Ethylene chemical structure.

Ethylene signaling pathway is a signal transduction in plant cells to regulate important growth and developmental processes.[1][2] Acting as a plant hormone, the gas ethylene is responsible for promoting the germination of seeds, ripening of fruits, the opening of flowers, the abscission (or shedding) of leaves and stress responses.[3] It is the simplest alkene gas and the first gaseous molecule discovered to function as a hormone.[4]

Most of the understanding on ethylene signal transduction come from studies on Arabidopsis thaliana.[5] Ethylene can bind to at least five different membrane receptors. Although structurally diverse, the ethylene receptors all exhibit similarity (homology) to two-component regulatory system in bacteria, indicating their common ancestry from bacterial ancestor.[6] Ethylene binds to the receptors on the cell membrane of the endoplasmic reticulum. Although homodimers of the receptors are required for functional state, only one ethylene molecule binds to each dimer.[7]

Unlike in other signal transductions, ethylene is the suppressor of its receptor activity. Ethylene receptors are active without ethylene due to binding with other enzymatically active co-receptors such as constitutive triple response 1 (CTR1) and ethylene insensitive 2 (EIN2). Ethylene binding causes EIN2 to split in two, of which the C-terminal portion of the protein can activate different transcription factors to bring about the effects of ethylene. There is also non-canonical pathway in which ethylene activates cytokinin receptor, and thereby regulate seed development (stomatal aperture) and growth of root (the apical meristem).[1]

Ethylene receptors

Isoforms of ethylene receptor in Arabidopsis thaliana.

Ethylene binds to it specific transmembrane receptor present on the cell membrane of endoplasmic reticulum.[8][9] There are different ethylene receptor isoforms. Five isoforms are known in Arabidopsis thaliana which are named ethylene response/receptor 1 (ETR1), ethylene response sensor 1 (ERS1), ETR2, ERS2, and ethylene insensitive 4 (EIN4).[10] The ETR1 is similar (conserved sequence) in different plants but with slight amino acid differences.[11][12] A. thaliana receptors are classified into two subfamilies based on genetic relationship and common structural features, namely subfamily 1 that includes ETR1 and ERS1, and subfamily 2 that consists of ETR2, ERS2, and EIN4.[13] In tomato there are seven types of ethylene receptors named SlETR1, SlETR2, SlETR3, SlETR4, SlETR5, SlETR6, and SlETR7 (Sl for Solanum lycopersicum, the scientific of tomato).[14]

All ethylene receptors have similar organisation: a short N-terminal domain, three conserved transmembrane domains towards the N-terminus, followed by a GAF domain of unknown function, and then signal output motifs in the C-terminal region.[10] The N-terminus is exposed on the lumen of the endoplasmic reticulum, and the C-terminus that is exposed to the cytoplasm of the cell. The N-terminus contains the sites for binding of ethylene, dimerization and membrane localization.[15][16] Two similar receptors combine to form a homodimer through a disulfide bridge forming a cysteine-cysteine interaction.[17] However, the main membrane localization is done by the transmembrane domain, which can also bind ethylene with the help of copper as a cofactor.[7] Copper ion is supplied by a transmembrane protein responsive-to-antagonist 1 (RAN1) from antioxidant protein 1 (ATX1) via tiplin,[18] or directly by copper transport protein.[19]

Although the receptors are functionally active as dimers, only one copper ion binds to such dimer, indicating that one receptor dimer binds only one ethylene molecule.[7] Mutations in the binding sites stop ethylene binding and also make plants insensitive to ethylene.[20] Cys-65 in the protein helix 2 is particularly important as the binding site of copper ion as mutation in it stops copper and ethylene binding.[1] The C-terminus is basically a bacterial two-component system with kinase activity and response regulator.[15] ETR1 has histidine kinase activity, whereas ETR2, ERS2, and EIN4 have serine/threonine kinase activity, and ERS1 has both.[1] The histidine kinase in ETR1 is not required for ethylene signaling.[21]

Origin and evolution

Ethylene receptors are functionally similar to bacterial two-component system which has two activation sites named response regulator and histidine kinase. The cytoplasmic carboxy-terminal part of ethylene receptor is similar in amino acid sequence to these response regulator and histidine kinase in bacteria; although the N-terminal region is altogether different.[22] Such genetic and protein relationships indicate that receptors and bacterial two-component receptors as well as phytochromes and cytokinin receptors in plants evolved from and were acquired by plants from a cyanobacterium that gave rise to plastids, the power organelles in plants and protists.[23][24]

Phylogenetic analysis also shows the common origin of the ethylene receptor in plants and ethylene-binding domain in cyanobacteria.[6] In 2016, Randy F. Lacey and Brad M. Binder at the University of Tennessee discovered that a cyanobacterium, Synechocystis sp. PCC 6803 response to ethylene signal and has a functional ethylene receptor, which they named Synechocystis Ethylene Response1 (SynEtr1).[25] They further showed that SynEtr1 acts similar to plant ethylene receptor in binding ethylene,[26] indicating the origin of ethylene receptor from Synechocystis-related cyanobacterium.[1] The functional difference however is that kinase activity is not compulsory for ethylene binding in plants, but is the key role of SynEtr1.[25]

Signal transduction

Ethylene signaling pathway in Arabidopsis thaliana.

