Arabidopsis Seedling Growth Response and Recovery to Ethylene. A Kinetic Analysis

Abstract

Responses to the plant hormone ethylene are mediated by a family of five receptors in Arabidopsis that act in the absence of ethylene as negative regulators of response pathways. In this study, we examined the rapid kinetics of growth inhibition by ethylene and growth recovery after ethylene withdrawal in hypocotyls of etiolated seedlings of wild-type and ethylene receptor-deficient Arabidopsis lines. This analysis revealed that there are two phases to growth inhibition by ethylene in wild type: a rapid phase followed by a prolonged, slower phase. Full recovery of growth occurs approximately 90 min after ethylene removal. None of the receptor null mutations tested had a measurable effect on the two phases of growth inhibition. However, loss-of-function mutations in ETR1, ETR2, and EIN4 significantly prolonged the time for recovery of growth rate after ethylene was removed. Plants with an etr1-6;etr2-3;ein4-4 triple loss-of-function mutation took longer to recover than any of the single mutants, while the ers1;ers2 double mutant had no effect on recovery rate, suggesting that receiver domains play a role in recovery. Transformation of the ers1-2;etr1-7 double mutant with wild-type genomic ETR1 rescued the slow recovery phenotype, while a His kinase-inactivated ETR1 construct did not. To account for the rapid recovery from growth inhibition, a model in which clustered receptors act cooperatively is proposed.Ethylene regulates a number of developmental processes in higher plants, including growth in etiolated seedlings. Inhibition of growth in etiolated seedlings by ethylene is a convenient and useful bioassay that has been used to quantify the dose-response characteristics of ethylene (Chen and Bleecker, 1995) and, in mutant screens, to identify components in the ethylene signal transduction pathway (Bleecker et al., 1988; Guzman and Ecker, 1989). Mutational analysis of the ethylene signaling pathway has led to an increasingly refined model for signaling (Guo and Ecker, 2004). According to this model, responses to ethylene are mediated by a family of five receptors in Arabidopsis that are related to bacterial two-component receptors (Chang et al., 1993;Sakai et al., 1998). The ethylene receptors are thought to transduce signal via Ser/Thr kinase activity in CTR1 (Kieber et al., 1993;Huang et al., 2003). CTR1 may negatively regulate the ethylene response pathway by inhibiting activity of an Nramp-related protein, EIN2, which is required for responses to ethylene (Alonso et al., 1999). Ethylene binding to the receptors reduces the activity of the receptors, leading to reduced activity of CTR1 protein and an increase in activity of EIN2 protein along with subsequent signaling associated with it. At least some responses to ethylene, including the seedling growth response, are mediated by activation of a transcriptional cascade (Solano et al., 1998), suggesting that ethylene responses are mediated by differential gene activation and inactivation. In support of this, a number of genes have been shown to be ethylene res...

Ethylene Regulates Levels of Ethylene Receptor/CTR1 Signaling Complexes in Arabidopsis thaliana

“…The production of new receptors in response to ethylene has been proposed as a mechanism to reset the ethylene sensitivity of the plant (3,40). Thus, the increase in levels of receptor/CTR1 complexes with ethylene concentrations up to 1 l/liter would serve to desensitize the plant to ethylene, thereby facilitating the adaptation response to ethylene.…”

“…7. Ethylene perception and signal transduction induce the production of new ethylene receptors, ERS1, ETR2, and ERS2 all being primary response genes whose transcription is rapidly induced in response to ethylene (40,41). The increase in overall ethylene receptor levels at the membrane results in a concurrent increase in membrane-associated CTR1 levels due to the physical association of CTR1 with the receptors.…”

Modulation of Ethylene Responses Affects Plant Salt-Stress Responses

“…Other Arabidopsis ethylene receptors had Ser kinase activity (Moussatche and Klee, 2004). The functions of the ETR1 kinase domain and its kinase activity are relatively weak in induction of ethylene response and seedling growth recovery after ethylene removal, and the signal output by the ETR N terminus is dependent on subfamily I members Binder et al, 2004;Qu and Schaller, 2004;Xie et al, 2006). The ETR1 receptor function can be regulated by RTE1 or the copper transporter RAN1 Resnick et al, 2006).…”

The Copper Transporter RAN1 Is Essential for Biogenesis of Ethylene Receptors in Arabidopsis

“…The receptors have homology to bacterial two-component receptors that transduce signals via the autophosphorylation of a His residue in the kinase domain, followed by the transfer of phosphate to a conserved Asp residue in the receiver domain of a response regulator protein (9). Although some of the receptors are capable of His kinase activity (10,11) and ethylene may inhibit this activity in ETR1 (12), kinase activity is not needed for signaling (13)(14)(15). The N terminus of these receptors contains the novel ligand-binding domain (16 -18), and a number of studies have now identified amino acid residues in the three ␣-helices of ETR1 that are important for ethylene binding and signal transduction (16, 19 -21).…”