Plants & Human Affairs

Plants & Human Affairs (BIOL106) - Stephen G. Saupe, Ph.D.; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; ssaupe@csbsju.edu; http://www.employees.csbsju.edu/ssaupe

PLANT REPRODUCTION

I. Reproduction
In an abstract sense, reproduction serves as the mechanism by which an individual passes genetic information through time. According to Harold Morowitz, one of my favorite science writers (he’s a biochemist), "an individual is the transient caretaker of the genetic instructions that must be conveyed down the generations". So, how do plants convey instructions from one generation to the next?

II. Plants can produce offspring without sex (asexual reproduction)
"A" means without; therefore asexual literally translates into reproduction without sex. But, exactly what is "sex"? - biologically "sex" is defined as the process that involves meiosis and fertilization. Thus, during asexual reproduction there is no production of gametes, no fertilization and hence, no genetic recombination. Obviously the offspring will be genetically identical to the parents (clones). Some points to ponder:

Plants excel at reproduction without sex. Asexual reproduction is much less common in animals.
Plants can reproducing asexually by: (1) producing new "offspring" directly from buds (meristematic regions) on stems (i.e., rhizomes, stolons, tubers, bulbs, corms), roots or leaves (i.e., maternity plant); (2) parthenogenesis - fruit development without pollination and/or fertilization; (3) apomixis - seed production without sex; and (4) fragmentation - cells in plant tissues can differentiate into new individual (i.e., leaf cuttings).
The offspring are "clones" or genetic carbon copies of the parents.
Plants practice "safe sex".
In other words, asexual reproduction is a much safer bet for producing an offspring because an individual doesn't have to rely on the "chanciness" of the sexual process.
One potential disadvantage.
In theory, asexual reproduction should limit genetic variation in a population, and hence, would not be a disadvantage in the long-term evolutionary sense.
Taking advantage of asexual reproduction.
Gardeners have long taken advantage of the asexual reproduction process in plants. If you’ve ever taken a cutting from a plant or grafted a plant, you have, too. Tissue culture is a high-tech version of the same process. Individuals cells of a plant can be grown aseptically in culture medium. Initially the cells form an undifferentiated cluster of cells, called a callus. Essentially it's like a plant "cancer". These clumps can be divided up transferring them to other cultures producing literally thousands of genetically identical cell masses from an original cell. Then, when given the appropriate conditions and hormones, these calluses will differentiate into an entire new plant! Cool, eh? Using various techniques (a DNA gun or, a bacterial vector) DNA carrying genes of interest can be inserted into the plant cells and new plants grown. My favorite example is the tobacco plant that had the gene(s) for firefly luciferase. The plants glowed in the dark when watered with ATP. More on this to come.

III. Plants have sex, too

Sexual Reproduction
Iinvolves the production of haploid gametes followed by fertilization to produce a zygote which in turn develops into an adult that produces more gametes. This process gives rise to genetic variation; offspring are genetically different than either parent. There are many modifications of this basic process.
The basic sexual life cycle: adult (2n) → meiosis → gametes (n) → fertilization → zygote (2n) → embryo → adult.

IV. Plants have "kinky" sex

Plants exhibit a unique variation of the generalized sexual life cycle termed "alternation of generations".
In other words, they alternate between a diploid and haploid phase in the life cycle. The diploid phase is termed the sporophyte phase and the haploid phase is the gametophyte phase. The sporophyte produces spores (microspores or megaspores) by meiosis. The gametophyte produces gametes by mitosis (yes, mitosis!). The sporophyte and gametophyte phases may be completely separate (as in ferns), or the sporophyte may be "parasitic" (nutritionally dependent) on the gametophyte (as in mosses) or the gametophyte may be parasitic on the sporophyte (as in flowering plants).
The plant way: adult (2n) → meiosis → spores (microspores, megaspores) → gametophyte (microgametophyte, megagametophyte) → gametes (n) → fertilization → zygote (2n) → embryo → adult
Gink & Go learn about the birds and bees

V. Flowers are the sex organs of angiosperms (flowering plants) (note this was covered in lab)
The flower is the key feature of angiosperms. It is a complex organ, that produces the male and female gametes. Evolutionarily, the flower is a short shoot. Thus, it is made of modified leaves and stem. The "leafy" nature of the flower can be readily seen in many of the floral structures. The parts of an "idealized" flower are:

Stem Axis.
The pedicel is the stalk that supports the flower. The receptacle is the swollen or enlarged terminal portion of the pedicel to which the other floral organs are attached.
Sepals.
Outermost layer. Collectively called calyx. Sterile. Often green (photosynthetic), but may be colored or petaloid. Enclose flower in bud and protect developing inner parts.
Petals.
Collectively termed the corolla, interior to the sepals. Usually colored, larger than sepals, delicate. The primary function is to attract pollinators. Pollination - transferring pollen from stamens to carpels. Note: pollination is different than fertilization.
Stamens
Fertile. Collectively called androecium. Male reproductive organ. Interior to petals. Function is to produce pollen (male gametophyte). Comprised of a(n): (a) anther - microsporangium or pollen sac, usually 2 or 4 chambered, in which pollen is produced. Pollen results from the meiotic division of a microspore parent cell in the anther; (b) filament - stalk that attaches anther to receptacle.
Carpels
Collectively gynoecium. Female reproductive organ. Inner most floral structures. Function to produce female gametophyte, which produces the egg. A carpel is differentiated into: (a) stigma - pollen receptive surface; (b) style - stalk; (c) ovary - enlarged basal portion that contains ovules.

