The lines shown in c, e, and f are promoter: GUS reporters and the line shown in e is a promoter: GFP reporter. Anyone you share the following link with will be able to read this content: Sorry, a shareable link is not currently available for this article. Two other studies in B. napus examined gene expression profiles in embryos at a slightly later stage of development, starting from 2 to 3days of culture, when induced microspores had already gone through a few sporophytic cell divisions (Joosen et al. Once the exine breaks, the main tissue layers of the embryo are formed, which include the protoderm, the procambium and the ground tissue layers that will form, respectively, the epidermis, the vascular tissue and the parenchyma. Meanwhile, other microspores are directly committed to embryogenesis and undergo numerous changes at different levels to become microspore-derived embryos. While the details may vary between species, the overall development of the female gametophyte has two distinct phases. Planta 208:383391, Sols M-T, Pintos B, Prado M-J, Bueno M-, Raska I, Risueo M-C, Testillano PS (2008) Early markers of in vitro microspore reprogramming to embryogenesis in olive (Olea europaea L.). Plant Cell Rep 28:551559, Raghavan V (1986) Embryogenesis in angiosperms. In: Agrawal GK, Rakwal R (eds) Seed development: OMICS Technologies toward improvement of seed quality and crop yield. 1996). The study by Maraschin et al. Spores work by either developing into a new individual or protecting the genetic material from unfavorable weather conditions. 1996). What do microspores then develop into? 1988; Massonneau et al. Plant Cell Rep 18:858862, Article 1999). 2013). d Callus-like structure; e symmetrically divided microspore with two equally sized nuclei; f multinucleate structure lacking clear organization that is still enclosed within the exine; g globular stage embryo with a well-defined protoderm; h suspensor-bearing embryo; i star-like morphology after stress treatment; j multicellular structure with two distinct domains; k multicellular structure breaking out of the exine; and l microspore-derived embryo at the cotyledon stage. Omissions? The utilization of microspore embryogenesis as a biotechnology tool has been extended to a relatively diverse range of plants (Ferrie and Caswell 2011; Maluszynski et al. CAS In tobacco, 35SCaMV::GUS reporter marked an early stage of sporophytic development prior to embryo development. Develop within the walls of spores 3. In a plants male reproductive organs, development of pollen takes place in a structure known as the microsporangium (Figure 32.6). [3] The only heterosporous ferns are aquatic or semi-aquatic, including the genera Marsilea, Regnellidium, Pilularia, Salvinia, and Azolla. These observations suggest that in some cases, an ordered series of cell division is not required for differentiation (Torres-Ruiz and Jurgens 1994). Development of microspores from microsporocytes is termed microsporogenesis.There are two basic types of microsporogenesis as determined by the timing of cytokinesis, which is the formation of a plasma membrane and cell wall that divides one cell into two (Figure 11.6A).If cytokinesis occurs after meiosis I, then microsporogenesis is successive (Figure 11.6B). Annu Rev Plant Biol 41:317338, Mascarenhas JP (1993) Molecular mechanisms of pollen tube growth and differentiation. These patches of tissue contain many diploid microsporocyte cells, each of which undergoes meiosis producing a quartet of microspores. Sporopollenin allows the pollen to survive under unfavorable conditions and to be carried by wind, water, or biological agents without undergoing damage. 2012; Supena et al. Alternatively, the competence for embryo induction around PMI could be explained by the ability of the microspore or immature pollen grain to divide; microspores near PMI can proceed with division under stress, while younger and older stages cannot, respectively, enter or re-enter the division phase (Gimnez-Abin et al. 2005; Stirn et al. So, Solve any question of Sexual Reproduction in Flowering Plants with:- Patterns of problems > Was this answer helpful? Sex Plant Reprod 24:161169, Zhao J-P, Simmonds DH, Newcomb W (1996) Induction of embryogenesis with colchicine instead of heat in microspores of Brassica napus L. cv.Topas. There are two basic types of microsporogenesis as determined by the timing of cytokinesis, which is the formation of a plasma membrane and cell wall that divides one cell into two ( Figure 11.6A ). Cambridge University Press, Cambridge, Ramesar-Fortner NS, Yeung EC (2006) Physiological influences in the development and function of the shoot apical meristem