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Figure 1. 

Model of ethylene signal transduction in Arabidopsis and rice. In Arabidopsis, ethylene receptors, AtCTR1, and AtEIN2 form a complex on the endoplasmic reticulum membrane. Ethylene binding inactivates the receptor/CTR1 complex, relieving repression on AtEIN2. The cleaved EIN2-CEND then follows two pathways: one part enters the P-body, where it inhibits AtEBF1/2 mRNA translation via AtEIN5, while the other translocates to the nucleus to regulate downstream gene expression with AtEIN3/EIL1, ENAP, PDC, and SRT1/2. AtRTE1 and AtETP1/2 act as negative regulators of ethylene signaling, targeting AtETR1 and AtEIN2, respectively. AtCTR1 also can enter the nucleus to repress AtEBF1/2, exerting a positive regulatory role. Also, a non-canonical pathway exists in which AtETR1 signals to histidine-containing AHP, which subsequently activates ARR to modulate ethylene-mediated growth traits. The ethylene receptor-OsCTR1/2-OsEIN2-OsEIL1/2 pathway in rice is highly conserved compared to Arabidopsis. There also exists a distinct MHZ1 pathway independent of OsEIN2. Furthermore, several new regulatory components have been identified: MHZ11 regulates signaling by inhibiting the activation of the ethylene receptor/OsCTR2 complex, while MHZ3 not only stabilizes OsEIN2 but also enhances the activity of the ethylene receptor/OsCTR2 complex. MHZ9 is involved in the translation repression of OsEBF1/2 mRNA in the P-body. The ABA and auxin signaling pathways also interact with the ethylene signaling pathway to co-regulate double responses in rice. The light blue arrows represent the canonical ethylene signaling pathway, while the black arrows indicate non-canonical ethylene signaling pathways or regulation of the components of the canonical ethylene signaling pathway. The 'T' blunt ends indicate negative regulation.

Figure 2. 

Ethylene perception complex (EPC) in rice. In the absence of ethylene (left), an EPC is formed with ethylene receptors at its core, preventing signaling activation. MHZ3 interacts with OsETR2 and OsERS2 to stabilize OsCTR2 on the ER membrane, keeping it autophosphorylated and inhibiting the canonical pathway. The GAF domains of ethylene receptors also bind histidine kinase MHZ1, suppressing its activity and blocking the phospho-relay pathway. Upon ethylene perception by the receptors (right), the EPC dissociates, triggering the activation of ethylene signaling. Ethylene binding weakens the interaction between the receptors and MHZ3, leading to the dissociation of OsCTR2. An unknown specific phosphatase may dephosphorylate OsCTR2, inactivating its inhibition of the canonical pathway. Meanwhile, MHZ1 dissociates from the EPC, activating the MHZ1-mediated phospho-relay pathway, transferring phosphate groups to OsAHP1/2, and activating the non-canonical ethylene signaling pathway. Under the influence of ethylene, MHZ11 converts membrane sterols into sterol esters, potentially increasing membrane fluidity and promoting the dissociation of the EPC. The arrows indicate biological processes. The 'T' blunt end indicates negative regulation.

Figure 3. 

Signal transduction complex (STC) in rice ethylene signaling. In the absence of ethylene (left), the STC centered on OsEIN2 is unstable. OsEIN2 undergoes phosphorylation by the serine/threonine kinase OsCTR2 and ubiquitination by potential F-box protein OsETPs. These modifications lead to the inactivation and degradation of OsEIN2 via the 26S proteasome, thereby preventing OsEIN2-mediated ethylene signaling. Upon ethylene binding (right), OsCTR2 loses activity and is unable to phosphorylate OsEIN2. Meanwhile, the membrane protein MHZ3 binds to OsEIN2, inhibiting its ubiquitination and subsequent degradation. The stabilized OsEIN2 is cleaved to produce an active C-terminal fragment, which is partially directed to the P-body and partially translocated to the nucleus, thereby completing ethylene signal transduction. Red arrows indicate biological processes, while the black arrows indicate subcellular translocation.

Figure 4. 

Translation regulatory complex (TRC) in rice ethylene signaling. In the processing body (P-body), the C-terminal of OsEIN2 (EIN2-CEND), along with MHZ9, OsEBF1/2 mRNA, and other components, forms the TRC, which is involved in the translational inhibition of the ethylene signaling negative regulators, OsEBF1/2 proteins. In the absence of ethylene (left), MHZ9 binds OsEBF1/2 mRNA with low affinity, allowing normal translation and accumulation of the negative regulators OsEBF1 and OsEBF2, which deactivates the ethylene response. In the presence of ethylene (right), the C-terminal region of MHZ9 interacts with OsEIN2-CEND, facilitating the binding of its N-terminal region to the 3' UTR of OsEBF1/2 mRNA, leading to translational repression in the P-body. This prevents OsEBF1/2 accumulation and activates a nuclear ethylene signaling cascade. Dark red arrows indicate biological processes and black arrows indicate subcellular translocation. The 'T' blunt end indicates negative regulation.

Figure 5. 

Nuclear regulatory complex (NRC) in rice ethylene signaling. The core transcription factor OsEIL1 can recruit different interacting components to form a NRC, participating in the regulation of ethylene-responsive genes in rice. In the absence of ethylene (left), the F-box proteins OsEBF1 and OsEBF2 promote the ubiquitination and subsequent degradation of the OsEIL1 transcription factor, thereby inhibiting the activation of ethylene-responsive genes. Low levels of OsEIL1 may interact with auxin repressors OsIAA21 and OsIAA31, repressing downstream genes by recruiting co-repressor TOPLESS (TPL) and histone deacetylases (HDACs), leading to chromatin condensation. Additionally, OsIAA21 and OsIAA31 may attenuate the OsEIL1-OsIAA1/9 complex, maintaining basal biosynthetic processes for normal metabolic functions. In the presence of ethylene (right), OsEIN2-CEND transduces the ethylene signal to the nucleus, where OsIAA1/9 recruits the histone acetyltransferase OsGCN5 to mediate histone acetylation and chromatin decondensation. OsIAA1/9 interacts with OsEIL1, thereby activating the expression of downstream genes. OsEIL1 directly binds to the promoters of MHZ10/OsTAR2, MHZ1/OsHK1, OsCLSC2, OsHKT2;1, and others, playing a role in regulating various biological processes. The response regulator OsRR21, receiving phosphoryl groups from the MHZ1-OsAHP1/2 phospho-relay, activates and amplifies the ethylene signal. Red arrows indicate activation and/or biological process and black arrows indicate subcellular translocation. The 'T' blunt end indicates negative regulation.