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Overview of Somatic Embryogenesis

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Somatic Embryogenesis

Part of the book series: Methods in Molecular Biology ((MIMB,volume 2527))

Abstract

Somatic embryogenesis is a natural phenomenon through which somatic embryos are produced from somatic cells although. It is considered the most efficient morphogenic pathways for plant multiplication. One of the key features of somatic embryogenesis is the use of cellular totipotency, where dedifferentiation is induced to foster cell proliferation, followed by the induction of differentiation using plant growth regulators to produce new plants. There is a cell group with the potential to undergo the somatic embryogenesis pathway through adequate stimulation (plant growth regulators, incubation conditions, and supplementation of the culture medium). There are two somatic embryogenesis pathways in plants: direct and indirect embryogenesis. Direct somatic embryogenesis consists of the formation of embryos directly from isolated cells, without the formation of “callous” tissue. Indirect somatic embryogenesis is characterized by the formation of a callus as a stage that precedes the formation of somatic embryos. It should be stressed that not all plant cells have this morphogenic capacity; consequently, determining the type of factors that drive this type of response has been challenging. This book provides the reader with updated available information on the techniques, relevant protocols, and tools to perform somatic embryogenesis in different plant species for economic purposes.

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References

  1. Rose RJ, Song Y (2017) Somatic embryogenesis, 2nd edn. Elsevier, Amsterdam

    Google Scholar 

  2. von Arnold S, Sabala I, Bozhkov P, Dyachok J, Filonova L (2002) Developmental pathways of somatic embryogenesis. Plant Cell Tissue Organ Cult 69:233–249. https://doi.org/10.1023/A:1015673200621

    Article  Google Scholar 

  3. Zavattieri MA, Frederico AM, Lima M, Sabino R, Arnholdt-Schmitt B (2010) Induction of somatic embryogenesis as an example of stress-related plant reactions. Electron J Biotechnol 13:1–9. https://doi.org/10.2225/vol13-issue1-fulltext-4

    Article  CAS  Google Scholar 

  4. Gaj MD (2001) Direct somatic embryogenesis as a rapid and efficient system for in vitro regeneration of Arabidopsis thaliana. Plant Cell Tissue Organ Cult 64:39–46. https://doi.org/10.1023/A:1010679614721

    Article  Google Scholar 

  5. Horstman A, Bemer M, Boutilier K (2017) A transcriptional view on somatic embryogenesis. Regeneration 4:201–216. https://doi.org/10.1002/reg2.91

    Article  PubMed  PubMed Central  Google Scholar 

  6. Fehér A (2019) Callus, dedifferentiation, totipotency, somatic embryogenesis: what these terms mean in the era of molecular plant biology? Front Plant Sci 10:536. https://doi.org/10.3389/fpls.2019.00536

    Article  PubMed  PubMed Central  Google Scholar 

  7. Rose RJ (2019) Somatic embryogenesis in the Medicago truncatula model: cellular and molecular mechanisms. Front Plant Sci 10:267. https://doi.org/10.3389/fpls.2019.00267

    Article  PubMed  PubMed Central  Google Scholar 

  8. Wu GY, Wei XL, Wang X, Wei Y (2020) Induction of somatic embryogenesis in different explants from Ormosia henryi Prain. Plant Cell Tissue Organ Cult 142:229–240. https://doi.org/10.1007/s11240-020-01822-5

    Article  CAS  Google Scholar 

  9. Asthana P, Rai MK, Jaiswal U (2017) Somatic embryogenesis from sepal explants in Sapindus trifoliatus, a plant valuable in herbal soap industry. Ind Crop Prod 100:228–235. https://doi.org/10.1016/j.indcrop.2017.02.034

    Article  CAS  Google Scholar 

  10. Ikeuchi M, Sugimoto K, Iwase A (2013) Plant callus: mechanisms of induction and repression. Plant Cell 25:3159–3173. https://doi.org/10.1105/tpc.113.116053