You are probably familiar with the term "pistil" that has traditionally been used to refer to the structure made of stigma, style and ovary. Botanists prefer to refer this structure as the carpel(s) for technical reasons, one of which is that carpel highlights the evolutionary origin of these structures. If you use the term pistil, make sure you spell it correctly - it's not a weapon.

V. Plants like sexual variety - or, there is no such thing as a "typical" flower
Flowers can be highly modified from the basic floral plan described above. In fact, finding a flower that looks like the "textbook diagrams" is somewhat difficult. Most flowers are modified, to a greater or lesser extent, from the basic pattern.

The fact that all flowers share the same basic floral plan is good evidence for evolution. If the flowering plants didn't share a common ancestry, then there is no reason to expect that all flowers would have the same plan. If different flowering plants were created separately, by individual acts of creation, then we would expect many different floral designs.

Stephen J. Gould describes the petals of some orchid flowers as imperfections of nature, meaning that they are not the "best" or most "perfect" design for solving the problem of pollination. However, orchids and all organisms are constrained by their evolutionary history. Thus, these imperfections show the tracks of evolution. Check out Gould's essay, "The Panda's Thumb."

One common floral modification is to lack one or more sets of floral organs. If a flower possesses all four sets of floral parts it is said to be complete. If it lacks one or more it is incomplete. A flower with both androecium and gynoecium is termed perfect; if missing male or female parts it is called imperfect and the flowers are considered to be unisexual. Note that by definition a complete flower must be perfect; but a perfect flower may be incomplete. The terms monoecious (unisexual male and female flowers on the same individual such as in oak and birch) and dioecious (unisexual male and female flowers on different individuals such as in willow and poplars) refer to how flowers are distributed on different plants.

VI. Meiosis occurs in the anther (stamen) and ovule (carpel)
Spore mother cells (which are diploid) divide meiotically to produce haploid spores (microspores - male; megaspore - female). In turn, the spores divide mitotically and develop into the male and female gametophytes, respectively. The microspores undergo one division to produce a two nucleate stage (tube and generative nuclei). The megaspore undergoes three mitotic divisions resulting in an eight nucleate stage (egg, 3 antipodals, 2 synergids, 2 polar nuclei).

VII. Pollen is the male gametophyte; actually the immature gametophyte
The germinated pollen grain, with its pollen tube and two sperm nuclei represents the mature male gametophyte. Two sperms develop in the germinating pollen grain. The pollen tube follows chemical signals on its trip through the stigma and style to the ovule.

VIII. The female gametophyte is housed in the ovule
There may be from one (i.e., cherry) to many (i.e., watermelon) ovules per flower, depending on the species. In one type of female gametophyte, there are several cells, with a total of 8 nuclei. One nucleus, near the micropyle (opening into ovule), serves as the egg, and two others in the middle of the gametophyte (also called embryo sac) are called polar nuclei.

IX. Pollination - the plant version of copulation

Pollination is transferring pollen from stamen (anther) to carpel (stigma)
Plants can have sex with themselves (self pollination) or with other individuals (cross pollination). There are advantages and disadvantages of each. There are mechanisms to prevent self fertilization (incompatibility mechanism involving a gene, S, with several alleles). Pollen will not germinate or the pollen tube will stop growth if both pollen and stigma have the same S allele.
Plants involve others in their sexual escapades. In other words, plants rely on a variety of different vectors to accomplish pollen transfer. These include (a) wind; (b) water; (c) animals (birds, bees, moths, butterflies, giraffes, etc.); and even (d) fungi (a recent report suggests that a fungus grows over the flower which attracts flies that eat it thus transferring pollen).
A flower is evolutionarily adapted for its mode of pollination. For example, wind pollinated flowers have little nectar or odor, reduced petals and produce copious pollen. There is intimate coevolution between flower and pollinator.

X. Fertilization
Refers to the fusion of sperm and egg, occurs in ovule. Pollen germinates on the stigma, the pollen tube grows out and grows through the style toward ovule. The pollen tube grows into ovule (through a gap in ovule covering called micropyle) and releases two sperm. One sperm fuses with egg to produce the zygote. The other sperm fuses with other nuclei to produce a triploid (or pentaploid, depending on species) cell that produces endosperm. The endosperm is a nutritive tissue that will be saved primarily for the seed to be used when it is germinating.

XI. Pollination and fertilization are different

XII. Plant sex is twice as much fun
In other words, there are two fertilization events (one to produce the embryo and the other to produce food storage tissue, the endosperm). This process, double fertilization, is unique to flowering plants.

