Such changes can increase harvest production or help a plant survive in a specific environment. Pollen is a fine to coarse powdery substance comprising pollen grains which are male microgametophytes of seed plants, which produce male gametes sperm cells.
Pollen in plants is used for transferring haploid male genetic material from the anther of a single flower to the stigma of another in cross-pollination. In terms of cellular complexity, seeds are superior because they're multicellular, while spores are unicellular. A seed also has more facilities for plant survival than a spore. Seeds are located either in the fruit or flower of flowering plants, while spores are located underneath the leaves of non-flowering plants.
The functions of the gametophytes are the production of the 'sperm cells and the female cells, and their union in fertilization. In flowering plants, the pollen grain is the male gametophyte and the embryo sac is the female gametoph yte. The male gametophyte completes its early development within the anther. Pollen , a mass of microspores in a seed plant appearing usually as a fine dust. Each pollen grain is a minute body, of varying shape and structure, formed in the male structures of seed-bearing plants and transported by various means wind, water, insects, etc.
Pollen is the grainy stuff inside a flowering plant that makes it possible for the plant to reproduce. Insects, birds, people, and the wind help to spread pollen between plants. When pollen spreads to the female part of a plant, it germinates, or begins the process of growing a new plant.
A person can easily inadvertently pick up pollen from a crime scene , whether it be in mud on their shoes or on their clothes from directly brushing against a plant in the area.
With this in mind, a primary use of palynology in a forensic investigation is to establish a link between two places, objects or people. Ferns do not bear pollen. They reproduce by spores. Spores are normally asexual, giving rise not to new ferns , but to a different type of organism called a gametophyte or prothallus, resembling a small liverwort. Pollen Fingerprint. The number and type of pollen grains found in a geographic area at a particular time of year pollen profile Pollen Grain.
A reproductive structure that contains the male gametes of seed plants. All conifers produce cone shaped strobili, both male cones often called pollen cones and female cones often called seed cones or ovulate cones. Each pollen grain consists of only four cells. Once they take a core sample, the scientists isolate the pollen and spores from the sediments and rocks using both chemical and physical means.
Not only can pollen records tell us about the past climate, but they can also tell us how we are impacting our climate. Once a pollen grain settles on a compatible pistil, it may germinate in response to a sugary fluid secreted by the mature stigma. Here, a multicellular diploid generation, known as the sporophyte generation, alternates with a multicellular haploid generation, known as the gametophyte generation.
Meiosis does not directly produce gametes. Rather, cells of the sporophyte generation undergo meiosis to produce haploid spores, which in turn divide mitotically to give rise to the gamete-producing gametophyte generation.
The embryo sac, which bears the ovule and is embedded in the flower, constitutes the female gametophyte generation. The pollen grain is the male gametophyte generation produced from microspores in the anthers of flowering plants, and consists of a vegetative cell and a generative cell.
Either in the grain or during pollen tube growth, depending on the species, the generative cell undergoes mitosis to give rise to two sperm cells Raven et al. Double fertilization is a hallmark of flowering plants Dresselhaus In addition to the fusing of one sperm cell with an egg cell to give rise to an embryo, the second sperm cell fuses with the two polar nuclei in the central cell of the female gametophyte to produce the nutritive triploid tissue of the endosperm. The embryo and endosperm are packaged as a seed, which becomes encased in a fruit formed from the 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 also essential for the production of much of the food that we eat, including nuts, seeds, grains, and fruits. Because the sperm of flowering plants have no flagella, they do not depend on water to transport them to the ovule, as do the sperm of protists, bryophytes, ferns, and some gymnosperms. Instead, the pollen grain may travel a great distance, transported by wind or an animal carrier e.
The sperm cells then travel in the cytoplasm of the large vegetative cell of the pollen tube to their target. A pollen tube is thus the tricellular male gametophyte generation that emerges from a pollen grain to bring about double fertilization.
Pollen tube growth is fast and highly polarized, with new material being added at the tip, which is called the apex. Secretory vesicles inside the pollen tube transport the cellular building blocks required for growth to the apex, where they are incorporated into the extending pollen tube by exocytosis Hepler et al.
With the added realization that cell elongation can be visualized under carefully controlled in vitro conditions, the pollen tube becomes an excellent model system for studies of plant cell growth. Clearly, it is ideal for enhancing our understanding of other cells undergoing tip growth, such as root hairs, fungal hyphae, and fern and moss protonemata, but the pollen tube may also serve as an effective model for the many plant cells that exhibit diffuse growth.
