Chapter 2: reproduction in plants: the birds and the bees, fruits, and seeds.
caryopsises dehiscent DNA fingerprinting drupe endocarp exocarp [F.sub.1] hybrid gamete gene homozygote inbred indehiscent mesocarp nut open-pollinated seeds parthenocarpy perfect pericarp pollen sac quantitative trait self-incompatible self-sterile tube nucleus zygote
People are often surprised to learn that plants have sexual organs that correspond to those on animals. Even some of the terminology is the same. For example, the reproductive cells, or gametes, of plants are known as egg and sperm cells. They are female and male, as they are in animals. Although there are many differences in reproductive systems between plants and animals, one of the most obvious is that many plants are hermaphroditic, meaning they have both male and female structures within each flower. The botanical term for this is perfect. This chapter reveals these details and others concerning sexual reproduction of plants.
PLANT REPRODUCTIVE ANATOMY
A typical perfect flower is composed of four floral organs: sepals, petals, stamens, and pistil(s). Of these, the stamen and pistil are involved in sexual reproduction. The pistil is composed of the stigma, style, and ovary. The ovary is the bulbous basal portion of the female floral organ, the pistil. One egg cell (or egg nucleus) and seven accompanying nuclei develop inside ovules, which are located inside the ovary (Fig. 2-1a). Sperm cells develop inside pollen sacs. These are inside the anthers on the male floral organ, the stamen (Fig. 2-1b).
DEVELOPMENT OF FLORAL ORGANS
Flowers develop in the shoot meristem when the plant has reached reproductive status. This can occur a few weeks after germination in some species or as long as 10 years or more for some woody plants. In a typical herbaceous plant, observable changes occur in the meristem as it develops from a vegetative structure producing leaves to a reproductive one producing flowers. (Fig. 2-2). The last stage of the floral development process is anther dehiscence, or splitting open, and pollen release. This initiates the pollination process.
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POLLEN TRANSFER: POLLINATORS AND MODES OF POLLEN TRANSPORT
The birds and the bees are only two groups that contribute to the cycle of life with regard to plant reproduction by transferring pollen from flower to flower. The wind plays an important role for many species. Other pollinators include bats, butterflies, and moths and, in some cases, monkeys. Flowers have various means of attracting pollinators and still others for ensuring the transfer of pollen (Fig. 2-3).
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Fragrance attracts butterflies, whereas bright colors attract hummingbirds and bees. Nectar is a reward for bees, hummingbirds, ants, and others. Some orchids mimic a female insect to such a degree that the male of the species will attempt to mate with the flower and get loaded with pollen instead. When he makes the same mistake on another flower, the pollen transfer is completed!
Pollen may be carried on special appendages on bees' legs or on the hairs of a butterfly. Some pollen is sticky and will not travel well in the wind, whereas other pollen is very light and can travel for many miles on the wind. Pollen is a form of storage for the genetic material of a plant. As such, it has developed to withstand adverse environmental conditions to survive until it lands on the appropriate flower and can germinate. Very old pollen can often be found at archeological sites and is a useful indicator of the types of plants humans used at the time.
POLLINATION AND DOUBLE FERTILIZATION
Plants reproduce sexually during the process of double fertilization. It is double because two sperm cells are involved in fertilization. If this is not confusing enough, three female cells are fertilized. Only one of the female cells is an egg cell. The others are called polar nuclei, which have become enclosed in a membrane together, forming the endosperm mother cell.
The process is as follows: A pollen grain lands on the stigmatic surface of the pistil. A pollen tube begins to grow from a pore in the pollen and a tube nucleus and two sperm nuclei travel along the tube as it grows until it reaches the micropyle, the opening into the ovule cavity. At this point the tube nucleus disintegrates and one sperm nucleus fertilizes the egg nucleus and the other sperm nucleus fertilizes the endosperm mother cell (Fig. 2-4). The fertilization of egg by sperm results in a diploid zygote, whereas the union of sperm and endosperm mother cell results in a triploid primary endosperm cell. Together these will develop into a seed.
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DEVELOPMENT OF SEEDS FROM OVULES AND FRUITS FROM OVARIES
After double fertilization, the ovule contains both embryo and endosperm. In seed development, mitotic cell division contributes to rapid growth in the endosperm and embryo. A fully developed seed usually has three components: embryo, endosperm, and seed coat (Fig. 2-5).