of microspore-derived embryos of Brassica napus Topas. Classical cell biology studies have helped to define some of the cellular characteristics of embryogenic cells, although a direct link between cellular changes and cell fate is difficult to establish, as these studies are invariably performed on fixed material (Simmonds and Keller 1999; Zaki and Dickinson 1991). Plant Reprod (this issue). Given the sporophyte-like identity of the microspore and bicellular pollen grain, de novo expression of these transcription factors in response to stress could be sufficient to induce a switch to totipotent growth. The exine contains sporopollenin, a complex waterproofing substance supplied by the tapetal cells. The regenerative competence of plant cells is widely exploited at a practical level, but a deeper mechanistic understanding of the molecular basis for plant totipotency is lacking. J Exp Bot 63:739755, Hause B, Hause G, Pechan P, van Lammeren AAM (1993) Cytoskeletal changes and induction of embryogenesis in microspore and pollen cultures of Brassica napus L. Cell Biol Int 17:153168, Hause B, van Veenendaal WLH, Hause G, van Lammeren AAM (1994) Expression of polarity during early development of microspore-derived and zygotic embryos of Brassica napus L. cv. 1995) (Fig. Each pollen grain has two coverings: the exine (thicker, outer layer) and the intine (Figure 32.7). These are fertilized by sperm produced by the male gametophyte . Pollen from male cones blows up into upper branches, where it fertilizes female cones. 2005; Olsen 1987). A compromise between a low degree of differentiation and stress resistance might be necessary to induce embryogenesis. Describe the components of a complete flower Describe the development of microsporangium and megasporangium in gymnosperms Sexual reproduction takes place with slight variations in different groups of plants. Sex Plant Reprod 4:4855, Zhang Z, Laux T (2011) The asymmetric division of the Arabidopsis zygote: from cell polarity to an embryo axis. The two spores include megaspores which develop into the female gametophyte while microspores develop into the male gametophyte. Expression of suspensor and embryo markers identified in B. napus microspore culture. In botany, a microspore refers to the spore produced by sporophytes. Initially, the vast majority of cells in culture follow the gametophytic pathway, but around 56days of culture, the pollen grains burst open and die. noun, plural: microspores These are found in the anther, which is at the end of the stamenthe long filament that supports the anther. 1995; Fig. Premature exine rupture seems to be detrimental to further embryo growth. The androecium has stamens with anthers that contain the microsporangia. Pollen Formation. In somatic embryos, the surrounding tissues (when present) can provide positional clues, but polarity can also be established in the absence of such tissue. Conifer sperm do not have flagella but rather move by way of a pollen tube once in contact with the ovule. The female cone also has a central axis on which bracts known as megasporophylls (Figure 32.10) are present. However, unlike cereals, embryogenic structures in B. napus are usually uniform clusters of cells in which no distinct domains can be distinguished (Fan et al. In barley microspores, which have only one aperture, the embryo consistently breaks out of the exine at the side opposite to the aperature. 2005c; Telmer et al. There are two types of incomplete flowers: staminate flowers contain only an androecium, and carpellate flowers have only a gynoecium (Figure 32.4). This phenomenon is referred to as totipotency and may be expressed as part of the normal development of some plants, as in apomixis or may be induced in tissue culture. Topas. Development 119:823831, Becker JD, Boavida LC, Carneiro J, Haury M, Feij JA (2003) Transcriptional profiling of Arabidopsis tissues reveals the unique characteristics of the pollen transcriptome. PubMed Springer, Dordrecht, pp 3546, Chapter Planta 225:641652, Twell D, Park SK, Lalanne E (1998) Asymmetric division and cell-fate determination in developing pollen. The question that remains is how this cell death process is regulated, i.e., whether it is the cause or consequence of exine rupture. A few studies have identified differentially expressed sequences using methods to subtract pollen-expressed genes (Joosen et al. 2007), targeted expression analysis of candidate genes (Snchez-Daz et al. 2007). Each of the microsporocytes in the microsporangia undergoes meiosis, producing four haploid microspores. Mature pollen grains contain two cells: a generative cell and a pollen tube cell. Overexpression of STM induced expression of known embryogenesis regulatory genes and also reduced reactive oxygen species (ROS) by the increase in scavenging enzyme activity and by increased ascorbic acid (Elhiti et al. 2002). In ferns, the gametophyte is free-living and very distinct in structure from the diploid sporophyte. 