In defense of this assertion, we note the similarity in cell wall structure and composition and membrane trafficking machinery between pollen tubes and other plant cells. In this article we describe the structure and physiology of a growing pollen tube, focusing on several processes believed to be key in the regulation of growth. We exploit the fact that pollen tube growth oscillates, as do many underlying physiological processes and structural elements. Through an analysis of the temporal and spatial ordering of these many factors, we provide insight into the basic regulatory events that control pollen tube growth.
Although we include data from pollen tubes of different species e. As the pollen tube grows, callose plugs are laid down in the shank in such a manner that the cytoplasm remains in the apical end of the growing tube. Because of this mechanism, pollen tubes have been described as moving cells, in that a fixed plug of cytoplasm containing floating sperm cells moves forward as the cell wall extends Sanders and Lord Both light and electron microscopy reveal that organelles have a particular arrangement inside a pollen tube figures 1 , 2 ; Hepler et al.
Most of the pollen tube is granular in appearance because of a rich supply of starch-containing plastids, or amyloplasts figure 1. However, amyloplasts and vacuoles are excluded from the extreme apex, creating the so-called clear zone figure 1. Although lacking certain inclusions, the extreme apex contains numerous secretory vesicles, as well as elements of the endoplasmic reticulum ER , mitochondria, and Golgi dictyosomes figure 2.
These vesicles are of particular interest because they contain the cell wall precursors, which are delivered to the apex of the pollen tube, where their contents are secreted to the wall, allowing the cell perimeter to extend. Although mitochondria, Golgi dictyosomes, and elements of the ER are abundant in the clear zone, they are not confined to this region; they are present throughout the pollen-tube shank Parton et al. Cytoskeletal elements, including both actin microfilaments and microtubules, are ubiquitous components of the pollen tube Hepler et al.
In lily pollen tubes, microtubules form a prominent collar at the base of the clear zone Lovy-Wheeler et al. The depolymerization of microtubules with oryzalin has no effect on pollen tube growth or organelle positioning in vitro Lovy-Wheeler et al.
Nevertheless, recent studies show that some cytoplasmic components, including mitochondria and Golgi vesicles, move slowly along microtubules, indicating that these cytoskeletal elements may indeed contribute to the control of pollen tube growth Romagnoli et al. In contrast to microtubules, a dynamic actin cytoskeleton is widely acknowledged to be essential for pollen tube growth Hepler et al.
Although there is agreement that the shank of the pollen tube contains longitudinal bundles of actin parallel to the axis of growth, the organization of these microfilaments in the clear zone has been widely debated.
Lovy-Wheeler and colleagues resolved this issue by employing an improved method of fixation, followed by labeling with antiactin antibodies, and analysis with confocal microscopy.
This study confirmed that a cortical fringe of actin is a consistent feature of the clear zone, and that the extreme apex contains relatively few actin filaments figure 3 ; Lovy-Wheeler et al. The actin cytoskeleton serves three important functions in growing pollen tubes. First, actin microfilaments, together with the motor protein myosin myosin XI in lily , drive cytoplasmic 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 Like the secretory vesicles, all organelles, including amyloplasts, dictyosomes, ER, mitochondria, and vacuoles, are propelled by the actomyosin system and are in constant motion.
A second function of the actin cytoskeleton is controlling the position of organelles within the pollen tube. Under normal conditions, the polarized distribution of organelles, including notably the detailed morphology of the clear zone, is maintained despite the fact that the contents are constantly in motion.
However, this polarized distribution of organelles is quickly and dramatically disrupted by antimicrofilament agents e. By some process that is not well understood, the actin cytoskeleton appears to create a filter that allows certain organelles such as the vesicles, mitochondria, Golgi dictyosomes, and ER to flow into the clear zone while preventing the incursion of amyloplasts and vacuoles.
A third function of actin relates to its direct role in the control of tube elongation. Researchers initially assumed that actin contributed to growth indirectly, through the control of cytoplasmic streaming and the consequent delivery of secretory vesicles to the apex. However, studies with agents that control actin polymerization e. These results strongly support the idea that actin polymerization contributes directly to pollen tube growth.
Although much more work is needed to completely unravel the mechanisms of actin's contribution to cell growth, we increasingly realize that actin has a profound effect on cell elongation. The cell wall is another important structural feature of growing pollen tubes Geitmann and Steer The freshly secreted pollen tube wall in the apex consists of methyl-esterified pectins, which are displaced to the shank of the pollen tube as the cell grows.
