Regulation of ethylene gas biosynthesis by the Arabidopsis ETO1 protein

Abstract

Ethylene gas is used as a hormone by plants, in which it acts as a critical growth regulator. Its synthesis is also rapidly evoked in response to a variety of biotic and abiotic stresses. The Arabidopsis ethylene-overproducer mutants eto2 and eto3 have previously been identified as having mutations in two genes, ACS5 and ACS9, respectively; these encode isozymes of 1-aminocyclopropane-1-carboxylic acid synthase (ACS), which catalyse the rate-limiting step in ethylene biosynthesis. Here we report that another ethylene-overproducer mutation, eto1, is in a gene that negatively regulates ACS activity and ethylene production. The ETO1 protein directly interacts with and inhibits the enzyme activity of full-length ACS5 but not of a truncated form of the enzyme, resulting in a marked accumulation of ACS5 protein and ethylene. Overexpression of ETO1 inhibited induction of ethylene production by the plant growth regulator cytokinin, and promoted ACS5 degradation by a proteasome-dependent pathway. ETO1 also interacts with CUL3, a constituent of ubiquitin ligase complexes in which we propose that ETO1 serves as a substrate-specific adaptor protein. ETO1 thus has a dual mechanism, inhibiting ACS enzyme activity and targeting it for protein degradation. This permits rapid modulation of the concentration of ethylene.

Ethylene-mediated cross-talk between calcium-dependent protein kinase and MAPK signaling controls stress responses in plants

“…Tomato LeACS2 was reported to be phosphorylated by a CDPK and the identified phosphorylation site in LeACS2 is conserved in some other ACS members (39,40). Activation of Arabidopsis ACS5 required the dissociation of ETO1 protein, a process that likely involves phosphorylation at a site differing from a MAPK phosphorylation motif (41,42). Conversely, a MAPK cascade activating MPK6 was shown to regulate ACS6 activity in Arabidopsis, and the expression of gain-of-function MAPK kinase mutants causing constitutive activation of respective MAPKs resulted in constitutive ethylene biosynthesis (10).…”

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

“…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.…”

The POLARIS Peptide of Arabidopsis Regulates Auxin Transport and Root Growth via Effects on Ethylene Signaling

“…For each assay, ;50 seedlings were grown in air on half-strength Murashige and Skoog medium containing 1% sucrose in the dark for 72 h in 22-mL vials for ethylene measurement by gas chromatography as described previously (Wang et al, 2004). Each assay represents an average number from six samples/vials of the same lines and was duplicated.…”