene overproducer eto1 also shows reduced lateral root formation, whereas ethylene-insensitive mutants such as ein2 and ert1 produce more lateral roots than the wild type (Negi et al, 2008).…”

“…The fact that XBAT32 can interact with and ubiquitinate ACS4 and ACS7 provides further evidence for a role for XBAT32 in ethylene biosynthesis. The abundance of the type 2 ACSs, ACS5 and ACS9, is regulated by ETO1 and ETO1-like (EOL1) broad complex/tramtrack/bric-a-brac proteins, which are substrate-recruiting components of cullin-based RING E3 ligases (Vogel et al, 1998;Chae et al, 2003;Wang et al, 2004;Yoshida et al, 2005;Christians et al, 2009). Similarly, XBAT32 may negatively regulate the abundance of ACSs to modulate ethylene production.…”

“…The rate-limiting conversion of SAM to ACC by ACS is regulated by ubiquitin-dependent proteasomal degradation (Vogel et al, 1998;Chae et al, 2003;Wang et al, 2004;Yoshida et al, 2005;Christians et al, 2009). XBAT32, a functional RING-type E3 ligase capable of E2-dependent protein ubiquitination (Nodzon et al, 2004), may regulate the abundance of one or more of the ACS enzymes.…”

Seed-Specific Expression of a Feedback-Insensitive Form of CYSTATHIONINE-γ-SYNTHASE in Arabidopsis Stimulates Metabolic and Transcriptomic Responses Associated with Desiccation Stress

“…Increased expression was also observed for two ET transcription factors, ETHYLENE INSENSITIVE3 (At3g20770) and ENHANCED ETHYLENE RESPONSE5 (At2g19560; Table II). The first is a well-characterized transcription factor that initiates downstream transcriptional cascades for ET responses, and the second is involved in ET signaling and response (Christians et al, 2009). Interestingly, another two transcription factors that function as active repressors of ET biosynthesis, ETHYLENE OVERPRODUCER-LIKE1 (At4g02680) and ERF7 (At3g20310; Christians et al, 2009), were down-regulated by 1.61-fold (Table II).…”