Fig. 2

Soybean Model of Ethylene Signal Transduction. In silico experiments identified 68 proteins that could be involved directly or indirectly in soybean signal transduction initiated by ethylene. In this putative model, brown rectangles show the route-identified proteins in A. thaliana, and white rectangles show the soybean genes that encode proteins homologous to this plant model; orange rectangles illustrate membrane sensors that respond to biotic and abiotic stress in addition to receptors/sensors for endogenous signals (i.e., other phytohormones); the purple rectangle represents mRNAs related to ETP proteins; the rectangle with dotted outline (accompanied by a question mark) represents a protein in this pathway that has not been identified in the studied plants; blue and purple hexagons represent ACSs types I and II, respectively; black and red circles correspond to ubiquitin and phosphate groups, respectively; gray arrows correspond to routes that occur in the presence of ethylene and/or biotic/abiotic stress; dotted arrows in red and gray represent pathways that occur in the absence of this hormone and routes that culminate in ethylene biosynthesis, respectively; black lines indicate interactions among proteins. Cellular compartments represented: endoplasmic reticulum (beige), Golgi complex (green), nucleus (white) and cytoplasm (blue). Symbols: ACS: 1-aminocyclopropane-1-carboxylic acid synthase; CPK (or CDPK): calcium-dependent protein kinase; CTR: constitutive triple response protein; EBF: EIN3 binding F-Box protein; EIL: EIN protein like; EIN: ethylene insensitive; EOL: ETO protein like; ERF: ethylene response factor; ETP: EIN2 targeting protein; ETO: ethylene overproducer; MKKK (or MAPKKK): MAP kinase kinase kinase; MKK (or MAPKK): MAP kinase kinase; MPK (or MAPK): mitogen-activated protein kinase; RAN: responsive to antagonist; RAV: related to ABI3/VP1; RTE: reversion to ethylene sensitivity. The route of intracellular signal transduction is initiated by the interaction of ethylene with a membrane receptor (encoded by ETR genes) and through the modulation of CTR activity, which regulates the activity of several genes, such as EIN3. The receptors with CTR (similar to the protein kinase RAF - MKKK) work similarly to negative regulators of the pathway and, in the absence of ethylene, suppress downstream positive components of signal transduction. The hormone binding blocks the receptors in an inactive conformation, reducing the repression of metabolic pathway-positive regulators [11]. In the absence of ethylene, CTR phosphorylates the EIN2 C-terminal domain, promoting its interaction with ETP F-box protein (not identified in soybean) and its subsequent degradation via proteasome 26S [30]. In the absence of EIN2 C-terminal phosphorylation (presence of the hormone), this domain is cleaved and moves to the nucleus, where it stimulates EIN3/EIL activity by EBF repression (stimulating the degradation of this F-box protein by unknown mechanisms), which in turn induces target genes transcription through some members of the AP2/ERF superfamily of transcriptional factors [31]. In addition to the interaction with the C-terminus of EIN2, EIN3/EIL activity can be influenced by the MKK4-5-9 → MPK3-6 phosphorylation cascade, which is CTR/EIN2-independent. In the presence of a signal, the EIN3/EIL transcriptional factors are phosphorylated by MPK3-6, preventing the interaction with EBF and their degradation via the 26S proteasome. Thus, EIN3 and EIL accumulate in the nucleus, interact with gene target promoters and trigger ethylene responses [33]. Another positive regulator is EIN5, a 5’-3’-exoribonuclease that promotes EBF mRNA decay, increasing the levels of EIN3/EIL in the nucleus [34]. Additionally, ethylene biosynthesis is also regulated. Possible receptors for endogenous signals (i.e., other phytohormones) can induce the secondary metabolites accumulation (i.e., calcium) in an intracellular environment and activate protein kinases (i.e., CPK2), culminating in the stabilization of type II ACSs, an important enzyme in ethylene biosynthesis. Then, type II ACSs (in A. thaliana AtACS5 and AtACS9) are phosphorylated by CPK2, which prevents the interaction of these enzymes with ETO/EOL and their subsequent degradation by the 26S proteasome. This event induces an increase in ethylene production and the activation of signal transduction pathways [109]. Moreover, various stress conditions (biotic and abiotic) induce the activation of MAPK modules (in Arabidopsis thaliana MKK4-5-9 and MPK3-6). The MPK3 and MPK6 kinases are able to phosphorylate the C-terminal type I ACSs (in A. thaliana AtACS2 and AtACS6), which stabilize and protect these enzymes against 26S proteasome degradation [21]. There is no consensus regarding the direct participation of CTR in a route involving MPK3-6 [39]. The receptor activity is associated with two proteins: RAN, a copper carrier protein (copper is an important cofactor in receptor activity) [110]; and RTE, a protein with an unknown mechanism of action that facilitates the transition among active and inactive states of one receptor, ETR1 [33, 111]. Each protein is represented by a generic name: EIN2: GmEIN#002, GmEIN#004 and GmEIN#007; EIN3: GmEIN#001, GmEIN#005, GmEIN#006, GmEIN#008 and GmEIN#010; EIN5: GmEIN#003, GmEIN#009 and GmEIN#011; MKK4: GmMKK#001 and GmMKK#003; MKK5: without representatives identified in soybean; MKK9: GmMKK#002 and GmMKK#004; MPK3: GmMPK#003 and GmMPK#004; MPK6: GmMPK#001 and GmMPK#002; Receptors : EIN4: GmETR#002, GmETR#004, GmETR#008 and GmETR#011; ERS1: GmETR#001 and GmETR#007; ERS2: without representatives identified in soybean; ETR1 : GmETR#003 and GmETR#006; ETR2: GmETR#005, GmETR#009 and GmETR#010 (Additional file 2: Table S6)