ISSN 00310301, Paleontological Journal, 2014, Vol. 48, No. 12, pp. 1324–1329. © Pleiades Publishing, Ltd., 2014.
Important Stages in the Development of Different Layers and Areas of Sporoderm in Angiosperms S. V. Polevova Biological Faculty, Moscow State University, Moscow, 119992 Russia email:
[email protected] Received September 11, 2013
Abstract—The morphological patterns of sporoderm development are illustrated by the example of pollen grains with different aperture types based on published results of transmission electron microscopy (TEM). It is noted that all studied pollen grains are characterized by the development of the periplasmic space between the plasmalemma and callose, where the primexine matrix and primexine begin to form. The aper ture areas remain almost invariable during marking and formation of ectexine. The aperture plug and endex ine are formed before dissolution of the callose. As the callose is dissolved, microspore is surrounded by a young sporoderm throughout the perimeter. Intine begins to form in the aperture with the pollen grain mat uration. Intine displaces the aperture plug from the aperture center (pollen tube exit site) by the time of anthers opening. Direct conversion of the aperture plug into a complex multilayer intine occurs only during the formation of the outer layer of intine of some monocots. Keywords: Angiosperms, sporoderm, pollen grains, aperture plug, aperture, sporoderm development DOI: 10.1134/S0031030114120077
INTRODUCTION
until the first mitosis and dehydration of the pollen grain (Fig. 1).
To date, sporoderm development is rather thor oughly studied in a number of angiosperms. Generali zation of available data will provide elaboration of a uniform terminology for the development of the pol len grain membrane to form a basis for a more fruitful use of available palynological characteristics for tax onomy and phylogeny. In the present paper, published data regarding the formation of monad pollen sporo derm, excluding features of aggregate pollen (dyads, tetrads, polyads, pollinia, and pseudotetrad) are con sidered. The emphasis is placed on the development of typical sporoderm, which occurs in majority of studied angiosperm species with colpate, colporate, porate, sulcate, and omniaperturate (inaperturate) pollen grains. The development of male gametophyte of angiosperms (pollen grain) includes the following stages: the formation of a special callose wall by the microspore mother cell; meiosis of the microspore mother cell; formation of tetrads; dissolution of the callose and release of microspores from the tetrad; microspore vacuolization; resorption of the central vacuole; the first mitosis of the microspore; the second mitosis of the microspore; dehydration of the pollen grain and formation of spermia; pollen tube growth (Clement et al., 1998; Ariizumi and Toriyama, 2011; Firon et al., 2012). Sporoderm development lasts from the meiosis and formation of tetrads of microspores
Microspore mother cells. As anther tissues are differ entiated, the innermost cells begin to encircle them selves (outwards the plasmalemma) with special callose wall at a certain point. As a result, each of microspore mother cells loses contact with neighboring cells and turn into a sphere (becomes rounded in section). Meio sis occurs in microspore mother cells under the protec tion of callose. At the same time, microsporogenesis may follow one way or another (simultaneous, succes sive, intermediate); as a result, a tetrad of microspores (tetrahedral, square, linear, Tshaped, or of other types) is formed. Callose continues depositing throughout this time.It is often possible to recognize by the electron density the callose of microspore mother cell and cal lose of the microspore (Kreunen and Osborn, 1999; Zolala and Polevova, 2009). Tetrad period (Fig. 1, Table 1). The periplasmic space between the callose and plasmalemma becomes visible after each microspore has surrounded itself with a callose layer. Initially, this space is narrow and electrontransparent, much lighter than even the elec trontransparent callose. Then, granularity is recog nized, displaying electrondense granules of the primexine and layered and/or spotted primexine matrix, which enclose these granules (Takahashi and Kouchi, 1988; Takahashi, 1989b). Granules vary in size, shape, and distance between adjacent elements.