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  11. Lelu-Walter MA, Gautier F, Eliášová K, Sanchez L, Teyssier C, Lomenech AM, Le Metté C, Hargreaves C, Trontin JF, Reeves C (2018) High gellan gum concentration and secondary somatic embryogenesis: two key factors to improve somatic embryo development in Pseudotsuga menziesii [Mirb.]. Plant Cell Tissue Organ Cult 132:137–155. https://doi.org/10.1007/s11240-017-1318-0

    Article  CAS  Google Scholar 

  12. Vinoth A, Ravindhran R (2018) In vitro morphogenesis of woody plants using Thidiazuron. In: Ahmad N, Faisal M (eds) Thidiazuron: from urea derivative to plant growth regulator. Springer Nature Singapore Pte Ltd., Nungambakkam, Chennai, India, pp 211–229

    Chapter  Google Scholar 

  13. Venkataiah P, Bhanuprakash P, Suman S (2016) Somatic embryogenesis and plant regeneration of Capsicum baccatum L. J Genet Eng Biotechnol 14(1):55–60. https://doi.org/10.1016/j.jgeb.2016.02.001

    Article  PubMed  PubMed Central  Google Scholar 

  14. Ahmad N, Faisal M, Anis M, Aref IM (2010) In vitro callus induction and plant regeneration from leaf explants of Ruta graveolens L. South African J Bot 76:597–600. https://doi.org/10.1016/j.sajb.2010.03.008

    Article  CAS  Google Scholar 

  15. Kumar S, Kumar A, Sahin G, Uslu E (2016) Influence of nutrient media on callus induction, somatic embryogenesis and plant regeneration in selected Turkish crocus species. Biotechnol Rep 10:66–74. https://doi.org/10.1016/j.btre.2016.03.006

    Article  Google Scholar 

  16. Lema-Ruminska J, Kulus D (2012) INDUCTION OF SOMATIC EMBRYOGENESIS IN Astrophytum asterias (Zucc.) Lem. IN THE ASPECT OF LIGHT CONDITIONS AND AUXIN 2, 4-D CONCENTRATIONS. Acta Sci Pol Hortorum Cultus 11:77–87

    Google Scholar 

  17. Nasab AM, Azar AM, Movafeghi A, Dadpour M (2011) Callus Induction and Embryogenesis of Alfalfa ( Medicago sativa L .) Using Hypocotyl Thin Cell Layer Culture 1. Russ Agric Sci 37:303–306. https://doi.org/10.3103/S1068367411040148

    Article  Google Scholar 

  18. Zimmerman JL (1993) Somatic embryogenesis: a model for early development in higher plants. Plant Cell 5:1411–1423. https://doi.org/10.1105/tpc.5.10.1411

    Article  PubMed  PubMed Central  Google Scholar 

  19. Grout B (2017) General principles of tissue culture, 2nd Edició. Elsevier, Copenhagen, Taastrup, Dinamarca

    Google Scholar 

  20. Neumann K-H, Kumar A, Imani J (2020) Plant propagation: meristem cultures, Somatic Embryogenesis Micropropagation, and Transformation of Somatic Embryos in Bioreactors. In: Plant Cell and Tissue Culture – A Tool in Biotechnology. Springer, New York

    Chapter  Google Scholar 

  21. Wójcik AM, Wójcikowska B, Gaj MD (2020) Current perspectives on the auxin-mediated genetic network that controls the induction of somatic embryogenesis in plants. Int J Mol Sci 21:1333. https://doi.org/10.3390/ijms21041333

    Article  CAS  PubMed Central  Google Scholar 

  22. Hazubska-Przybył T, Ratajczak E, Obarska A, Pers-Kamczyc E (2020) Different roles of auxins in somatic embryogenesis efficiency in two picea species. Int J Mol Sci 21:3394. https://doi.org/10.3390/ijms21093394

    Article  CAS  PubMed Central  Google Scholar 

  23. Sabooni N, Shekafandeh A (2017) Somatic embryogenesis and plant regeneration of blackberry using the thin cell layer technique. Plant Cell Tissue Organ Cult 2:313–321. https://doi.org/10.1007/s11240-017-1225-4

