Ethylene, a Signaling Compound Involved in Seed Germination and Dormancy

Abstract

The present review is focused on current findings on the involvement of ethylene in seed biology. The responsiveness of seeds to ethylene depends on the species and the dormancy status, improving concentrations ranging from 0.1 to 200 μL L^-1. The signaling pathway of ethylene starts with its binding to five membrane-anchored receptors, which results in the deactivation of Constitutive Triple Response 1 (CTR1, a protein kinase) that does not exert its inhibitory effect on Ethylene Insensitive 2 (EIN2) by phosphorylating its cytosolic C-terminal domain. An analysis of germination in the presence of inhibitors of ethylene synthesis or action, and using seeds from mutant lines altered in terms of the genes involved in ethylene synthesis (acs) and the signaling pathway (etr1, ein2, ein4, ctr1 and erf1), demonstrates the involvement of ethylene in the regulation of seed dormancy. The promoting effect of ethylene is also regulated through crosstalk with abscisic acid (ABA) and gibberellins (GAs), essential hormones involved in seed germination and dormancy, and Reactive Oxygen Species (ROS). Using a mutant of the proteolytic N-degron pathway, Proteolysis (PRT6), the Ethylene Response Factors (ERFs) from group VII (HRE1, HRE2, RAP 2.2, RAP2.3 and RAP 2.12) have also been identified as being involved in seed insensitivity to ethylene. This review highlights the key roles of EIN2 and EIN3 in the ethylene signaling pathway and in interactions with different hormones and discusses the responsiveness of seeds to ethylene, depending on the species and the dormancy status.

Keywords: crosstalk between ethylene, ABA, Gas and ROS; ethylene; ethylene response factors (ERFs); ethylene signaling pathway; proteolytic N-degron pathway; seed germination and dormancy.

Figure 1

Ethylene (C₂H₄) biosynthesis pathway. ACC, 1-Aminocyclopropane-1-carboxylic acid; ACO, ACC oxidase; ACS, ACC synthase; KMB, 2-keto-4-methylthiobutyrate; MACC, Malonyl-ACC; MTA, 5-Methylthioadenosine; MTR, 5-Methylthioribose; S-AdoMet, S-adenosylmethionine. Cold differentially inhibits ACS and ACO activities, resulting in ACC accumulation during cold treatment and in an ethylene burst after transfer of the organs at warmer temperatures. Hypoxia inhibits ACO activity, which is nil in anoxia, with oxygen being required for ACO activity. From [28,30,31,32,92].

Figure 2

Schematic model of the ethylene signaling pathway in the presence (A) and in the absence of ethylene (B). The ethylene receptors localized at the endoplasmic reticulum (ER) are divided into two subfamilies based on the feature of the histidine kinase motif; subfamily I includes ETR1 and ERS1, while ETR2, ERS2 and EIN4 fall into subfamily II. CTR1 interacts with the five receptors, but it is presented separately in order to indicate its state (active or inactive). In the presence of C₂H₄ (A), the receptors deactivate CTR1, which, in turn, activates EIN2 that interacts with EIN3 and EIL1, allowing it to activate ethylene response factors (ERFs), resulting in the expression of ethylene responsive genes. EBF 1 and 2 (EIN3-binding F box protein 1 and 2) are able to inhibit EIN3 and EIL1. Without ethylene (B), the receptors-CTR1 complexes maintain CTR1 in an active state, allowing it to phosphorylate EIN2, thus preventing ethylene response through EIN2. In the nucleus, the EIN3/EIL1 are degraded. The turnover of EIN2 is regulated by ETP and the degradation of EIN3 is controlled by EBF. CTR1, Constitutive Triple Response 1; EBF, EIN3 Binding F-Box; EIL3, Ethylene Insensitive-Like protein; EIN2, Ethylene Insensitive 2; EIN3, Ethylene Insensitive 3; EIN4, Ethylene Insensitive 4; ERBPS, Ethylene Responses Element Binding Proteins; ERFs, Ethylene Response Factor; ERS1, ERS2, Ethylene Response Sensor 1 and 2; ETP, EIN2 Targeting protein; ETR1, ETR2, Ethylene Resistant 1 and 2; RAN, Response to Antagonist 1, a copper cofactor. An activated component is shown by color, while an inactivated or repressed component is shown in grey. Modified from [128,130,131,132,133].

Figure 3

Ethylene signaling transduction pathways. A, the canonical ethylene signaling (cf. Figure 2); B, EIN2 independent signaling pathway involving a MAPK cascade; C, CTR1 independent signaling pathway involving RTE1; D, two-component signaling that relies on the histidine kinase activity of the receptor. AHPs, Arabidopsis Histidine-containing Phosphotransmitter; ARRs, Arabidopsis Response Regulators; CTR1, Constitutive Triple Response; EIN2, Ethylene Insensitive 2; EIN3, Ethylene Insensitive 3; EILs, EIN Like; ETR1, Ethylene Response 1; MKK9, Mitogen-Activated Protein Kinase Kinase; MPK3/6 Protein Kinase 3/6; RTE1, Reversion To Ethylene sensitivity 1. Modified from [91,130,159,165].

Figure 4

Interactions between ethylene, abscisic acid, gibberellins, and ROS in the regulation of seed germination. This scheme is based on genetic, molecular, and physiological studies cited in the text on seed responsiveness to C₂H₄, ABA, GAs, and ROS. Ethylene down-regulates ABA level by inhibiting its synthesis, in particular through NCED, and promoting its catabolism through CYP707. It also down-regulates ABA signaling through ABI5. In addition, ABA inhibits ethylene biosynthesis by reducing ACS and ACO activities. Ethylene also improves GA metabolism, through GA3ox and GA20ox, and GAS signaling, through RGL2 and RGA. ROS produced in relation with stress environments enhance ABA catabolism and both C₂H₄ and GAs signaling pathways (black arrows). (+) and (−) indicate positive and negative interactions between the components of the signaling cascade. The interrelations between ABA and ethylene, and GAs and ethylene, are in red and blue, respectively. ABA, abscisic acid; ABF, ABA-response element-(ABRE) binding factors; ABI5, ABA INSENSITIVE 5; ACC, 1-aminocyclopropane-1-carboxylic acid; ACO, ACC oxidase; ACS, ACC synthase; CTR1, Constitutive Triple Response 1; CYP707, cytochrome P450-dependent mono-oxygenase; EIN2, Ethylene Insensive 2; EIN3, Ethylene Insensitive 3; ERFs, Ethylene Response Factors; ERS1, Ethylene Response Sensor; ETR1, Ethylene Response 1; GAs, gibberellins; GA3ox, GA3-oxidase; GA20ox, GA20-oxidase; GID1, receptor GIBBERELLIN INSENSITIVE DWARF1; NCED, 9-cis-epoxycarotenoid dioxygenase; PA, phaseic acid; PP2C, phosphatase 2C proteins; PYL, PYR1-like; PYR, pyrabactin resistance; RCAR, regulatory components ABA receptor; RGA, REPRESSON OF ga-1-3; RGL2, RGA LIKE 2; SnRK2, subfamily 2 SNF1-related kinase. Modified from [31].