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2
3
4
5
6
7
8
1325
9
Development of sporoderm
a
b
c
d
e
g
Functioning of aperture plug Fig. 1. Periodization scheme of formation and functioning of the male gametophyte of angiosperms and formation of sporoderm and colporate aperture; stages: (1) microspore mother cells (MMC); (2) initial deposition of callose envelope and acquisition of the spherical shape by microspore mother cell (rounding and loss of contacts between adjacent MMC); (3) meiosis; (4) microspore tetrads surrounded by callose (tetrad period); (5) callose dissolution, disintegration of microspore tetrad (post tetrad/free spore period); (6) vacuolization, formation and disorganization of the central vacuole (tonoplast); (7) first mitosis, conversion of the microspore into young male gametophyte (pollen grain); the binuclear pollen grain releases form the anther cavity in some species; (8) second mitosis, trinuclear pollen grain; (9) germination of the pollen tube. Stages 4–8 include sporo derm development. The formation of the primexine and marking of sites of future apertures falls on the early tetrad stage (a). Ini tiation of the aperture plug occurs in the primexine matrix in the middletetrad period (b). The formation of the aperture plug and initiation of endexine in the form of lamellae with white lines occur during dissolution of callose at the late tetrad and early posttetrad stage (c). Intine is initiated in the middleposttetrad period, when the first mitosis occurs and the first intercellular wall is formed (d). Overgrowth of intine over the entire pollen grain surface and thickening under apertures (e) can be a long pro cess. Rupture of the aperture plug of intine in the aperture center and formation of the colporate aperture (g) is only characteristic of this aperture type. Aperture plug is morphologically distinct and works actively at stages 4–7 and from (b) to (e). Legend: (1–9) black color designates nuclei, light gray is protoplasts, white is callose and vacuole; (a–g) dark gray is ectexine, gray is endexine and primexine, light gray is protoplast cells, and white is intine.
This corresponds to the alignment of the procolumel lae in the forming exine. The development of the primexine in the studied plants with underwater pollination (and, hence, with reduced exine) stops at the stage of early tetrad. Later, this primexine matrix becomes slightly thicker over the entire surface of the microspore and converts into intine; the sporoderm does not contain sporopollenin. The entire pollen grain is regarded as omniaperturate and exineless (Osborn et al., 2001). It is noted in many papers that the periplasmic space is noticeably thinner or even indistinguishable in the future apertural region, unlike inapertural areas. A thin periplasmic space with primexine elements is detected in the apertural region of pollen grains with an aperture, extending over almost the entire surface of the distal hemisphere (Owens and Dickinson, 1983; Gabaraeva and Grigorjeva, 2010a). Procolumellae are not formed in apertures. If initiation of the procol umellae occurs over the entire surface of the pollen grain, ectoapertures are not formed. However, this does not prevent the development of endoapertures and cryptoapertures in the future (Zavada and Ander son, 1997). The next important stage in sporoderm develop ment is the emergence of lamellae with a white line. PALEONTOLOGICAL JOURNAL
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They appear in the shape of regions of compressed plasmalemma with a bright white space between dark and slightly thickened layers (Gabaraeva, 1987, 1997) and mark the halfway point of the tetrad period and the beginning of endexine formation. The formation of the aperture also falls in this period. Sometimes, inward deviation of plasmalemma (a fold in place of the future aperture) around the future apertural region (Horner and Pearson, 1978; Gabaraeva 1987, 1997; MeyerMelikyan et al., 2004; Gabaraeva et al., 2012a) or light periplasmic space expanding into a lenticular body is recorded (Takahashi and Kouchi, 1988; Gabaraeva and Grigorjeva, 2010b). A wide cavity, where the elements of the primexine do not reach the sporoderm, is formed; the lamellae with white lines appear later (Gabaraeva et al., 2009a, 2009b). In some objects, the first lamellae with white lines are confined to the periphery of the apertural region (Horner and Pearson, 1978; Takahashi, 1987; Kreunen and Osborn, 1999; MeyerMelikyan et al., 2004). In other taxa, the lamellae with white lines are formed throughout the microspore surface (Hess and Frosch, 1994; Gabarayeva and Grigorjeva, 2010a) or only out side the aperture (Taylor and Osborn, 2006). All processes of marking and formation of the ect exine pattern are completed before the callose dissolu
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Table 1. Aperture plugs and apertures of some plant species according to TEM data Species
Initiation of aperture
Aperture plug
Almost mature aperture
Alliaria petiolata (M. Bieb.) Cavara et Grande
1 µm
1
2 µm
6
Myosotis palustris (L.) Nath.
0.5 µm
2
0.5 µm
3
Nicotiana tabacum L.
1 µm
4
0.5 µm
5
0.5 µm
7
1 µm
8
0.5 µm
9 0.5 µm
Pratia begoniifolia (Wall.) Lindl.
Quercus robur L.
10
Symphytum officinale L.