    Article  CAS  Google Scholar 

  24. Bonga JM, Klimaszewska KK, von Aderkas P (2010) Recalcitrance in clonal propagation, in particular of conifers. Plant Cell Tissue Organ Cult 100:241–254. https://doi.org/10.1007/s11240-009-9647-2

    Article  Google Scholar 

  25. Sivanesan I, Park SW (2014) The role of silicon in plant tissue culture. Front Plant Sci 5:2012–2015. https://doi.org/10.3389/fpls.2014.00571

    Article  Google Scholar 

  26. Ali A, Zhang J, Zhou M, Chen T, Shah L, Rehman SU, Hayat S, Shi J, Chen J (2021) Chitosan oligosaccharides stimulate the efficacy of somatic embryogenesis in different genotypes of the liriodendron hybrid. Forests 12:557. https://doi.org/10.3390/f12050557

    Article  Google Scholar 

  27. Ahmadi B, Shariatpanahi ME (2015) Proline and chitosan enhanced efficiency of microspore embryogenesis induction and plantlet regeneration in Brassica napus L. Plant Cell Tissue Organ Cult 123:57–65. https://doi.org/10.1007/s11240-015-0814-3

    Article  CAS  Google Scholar 

  28. Tran Than Van M (1973) In vitro control of de novo flower, bud, root, and callus differentiation from excised epidermal tissues. Nature 246:44–45. https://doi.org/10.1038/246044a0

    Article  Google Scholar 

  29. Nhut DT, Van Le B, Minh NT, de Teixeira SJ, Fukai S, Tanaka M, Van KTT (2002) Somatic embryogenesis through pseudo-bulblet transverse thin cell layer of Lilium longiflorum. Plant Growth Regul 37:193–198

    Article  CAS  Google Scholar 

  30. van Le B, de Carvalho MHC, Zuily-Fodil Y, ATP T, KTT V (2002) Direct whole plant regeneration of cowpea [Vigna unguiculata (L.) Walp] from cotyledonary node thin cell layer explants. J Plant Physiol 159:1255–1258

    Article  Google Scholar 

  31. Ramírez-Mosqueda MA, Iglesias-Andreu LG, Armas-Silva AA, Cruz-Gutiérrez EJ, de la Torre-Sánchez JF, Leyva-Ovalle OR, Galán-Páez CM (2018) Effect of the thin cell layer technique in the induction of somatic embryos in Pinus patula Schl. et Cham. J Forestry Res 30:1535–1539. https://doi.org/10.1007/s11676-018-0663-0

    Article  CAS  Google Scholar 

  32. Soonthornkalump S, Nakkanong K, Meesawat U (2019) In vitro cloning via direct somatic embryogenesis and genetic stability assessment of Paphiopedilum niveum (Rchb.F.) stein: the endangered Venus’s slipper orchid. In Vitro Cell Dev Biol—Plant 55:265–276. https://doi.org/10.1007/s11627-019-09981-7

    Article  CAS  Google Scholar 

  33. Mahendran G, Narmatha Bai V (2016) Direct somatic embryogenesis of Malaxis densiflora (a. rich.) Kuntze. J Genet Eng Biotechnol 14:77–81. https://doi.org/10.1016/j.jgeb.2015.11.003

    Article  CAS  PubMed  Google Scholar 

  34. Tang W, Guo Z (2001) In vitro propagation of loblolly pine via direct somatic organogenesis from mature cotyledons and hypocotyls. Plant Growth Regul 33:25–31. https://doi.org/10.1023/A:1010764816523

    Article  Google Scholar 

  35. Monja-Mio KM, Robert ML (2013) Direct somatic embryogenesis of Agave fourcroydes Lem. Through thin cell layer culture. In Vitro Cell Dev Biol—Plant 49:541–549. https://doi.org/10.1007/s11627-013-9535-7