The seed development process requires a great deal of food and energy. Endosperm is nutritive tissue to support either the developing embryo or the germination process (Fig. 2-6). Endosperm often has high levels of nutrients for animal and human consumption as well. For example, oilseed crops have high levels of oils stored in endosperm tissue. Other seeds, such as peanuts, corn, and walnuts have high levels of carbohydrates that are valued as food and feed sources.
Seeds develop from ripened ovules and fruit develops from a ripened ovary (Figs. 2-7 and 2-8). During seed development, the ovary wall matures into fruit. Although we are most familiar with fleshy fruits such as grapes and watermelon, some fruits are dry. Dry fruits include true nuts, such as acorns, or pods, such as those found on peanuts (See Figures 2-9a-p for examples and descriptions of each fruit type.).
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Fruits may be simple or compound or fleshy or dry. Compound fruits are either multiple or aggregate. Simple fruits form from a single ovary. Multiple fruits derive from separate, but closely spaced, flowers on an inflorescence. A good example of this is pineapple. Aggregate fruits derive from a single flower that has numerous pistils. Raspberries and strawberries are examples. Dry fruits may be dehiscent (split open when ripe) or indehiscent (do not split open).
When the ovary wall develops into fruit, it may have distinct layers that together comprise the pericarp (peri- surrounds + carp carpel). The layers are exocarp (exo- outer), mesocarp (meso- middle), and endocarp (endo- inner). When the endocarp of a fleshy fruit is hard, or stony, the fruit is classified as a drupe. Seeds of dry fruits may be fused to the ovary wall, forming a single unit, as in true nuts and caryopsises.
Occasionally, fruit develops in the absence of pollination and fertilization. The process is called parthenocarpy. This is a rare occurrence and is only known in a handful of species. In other cases, pollination occurs, and fruit develops but seeds fail to develop. For example, bananas and seedless watermelons do not have seeds. In seedless grapes, seeds begin to develop, but their development is aborted early in the process, while the fruit continues to develop. In fruits containing multiple seeds (ovules), deformed fruit may develop if all the ovules are not fertilized or if development of some seeds is arrested. This may happen in strawberries, green peppers, and pineapples.
PROBLEMS WITH POLLINATION: SELF-INCOMPATIBILITY, POLLEN STERILITY
Some plants will not set seed when they are self-pollinated, and they are said to be self-sterile. The pollen is self-incompatible with the pistil. Apples and plums are self-incompatible plants, among others. Occasionally, pollen sterility occurs. This can be a useful trait for seed producers who want to prevent self-pollination and enforce cross-pollination.
Plant breeding has been practiced by humans for centuries. The person famous for his discovery of genetic inheritance was Gregor Mendel (see box). Perhaps the most prolific American plant breeder was Luther Burbank (see box). Thomas Jefferson supported the right of individuals to obtain patents for improved cultivars of plants. He said: "The greatest service which can be rendered any country is to add a useful plant to its culture." Nevertheless, patenting of plants was not allowed until 1930 when The Plant Patent Act was enacted, and, even then, patents were only applicable to asexually reproduced cultivars (except tubers). Sexually reproduced plants were not legally protected in the United States until 1970 in The Plant Variety Protection Act (PVP Act).
Inbred plants result from natural or forced self-pollination. Plants differ from animals in that they can sometimes tolerate inbreeding without deleterious effects. However, sometimes inbreeding results in nonviable seed, diminished growth and performance, or plant death because of so-called deleterious genes.
Many plants are available as F1 hybrids that offer improved flavor, color, height, disease tolerance, and pest resistance. The first filial generation (offspring) between two inbreds (homozygotes) is designated an F1 hybrid. Many horticultural plants, including geraniums, tomatoes, potatoes, broccoli, and snapdragons, have been improved in this way. Typically, F1 hybrids display a high level of genetic similarity.
When plants freely cross-pollinate with any number of other plants within their species and set seed, they produce open-pollinated seeds or O.P. seed. The resultant plants will often contain wide genetic diversity. Offspring may vary in traits such as height, flower color, tolerance to cold or heat, disease and pest resistance, and many other factors. Heirloom seeds are often open-pollinated lines or strains that gardeners collect and save each year. This is a common practice in many areas of the world, but in developed countries, people often purchase new seeds each year because they want the improved yield and quality that new, improved cultivars can produce. But open-pollinated seeds have the advantage of being free while still producing a crop with known characteristics.
