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«Expt. How do flowering plants do it without flagella? The journey to find an egg. What causes pollen grain germination and tube growth? File: ...»

2013 Plant Physiol Lab. Howard U (Sze) 1

Expt. How do flowering plants do it without flagella? The journey to find an

egg. What causes pollen grain germination and tube growth?

____________________________________________________________________________

File: F12-07_pollen Modified from E. Moctezuma & others for BSCI 442 (H. Sze, Univ Maryland)

Objectives:

 Observe. What are the parts of a flower?

What does each part look like and what is its job? Where are male and female organs?

What are pollen grains? What do they do?

 Test: what is needed for pollen to germinate and to grow a tube growth?

 How can one observe and measure germination and tube growth? Analyze results?

 Learn to present a short oral report to the class and participate in a group discussion.

Introduction Ancestors of land plants lived in marine or aquatic environment. These ancestors reproduced sexually by motile sperms that could swim to the egg. As plants colonized land, the reproductive structures evolved as fertilization could no longer depend on water. The largest and most successful plant group on land is the flowering plants, or angiosperms. From giant eucalyptus trees to minuscule duckweeds, from desert cacti to water lilies, these familiar plants dominate the world. They play a significant role in providing food and shelter for animals and other organisms. Angiosperms derive their name from the Greek words angion (vessel) and sperma (seed). The flower contains the vessel that houses the seed. It is also the organ for sexual reproduction.

How do land plants reproduce? Early land plant, like a moss, developed structures to protect gametes from drying. The egg is formed inside a protective vase-like structure with a long neck and a small opening at the top. Sperms formed in the male organ could swim down the neck to find the egg.

Other land plants evolved so the egg was formed and completely protected from the environment by layers of cells that sealed the egg from the outside. How can sperms reach such eggs? How can sperms from one plant fertilize another when they are physically apart. Answer: sperms are delivered to another plant inside a pollen grain, a structure that protects the sperms from drying out as they are carried over long distances. To break the seal that entombs the egg, each pollen grain grows a tube that can navigate through the female tissues and then target an ovule containing an egg.

To understand this process, look at a flower. A typical flower is composed of modified leaves arranged in four concentric circles on the end of an

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Pollen Development The diploid male sex organ of the flower, the anther, encloses four compartments containing microspore mother cells. Each one of these cells undergoes meiosis to produce haploid microspores (1n). The haploid microspores grow and mature into haploid male gametophytes, known as pollen grains, which are released and travel to the female. The pollen houses the sperm gametes.

Embryo Sac Development The female ovary contains the diploid female sex organs of the flower called the ovules.

Specific cells in the ovules undergo meiosis to produce haploid megaspores. One of these megaspores grows and develops into the female gametophyte or embryo sac within which develops an egg cell.

Flowering Plant Pollination Pollination, the transfer of pollen from the male anther to the female stigma, must occur before seed plants can reproduce sexually. Pollen grains have a limited life span. Depending on the species, pollen may be viable for only a few hours up to several weeks after it is released from the anther. Many plants rely on wind to aid in pollination. However, most flowering plants use animal agents to transfer pollen.

A primary function of flowers is to attract pollinators with colorful petals, scent, nectar, and pollen. In exchange for moving their pollen around, flowers have evolved many rewards to attract pollinators. The most common attraction is food, either nectar (a sugar solution) or pollen, which is high in protein. In many plants, nectar is produced in nectaries most commonly found in the flowers.

2013 Plant Physiol Lab. Howard U (Sze) 3 Pollen tube growth and Fertilization After landing on the right stigma, the pollen grain germinates. The sperm in the pollen grain must be delivered to the egg, located in the ovary. The pollen grain grows a tube from the stigma, through the style, and down to the ovary. When the tube finds and reaches the ovule, the tip of the tube bursts. The two sperms are released into the ovule. One sperm fertilizes the egg in the embryo sac to produce a diploid zygote (2n), which grows into an embryo enclosed in a seed. The other sperm fuses with the two polar nuclei to form the endosperm that provides food for the developing embryo. This event is called double fertilization and is unique to flowering plants. Thus, seeds are the mature ovules that contain the embryos and the ovary develops into the fruit, a structure for seed dispersal.

When the seed germinates in the soil, the young embryo uses the energy stored in the cotyledons to start growth. One of the reasons that flowering plants are so successful today is that their seeds and fruits are adapted for dispersal in many different ways.

Materials Lilies are in bloom – tiger, yellow, red all v. good

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Nutrients:

1. Prepare the test solutions 20 ml/each. Each group will need a) complete medium, and a medium lacking one nutrient. Each group will need 4 petri plates.

2. Take 2 tiny (35 mm) petri plates. Dust pollen over the bottom of the plate.

3. Add drops of the control solution onto the pollen of one dish, and a test solution in the second dish. Look at the dishes using the scanning (4X or 10 x) objective and find a spot where there is a lot of pollen. [or Use dissecting scope]

4. Examine at low magnification. Change to the low power objective (10X) and watch the pollen.

After about 15 minutes, you should see pollen tubes starting to form. The first evidence of this is a small bump on the side of a pollen grain.

5. Examine at higher magnification. When germination starts, take out a sample and place on a slide with a coverslip so you can look at it with 40x. If you see a pollen grain germinating, center it in your field of vision and change to the high-power objective (40X) to watch the tube grow. You can probably see the cytoplasmic streaming within the tube. [Many pollen tubes will break apart (lyse) because the tube membrane is too thin to hold the cell together in this sucrose solution. However, it would not lyse if it were growing in a flower’s style.]

6. Analyze results a. Determine % germination. (Time at 30 min). In a field, count the number of total pollen grains, and the number of those showing emerging tube. (total count 20) Express results as % grains germinated at __ specified time.

Put in a table showing % Germination b. Measure tube lengths of pollen tubes. (Time at 1- 1.5 h) Measure the relative length of tubes (count 20 tubes for controls, and 20 for a treatment).

*Hint: Take pictures. Save and measure according to pixels with Image J.

Record each length and show range of lengths. Indicate frequency of each length.

–  –  –

Discuss & include in your report:

1. What do results show? How does the absence of sucrose, nitrate, calcium or B affect pollen germination and tube growth. How do you interpret the results?

2. What purpose(s) does sucrose serve? Give at least 2 distinct examples.

In the plants, where does the sucrose in the flower come from?

3. What is a likely reason for NO3- requirement?

Propose a likely fate of nitrate from its uptake to its incorporation into a macromolecule. Show specific reactions.

4. Pollen tube growth is fairly fast, and reaches the ovule in a few h or less. Why?

Ref.

Taiz & Zeiger 2006 or 2010. ‘Plant Physiology’ book (Sinauer) chap. 16 Fig. 2. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates and WH Freeman Moctezuma E (2008). Manual for Plant Biology Lab (U. Maryland, College Park)



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