    Article  Google Scholar 

  36. Kaur A, Reddy MS, Kumar A (2018) Direct somatic embryogenesis of potato [Solanum tuberosum (L.)] cultivar ‘Kufri Chipsona 2. Plant Cell Tissue Organ Cult 134:457–466. https://doi.org/10.1007/s11240-018-1435-4

    Article  CAS  Google Scholar 

  37. Du Y, Cheng F, Zhong Y (2020) Induction of direct somatic embryogenesis and shoot organogenesis and histological study in tree peony (Paeonia sect. Moutan). Plant Cell Tissue Organ Cult 141:557–570. https://doi.org/10.1007/s11240-020-01815-4

    Article  CAS  Google Scholar 

  38. Midhu CK, Hima S, Binoy J, Satheeshkumar K (2019) Influence of incubation period on callus tissues for plant regeneration in Ophiorrhiza pectinata Arn. Through Somatic Embryogenesis. Proc Natl Acad Sci India Sect B - Biol Sci 89:1439–1446. https://doi.org/10.1007/s40011-018-01061-x

    Article  CAS  Google Scholar 

  39. Murvanidze N, Nisler J, Leroux O, Werbrouck SPO (2021) Cytokinin oxidase/dehydrogenase inhibitors stimulate 2iP to induce direct somatic embryogenesis in Coffea arabica. Plant Growth Regul 94:195–200. https://doi.org/10.1007/s10725-021-00708-6

    Article  CAS  Google Scholar 

  40. Bertero VG, Beznec A, Faccio P, Auteri M, Arteaga M, Bonafede M, Bossio E (2020) High-efficiency direct somatic embryogenesis and plant regeneration from leaf base explants of “peperina” (Minthostachys verticillata). In Vitro Cell Dev Biol—Plant 56:915–919. https://doi.org/10.1007/s11627-020-10098-5

    Article  CAS  Google Scholar 

  41. Hu R, Sun Y, Wu B, Duan H, Zheng H, Hu D, Lin H, Tong Z, Xu J, Li Y (2017) Somatic embryogenesis of immature Cunninghamia lanceolata (lamb.) hook zygotic embryos. Sci Rep 7:1–14. https://doi.org/10.1038/s41598-017-00156-1

    Article  CAS  Google Scholar 

  42. Kashyap A, Penak SM, Saha A, Singh BR (2018) In vitro plant development of Eleusine coracana via indirect organogenesis and somatic embryogenesis using mature seeds as explants. Curr Sci 115:91–98. https://doi.org/10.18520/cs/v115/i1/91-98

    Article  CAS  Google Scholar 

  43. Seijo MF (2003) Aspectos básicos de la embriogénesis somática. Biotecnol Veg 3:195–209

    Google Scholar 

  44. Śliwińska AA, Białek A, Orłowska R, Mańkowski D, Sykłowska-Baranek K, Pietrosiuk A (2021) Comparative study of the genetic and biochemical variability of Polyscias filicifolia (Araliaceae) regenerants obtained by indirect and direct somatic embryogenesis as a source of triterpenes. Int J Mol Sci 22:5752. https://doi.org/10.3390/ijms22115752

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  45. Raji MR, Lotfi M, Tohidfar M, Zahedi B, Carra A, Abbate L, Carimi F (2018) Somatic embryogenesis of muskmelon (Cucumis melo L.) and genetic stability assessment of regenerants using flow cytometry and ISSR markers. Protoplasma 255:873–883. https://doi.org/10.1007/s00709-017-1194-9

    Article  CAS  PubMed  Google Scholar 

  46. Lema-Rumińska J, Kulus D, Tymoszuk A, Varejão JMTB, Bahcevandziev K (2019) Profile of secondary metabolites and genetic stability analysis in new lines of Echinacea purpurea (L.) Moench microprfopagated via somatic embryogenesis. Ind Crops Prod 142:111851. https://doi.org/10.1016/j.indcrop.2019.111851