2005b). Embryos that contain degenerated meristems cannot be converted directly into plants. Two of the nucleithe polar nucleimove to the equator and fuse, forming a single, diploid central cell. The second whorl is comprised of petalsusually, brightly coloredcollectively called the corolla. Plant Physiol 144:134154, Maluszynski M, Kasha KJ, Forster BP, Szarejko I (2003) Doubled haploid production in crop plants: a manual. 2013) have shown that the transcriptomes of microspores and bicellular pollen are highly similar. These suspensor-like structures comprise clusters of larger cells, short rudimentary filaments or uniseriate filaments attached to the root pole of the embryo. We summarize the major concepts that have arisen from decades of cell and molecular studies on microspore embryogenesis and put these in the context of recent experiments, as well as results obtained from the study of pollen and zygotic embryo development. Box 619, 6700 AP, Wageningen, The Netherlands, You can also search for this author in 1999; Fan et al. Further mitosis of the microspore produces two nuclei: the generative nucleus, and the tube nucleus. The development of the globular structure begins to mimic that of zygotic embryos once the embryos break out of the exine and is marked by the establishment of a protoderm layer (Telmer et al. Overall, the change in redox status during embryo development produces a metabolic switch needed for the embryos to reach maturity. In angiosperms, the female gametophyte exists in an enclosed structurethe ovulewhich is within the ovary; in gymnosperms, the female gametophyte is present on exposed bracts of the female cone. The OpenStax name, OpenStax logo, OpenStax book covers, OpenStax CNX name, and OpenStax CNX logo Although ESTs for these genes were only detected after 7days of culture, their expression could be detected by RT-PCR much earlier, at 12days of culture, suggesting that embryo cell identity is established as early as the first few sporophytic cell divisions. Cell 112:219230, George EF, Hall MA, De Klerk G-J (2008) Somatic embryogenesis plant propagation by tissue culture. When an egg and pollen meet, fertilization occurs and a new. Embryogenic microspores show variability in their ability to undergo further growth and differentiation. The development of high-quality, histodifferentiated embryos with functional meristems is of major importance for the regeneration of DH plantlets and can be a limiting step in embryo production in some species and genotypes. In support of this, expression of protein synthesis machinery genes increased in 5- and 7-day-old cultures (Joosen et al. The production of this type of embryo has been optimized in B. napus (Joosen et al. Also, gradients of exogenously applied plant hormones can be established and direct embryo growth and division (Friml et al. 2a), marks embryogenic microspores in culture in a poorly responding genotype as early as 3days after the start of culture (Fig. Because the gametophytes mature within the spores, they are not free-living, as are the gametophytes of other seedless vascular plants. In cereals, a high percentage of the microspores divide sporophytically, but form callus rather than embryos (Castillo et al. J Exp Bot 52:12271238, Kim M, Kim J, Yoon M, Choi D-I, Lee K-M (2004) Origin of multicellular pollen and pollen embryos in cultured anthers of pepper (Capsicum annuum). Plant Cell Rep 13:267271, Custers JBM, Snepvangers SCHJ, Jansen HJ, Zhang L, van Lookeren Campagne MM (1999) The 35S-CaMV promoter is silent during early embryogenesis but activated during nonembryogenic sporophytic development in microspore culture. Finally, the studies on the molecular changes during microspore embryo induction will be put in context of male gametophytic development. In B. napus, isolated microspores develop into embryos after exposure to heat stress. These plants have two spore types, megaspores and microspores. Also shown is the lower stem and root. 2cf). Plant Sci 106:195206, Stone SL, Kwong LW, Yee KM, Pelletier J, Lepiniec L, Fischer RL, Goldberg RB, Harada JJ (2001) LEAFY COTYLEDON2 encodes a B3 domain transcription factor that induces embryo development. For example, in the model B. napus line Topas DH4079, around 40% of the initial population divides sporophytically, while the remaining 60% has a gametophytic identity. The decisive tissue culture parameter required to induce embryogenic growth is the application of a stress treatment, usually temperature, nutrient or osmotic stress, either alone or in combination (reviewed by Islam and Tuteja 2012; Shariatpanahi et al. 2001) and barley (Pulido et al. 2007; Tsuwamoto et al. 2001). 