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The Control of Pollen Tube Growth Downloaded from by Guest on 02 October 2021 KATHLEEN L Articles Sperm Delivery in Flowering Plants: The Control of Pollen Tube Growth Downloaded from https://academic.oup.com/bioscience/article/57/10/835/232382 by guest on 02 October 2021 KATHLEEN L. WILSEN AND PETER K. HEPLER Although most people think of pollen merely as an allergen, its true biological function is to facilitate sexual reproduction in flowering plants. The angiosperm pollen grain, upon arriving at a receptive stigma, germinates, producing a tube that extends through the style to deliver its cargo to the ovule, thereby fertilizing the egg, and completing the life cycle of the plant. The pollen tube grows rapidly, exclusively at its tip, and produces a cell that is highly polarized both in its outward shape and its internal cytoplasmic organization. Recent studies reveal that the growth oscillates in rate. Many underlying physiological processes, including ionic fluxes and energy levels, also oscillate with the same periodicity as the growth rate, but usually not with the same phase. Current research focuses on these phase relationships in an attempt to decipher their hierarchical sequence and to provide a physiological explanation for the factors that govern pollen tube growth. Keywords: actin cytoskeleton, ion dynamics, molecular switches, oscillatory growth, pollen tube n animals, meiosis produces gametes that fuse to do the sperm of protists, bryophytes, ferns, and some gymno- Iform a zygote during sexual reproduction. The life cycle of sperms. Instead, the pollen grain may travel a great distance, flowering plants, referred to as an alternation of generations, transported by wind or an animal carrier (e.g., bird, bat, is more complex. Here, a multicellular diploid generation, insect), before alighting on a receptive stigma and germi- known as the sporophyte generation, alternates with a multi- nating. The sperm cells then travel in the cytoplasm of the cellular haploid generation, known as the gametophyte gen- large vegetative cell of the pollen tube to their target. A eration. Meiosis does not directly produce gametes. Rather, pollen tube is thus the tricellular male gametophyte gener- cells of the sporophyte generation undergo meiosis to produce ation that emerges from a pollen grain to bring about haploid spores, which in turn divide mitotically to give rise double fertilization. to the gamete-producing gametophyte generation. The em- Pollen tube growth is fast and highly polarized, with new bryo sac, which bears the ovule and is embedded in the material being added at the tip, which is called the apex. flower, constitutes the female gametophyte generation. The Secretory vesicles inside the pollen tube transport the cellu- pollen grain is the male gametophyte generation produced lar building blocks required for growth to the apex, where they from microspores in the anthers of flowering plants, and are incorporated into the extending pollen tube by exocyto- consists of a vegetative cell and a generative cell. Either in the sis (Hepler et al. 2006). This process is so efficient that a grain or during pollen tube growth, depending on the species, pollen tube from Zea mays (corn) can attain growth rates of the generative cell undergoes mitosis to give rise to two sperm up to 1 centimeter (cm) per hour, or approximately 3 micro- cells (Raven et al. 1999). meters (µm) per second, making it one of the fastest-grow- Double fertilization is a hallmark of flowering plants ing cells known. With the added realization that cell elongation (Dresselhaus 2006). In addition to the fusing of one sperm cell can be visualized under carefully controlled in vitro conditions, with an egg cell to give rise to an embryo, the second sperm the pollen tube becomes an excellent model system for stud- cell fuses with the two polar nuclei in the central cell of the ies of plant cell growth. Clearly, it is ideal for enhancing our female gametophyte to produce the nutritive triploid tissue understanding of other cells undergoing tip growth, such as of the endosperm. The embryo and endosperm are packaged root hairs, fungal hyphae, and fern and moss protonemata, as a seed, which becomes encased in a fruit formed from the but the pollen tube may also serve as an effective model for ovary and, in some instances, from additional floral parts. It will be apparent, therefore, that double fertilization is not only necessary for sexual reproduction in flowering plants but Kathleen L. Wilsen (e-mail: [email protected]) works in the School also essential for the production of much of the food that we of Biological Sciences at the University of Northern Colorado in Greeley. eat, including nuts, seeds, grains, and fruits. Peter K. Hepler (e-mail: [email protected]) works in the Biology and Plant Because the sperm of flowering plants have no flagella, they Biology Graduate Program at the University of Massachusetts in Amherst. do not depend on water to transport them to the ovule, as © 2007 