A pollen tube is a tubular structure produced by the male gametophyte of seed plants when it germinates. Pollen tube elongation is an integral stage in the plant life cycle. The pollen tube acts as a conduit to transport the male gamete cells from the pollen grain—either from the stigma (in flowering plants) to the ovules at the base of the pistil or directly through ovule tissue in some gymnosperms. In maize, this single cell can grow longer than to traverse the length of the pistil.
Pollen tubes were first discovered by Giovanni Battista Amici in the 19th century.
They are used as a model for understanding plant cell behavior. Research is ongoing to comprehend how the pollen tube responds to extracellular guidance signals to achieve fertilization.
Pollen tubes are unique to seed plants and their structures have evolved over their history since the Carboniferous period. Pollen tube formation is complex and the mechanism is not fully understood.
Angiosperms
The male reproductive organ of the flower, the stamen, produces pollen. The opening of anthers makes pollen available for subsequent pollination (transfer of pollen grains to the pistil, the female reproductive organ). Each pollen grain contains a vegetative cell, and a generative cell that divides to form two sperm cells. Abiotic vectors such as wind, water, or biotic vectors such as animals carry out the pollen distribution.
Once a pollen grain settles on a compatible pistil, it may germinate in response to a sugary fluid secreted by the mature stigma. Lipids at the surface of the stigma may also stimulate pollen tube growth for compatible pollen. Plants that are self-sterile often inhibit the pollen grains from their own flowers from growing pollen tubes. The presence of multiple grains of pollen has been observed to stimulate quicker pollen tube growth in some plants. The vegetative cell then produces the pollen tube, a tubular protrusion from the pollen grain, which carries the sperm cells within its cytoplasm. The sperm cells are the male gametes that will join with the egg cell and the central cell in double fertilization. The first fertilization event produces a diploid zygote and the second fertilization event produces a triploid endosperm.
The germinated pollen tube must drill its way through the nutrient-rich style and curl to the bottom of the ovary to reach an ovule. Once the pollen tube reaches an ovule, it bursts to deliver the two sperm cells. One of the sperm cells fertilizes the egg cell which develops into an embryo, which will become the future plant. The other one fuses with both polar nuclei of the central cell to form the endosperm, which serves as the embryo's food supply. Finally, the ovary will develop into a fruit and the ovules will develop into seeds.
Gymnosperms
Gymnosperm pollen is produced in microsporangia borne on the scales of the male cone or microstrobilus. In most species, the plants are wind-pollinated, and the pollen grains of conifers have air bladders that provide buoyancy in air currents. The grains are deposited in the micropyle of the ovule of a female cone or megastrobilus, where they mature for up to a year. In conifers and gnetophytes, the pollen germinate to produce a pollen tube that penetrates the megasporangium or nucellus carrying with it sperm nuclei that are transferred to the egg cell in the developing archegonia of the female plant.
Mechanism of pollen tube growth
Recognition
The female sporophyte must recognize the pollen stuck to the stigma. Often, only pollen of the same species can successfully grow. Outcrossed pollen grows more successfully. With self-incompatibility systems, outcrossed pollen grows and outcompetes self pollen. The interaction between the style and the pollen detects compatibility and influences growth rate of the pollen tube. This selection process relies on gene level regulation in which gene loci of the gynoecium allow either self-pollen to grow slowly, stop growing or burst while faster growth of outcrossed pollen occurs. Self-incompatibility systems maintain genetic diversity. As for gymnosperms, they do not contain a pistil with a stigma. Therefore, pollen must submerge into the pollination droplet, bringing the male gametophyte to the egg of the exposed ovule. However, pollen of different species will not submerge into the droplet; the pollen is left floating on top, while the droplet retracts back into the micropyle.
Initiation
Once the pollen grain is recognized and hydrated, the pollen grain germinates to grow a pollen tube. There is competition in this step as many pollen grains may compete to reach the egg. The stigma plays a role in guiding the sperm to a receptive ovule, in the case of many ovules. Only compatible pollen grains are allowed to grow as determined by signaling with the stigma.
In the pollen grain, the generative cell gives rise to the sperm, whereas the vegetative cells have a tube cell that grows the pollen tube. Some plants have mechanisms in place to prevent self pollination, such as having stigma and anther mature at different times or being of different lengths, which significantly contributes to increasing genetic diversity of the next generation.
