STUDY OF POLLEN GRAIN, POLLEN MORPHOLOGY, POLLEN GERMINATION, AND POLL

Views:
 
     
 

Presentation Description

No description available.

Comments

Presentation Transcript

PowerPoint Presentation:

STUDY OF POLLEN GRAIN, POLLEN MORPHOLOGY, POLLEN GERMINATION, AND POLLEN VIABILITY

Pollen grain :

Pollen grain Introduction Pollen is a fine to coarse powder containing the micro gametophytes seed plants, which produce the male gametes (sperm cells). Pollen grains have a hard coat that protects the sperm cells during the process of their movement between the stamens to the pistil of flowering plants or from the male cone to the female cone of coniferous plants. When pollen lands on a compatible pistil of flowering plants, it germinates and produces a pollen tube that transfers the sperm to the ovule of a receptive ovary. The individual pollen grains are small enough to require magnification to see detail.

Structure of pollen:

Structure of pollen Pollen itself is not the male gamete, but each pollen grain contains vegetative (non-reproductive) cells (only a single cell in most flowering plants but several in other seed plants) and a generative (reproductive) cell containing two nuclei: a tube nucleus (that produces the pollen tube) and a generative nucleus (that divides to form the two sperm cells). The group of cells is surrounded by a cellulose-rich cell wall called the intine, and a resistant outer wall composed largely of sporopollenin called the exine.

PowerPoint Presentation:

Pollen is produced in the microsporangium (contained in the anther of an angio spermflower, male cone of a coniferous plant, or male cone of other seed plants). Pollen grains come in a wide variety of shapes (most often spherical), sizes, and surface markings characteristic of the species (see electron micrograph at top right). Pollen grains of pines,firs , and spruces are winged. The smallest pollen grain, that of the Forget-me-not ( Myosotis spp .), is around 6 µm (0.006  mm) in diameter. Wind-borne pollen grains can be as large as about 90-100 µm. The study of pollen is called palynology and is highly useful in paleoecology , paleontology, archeology, and forensics.

PowerPoint Presentation:

In angiosperms, during flower development the anther is composed of a mass of cells that appear undifferentiated, except for a partially differentiated dermis. As the flower develops, four groups of sporogenous cells form with in the anther, the fertile sporogenous cells are surrounded by layers of sterile cells that grow into the wall of the pollen sac, some of the cells grow into nutritive cells that supply nutrition for the microspores that form by meiotic division from the sporogenous cells. Four haploid microspores are produced from each diploid sporogenous cell called a microsporocyte, after meiotic division.

PowerPoint Presentation:

After the formation of the four microspores, which are contained by callose walls, the development of the pollen grain walls begins. The callose wall is broken down by an enzyme called callase and the freed pollen grains grow in size and develop their characteristic shape and form a resistant outer wall called the exine and an inner wall called the intine. The exine is what is preserved in the fossil record.

PowerPoint Presentation:

The pollen wall protects the sperm nucleus while the pollen grain is moving from the anther to the stigma, it protects the vital genetic material from drying out and solar radiation. The pollen grain surface is covered with waxes and proteins, which are held in place by structures called sculpture elements on the surface of the grain. The outer pollen wall prevents the pollen grain from shrinking and crushing the genetic material during desiccation and it is composed of two layers. These two layers are the tectum and the foot layer, which is just above the intine . The tectum and foot layer are separated by a region called the columella , which is composed of strengthening rods. The outer wall is constructed with a resistant biopolymer called sporopollenin . The pollen tube passes through the wall by way of structures called apertures.

PowerPoint Presentation:

Pollen apertures are any modification of the wall of the pollen grain. These modifications include thinning, ridges and pores, they serve as an exit for the pollen contents and allow shrinking and swelling of the grain caused by changes in moisture content. The elongated apertues / furrows in the pollen grain are called colpi ( s. colpus ) which along with pores, are a chief criteria for the identifying pollen classes. Pollen grains may have furrows, the orientation of which (relative to the original tetrad of microspores) classify the pollen as colpate or sulcate . The number of furrows or pores helps classify the flowering plants, with eudicots having three colpi ( tricolpate ), and other groups having one sulcus .

PowerPoint Presentation:

Except in the case of some submerged aquatic plants, the mature pollen-grain has a double wall, a thin delicate wall of unaltered cellulose (the endospore or intine) and a tough outer cuticularized exospore or exine. The exine often bears spines or warts, or is variously sculptured, and the character of the markings is often of value for identifying genus, species, or even cultivar or individual. In some flowering plants, germination of the pollen grain often begins before it leaves the microsporangium, with the generative cell forming the two sperm cells.