GENETICALLY MODIFIED PLANTS
The molecule that carries the genetic code-deoxyribonucleic acid, or DNA--was identified by Watson, Crick, and Franklin in 1953. Since then, we have come to understand the genetic recombination process much more completely. One outcome is that we can reduce the amount of time required to make progress in plant improvement. For example, we now know how to locate a segment of DNA responsible for a particular trait (a gene), excise it from the DNA, grow more of it in vitro, and reintroduce it into another plant. When that gene is expressed in the plant into which it has been introduced, the result is a genetically modified plant that is often of increased commercial value. (Table 2-1 lists commercially important horticultural crops with altered genes.)
Another outcome of our understanding of the molecular basis of genetics is the use of DNA fingerprinting as a tool for selection, especially of quantitative traits, or those that are determined by more than one or two genes, and in seedlings or juvenile plants. Seedlings or juvenile plants that resulted from controlled crosses can now be evaluated at a very early stage on a molecular basis rather than waiting to see what traits the mature plant will exhibit. This early evaluation can save valuable time and the expense of growing plants in the field. Much research remains to be done in this important area.
Plants reproduce sexually through the floral organs stamens and pistils. Stamens produce pollen and are considered to be the male part of a flower. Pistils are female and contain eggs in their ovule, located at their base. The result of pollination and fertilization is a seed, or ripened ovule. Seeds are contained within the ripened ovary, or fruit. Insects and other animals, wind, and even water facilitate the transfer of pollen. Plants that will not self-pollinate may be self-incompatible or self-sterile. They may also have sterile pollen.
Plant breeding has been practiced for many years, with increased understanding of the genetic processes occurring over the last century. The results of these advances are improved hybrids and genetically modified plants.
* Using large flowers, such as lilies, dissect the flowers and observe the various floral organs. Mount them with tape to card stock and then label the important parts.
* Observe pollen under a microscope. Compare different types of pollen. Identify the surface features and pores. Is all the pollen from different plants of the same species alike?
* Germinate pollen grains on an agar medium in a Petri dish. Try several different kinds of plants and see if some germinate better than others. Are there differences in the amount of time required for germination? Does the pollen from some plants fail to germinate in the Petri dish?
1. What are reproductive cells also called? What are male and female cells also called?
2. How many main floral organs are there? Name them.
3. How many nuclei form in a pollen grain and what are they called?
4. How many nuclei form in an ovule and what are they called?
5. Discuss how pollen is dispersed and differences in pollen that result in different modes of dispersal.
6. What are the three basic components of a seed?
7. What floral organ or tissue develops into a seed? Fruit?
8. What type of seed exhibits wide genetic diversity? What type exhibits a high level of similarity?
9. List three modes used to prevent inbreeding in plants and discuss how they work.
10. Contrast and compare qualitative and quantitative traits in plants.
Allard, R. W. (1999). Principles of plant breeding. New York: Wiley.
Attenborough, D. (Writer) (1996). The birds and the bees [videocassette]. In BBC (Producer), The private life of plants (Vol. 3). London BBC.
Briggs, F. N., & Knowles, P. F. (1967). Introduction to plant breeding. Davis, CA: Reinhold.
Copeland, L. L., & Armitage, A. M. (2001). Legends in the garden. Atlanta, GA: Wings.
Henig, R. M. (2000). The monk in the garden. New York: Houghton-Mifflin.
Two Famous Men: Gregor Johann Mendel and Luther Burbank
Gregor Johann Mendel
Born Johann Mendel in 1822 in Heizendorf, Austria, to peasant farmers, Mendel attended elementary school and proceeded on to college where he studied mathematics and biology. He was ordained into the Augustinian order in 1847 and took the name Gregor. Although he failed the teacher certification examination, some attribute this failure, in part at least, to test anxiety. He taught natural science to high school students at the school run by the monastery. Mendel was clearly a lover of plants. In most of his official monastery photographs he is holding a flower-usually a fuchsia, his favorite. He was also interested in meteorology and theories of evolution. He was familiar with Darwin's work and may have sent Darwin a reprint of his paper (the English translation is titled "Experiments in Plant Hybridization"), but Darwin apparently never read it. (Darwin's own Origin of Species was published in 1859.) In the years from 1856 to 1863 Mendel made his now-famous controlled crosses on peas in the monastery courtyard garden. He was trying to discover whether traits were stable from one generation to the next. He made two important discoveries: the Law of Segregation and the Law of Independent Assortment. His strength in mathematics allowed him to see the pattern that emerged from his controlled crosses. He presented the results of his research on peas in 1865 at two meetings of the Brunn Society for the Study of Natural Sciences. His research findings were published by the Society in its official Proceedings in 1866. In 1868 he was appointed Abbot at The St. Thomas Augustinian Monastery in Brunn, Austria. In 1884, Mendel died.