    Article  CAS  Google Scholar 

  47. Egertsdotter U, Ahmad I, Clapham D (2019) Automation and scale up of somatic embryogenesis for commercial plant production, with emphasis on conifers. Front Plant Sci 10:109. https://doi.org/10.3389/fpls.2019.00109

    Article  PubMed  PubMed Central  Google Scholar 

  48. Henao Ramírez AM, de la Hoz VT, Ospina Osorio TM, Garcés LA, Urrea Trujillo AI (2018) Evaluation of the potential of regeneration of different Colombian and commercial genotypes of cocoa (Theobroma cacao L.) via somatic embryogenesis. Sci Hortic (Amsterdam) 229:148–156. https://doi.org/10.1016/j.scienta.2017.10.040

    Article  CAS  Google Scholar 

  49. Gatica-Arias AM, Arrieta-Espinoza G, Espinoza Esquivel AM (2008) Plant regeneration via indirect somatic embryogenesis and optimisation of genetic transformation in Coffea arabica L cvs Caturra and Catuaí. Electron J Biotechnol:11. https://doi.org/10.2225/vol11-issue1-fulltext-9

  50. Zayed ZE (2017) Enhanced indirect somatic embryogenesis from shoot-tip explants of date palm by gradual reductions of 2,4-D concentration. In: Al-Khayri JM (ed) Date palm biotechnology protocols volume 1: tissue culture applications, methods in molecular biology. Springer Science + Bussiness media, New York, pp 77–88

    Chapter  Google Scholar 

  51. Krishnan SRS, Siril EA (2017) Auxin and nutritional stress coupled somatic embryogenesis in Oldenlandia umbellata L. Physiol Mol Biol Plants 23:471–475. https://doi.org/10.1007/s12298-017-0425-z

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  52. Yang J, Yang D, Lü W, Zhang X, Ma M, Liu G, Jiang J, Li C (2021) Somatic embryogenesis and plant regeneration in Betula platyphalla. J For Res 32:937–944. https://doi.org/10.1007/s11676-020-01131-9

    Article  CAS  Google Scholar 

  53. Fki L, Kriaa W, Nasri A, Baklouti E, Chkir O, Msmoudi RB, Rival A, Drira N (2017) Indirect somatic embryogenesis of date palm using juvenile leaf explants and low 2,4-D concentration. In: Al-Khayri JM (ed) Date palm biotechnology protocols volume 1: tissue culture applications, methods in molecular biology. Springer Science + Bussiness Media, New York, pp 44–45

    Google Scholar 

  54. Keshvari T, Najaphy A, Kahrizi D, Zebarjadi A (2018) Callus induction and somatic embryogenesis in Stevia rebaudiana Bertoni as a medicinal plant. Cell Mol Biol 64:46–49. https://doi.org/10.14715/cmb/2018.64.2.9

    Article  PubMed  Google Scholar 

  55. Solórzano-Cascante P, Sánchez-Chiang N, Jiménez VM (2018) Explant type, culture system, 6-benzyladenine, meta-topolin and encapsulation affect indirect somatic embryogenesis and regeneration in carica papaya l. Front Plant Sci 871:1–12. https://doi.org/10.3389/fpls.2018.01769

    Article  Google Scholar 

  56. Vargas JLO, Fernández JS, Vega MEA (2018) Regeneration of Plants from Embryogenic Cell Suspensions of cv. “Datil” (Musa AA): Morphological Evaluation of Plants in the Field. J Agric Sci Technol B 8:29–41. https://doi.org/10.17265/2161-6264/2018.01.003

    Article  CAS  Google Scholar 

  57. Sathish S, Venkatesh R, Safia N, Sathishkumar R (2018) Studies on growth dynamics of embryogenic cell suspension cultures of commercially important indica rice cultivars ASD16 and Pusa basmati. 3 Biotech 8:1–9. https://doi.org/10.1007/s13205-018-1213-3

