Plant embryogenesis is a process that occurs after the fertilization of an ovule to produce a fully developed plant embryo. This is a pertinent stage in the plant life cycle that is followed by dormancy and germination.[1] The zygote produced after fertilization, must undergo various cellular divisions and differentiations to become a mature embryo.[1] An end stage embryo has five major components including the shoot apical meristem, hypocotyl, root meristem, root cap, and cotyledons.[1] Unlike animal embryogenesis, plant embryogenesis results in an immature form of the plant, lacking most structures like leaves, stems, and reproductive structures[2]
Morphogenic events
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Embryogenesis occurs naturally as a result of single, or double fertilzation, of the ovule, giving rise to two distinct structures: the plant embryo and the endosperm which go on to develop into a seed.[3] The zygote goes through various cellular differentiations and divisions in order to produce a mature embryo. These morphogenic events form the basic cellular pattern for the development of the shoot-root body and the primary tissue layers; it also programs the regions of meristematic tissue formation. The following morphogenic events are only particular to eudicots, and not monocots.
Two cell stage
editFollowing fertilization, the zygote and endosperm are present within the ovule. Stage I, in the photograph on this page, represents what the ovule looks like after fertilization occurs. 1 indicates the endosperm and 2 indicates the single celled zygote. After fertilization, the zygote undergoes an asymmetric transverse cell division that gives rise to two cells - an apical cell and a basal cell.[4] These two cells are very different, and give rise to different structures, establishing polarity in the embryo. Polarity is an important property of plant embryogenesis that is necessary for the rest of the plant life cycle.[5] The apical cells lies above the basal cell and is much smaller. The important aspect of the apical cell, is that it contains most of the cytoplasm from the original zygote.[6] Cytoplasm is the substance that contains all of the organelles; which indicates that the apical region gives rise to more advanced strucutres, like the hypocotyl, shoot apical meristem, and cotyledons.[6] The basal cell resides underneath the apical cell and is much larger. While the apical contains more cytoplasm, the basal cell consists of a large vacuole.[6] This region gives rise to the supporting structures, like the hypophysis and the suspensor complex.[4]
Eight cell stage
editAfter two rounds of longitudinal division, and one round of transverse division, an eight-celled embryo is the result.[5] Stage II, in the photograph above, indicates what the embryo looks like during the eight cell stage. 1 refers to the endosperm, 3 indicates the embryo proper, and 4 indicates the suspensor. According to Laux et al., there are four distinct domains during the eight cell stage.[7] The first two domains contribute to the embryo proper. The apical embryo domain, gives rise to the shoot apical meristem and cotyledons. The second domain, the central embryo domain , gives rise to the hypocotyl, root apical meristem, and parts of the cotyledons. The third domain, the basal embryo domain, contains the hypophysis. The hypophysis will later give rise to the radicle and the root cap. The last domain, the suspensor, is the region at the very bottom, which connects the embryo to the endosperm for nutritional purposes.
Sixteen cell stage
editAdditional cell divisions occur, which leads to the sixteen cell stage. The four domains are still present, but they are more defined with the presence of more cells. The important aspect of this stage is the introduction of the protoderm, which is meristematic tissue that will give rise to the epidermis.[5] The protoderm is the outermost layer of cells in the embryo proper.[5]
Globular stage
editThe name of this stage is indicative of the embryo's appearance at this point in embryogenesis; it is spherical or globular. Stage III, in the photograph above, depicts what the embryo looks like during the globular stage. 1 is indicating the location of the endosperm. The important component of the globular phase is the introduction of the rest of the primary meristematic tissue. The protoderm was already introduced during the sixteen cell stage. According to Evert and Eichhorn, the ground meristem and procambium are initiated during the globular stage.[5] The ground meristem will go on to form the ground tissue, which includes the pith and cortex. The procambium will eventually form the vascular tissue, which includes the xylem and phloem.
Heart stage
editAccording to Evert and Eichhorn, the heart stage is a transition period where the cotyledons finally start to form and elongate.[5] It is given this name in eudicots because most plants from this group have two cotyledons, giving the embryo a heart shaped appearance. Between the cotyledons is where the shoot apical meristem lies. Stage IV, in the photograph above, indicates what the embryo looks like at this point in development. 5 indicates the position of the cotyledons.
Torpedo stage
editThis stage is defined by the continued growth of the cotyledons and axis elongation.[5] In addition, programmed cell death must occur during this stage. This is carried out throughout the entire growth process, like any other development.[8] However, in the torpedo stage of development, parts of the suspensor complex must be terminated.[8] The suspensor complex is shortened because at this point in development most of the nutrition from the endosperm has been utilized, and there must be space for the mature embryo.[6] After the suspensor complex is gone, the embryo is fully developed.[7] Stage V, in the photograph above, indicates what the embryo looks like at this point in development. 1 indicates the endosperm, 5 indiciates the cotyledons, 6 indicates the shoot apical meristem, and 7 indicates the root apical meristem.
The second phase, or postembryonic development, involves the maturation of cells, which involves cell growth and the storage of macromolecules (such as oils, starches and proteins) required as a 'food and energy supply' during germination and seedling growth. The appearance of a mature embryo is seen in Stage VI, in the photograph above.