In the Law of Segregation, Mendel proposed that, somehow, the traits to be passed from parents to offspring were reduced from two copies to one. In that way, offspring received one copy of each factor determining a trait. Cell division resulting in egg and sperm cells was not understood for another 25 years. In the Law of Independent Assortment, Mendel proposed that each factor for a trait was passed on independently from other factors. Chromosomes were proven to be the material for independent assortment in 1905, 40 years after Mendel presented his work and 21 years after he died.
Luther Burbank was a contemporary of Mendel's, but his life was very different. Yet, the two men shared one love: a love of plants. Burbank, born in 1849 in Massachusetts, began experimenting with plants at a young age. His first commercial development, a new russet-type potato that was a particularly good baking potato, is still popular today! (In fact, these potatoes are still called russet Burbanks in many stores.) He sold all the seed potatoes from it to a salesman for $150.00 and used this money to finance his trip to California. He settled in Santa Rosa, where he set up a farm for breeding plants. He worked on a large number of different species of plants, including peaches, berries, and lilies. Burbank was not a good record-keeper, and, perhaps for this reason, a complete list of his inventions does not exist. At least 800 improved cultivars are attributed to Burbank. He invented the 'Shasta' daisy, 'Santa Rosa' plum, and the 'Burbank' rose. Many of his inventions had staying power and became standards in the field. Some are still grown commercially today.
Burbank knew and met with other leaders of his day. He was friends with Henry Ford and Thomas Edison. He was outgoing and energetic, compared with Gregor Mendel, who was more reticent and self-effacing. Burbank was also familiar with Darwin's theories on evolution, and he agreed with them, even though they were still controversial at that time. (The Scopes trial was conducted in 1925.) Burbank died in 1926. In the eulogy given by Judge Lindsey, these words were used to describe Burbank: "We have advanced on the road of progress and now he is to be forever known as one of the world's greatest benefactors. In the creative improvement of plant life, he is our greatest genius." Because of the lack of intellectual rights protection for plant breeders at the time, Burbank held no patents and received no royalties for the contributions he made. His Santa Rosa home is now open to visitors.
Dr. Marietta Loehrlein currently teaches horticulture classes at Western Illinois University in Macomb, Illinois. She earned both her bachelor's degree in Agronomy and her master's degree in Plant Genetics at The University of Arizona. Her master's research project was concerned with germination problems associated with triploid seeds, from which seedless watermelons grow. Following that she worked for 5 years in a breeding and research program for Sunworld, International near Bakersfield, California. She worked with peaches, nectarines, plums, apricots, and cherries. Then she returned to school to earn her Ph.D. in Horticultural Genetics at The Pennsylvania State University. Her Ph.D. research focused on flowering processes in regal pelargonium (also called Martha Washington geraniums). While at The Pennsylvania State University, she bred a new cultivar of regal pelargonium, "Camelot." At Western Illinois University, Dr. Loehrlein teaches nine courses: Greenhouse and Nursery Management, Introductory Horticulture, Landscape Design, Landscape Management, Home Horticulture, Plant Propagation, Turf Management, and two courses in Plant Identification.
TABLE 2-1 List of Genetically Modified Horticultural Crops of Commercial Importance CROP MODIFIED TRAIT Carnation Color-alteration (blue) Corn Insect/herbicide resistance, male sterile Papaya Virus resistance Potato Insect resistance, virus resistance Radicchio Male sterile Rice Iron fortification, provitamin A enriched Squash Virus resistance Tomato Modified fruit ripening, -carotene, lycopene enriched, insect resistance
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|Author:||Loehrlein, Marietta M.|
|Publication:||Home Horticulture: Principles and Practices|
|Date:||Jan 1, 2008|
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