On the flanks of the apical dome, esterified pectins are enzymatically demethylated, and the cell wall is greatly reinforced when calcium cross-links the acidic residues on the pectin chains Bosch and Hepler Thus, the pollen tube wall is stronger and more rigid in the shank of the tube than in the apex. This arrangement means that only the apical wall is plastic and able to undergo strain deformation in response to internal turgor pressure Bosch and Hepler These conditions contribute to the polarity of pollen tube growth.
For more than 40 years it has been recognized that calcium ions are required for pollen germination and pollen tube growth reviewed in Hepler et al. In addition to calcium being an essential component of the surrounding extracellular environment, an intracellular calcium gradient and extracellular fluxes associated with the pollen tube are essential for growth. However, within the cell the free concentration of this ion is substantially lower, with basal concentrations from to nanomolar nM Holdaway-Clarke and Hepler The underlying reason for the low concentration of intracellular calcium may derive from the physiological incompatibility of millimolar concentrations of phosphate and calcium, at which point these two ions would react to form an insoluble precipitate of calcium phosphate, and destroy phosphate-based energy metabolism.
Virtually all living cells thus evolved mechanisms for reducing the intracellular calcium ion concentration, leading to the enormous concentration difference between the cytosol and the extracellular environment.
It seems likely that during the process of evolution, this concentration difference was exploited for signaling purposes. For example, a primary messenger e. Calcium flows into the cell, generating a large local increase in its intracellular concentration. Calcium becomes the second messenger because it will now bind to and activate proteins e. In the pollen tube, there is direct evidence for a localized domain of elevated calcium see below , which we presume behaves as a second messenger and activates growth-dependent events, without compromising phosphate-based energy metabolism.
Several investigators report a region of high intracellular calcium in the apex of the pollen tube, immediately adjacent to the plasma membrane, where growth is maximal figure 4 , top row; Holdaway-Clarke and Hepler , Hepler et al.
Thus calmodulin, a ubiquitous calcium-binding protein that binds to and regulates numerous cellular targets, or key enzymes, such as the calcium-dependent protein kinases CDPKs commonly found in plants, may be saturated with calcium and maximally active in the apex of growing pollen tubes but switched off in the region just behind the apex Holdaway-Clarke and Hepler The observation that pollen tubes as well as virtually all other cells have low basal levels of calcium means that there are compensatory systems that respond to the ion, sequestering it or extruding it from the cell.
Indeed, within the pollen tube the region of elevated calcium is restricted to the apical portion of the clear zone, indicating that the removal system must be located there. The localized ER and mitochondria are strong candidates for the sequestering system because they occur at high density at the base of the clear zone; preliminary evidence also shows that the ER takes up calcium and transports it basally Wilsen Studies directed at the flow of ER and mitochondria in living pollen tubes reveal that they continually move into the clear zone, providing a fresh supply of uncharged elements that remove calcium ions and restrict the basal spread of the tip-focused gradient Lovy-Wheeler et al.
There are also calcium adenosine-triphosphatases ATPases on the plasma membrane; these are membrane-bound enzymes that use the energy of adenosine triphosphate ATP to move calcium against its concentration gradient Sze et al. Taken together, these lines of evidence make it clear that the pollen tube has overlapping components that respond to elevated calcium and restore the basal concentration. Extracellular calcium, as previously noted, is at much higher concentrations than are concentrations within the cell.
For this reason, an influx of extracellular calcium requires only the opening of selected pores or channels on the plasma membrane. Early predictions of an inward flow of calcium ions into the pollen tube apex from the surrounding environment were confirmed by high-resolution studies using the calcium-specific vibrating probe. This is an electrode that oscillates between two points close to the pollen tube, measuring the local calcium ion concentration.
The difference in ion concentration between these two points indicates both the direction and magnitude of flux. More recently, these stretch-activated calcium channels have been identified in protoplasts from Lilium longiflorum pollen tube tips and from regions of the pollen grain associated with the site of germination Dutta and Robinson Although it seems clear that calcium ions flow into the cell from the extracellular environment, it must also be recognized that a substantial amount of this apparent flow may be associated with binding to cell wall components.
The bulk of the cell wall material is secreted as methyl-esterified pectins. As previously noted, these are demethylated by the enzyme pectin methyl esterase, exposing acidic groups which then bind calcium in a process that cross-links these pectic residues and strengthens the cell wall Bosch and Hepler Nevertheless, the small amount that does cross the plasma membrane creates the apical gradient and is essential for pollen tube growth.
Although receiving much less attention than calcium, protons are also essential for pollen germination and pollen tube growth. In experiments with pollen tubes, the culture medium must be acidic, with growth being optimal at pH 5 to pH 6 Holdaway-Clarke et al. However, intracellular pH has proved to be more challenging to gauge than intracellular calcium because protons, which are considerably more mobile than calcium ions, are easily perturbed when probed by an indicator dye.