XIII. Plant embryos are not miniaturized adults
Embryogenesis refers to the sequence of developmental events that produce the embryo.

XIV. Embryo development in plants is discontinuous
In other words, after the embryo develops a while, it undergoes a dormant period. In animals, once fertilization occurs, development of the individual is continuous throughout the life cycle. Dormancy is built in to allow time for seed/fruit dispersal.

XV. After fertilization, the ovule develops into the seed
A seed is essentially a sleeping baby in a suitcase with his/her lunch. The three major parts of a seed are:

Seed coat
Is derived from the outer layers (integuments) of the ovule. Thus, it is derived from maternal tissue (the only part of the seed that is not part of the new generation). The seed coat is primarily for protection. The seed coat may have hairs (i.e., cotton) or other appendages. The hilum is the point where the seed coat detached from the ovary (like our bellybutton). The micropyle can often been seen (point of entry of the pollen tube into the ovule) near the hilum. The stalk that connects the seed to the fruit is the funiculus (like our umbilical cord).
Embryo - which is derived from the zygote
The embryo has a region that will give rise to roots (radicle); a region that will produce the shoots (epicotyl/plumule), a stem axis (hypocotyl) that connects the root end to the shoot end; and embryonic leaves called the cotyledons or seed leaves. The appearance of these structures vary with the species. There are two basic patterns; monocot (grass) embryos and dicot embryos. As the names suggest, perhaps the major difference is that monocots have a single cotyledon vs. two in dicots. Monocot embryos also have a covering over the immature leaves (coleoptile) and radicle (coleorhiza).
Food Source
The germinating seed is initially heterotrophic - it subsists off of its own stored food reserves until it becomes photosynthetically competent. Thus, it must contain a nutrient source. This nutrient source can take two forms: (1) endosperm (derived from the fusion of one of the sperm with the fusion nuclei). Endosperm essentially surrounds the embryo and is common in grasses; or (2) in other species, the endosperm is reabsorbed during development and stored as starch, protein or oil in the cotyledons (like beans).

To summarize, seeds contain: (1) an embryo (dicot or monocot type); (2) with its lunch (in the form of endosperm which may be present or the endosperm may be reabsorbed and stored in another form in cotyledons); (3) in a suitcase (seed coat).

XVI. Seeds are a hybrid of maternal and offspring tissue
The seed coat is derived from the integuments of the ovule.

XVII. The ovary develops into the fruit
A fruit is thus a ripened, mature ovary and its contents (seeds). Fruits are frequently associated with accessory structures (i.e., tissues other than those from just the ovary).

XVIII. What’s the difference between a fruit and a vegetable?
Botanically, fruits are derived from the ovary or reproductive parts of the plants. Vegetables are derived from "vegetative" parts, such as leaves, roots and stems. Simple, right? So, is a zucchini a fruit or vegetable? How about cauliflower? or tomato? or squash? Many foods that we call vegetables are actually fruits, and vice versa. Some "fruits" are really vegetables (such as rhubarb). The problem arises because "fruit" is commonly used to describe a food that is usually sweet and eaten as dessert or perhaps salad; whereas vegetable is used to describe a food, usually green, eaten during the main course of a meal.

XIX. Seeds vs. Fruits
These terms are also often incorrectly applied (from a strict botanical sense) by the non-botanist. For example, sunflower seeds and corn grains are actually fruits. Conversely, seeds like coconut, are often considered to be a fruit. Take-home-lesson: fruits have seeds, vegetables do not...the seeds are inside the fruit.

XX. The function of fruits (and seeds) is to send "the kids off to college"
Fruits protect the seeds and function in dispersal. They can be disseminated by wind, water, animals, etc. Note: fruits do not provide nourishment for the germinating seedling - they provide nourishment for dispersal agents.

XXI. The mathematics of plant sex
1 pollen grain/ovule; 2 sperms/ovule. How many ovules, sperms, eggs, ovaries, flowers were necessary to produce a watermelon? cherry?

Summary of plant reproduction: (a good overview)

Plants have sex (meiosis & fertilization)

Meiosis occurs in the stamen (anther) and pistil (ovule)

The female gametophyte is in the ovule, protected from the environment

Pollen is the male gametophyte

Pollination is the transfer of pollen from anther to stigma

Pollen only germinates on a compatible stigma

Two sperms develop in the germinating pollen grain

Fertilization occurs in the pistil (ovule)

Fertilization and pollination are different events

Plants have two fertilizations - one to produce the zygote (embryo) and the other to give rise to food storage tissue (endosperm)

Plant embryos are not miniaturized adults (as in animals)

Plant embryo development is discontinuous (to allow time for dispersal)

Plants exhibit an alternation of generations having a distinct haploid (gametophyte) and diploid (sporophyte) stage

The gametophyte phase of flowering plants is short-lived; the sporophyte is the dominant phase

After fertilization, the ovule develops into a seed

The ovary develops into a fruit

For every seed there was one ovule in the ovary

Each seed required one pollen grain for development

Each seed requires 2 sperm (present inside the single pollen grain) for development