0.5 µm
11
0.2 µm
12
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0.5 µm
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IMPORTANT STAGES IN THE DEVELOPMENT OF DIFFERENT LAYERS AND AREAS
tion at the late tetrad stage. The tectum, columellae, foot layer, and supratectal sculpture elements already exist; subsequently, they only extensively grow at the expense of sporopollenin, which comes from the anther cavity after dissolution of the callose. The most important events of the sporoderm formation are con fined to the inner layers of the sporoderm. The lamellae with white lines underlie the entire ectexine and grow especially thick in the apertural region to prevent plasmalemma baring during the stressful for young microspores time of callose dissolv ing. The body lenticular in section (aperture plug) occupies all space without ectexine. The lenticular body is formed even in the absence of free spaces in the ectexine (apertures); the endoaperture develops in the regions corresponding to the aperture type of the cryp toapertural pollen grain (Zavada and Anderson, 1997). Species differ in consistence and size of the aperture plug according to the aperture type. It is usu ally light, with many electrondense lamellae with white lines (Horner and Pearson, 1978; Takahashi, 1989a; Rowley et al., 1999; ElGazaly et al., 2001; SaadLimam et al., 2002; MeyerMelikyan et al., 2004; Rowley and Skvarla, 2004; Gabaraeva et al., 2012b). Lamellae consolidate in the electrondense layer of the future endexine along the apertural edges. Exine elements, corresponding to the granules, spines, operculums or other sporopollenincontaining ele ments on the aperture membrane, can be observed on the surface of the aperture plug. The aperture plug may be composed of homogeneous substance of gray elec tron density (Takahashi and Kouchi, 1988; Takahashi, 1989; Jimenez et al., 1996; Gabaraeva and Grigorjeva, 2010b). Posttetrad period (Fig. 1, Table 1). At the time of callose dissolution and release of young microspores from the tetrad, the microspore coat is sufficiently strong over the entire microspore surface and it is fre quently much thicker in the future apertural region due to the aperture plug. The onset of the posttetrad period is characterized by the increasing growth of the microspore and its sporoderm (Firon et al., 2012). The ectexine rapidly thickens, accumulates sporopollenin, and acquires all characteristics of a mature ectexine. The endexine in the nonapertural regions also acquires features characteristic of the mature sporo derm. It becomes consolidated and compressed (if it is rather extensive in the mature sporoderm) or lighter (if it is thin or absent in the mature sporoderm). The next stage of sporoderm rearrangement occurs in apertures; at the periphery of the aperture plug, the plasmalemma moves again inwards. The pollen grain begins to form intine during the first mitosis and devel opment of male gametophyte from the microspore (Polowick and Sawnhey, 1992; Romero and Fernan dez, 2000; MeyerMelikyan et al., 2004; Taylor and Osborn, 2006; Matveeva et al., 2012). The layer of electrontransparent intine rapidly thickens under the aperture and extends around the perimeter of the pol PALEONTOLOGICAL JOURNAL
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len grain. Final maturation of the pollen grain is marked in the sporoderm structure by the second crit ical point, i.e., rupture of the aperture plug by thicken ing intine in the pollen tube exit site (at the center of simple apertures –and in the intersections of ecto and endoapertures) (Rowley and Skvarla, 2007). At this time, the shape and dimensions of the endoaperture (if available) are finally formed (ElGazaly et al., 2001; MeyerMelikyan et al., 2004; Matveeva et al., 2012). At this stage, intine thickening in the apertures distin guishes them from pseudoapertures in heteroapertural pollen grains (Volkova et al., 2012), where endexine is consolidated and compressed, but still remains thick, while intine is thin. Endexine of many mature pollen grains is only preserved in the aperture (Weber and Ulrich, 2010). The development of the aperture mem brane differs significantly from the scenario described in pollen grains, with a large sulcus and multilayer intine. In this pollen, the layer with lamellae with white lines becomes much thicker around the microspore perimeter, as the callose is dissolved. Then, it is differentiated into the outer part with lamellae remains with white lines and electrontrans parent interior part. The outer part is compressed