plants Review Pollen Germination and Pollen Tube Growth in Gymnosperms Maria Breygina * , Ekaterina Klimenko and Olga Schekaleva Department of Plant Physiology, Biological Faculty, Lomonosov Moscow State University, 119991 Moscow, Russia; [email protected] (E.K.); [email protected] (O.S.) * Correspondence: [email protected]; Tel.: +7-499-939-1209 Abstract: Pollen germination and pollen tube growth are common to all seed plants, but these processes first developed in gymnosperms and still serve for their successful sexual reproduction. The main body of data on the reproductive physiology, however, was obtained on flowering plants, and one should be careful to extrapolate the discovered patterns to gymnosperms. In recent years, physiological studies of coniferous pollen have been increasing, and both the features of this group and the similarities with flowering plants have already been identified. The main part of the review is devoted to physiological studies carried out on conifer pollen. The main properties and diversity of pollen grains and pollination strategies in gymnosperms are described. Keywords: plant reproduction; pollen germination; pollen tube growth; gymnosperms; conifers; Picea; Pinus; male gametophyte; polar growth 1. Introduction Citation: Breygina, M.; Klimenko, E.; Schekaleva, O. Pollen Germination Gymnosperms are amazing representatives of the flora. On the one hand, they are and Pollen Tube Growth in ancient plants with primitive characteristics of anatomical structure; on the other hand, they Gymnosperms. Plants 2021, 10, 1301. are perfectly adapted to their habitat and are the dominant species in many ecosystems due https://doi.org/10.3390/ to their impressive size and longevity, with their reproductive system being of particular plants10071301 interest. It has progressive features, because in this group the reduced male gametophyte— pollen grain—first appeared, as well as the ability to form seeds. In addition, this group Academic Editors: Cristina Ferrandiz still represents a wide variety of reproductive patterns, strategies, and relationships. For and Concha (Concepción) example, the degree of gametophyte reduction varies, there are both zooidogamy and Gómez-Mena siphonogamy, and the reproductive process can both be relatively fast and last over several years. Botanical and evolutionary aspects, however, have already been described in Received: 26 May 2021 sufficient detail in other reviews [1–4], whereas here we focus on the physiological data Accepted: 23 June 2021 accumulated during the study of pollen germination in conifers; an informative overview Published: 26 June 2021 of this topic was published in 2005 [5]. Here, we mainly consider new data accumulated over the past 16 years, as well as some important aspects that were outside the focus of the Publisher’s Note: MDPI stays neutral previous review. with regard to jurisdictional claims in published maps and institutional affil- 2. Features of Gymnosperm Pollen Grains iations. The transition from zooidogamy to siphonogamy, that is, to fertilization by immobile gametes instead of motile spermatozoa, occurred during the evolution of gymnosperms, and nowadays both options are found in this taxon [1]. Extant gymnosperms include 12 main families and 83 genera. The term “conifers” refers to a group of gymnosperms Copyright: © 2021 by the authors. that make up the division Pinophyta. To date, physiological studies of gymnosperms were Licensee MDPI, Basel, Switzerland. carried out almost exclusively on conifers (mainly pine and spruce, rarely cypress, fir, and This article is an open access article others), and all of them are characterized by typical siphonogamy, that is, fertilization with distributed under the terms and immotile gametes. The physiology of conifer pollen is discussed in the following sections. conditions of the Creative Commons The male gametophyte of gymnosperms is generally more complex than that of Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ flowering plants; it develops and germinates more slowly; and it is formed through three 4.0/). to five mitoses, which in different groups taxa occur at different stages—before or after Plants 2021, 10, 1301. https://doi.org/10.3390/plants10071301 https://www.mdpi.com/journal/plants PlantsPlants 2021 2021, 10, 10, x, xFOR FOR PEER PEER REVIEW REVIEW 2 2of of 17 17 Plants 2021, 10, 1301 2 of 16 floweringflowering plants plants; it; it develops develops and and germinates germinates more more slowly slowly; and; and it it is is formed formed through through three three toto five five mitoses, mitoses, which which in in different different groups groups taxa taxa occur occur at at different different stages stages——beforebefore or or after after pollinationpollinationpollination [6]. [[6].6]. In InIn this thisthis regard, regard,regard, pollen pollenpollen released releasedreleased from fromfrom microsporangia microsporangiamicrosporangia consists consistsconsists of ofof different differentdifferent numbnumbernumberer of ofof cells cellscells (Figure (Figure(Figure 1):1 ):1): four four cells cells in in pine pine and and ginkgo ginkgo (two (two prothallial prothallial cells, cells, antheridial antheridial cellcellcell and andand tube tubetube cell), cell),cell), two twotwo in inin sequoia, sequoia,sequoia, and andand one oneone in inin juniper juniperjuniper [6 [[66––8].