Two proteins are crucial for interacting ethylene with the receptors, namely constitutive triple response 1 (CTR1) and ethylene insensitive 2 (EIN2). CTR1 is a serine/threonine protein kinase that functions as a negative regulator of ethylene signalling. It is a member of the signaling protein mitogen-activated protein kinase (MAPK) kinase kinase.[10] EIN2 is required for ethylene signalling and is part of the NRAMP (natural resistance-associated macrophage protein) family of metal transporters; it comprises a large, N-terminal portion containing multiple transmembrane domains (EIN2-N) in the ER membrane and a cytosolic C-terminal portion (EIN2-C).[1] Other proteins such as reversion to ethylene sensitivity 1 (RTE1), cytochrome b5 and tetratricopeptide repeat protein 1 (TRP1) also play important roles in ethylene signaling. RTE1 is a highly conserved proteins in plants and protists but absent in fungi and prokaryotes.[27] TRP1 is genetically related to transmembrane and coiled-coil protein 1 (TCC1) in animals that is involved F actin function and competes with Raf-1 for Ras binding.[28]

Unlike in most signal transductions where the ligands activate their receptors to relay their signals, ethylene acts as the suppressor of its receptor, and the receptor being the negative regulator in ethylene responses. Ethylene receptor is active in the absence of ethylene. Without ethylene, the receptor binds to CTR1 at its C-terminal kinase domain. The kinase activity of CTR1 becomes activated and phosphorylates the neighbouring EIN2.[1] As long as EIN2 remains highly phosphorylated, it remains inactive and there never is an ethylene signal relay. In ETR1, the receptor histidine kinase is required for binding with EIN2.[29] RTE1 can bind to and activate ETR1 independent of CTR1.[30] There is evidence that cytochrome b5 aids or acts similar to RTE1.[31]

Ethylene binding to the receptor disrupts the EIN2 phosphorylation. It does not cause any particular change in the structural feature of the receptor-CTR1-EIN2 complex or stop the phosphorylation. In fact, at low level of ethylene there is increased receptor-CTR1-EIN2 complexes, which is then reduced as ethylene level rises.[32] The turnover process is not yet fully understood. The only consequence of ethylene binding is reduced phosphorylation of EIN2. Under such condition EIN2 is activated and is cleaved to release EIN2-C from the membrane-bound EIN2-N portion. The enzyme that causes the cleavage is yet unknown.[1] The role of EIN2-N is also unknown in A. thaliana. But in rice, its homologue OsEIN2-N (Os for Oryza sativa, the scientific name for rice) interacts with another protein, mao huzi 3 (MHZ3), a mutation of which gives rise to insensitivity to ethylene.[33]

EIN2-C is the main component that mediates ethylene signal in the cell. It acts in two ways. In one, it binds the mRNAs that encode for EIN3-binding F-box proteins, EBF1 and EBF2 to cause their degradation.[34] In another, it enters the nucleus to bind with EIN2 nuclear associated protein 1 (ENAP1) to regulate transcriptional and translational activities of EIN3 and the related EIL1 transcription factor to cause most of the ethylene responses.[35]