  Pollen germination :

Pollen germination Another germination event during the life cycle of gymnosperms and flowering plants is the germination of a pollen grain after pollination. Like seeds, pollen grains are severely dehydrated before being released to facilitate their dispersal from one plant to another. They consist of a protective coat containing several cells (up to 8 in gymnosperms, 2-3 in flowering plants). One of these cells is a tube cell. Once the pollen grain lands on the stigma of a receptive flower (or a female cone in gymnosperms), it takes up water and germinates. Pollen germination is facilitated by hydration on the stigma, as well as by the structure and physiology of the stigma and style. Pollen can also be induced to germinate in vitro (in a Petri or test tube).

PowerPoint Presentation:

During germination, the tube cell elongates into a pollen tube. In the flower, the pollen tube then grows towards the ovule where it discharges the sperm produced in the pollen grain for fer The germinated pollen grain with its two sperm cells is the mature male microgametophyte of these plants

PowerPoint Presentation:

Self-incompatibility Since most plants carry both male and female reproductive organs in their flowers, there is a high risk of self-pollination and thus inbreeding. Some plants use the control of pollen germination as a way to prevent this self-pollination. Germination and growth of the pollen tube involve molecular signaling between stigma and pollen. In self-incompatibility in plants, the stigma of certain plants can molecularly recognize pollen from the same plant and prevent it from germinating.

PowerPoint Presentation:

Pollen tube growth and guidance Once the pollen interacts with the silk/stigma and begins to hydrate, a pollen tube is extruded through the pollen tube. At a growth rate of some 0.5 cm/h in maize, the pollen tube grows rapidly through the transmitting tissue aiming to reach the female gametophyte via the micropyle region of the ovary.

PowerPoint Presentation:

We are interested in genes regulating pollen tube growth and have applied a functional genomics approach to identify oligopeptides involved in guiding the tube to the female reproductive cells. Our recent experiments have shown, that peptides secreted by the female gametophyte are involved in the final stage of pollen tube attraction.

PowerPoint Presentation:

when pollen reaches the stigma, it grows a pollen tube to extend down the style. Two sperm nuclei enter the ovule: one fuses with the egg to produce the zygote, the other fuses with two polar nuclei to make the 3n endosperm . The endosperm is part of the seed and provides nutrition for the sporophyte embryo. The fusion of two sperm nuclei with megagametophyte nuclei is called double fertilization.

PowerPoint Presentation:

Germinatinpollen grainsg

Pollen development in flowering plant:

Pollen development in flowering plant Pollen grains develop in the anthers of the staminae. In the anthers (figure A and detailed view D) mostly four, but sometimes only two loculi are present. In the loculi sporogenic tissue (from the Greek spora = seed and the Latin generare = produce) can be found from which pollen develop. At the inner side of each loculus a layer of large, rectangular cells, the tapetum (from the Greek tapes = carpet; t in figure D) can be found. The tapetum serves for the nutrition of the developing pollen, the deposition of cell wall material in the outer part of the pollen grain and other compounds in and over the wall.

PowerPoint Presentation:

First, free pollenmothercells (PMC) are formed, which become spores by a meiotic division (see example of a meiotic stage in figure B). The meiosis involves two divisions, which lead to the formation of four daughter cells, the spores. Those four cells are originally still interconnected and are called tetrads (Greek Tetra = four; figure C). Later they come apart and the tapetum deposits the outer wall or exine (more on the pollen wall in pollen morphology). The exine protects the spore against dessication, mechanical pressure and ultraviolet radiation (figure F).

PowerPoint Presentation:

Sometimes the exine layer is covered by sticky substances which are also produced by the tapetum. This adhesive material facilitates the attachment of pollen grains to insects, and in this way also zoophilic pollination. It also plays an important role in the adhesion of pollen grains to the female stigma and in the recognition between pollen and pistil. Also substances responsible for pollen allergy are often products originating from the tapetum

PowerPoint Presentation:

A. Cross-section through an anther of Lilie ( Lilium )sp.) with on the left and the right side two loculi each. In the loculi sporemothercells (SMCs) can be seen from which the four spores develop through meiosis I and II. In between the loculi of each pair a thin layer of cells (arrow) is visible along which the loculus can burst open at maturity and release the pollen grains. In the middle the cross-sectioned filament (Fi) to which the anther is attached is indicated. In the upper part the vascular bundle (v) of the loculus can be distinguished. B. Loculus. The lumen contains developing pollen. On the inner wall (w) of the loculus a layer constitued of block-shaped single cells is present, the tapetum (t). The tapetum feeds the developing spore and -later- pollen.