Dormancy
editThe end of embryogenesis is defined by an arrested development phase, or stop in growth. This phase usually coincides with a necessary component of growth called dormancy. Dormancy is a period in which a seed cannot germinate, even under optimal environmental conditions, until a specific requirement is met.[9] Breaking dormancy, or finding the specific requirement of the seed, can be rather difficult. For example, a seed coat can be extremely thick. According to Evert and Eichhorn, very thick seed coats must undergo a process called scarification, in order to deteriorate the coating.[5] In other cases, seeds must experience stratification. This process exposes the seed to certain environmental conditions, like cold or smoke, to break dormancy and initiate germination.
The Role of Auxin
editAuxin is a hormone related to the elongation and regulation of plants.[10] It also plays an important role in the establishment polarity with the plant embryo. Research, conducted by Cooke, Racusen, and Cohen, has shown that the hypocotyl from both gymnosperms and angiosperms show auxin transport to the root end of the embryo[11] They hypothesized that the embryonic pattern is regulated by the auxin transport mechanism, and the polar positioning of cells within the ovule. The importance of auxin was shown, in their research, when carrot embryos, at different stages, were subjected to auxin transport inhibitors. The inhibitors that these carrots were subjected to made them unable to progress to later stages of embryogenesis. During the globular stage of embryogenesis, the embryos continued spherical expansion. In addition, oblong embryos continued axial growth, without the introduction of cotyledons. During the heart embryo stage of development there were additional growth axes on hypocotyls. Further auxin transport inhibition research, conducted on Brassica juncea, show that after germination, the cotyledons were fused and not two separate structures.
Alternative Forms of Embryogenesis
editSomatic Embryogenesis
editSomatic embryos are formed from plant cells that are not normally involved in the development of embryos, i.e. ordinary plant tissue. No endosperm or seed coat is formed around a somatic embryo.
Androgenesis
editThe process of androgenesis allows a mature plant embryo to form from a reduced, or immature, pollen grain.[12] Androgenesis usually occurs under stressful conditions.[12] Embryos that result from this mechanism can germinate into fully functional plants. As mentioned, the embryo results from a single pollen grain. Pollen grains consists of three cells - one vegetative cell containg two generative cells. According to Maraschin et al., androgenesis must be triggered during the asymmetric division of microspores.[12] However, once the vegetative cell starts to make starch and proteins, androgenesis can no longer occur. Maraschin et al., indicates that this mode of embryogenesis consists of three phases. The first phase is the acquisition of embryonic potential, which is the repression of gametophyte formation, so that the differentiation of cells can occur. Then during the initiation of cell divisions, multicellular structures begin to form, which are contained by the exine wall. The last step of androgenesis is pattern formation, where the embryo-like structures are released out of the exile wall, in order for pattern formation to continue.
After these three phases occur, the rest of the process falls in line with the standard embryogenesis events.
References
edit- ^ a b c Goldberg, Robert; Paiva, Genaro; Yadegari, Ramin (October 28, 1994). "Plant Embryogenesis: Zygote to Seed". Science. 266: 605–614 – via Google Scholar.
- ^ Jurgens, Gerd (May 19, 1995). "Axis formation in plant embryogenesis: cues and clues". Cell. 81: 467–470 – via Google Scholar.
- ^ Radoeva, Tatyana; Weijers, Dolf (November 2014). "A roadmap to embryo identity in plants". Trends in Plant Science. 19: 709–716 – via Google Scholar.
- ^ a b West, Marilyn A. L.; Harada, John J. (October 1993). "Embryogenesis in Higher Plants: An Overview". The Plant Cell. 5: 1361–1369 – via Google Scholar.
- ^ a b c d e f g h Evert, Ray F.; Eichhorn, Susan E. (2013). Raven Biology of Plants. New York: W. H. Freeman and Company. pp. 526–530.
- ^ a b c d Souter, Martin; Lindsey, Keith (June 2000). "Polarity and signaling in plant embryogenesis". Journal of Experimental Botany. 51: 971–983 – via Google Scholar.
- ^ a b Laux, T.; Wurschum, T.; Breuninger, Holger. "Genetic Regulation of Embryonic Pattern Formation". The Plant Cell. 6: 190–202 – via Google Scholar.
- ^ a b Bozhkov, P. V.; Filonova, L. H.; Suarez, M. F. (January 2005). "Programmed cell death in plant embryogenesis". Current Topics in Developmental Biology. 67: 135–179 – via Google Scholar.
- ^ Baskin, Jeremy M.; Baskin, Carol C. (2004). "A classification system for seed dormancy" (PDF). Seed Science Research. 14: 1–16 – via Google Scholar.
- ^ Liu, C; Xu, Z; Chua, N. "Auxin Polar Transport Is Essential for the Establishment of Bilateral Symmetry during Early Plant Embryogenesis". The Plant Cell. 5: 621–630 – via Google Scholar.
- ^ Cooke, T. J.; Racusen, R. H.; Cohen, J. D. (November 1993). "The role of auxin in plant embryogenesis". Plant Cell. 11: 1494–1495 – via Google Scholar.
- ^ a b c Maraschin, S. F.; de Priester, W.; Spaink, H. P.; Wang, M. (July 2005). "Androgenic switch: an example of plant embryogenesis from the male gametophyte perspective". Journal of Experimental Botany. 56: 1711–1726 – via Google Scholar.