References

  1. 1 2 3 4 5 6 7 8 Binder, Brad M. (2020). "Ethylene signaling in plants". The Journal of Biological Chemistry. 295 (22): 7710–7725. doi:10.1074/jbc.REV120.010854. PMC 7261785. PMID 32332098.
  2. ↑ Johnson, P. R.; Ecker, J. R. (1998). "The ethylene gas signal transduction pathway: a molecular perspective". Annual Review of Genetics. 32: 227–254. doi:10.1146/annurev.genet.32.1.227. PMID 9928480.
  3. ↑ Bleecker, A. B.; Kende, H. (2000). "Ethylene: a gaseous signal molecule in plants". Annual Review of Cell and Developmental Biology. 16: 1–18. doi:10.1146/annurev.cellbio.16.1.1. PMID 11031228.
  4. ↑ Bakshi, Arkadipta; Shemansky, Jennifer M.; Chang, Caren; Binder, Brad M. (2015). "History of Research on the Plant Hormone Ethylene". Journal of Plant Growth Regulation. 34 (4): 809–827. doi:10.1007/s00344-015-9522-9. S2CID 14775439.
  5. ↑ Gallie, Daniel R. (2015). "Ethylene receptors in plants - why so much complexity?". F1000Prime Reports. 7: 39. doi:10.12703/P7-39. ISSN 2051-7599. PMC 4479046. PMID 26171216.
  6. 1 2 Hérivaux, Anaïs; Dugé de Bernonville, Thomas; Roux, Christophe; Clastre, Marc; Courdavault, Vincent; Gastebois, Amandine; Bouchara, Jean-Philippe; James, Timothy Y.; Latgé, Jean-Paul; Martin, Francis; Papon, Nicolas (2017-01-31). "The Identification of Phytohormone Receptor Homologs in Early Diverging Fungi Suggests a Role for Plant Sensing in Land Colonization by Fungi". mBio. 8 (1). doi:10.1128/mBio.01739-16. ISSN 2150-7511. PMC 5285503. PMID 28143977.
  7. 1 2 3 Rodríguez, F. I.; Esch, J. J.; Hall, A. E.; Binder, B. M.; Schaller, G. E.; Bleecker, A. B. (1999). "A copper cofactor for the ethylene receptor ETR1 from Arabidopsis". Science. 283 (5404): 996–998. Bibcode:1999Sci...283..996R. doi:10.1126/science.283.5404.996. PMID 9974395.
  8. ↑ Chen, Yi-Feng; Randlett, Melynda D.; Findell, Jennifer L.; Schaller, G. Eric (2002). "Localization of the ethylene receptor ETR1 to the endoplasmic reticulum of Arabidopsis". The Journal of Biological Chemistry. 277 (22): 19861–19866. doi:10.1074/jbc.M201286200. PMID 11916973. S2CID 8256915.
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  12. ↑ Hall, M. A.; Connern, C. P.; Harpham, N. V.; Ishizawa, K.; Roveda-Hoyos, G.; Raskin, I.; Sanders, I. O.; Smith, A. R.; Turner, R.; Wood, C. K. (1990). "Ethylene: receptors and action". Symposia of the Society for Experimental Biology. 44: 87–110. PMID 2130520.
  13. ↑ Chen, Yi-Feng; Gao, Zhiyong; Kerris, Robert J.; Wang, Wuyi; Binder, Brad M.; Schaller, G. Eric (2010). "Ethylene receptors function as components of high-molecular-mass protein complexes in Arabidopsis". PLOS ONE. 5 (1): e8640. Bibcode:2010PLoSO...5.8640C. doi:10.1371/journal.pone.0008640. PMC 2799528. PMID 20062808.
  14. ↑ Chen, Yi; Hu, Guojian; Rodriguez, Celeste; Liu, Meiying; Binder, Brad M.; Chervin, Christian (2020). "Roles of SlETR7, a newly discovered ethylene receptor, in tomato plant and fruit development". Horticulture Research. 7: 17. doi:10.1038/s41438-020-0239-y. PMC 6994538. PMID 32025320.
  15. 1 2 Chen, Yi-Feng; Randlett, Melynda D.; Findell, Jennifer L.; Schaller, G. Eric (2002). "Localization of the ethylene receptor ETR1 to the endoplasmic reticulum of Arabidopsis". The Journal of Biological Chemistry. 277 (22): 19861–19866. doi:10.1074/jbc.M201286200. PMID 11916973.
  16. ↑ Grefen, Christopher; Städele, Katrin; Růzicka, Kamil; Obrdlik, Petr; Harter, Klaus; Horák, Jakub (2008). "Subcellular localization and in vivo interactions of the Arabidopsis thaliana ethylene receptor family members". Molecular Plant. 1 (2): 308–320. doi:10.1093/mp/ssm015. PMID 19825542.