PowerPoint Presentation:

C. Tetrad stage during pollen development. After the two meiotic divisions the four daughter cells are still interconnected and form a tetrad. They are still surrounded by the wall (arrow) of the original cell, the microspore mother cell (MMC). D. Mitotic division in the spore leading to the formation of a microgametophyte or pollen. Only the metaphase is shown here. The chromosomes lay in the equatorial plane of the cell. E. Nearly ripe pollen grain: visible are a vegetative cell with nucleus (VN), which later will form the pollen tube, and a generative cell with its own nucleus (GN), which later will divide into two sperm cells.

PowerPoint Presentation:

F. Ripe pollen grain in which the texture of the outer cell wall, the exine, can be recognized. The grainy dark purple structure in the middle of the pollen grain is the vegetative nucleus. G. Diagram in 3 parts: Ripe pollen grain consisting of the vegetative cell (VC) and therein the smaller generative cell (GC). After landing on the stigma (St) the pollen grain germinates and forms a pollen tube. In the pollen tube the generative cell divides into two sperm cells (SC). The pollen tube grows to the embryo sac (ES) and delivers the two sperm cells that are involved in double fertilization.

  Pollen viability :

Pollen viability Viability is defined as the ability to live, develop, or in the case of plants, to germinate when conditions favorable to the plant exist. Measuring Viability One method scientists use is to stain collected anthers, the male reproductive organ that holds the pollen, in aniline blue dye. The dye will be absorbed by the viable pollen grains. A slide is prepared and the dyed grains are counted under a microscope.

PowerPoint Presentation:

Pollen Size Scientists sometimes use electron particle counters to analyze pollen for viability. They have learned that viable pollen grains are larger than the sterile grains so the counters only count the particles that are of a certain size. Pollen Storage Though pollen has the best chance of being viable when it is fresh, some horticulturalists cultivate pollen and store it. Pollen intended for use outside of a one- or two-week window when the grains are considered viable are refrigerated

Assessing pollen viability :

Assessing pollen viability frequently used to assess pollen viability. methods, a more extensive overview has recently been provided by Dafni and Firmage The fastest way of analyzing pollen viability is using vital stains that react with pollen enzymes, thereby indicating the presence of intact cellular contents. The most commonly used vital stain is the fluorochromatic reaction (FCR) which reveals esterase activity in pollen with an intact plasma membrane This test can be performed very quickly in the laboratory, and often the pollen viability results correlate with seed set. However, in a number of experiments false 13positives were reported or no relation with seed set could be established. In general, vital stains frequently produce false positives leading to an overestimation of pollen viability.

PowerPoint Presentation:

Another frequently used method to assess pollen viability is in vitro germination. Because pollen grains of many species will easily germinate in a medium that contains boric acid and an osmoticum, this method is widely used (Taylor and Hepler, 1997). Furthermore, in recent years protocols have been developed that allow high germination frequencies of pollen of the more recalcitrant pollen. However, despite the simple basic requirements of pollen tube growth media, the optimal composition may vary from species to species and the use of suboptimal media may underestimate pollen viability.

PowerPoint Presentation:

Still, pollen germination rates usually provide more reliable data on pollen viability than vital stains. Finally, pollen viability may be measured after pollination, by analyzing germination on the stigma or seed set derived from that pollination. Both methods are time consuming and may lead to an overestimation of pollen viability if the pistil is overpollinated. Although all methods are valuable with respect to predicting seed set, they are prone to errors and results should be interpreted carefully. Furthermore, obtaining accurate pollen viability rates may depend strongly on storage conditions, protocols used and other factors. Because of these reasons, it is difficult to compare the results obtained in individual experiments.

PowerPoint Presentation:

Factor affecting pollen viability Relative humidity Low humidity during pollen storage usually has a positive effect on pollen viability. Notable exceptions include species of the Gramineae family Temperature Storage at low temperatures usually has a positive effect on pollen viability Atmosphere composition Increased CO has a positive effect of pollen viability after storage Oxygen pressure Decreased oxygen during storage has a positive effect on pollen viability

PowerPoint Presentation:

UV-B radiation The potential damage by UV-B radiation to the pollen during the transfer from the anther to the stigma may be reduced by flavonoids present in the pollen wall (Stapleton and Walbot, 1994). In addition, dispersal of pollen clustered in some way may prevent the inner pollen grains from radiation damage (Dafni and Firmage, 2000). Significant reductions in pollen germination rates were found for 5 species only, while more than 50% showed a significant reduction in pollen tube lengths.

PowerPoint Presentation:

Transport One of the crucial events in the fertilization process of higher plants is the transfer of pollen grains from the anther to the stigma, often of another flower. In the case of wind pollinators, the transport of pollen trough the atmosphere will probably have similar effects as exposure to the environment, which was shown to rapidly decrease viability of maize pollen (Luna et al., 2001; Aylor, 2003, 2004).

PowerPoint Presentation:

Thank you