    Article  Google Scholar 

  58. Kong EYY, Biddle J, Foale M, Adkins SW (2020) Cell suspension culture: a potential in vitro culture method for clonal propagation of coconut plantlets via somatic embryogenesis. Ind Crop Prod 147:112125. https://doi.org/10.1016/j.indcrop.2020.112125

    Article  CAS  Google Scholar 

  59. El-Sharabay S, El-Dawayati M (2019) Bioreactor steroid production and analysis of date palm embryogenic callus. In: Al-Khayri JM (ed) Date palm biotechnology protocols volume 1: tissue culture applications, methods in molecular biology. Springer Science + Bussiness Media, New York, pp 309–318

    Google Scholar 

  60. Woo HA, Ku SS, Jie EY, Kim HR, Kim HS, Cho HS, Jeong WJ, Park SU, Min SR, Kim SW (2021) Efficient plant regeneration from embryogenic cell suspension cultures of Euonymus alatus. Sci Rep 11:1–9. https://doi.org/10.1038/s41598-021-94597-4

    Article  CAS  Google Scholar 

  61. Tripathi MK, Tripathi N, Tiwari S, Tiwari G, Mishra N, Bele D, Patel RP, Sapre S, Tiwari S (2021) Optimization of different factors for initiation of somatic embryogenesis in suspension cultures in sandalwood (Santalum album l.). Horticulturae 7:1–15. https://doi.org/10.3390/horticulturae7050118

    Article  CAS  Google Scholar 

  62. Mondal TK, Bhattacharya A, Ahuja PS (2001) Induction of synchronous secondary somatic embryogenesis in Camellia sinensis (L.) O. Kuntze. J Plant Physiol 158:945–951. https://doi.org/10.1078/0176-1617-00179

    Article  CAS  Google Scholar 

  63. Bogdanović MD, Ćuković KB, Subotić AR, Dragićević MB, Simonović AD, Filipović BK, Todorović SI (2021) Secondary somatic embryogenesis in centaurium erythraea rafn. Plan Theory 10:1–20. https://doi.org/10.3390/plants10020199

    Article  CAS  Google Scholar 

  64. Ramirez MDA, Da Silva RF (2018) Morpho-anatomical characterization of secondary somatic embryogenesis in Azadirachta indica (Meliaceae). Acta Bot Mex 122:7–20. https://doi.org/10.21829/abm122.2018.1242

    Article  Google Scholar 

  65. Pérez-Núñez MT, Chan JL, Sáenz L, González T, Verdeil JL, Oropeza C (2006) Improved somatic embryogenesis from Cocos nucifera (L.) plumule explants. In Vitro Cell Dev Biol—Plant 42:37–43. https://doi.org/10.1079/IVP2005722

    Article  Google Scholar 

  66. Williams EG, Maheswaran G (1986) Somatic embryogenesis: factors influencing coordinated behaviour of cells as an embryogenic group. Ann Bot 57:443–462. https://doi.org/10.1093/oxfordjournals.aob.a087127

    Article  Google Scholar 

  67. Raemakers CJJM, Jacobsen E, Visser RGF (1995) Secondary somatic embryogenesis and applications in plant breeding. Euphytica 81:93–107. https://doi.org/10.1007/BF00022463

    Article  Google Scholar 

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  1. Facultad de Ciencias Biológicas y Agropecuarias, Universidad Veracruzana, Amatlán de los Reyes, Veracruz, Mexico

    Marco A. Ramírez-Mosqueda

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Correspondence to Marco A. Ramírez-Mosqueda .

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  1. Laboratorio de Micropropagación Vegetal, Facultad de Ciencias Biológicas y Agropecuarias, Universidad Veracruzana, Veracruz, Mexico

    Marco A. Ramírez-Mosqueda

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Ramírez-Mosqueda, M.A. (2022). Overview of Somatic Embryogenesis. In: Ramírez-Mosqueda, M.A. (eds) Somatic Embryogenesis. Methods in Molecular Biology, vol 2527. Humana, New York, NY. https://doi.org/10.1007/978-1-0716-2485-2_1

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