There is great variation in the rate of growth of pollen tubes and many studies have focused on signaling. The gene expression in the pollen grain has been identified as that of the gametophyte and not of the parental sporophyte, as it expresses its own unique mRNA and enzymes. In the peach tree, the style environment which the pollen tube grows through provides nutrition for the tube's growth to the ovule. Pollen tubes are tolerant and even pollen damaged by X-rays and gamma rays can still grow pollen tubes.
Growth and signaling
Pollen tube growth is influenced by the interaction between the stigma-style and the pollen grain. The elongation of the tube is achieved with elongation of the cytoskeleton and it extends from the tip, which is regulated by high levels of calcium in the cytosol. The calcium levels help the synaptic vesicles in the membranes grow and extend at the tip. Polypeptides found in the style also regulate growth of tube and specific peptides that play a role in signaling for growth have been identified.
The LURE peptides that are secreted from the synergids, which occupy the space adjacent to the egg cell, can use attractants. In mutant Arabidopsis plant embryos, specifically in those without the synergids, the pollen tubes were unable to grow. Pollen tube growth is toward eggs of the same species as the pollen. Intraspecific signaling helps fertilize egg and sperm of the same species. The signaling in the style is important as pollen tubes can grow without the presence of an embryo sac with just interaction with the style. Other parts in the ovary include cytoplasmic factors like miRNA and chemical gradients that attract the pollen tube to grow toward the synergids.
Calcium and ethylene in Arabidopsis thaliana are involved in termination of the pollen tube when it grows near the ovary. The increase in calcium allows the release of the two sperm cells from the tube as well as degeneration of a synergid cell. The chemical gradient of calcium can also contribute to termination early on in tube growth or at the appropriate time.
The length of the pollen tube varies by species. It grows in an oscillating fashion until it is ready to release the sperm near the egg for fertilization to take place. Some fast-growing pollen tubes have been observed in lily, tobacco, and Impatiens sultanii. The rate of growth confers advantage to the organism but it is not clear whether the variation in growth rate exists in the population or has been selected for over generations due to increased fitness.
Evolution
Many transitional features have been identified that show correlation between the evolution of the pollen tube with that of a non-motile sperm. Early seed plants like ferns have spores and motile sperm that swim in a water medium, called zooidogamy. The angiosperm pollen tube is simple, unbranched, and fast growing, however this is not the case for ancestral plants.
In gymnosperms like Ginkgo biloba and cycadophyta, a haustorial pollen tube forms. The tube simply soaks up nutrients from the female nucellus and grows in two stages. The pollen tube is highly branched and grows on the female sporophyte tissues. First, it grows the main tube followed by a more spherical tip at the end to allow the sperm to burst near the archegonia. The binucleated, multiflagellated sperm can then swim to the egg. Cycads have a less branched structured and the tip end swells the same way as in the ginkgo. In cycads, however, various enzymes have been identified in the pollen tube that direct growth and the nucellus tissues are more damaged with the tube growth.
In other phyla of gymnosperms, the Coniferophyta and Gnetophyta, the sperm is non motile, and the pollen tube delivers the sperm to the egg directly, in a process called siphonogamy. Conifers can be branched or unbranched and they cause degeneration of the female tissue as it grows through more tissue. Pines, for instance discharge cytoplasm of the sperm and union of the one sperm occurs as the other sperm degenerates. Yet, in Gnetophyta, there are features more similar to angiosperm pollen tubes where the tube reaches the egg with an early form of double fertilization. However, the endosperm does not form and the second fertilization is aborted.
In angiosperms, the mechanism has been studied more extensively as pollen tubes in flowering plants grow very fast through long styles to reach the well-protected egg. There is great variation in pollen tubes in angiosperms and many model plants like petunia, Arabidopsis, lily and tobacco plants have been studied for intraspecific variation and signaling mechanisms. In flowering plants, a phenomenon called polyamory can occur where many ovules are fertilized and overall fitness of the organism is yet to be studied with respect to rate of pollen tube growth.
Behavior
Pollen tubes are an excellent model for the understanding of plant cell behavior. They are easily cultivated in vitro and have a very dynamic cytoskeleton that polymerizes at very high rates, providing the pollen tube with interesting mechanical properties. The pollen tube has an unusual kind of growth; it extends exclusively at its apex. Extending the cell wall only at the tip minimizes friction between the tube and the invaded tissue. This tip growth is performed in a pulsating manner rather than in a steady fashion. The pollen tube's journey through the style often results in depth-to-diameter ratios above 100:1 and up to 1000:1 in certain species. In maize, this single cell can grow longer than to traverse the length of the pistil. The internal machinery and the external interactions that govern the dynamics of pollen tube growth are far from being fully understood.
Role of actin cytoskeleton