–88].]. Male MaleMale gametes gametesgametes in inin all allall cases casescases are areare formedformedformed after afterafter pollination. pollination.pollination. FigureFigureFigure 1. 1.1. CellularCellular Cellular composition compositioncomposition of ofof some somesome gymnosperm gymnospermgymnosperm pollen pollenpollen grains grainsgrains at at at dehiscence. dehiscence. dehiscence. Pollen grain morphology in gymnosperms is very diverse, as well as their structure, PollenPollen grain grain morphology morphology in in gymnosperms gymnosperms is is very very diverse, diverse, as as well well as as their their structure, structure, but several key patterns can be distinguished that are typical for cycads, ginkgo, gnetales, butbut several several key key patterns patterns can can be be distinguished distinguished that that are are typical typical for for cycads, cycads, ginkgo, ginkgo, gnetales gnetales, , and conifers (Figure2). andand conifers conifers (Figure (Figure 2). 2). Figure 2. SEM of gymnosperm pollen: (a) Cycas micholitzii Dyer (Cycadaceae); (b) Ginkgo biloba L. with a bulge in the aperture FigureFigure 2. 2. SE SEMM of of gymnosperm gymnosperm pollen: pollen: ( a()a )Cycas Cycas micholitzii micholitzii Dyer Dyer (Cycadaceae); (Cycadaceae); ( b(b) )Ginkgo Ginkgo biloba biloba L. L. with with a abulge bulge in in the the ap- ap- erturearea;erture (area;c )area;Picea ( c()c pungens)Picea Picea pungens pungensEngelm. Engelm. Engelm. with typicalwith with typical typical saccate saccate saccate morphology; morphology; morphology; (d) Gnetum ( d(d) )Gnetum Gnetum macrostachyum macrostachyum macrostachyumHook Hookwith Hook microechinatewith with microe- microe- chinatesculpture.chinate sculpture sculpture Scale bar:. Scale. Scale (a, bbar: ,bar:d)—3 (a ()a,) µ(, bm.(b),), and Pictures and ( d(d)—)— are3 3µm. fromµm. Pictures Pictures the following are are from from articles: the the following following (a,b)[9 ],articles: articles: (d)[10 ].( a()a and) and (b (b) )[9], [9], ( d(d) )[10]. [10]. Plants 2021, 10, 1301 3 of 16 Gametophyte cells are separated by thin walls, and from outside, as in flowering plants, are protected by inner cellulosic wall (the intine) and massive outer sporopollenin wall (exine) [11]. Apertures are specialized areas, where the exine is thinner and the intine is usually thickened, channeled, or forms multiple layers [12]. Pollen wall in many cases has polar structure, and the polarity of microspores of gymnosperms, such as angiosperms, is determined during meiosis [8]. The proximal pole (the one closer to the tetrad center) later becomes more convex, and the distal pole becomes flatter, for example, in Ginkgo biloba (Figure2b); exine is thicker on the proximal side, and the large aperture is located on the distal side. Cytological polarity is manifested in the cell arrangement: prothallial cells are located at the proximal end, and the large tube cell is located at the distal end [13–15]. In cycads, the aperture occupies almost half of the pollen grain surface, exine is the thickest in the proximal region and the thinnest in the distal one, intine is relatively thin, and exine is smooth (Figure2a) [ 16]. The question of the number of apertures and polarity in Gnetales has not yet been fully clarified. In Gnetum, pollen is apertureless with internal polarity, and exine is embossed (Figure2d) [10,17,18]. In most Pinaceae plants, pollen is saccate, which means, it has sacci (air sacs)—large hollow outgrowths formed by the exine (Figures1 and2c). The sacs are located at the distal end of the pollen grain, and the exine of the proximal part is thickened. The presence of air sacs is associated with the pollination mechanism in many conifers: pollen landing on a pollination drop. This is the liquid secretion of the receptive ovule, which forms a large drop and rises to the micropyle surface. The drop holds pollen and pulls it into the ovule, where it germinates [19,20]. As it was shown in a model experiment, the floating of pollen grains in pollinating droplets (due to the air sacs) is an effective mechanism for their retraction into micropyle; therefore, air sacs