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  19. ↑ Hoppen, Claudia; Müller, Lena; Hänsch, Sebastian; Uzun, Buket; Milić, Dalibor; Meyer, Andreas J.; Weidtkamp-Peters, Stefanie; Groth, Georg (2019). "Soluble and membrane-bound protein carrier mediate direct copper transport to the ethylene receptor family". Scientific Reports. 9 (1): 10715. Bibcode:2019NatSR...910715H. doi:10.1038/s41598-019-47185-6. PMC 6656775. PMID 31341214.
  20. ↑ Cancel, Jesse D.; Larsen, Paul B. (2002). "Loss-of-function mutations in the ethylene receptor ETR1 cause enhanced sensitivity and exaggerated response to ethylene in Arabidopsis". Plant Physiology. 129 (4): 1557–1567. doi:10.1104/pp.003780. PMC 166743. PMID 12177468.
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  26. ↑ Allen, Cidney J.; Lacey, Randy F.; Binder Bickford, Alixandri B.; Beshears, C. Payton; Gilmartin, Christopher J.; Binder, Brad M. (2019). "Cyanobacteria Respond to Low Levels of Ethylene". Frontiers in Plant Science. 10: 950. doi:10.3389/fpls.2019.00950. PMC 6682694. PMID 31417582.
  27. ↑ Resnick, Josephine S.; Wen, Chi-Kuang; Shockey, Jason A.; Chang, Caren (2006). "REVERSION-TO-ETHYLENE SENSITIVITY1, a conserved gene that regulates ethylene receptor function in Arabidopsis". Proceedings of the National Academy of Sciences of the United States of America. 103 (20): 7917–7922. doi:10.1073/pnas.0602239103. PMC 1458508. PMID 16682642.
  28. ↑ Lin, Zhefeng; Ho, Chin-Wen; Grierson, Don (2009). "AtTRP1 encodes a novel TPR protein that interacts with the ethylene receptor ERS1 and modulates development in Arabidopsis". Journal of Experimental Botany. 60 (13): 3697–3714. doi:10.1093/jxb/erp209. PMC 2736885. PMID 19567478.
  29. ↑ Bisson, Melanie M. A.; Groth, Georg (2011). "New paradigm in ethylene signaling: EIN2, the central regulator of the signaling pathway, interacts directly with the upstream receptors". Plant Signaling & Behavior. 6 (1): 164–166. doi:10.4161/psb.6.1.14034. PMC 3122035. PMID 21242723.
  30. ↑ Qiu, Liping; Xie, Fang; Yu, Jing; Wen, Chi-Kuang (2012). "Arabidopsis RTE1 is essential to ethylene receptor ETR1 amino-terminal signaling independent of CTR1". Plant Physiology. 159 (3): 1263–1276. doi:10.1104/pp.112.193979. PMC 3387708. PMID 22566492.
  31. ↑ Chang, Jianhong; Clay, John M.; Chang, Caren (2014). "Association of cytochrome b5 with ETR1 ethylene receptor signaling through RTE1 in Arabidopsis". The Plant Journal: For Cell and Molecular Biology. 77 (4): 558–567. doi:10.1111/tpj.12401. PMC 4040253. PMID 24635651.
  32. ↑ Shakeel, Samina N.; Gao, Zhiyong; Amir, Madiha; Chen, Yi-Feng; Rai, Muneeza Iqbal; Haq, Noor Ul; Schaller, G. Eric (2015). "Ethylene Regulates Levels of Ethylene Receptor/CTR1 Signaling Complexes in Arabidopsis thaliana". The Journal of Biological Chemistry. 290 (19): 12415–12424. doi:10.1074/jbc.M115.652503. PMC 4424370. PMID 25814663.
  33. ↑ Ma, Biao; Zhou, Yang; Chen, Hui; He, Si-Jie; Huang, Yi-Hua; Zhao, He; Lu, Xiang; Zhang, Wan-Ke; Pang, Jin-Huan; Chen, Shou-Yi; Zhang, Jin-Song (2018). "Membrane protein MHZ3 stabilizes OsEIN2 in rice by interacting with its Nramp-like domain". Proceedings of the National Academy of Sciences of the United States of America. 115 (10): 2520–2525. Bibcode:2018PNAS..115.2520M. doi:10.1073/pnas.1718377115. PMC 5877927. PMID 29463697.
  34. ↑ Li, Wenyang; Ma, Mengdi; Feng, Ying; Li, Hongjiang; Wang, Yichuan; Ma, Yutong; Li, Mingzhe; An, Fengying; Guo, Hongwei (2015). "EIN2-directed translational regulation of ethylene signaling in Arabidopsis". Cell. 163 (3): 670–683. doi:10.1016/j.cell.2015.09.037. PMID 26496607.
  35. ↑ Zhang, Weiqiang; Hu, Yingxiong; Liu, Jian; Wang, Hui; Wei, Jihui; Sun, Pingdong; Wu, Lifeng; Zheng, Hongjian (2020). "Progress of ethylene action mechanism and its application on plant type formation in crops". Saudi Journal of Biological Sciences. 27 (6): 1667–1673. doi:10.1016/j.sjbs.2019.12.038. PMC 7253889. PMID 32489309.
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