and transforms into endexine; the internal part thickens, remains electrontransparent, and forms intine, which is often multilayer and complex, as, for example, in Ledebouria (Hess, 1993; Hess and Frosch, 1994). Intine is considerably thickened in the apertural region (i.e., almost over the entire distal hemisphere) due to intense exocytosis of vesicles in the form of prominences with electrontransparent content. Sub sequently, remains of these tubiform elements do not disappear; they mark thick exintine (outer intine) with characteristic radial channels, as in Chamaedorea and Ledebouria (Hess, 1993; Hess and Frosch, 1994; Gabarayeva and Grigorjeva, 2010). Inaperturate pol len grains are easily formed on the basis of this sporo derm ultrastructure. In this case, sparse elements of exine, arranged across the surface, camouflage sulcus, which is distinct at the distal pole by the presence of channeled exintine, as, e.g., in Trillium and Heliconia (Stone, 1979; Takahashi, 1987). CONCLUSIONS During the formation of microspore of mother cell and its further transformation in the pollen grain, the cell wall fulfils the most important functions (of the outer skeleton and conduction of nutrients), continu ously changing its structure and chemical composition. Meiosis occurs under the protection of callose. The ini tial stages of the microspore formation occur under cover of the plasmalemma and callose and then added by the primexine. As the callose is dissolved, the microspore coat is composed of wellformed ectexine and aperture plug, which is formed by the elements of endexine (matrix and lamellae with white lines). Intine adds to them at the first mitosis. Structural changes in
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the already deposited layers of the sporoderm (thicken ing of the ectexine elements, particularly supratectal sculptural elements, due to precipitation of sporopolle nin of the tapetal origin from the anther cavity; consol idation of the lamellae with white lines into continuous or discontinuous homogeneous endexine; extrusuion of the endexine lamellae by intine around the perimeter of the pollen grain, or only at the aperture center) occur during rapid growth, vacuolization, and dehydration of young pollen grains. ACKNOWLEDGMENTS I am grateful to N.I. Gabaraeva , V.V. Grigorjeva, N.P. Matveeva , O.V. Volkova, and H. Zolala for provid ing me with diverse material. Discussions with M.V. Tekleva and N. E. Zavialova were especially fruitful. The study was supported by the Russian Founda tion for Basic Research, project nos. 120401740a and 110400462a. REFERENCES Ariizumi, T. and Toriyama, K., Genetic regulation of sporopollenin synthesis and pollen development, Ann. Rev. Plant. Biol., 2011, vol. 62, pp. 1–24. Clement, C., Laporte, P., and Andran, J.C., The loculus content and tapetum during pollen development in Lilium, Sex. Plant. Rep., 1998, vol. 11, pp. 11–94. ElGazaly, G., Huysmans, S., and Smets, E.F., Pollen development of Rondeletia odorata (Rubiaceae), Am. J. Bot., 2001, vol. 88, no. 1, pp. 14–30. Firon, N., Nepi, M., and Pacini, E., Water status and asso ciated processes mark critical stages in pollen development and functioning, Ann. Bot., 2012a, vol. 109, pp. 1201–1213. Firon, N., Nepi, M., and Pacini, E., Water status and asso ciated processes mark critical stages in pollen development and functioning, Ann. Bot., 2012b, vol. 109, no. 7, pp. 1–13. Gabaraeva, N.I., Ultrastructure and development of endex tine lamellae in Manglietia tenuipes” (Magnoliaceae), Bot. Zh., 1987, vol. 72, no. 10, pp. 1310–1317. Gabaraeva, N.I., Ultrastructural bases of the development of sporoderm in members of the dicotyledon subclass Mag noliidae, Doctoral Dissertation in Biology, St. Petersburg, 1997. Gabaraeva, N.I. and Grigorjeva, V.V., Sporoderm develop ment in Acer tataricum (Aceraceae): An interpretation, Pro toplasma, 2010b, vol. 247, pp. 65–81. Gabaraeva, N., Grigorjeva, V., and Kosenko, Y., 1. Primex ine development in Passiflora racemosa Brot.: Overlooked aspects of development, Plant. Syst. Evol., 2012a, DOI 10.1007/s0060601307572 Print ISSN 03782697 Online ISSN 16156110. Gabaraeva, N., Grigorjeva, V., and Kosenko, Y., 2. Exine development in Passiflora racemosa Brot.: Overlooked aspects of development, Plant. Syst. Evol., 2012b, DOI 10.1007/s0060601307563 Print ISSN 03782697 Online ISSN 16156110. Gabaraeva, N., Grigorjeva, V., and Polevova, S., Exine and tapetum development in Symphytum officinale (Boragi
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Translated by A. Sokolova