2007), coinciding with the loss of pollen and increase in sporophytic growth. proposed that STM delays cellular differentiation through a decrease in ROS levels and by reducing cell wall rigidity. After PMII, the pollen grain enters a highly specialized transcriptional program that is different from that of both the microspore/binucleate pollen grain and other sporophytic tissues (see below) (Honys and Twell 2003). Key Terms The variability that seems to operate in different species should be explored to gain insight into the pathways that lead to plant and embryo polarity and self-organization. Trends Plant Sci 3:305310, Ueda M, Zhang Z, Laux T (2011) Transcriptional activation of Arabidopsis axis patterning genes WOX8/9 links zygote polarity to embryo development. The corn plant has both staminate (male) and carpellate (female) flowers. The flower is borne on a stalk known as a receptacle. Live cell imaging of immobilized microspores in wheat and barley showed that a star-like morphology is associated with cell division (Indrianto et al. In maize and barley, some microspores divide to produce embryogenic calli with varying degrees of regenerability (Massonneau et al. Carlsberg Res Commun 52:393404, Prem D, Sols M-T, Brny I, Rodrguez-Sanz H, Risueo MC, Testillano PS (2012) A new microspore embryogenesis system under low temperature which mimics zygotic embryogenesis initials, expresses auxin and efficiently regenerates doubled-haploid plants in Brassica napus. The microspores develop inside the microsporangium. Carpellate flowers are clustered in the immature ears. For example, knolle mutants, which lack an epidermal cell layer, cannot grow into a normal embryo and are defective in the establishment of the apicalbasal axis (Mayer et al. 2012). In pteridophytes, megaspores are typically larger than microspores, but the opposite is true in most seed plants. What specific impact would you expect this to have on fertilization? Plant Reprod 26, 181196 (2013). Initially, the zygote elongates and then divides asymmetrically to form a large basal cell that will become the suspensor and the hypophysis, and a smaller apical cell that will form the embryo proper. The application of chemical agents, such as colchicine, cytochalasin D or n-butanol, has shown that the rearrangement of the microtubule and actin networks plays a major role in cell fate decisions, since disruption of these networks enhances or is sufficient to trigger embryo formation in the absence of a stress treatment (Gervais et al. 2011, Raghavan 1986). These so-called doubled-haploid (DH) plants have been extensively exploited in plant breeding programs to increase the speed and efficiency with which homozygous lines can be obtained (reviewed in Forster et al. Plant Cell Tiss Org Cult 106:359362, Binarova P, Hause G, Cenklov V, Cordewener JH, Campagne ML (1997) A short severe heat shock is required to induce embryogenesis in late bicellular pollen of Brassica napus L. Sex Plant Reprod 10:200208, Bino R, Van Tuyl J, De Vries J (1990) Flow cytometric determination of relative nuclear DNA contents in bicellulate and tricellulate pollen. Jan 9, 2023 OpenStax. PubMedGoogle Scholar. The members of each part of the microspores separate from each other. 2012; Maraschin et al. Moreover, it was shown that early patterning in microspore-derived embryos that contain a suspensor is more similar to that of zygotic embryos, pointing to a novel function of the suspensor in supporting early cellular patterning in the embryo proper. The microspore divides to form a reduced gametophyte, merely a jacket of cells and a few sperm cells; the megaspore divides to form a mass of tissue and archegonia, each enclosing an egg. By the end of this section, you will be able to do the following: Sexual reproduction takes place with slight variations in different groups of plants. 2006). The two nucleus that are developed are the generative nucleus and the vegetative nucleus which are involved in how the pollen grain achieves fertilization. volume26,pages 181196 (2013)Cite this article. Although DH production is widely exploited, there are often one or more bottlenecks that need to be overcome before an efficient system can be established for a specific crop or genotype. Either 128 or 256 sperm cells with flagella are produced in each antheridium.
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