Nonetheless, it has been shown, using low dye concentrations, that growing pollen tubes exhibit an intracellular pH gradient figure 4 , bottom row.
The apex of the pollen tube is slightly acidic pH 6. In addition to the intracellular expression of pH noted above, studies with extracellular ion-selective electrodes reveal a pattern of fluxes that correlates with the pattern of intracellular pH.
These observations indicate that there is an influx of protons at the extreme apex of the tube, precisely at the intracellular acidic domain. This pattern of extracellular proton fluxes provides evidence for the presence of a current loop in the apical dome of the pollen tube, which we presume is driven by a plasma membrane—associated proton-ATPase.
The regulation of pH emerges as a key factor in the control of pollen tube growth. Pollen tube growth is also regulated by a complex network of signaling molecules and pathways. Two categories of molecules and their associated enzymes and cofactors have been localized to the apex of the pollen tube and appear to play central roles in regulating growth; these include the phosphoinositides and the small G-proteins, which are guanosine triphosphate GTP -binding proteins, such as ROP Rho family GTPases of plants.
The phosphoinositides are membrane lipids of which phosphatidyl inositol 4,5 bisphosphase PIP 2 has received the most attention. It is a multifaceted molecule that associates with certain actin-binding proteins, including actin depolymerizing factor ADF , profilin, and villin Martin , which are common in pollen tubes.
In addition, PIP 2 is enzymatically cleaved by phospholipase C PLC to produce two more signaling agents, diacylglycerol and inositol 1,4,5 trisphosphate. The latter diffuses in the cytoplasm and can release calcium from internal stores Franklin-Tong et al.
It is noteworthy, therefore, that PIP 2 localizes to the apical plasma membrane of the pollen tube figure 5 ; Kost et al. Observations support the importance of these phosphoinositides to pollen tube growth. For example, an increase in PIP 2 at the apex is associated with an increase in pollen tube growth Dowd et al. Further studies on PLC reveal that modulation of its activity also modulates growth; thus inhibition of PLC activity markedly blocks pollen tube growth Helling et al.
There is evidence that PLC activity may impinge most directly on the actin cytoskeleton, because cells compromised with an inactive form of the enzyme can be partially rescued with the actin polymerization inhibitor latruculin B Dowd et al. PLC activity thus restricts PIP 2 to the extreme apex of the tube and in so doing contributes centrally to the control of pollen tube growth polarity Dowd et al.
The small G-proteins are also potential central regulators of pollen tube growth Hwang and Yang ROPs constitute a subfamily of molecular switches, unique to plants, that function in developmental and signaling events.
In the cell, ROP toggles between an active GTP-bound form and an inactive guanosine diphosphate—bound form, under the influence of guanine nucleotide exchange factors, GTPase-activating proteins, and guanine nucleotide dissociation inhibitors. It has been shown that certain ROPs localize to the apical region of the plasma membrane of pollen tubes, the site of growth figure 6 ; Kost et al. As with PIP 2 , the structural studies on ROPs reveal the presence of regulating factors that inactivate these agents if they migrate from the extreme apex to the flank of the apical dome Gu et al.
These conditions ensure that the active species will be confined to the extreme apex of the tube, where they may participate in the control of pollen tube growth polarity. It seems important that both ROPs and PIP 2 localize to the same extreme apical domain and may act together in a common pathway Kost et al.
An exciting finding that has emerged in the last 15 years is that pollen tubes do not extend uniformly but exhibit pronounced oscillations in their growth rate. For example, in lily pollen tubes the growth rate oscillates between and nanometers per second, with a periodicity of about 20 to 50 seconds Holdaway-Clarke and Hepler Inevitably the question arises as to why pollen tube growth oscillates.
Although the answer cannot be stated with certainty, there are some important considerations that help us understand the matter. It has been known for many years that cultures of yeast cells generate oscillations in reduced nicotinamide adenine dinucleotide phosphate NAD[P]H , a coenzyme essential in numerous energetic reactions within the cell Goldbeter To the extent that pollen tubes produce and use NAD P H, we can expect similar oscillations in those cells.
But beyond these general considerations there are issues that pertain more specifically to pollen tube growth. In our studies of lily, we note that when the pollen grains first germinate, the emerging tubes exhibit fluctuations in their growth rate but not regular oscillations. Oscillatory growth thus is more rapid than nonoscillatory growth.
Abstract Sperm competition theory predicts that males should produce many, similar sperm. Publication types Research Support, Non-U. Gov't Research Support, U.
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