American Institute of Biological Sciences. www.biosciencemag.org November 2007 / Vol. 57 No. 10 • BioScience 835 Articles the many plant cells that exhibit diffuse growth. In defense of so-called clear zone (figure 1). Although lacking certain in- this assertion, we note the similarity in cell wall structure and clusions, the extreme apex contains numerous secretory vesi- composition and membrane trafficking machinery between cles, as well as elements of the endoplasmic reticulum (ER), pollen tubes and other plant cells. mitochondria, and Golgi dictyosomes (figure 2). These vesi- In this article we describe the structure and physiology of cles are of particular interest because they contain the cell wall a growing pollen tube, focusing on several processes believed precursors, which are delivered to the apex of the pollen to be key in the regulation of growth. We exploit the fact that tube, where their contents are secreted to the wall, allowing pollen tube growth oscillates, as do many underlying physi- the cell perimeter to extend. Although mitochondria, Golgi ological processes and structural elements. Through an analy- dictyosomes, and elements of the ER are abundant in the clear sis of the temporal and spatial ordering of these many factors, zone, they are not confined to this region; they are present we provide insight into the basic regulatory events that con- throughout the pollen-tube shank (Parton et al. 2003, Lovy- Downloaded from https://academic.oup.com/bioscience/article/57/10/835/232382 by guest on 02 October 2021 trol pollen tube growth. Although we include data from Wheeler et al. 2007). pollen tubes of different species (e.g., Arabidopsis and Nico- Cytoskeletal elements, including both actin microfilaments tiana), we focus in particular on those from the lily family (e.g., and microtubules, are ubiquitous components of the pollen the Easter lily, Lilium longiflorum), which, because of their tube (Hepler et al. 2001). In lily pollen tubes, microtubules relatively large size and their readiness to germinate in vitro form a prominent collar at the base of the clear zone (Lovy- and grow at in vivo rates, have been the subject of many Wheeler et al. 2005); their function, however, is not clear. The insightful studies. depolymerization of microtubules with oryzalin has no effect on pollen tube growth or organelle positioning in vitro (Lovy- Growing pollen tubes are highly organized Wheeler et al. 2007). Nevertheless, recent studies show that Whereas the diameter of a pollen grain is between 10 and 100 some cytoplasmic components, including mitochondria and µm (lily pollen is about 60 µm), the length of a pollen tube Golgi vesicles, move slowly along microtubules, indicating that is much greater, and can reach several centimeters (approx- these cytoskeletal elements may indeed contribute to the imately 10 cm in lily) as it grows through the tissues of the style control of pollen tube growth (Romagnoli et al. 2007). to deliver the sperm cells to the embryo sac. The growing lily In contrast to microtubules, a dynamic actin cytoskeleton pollen tube is around 15 µm in diameter (figure 1) and has is widely acknowledged to be essential for pollen tube growth an apical dome that is roughly hemispherical in shape. As the (Hepler et al. 2001). Although there is agreement that the pollen tube grows, callose plugs are laid down in the shank shank of the pollen tube contains longitudinal bundles of actin in such a manner that the cytoplasm remains in the apical end parallel to the axis of growth, the organization of these mi- of the growing tube. Because of this mechanism, pollen tubes crofilaments in the clear zone has been widely debated. have been described as moving cells, in that a fixed plug of Lovy-Wheeler and colleagues (2005) resolved this issue by em- cytoplasm containing floating sperm cells moves forward as ploying an improved method of fixation, followed by label- the cell wall extends (Sanders and Lord 1992). ing with antiactin antibodies, and analysis with confocal Both light and electron microscopy reveal that organelles microscopy. This study confirmed that a cortical fringe of actin have a particular arrangement inside a pollen tube (figures 1, is a consistent feature of the clear zone, and that the extreme 2; Hepler et al. 2001). Most of the pollen tube is granular in apex contains relatively few actin filaments (figure 3; Lovy- appearance because of a rich supply of starch-containing Wheeler et al. 2005). plastids, or amyloplasts (figure 1). However, amyloplasts and The actin cytoskeleton serves three important functions in vacuoles are excluded from the extreme apex, creating the growing pollen tubes. First, actin microfilaments, together with the motor protein myosin (myosin XI in lily), drive cyto- plasmic streaming. This process is thought to transport the secretory vesicles from their point of origin to the apical end of the pollen tube, where they ultimately empty their contents into the expanding cell wall (Yokota and Shimmen 2006).
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