Introduction to Organismal Biology (BIOL221) - Dr. S.G. Saupe; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; ssaupe@csbsju.edu; http://www.employees.csbsju.edu/ssaupe/

THE PLANT WAY OF LIFE, or ON BEING A PLANT

I. What is a plant?  By most definitions, a plant:

• is multicellular
• is non-motile
• has eukaryotic cells
• has cell walls composed of cellulose
• is autotrophic; and
• exhibits alternation of generations (has a distinctive diploid and haploid phase).

    Examples include the angiosperms (flowering plants), gymnosperms (cone-bearing plants), ferns, and bryophytes (mosses & liverworts). Recent classification systems suggest that these organisms, in addition to the red algae and green algae, should be classified in the Plant Kingdom (Plantae).

II. What is the single most important characteristic that distinguishes plants from other organisms? 
    Autotrophic nutrition!  That's my guess, too. We should recognize that a systematist (scientists who study classification systems) familiar with the most recent notions of classification might disagree since members of a "new" kingdom, Chromista, are also photosynthetic autotrophs. Nevertheless, both of these groups are related so we can still safely agree that autotrophism is important to the plant way of life.

A. Take-Home-Lesson 1: An autotroph makes its own food (energy-rich organic compounds) from simple, inorganic materials in the environment. Plants use light as their energy source, hence they are photosynthetic (vs. chemo-synthetic for certain bacteria). The general equation for photosynthesis is:

CO₂ + H₂O + light → (CH₂O)_n +O₂

    In contrast, animals are heterotrophic, meaning that they must obtain their food (pre-fabricated organic compounds) from the environment. They cannot manufacture their own food. Examples of heterotrophs include decomposers, carnivores, herbivores.

    However, this brings up some interesting questions - are carnivorous plants autotrophs or heterotrophs?  or how about the parasitic plants like mistletoe (Phoradendron) and dodder (Cuscuta)? or mycotrophic plants like Indian pipes (Monotropa)

B. Take-Home-Lesson 2: The autotrophic mode of nutrition evolved early in the evolution of life, ca. 3 billion years ago. This event set in motion the evolutionary events that culminated in modern plants. Therefore, plant form (characteristics) can be explained as a direct or indirect consequence of the autotrophic mode of nutrition.

III. Consequences of autotrophic nutrition 
    Plants required specialized structures adapted for the autotrophic mode of nutrition. Specialization occurs at all levels of biological organization (i.e., organ, tissue, cell, organelle). Specific problems, and their solutions, related to autotrophic nutrition are:

A.  Problem: Photosynthesis is a complicated biochemical process.
   In order for photosynthesis to function properly and efficiently, it was necessary to separate these reactions from the countless others that occur in the cell. This required the evolution of a specialized organelle for this process - chloroplasts. Even within the chloroplast, specialization was required. Recall that there are three major regions within the chloroplast - the stroma, inner membrane, and inter-membrane space. Each of these three regions are important for the functioning of photosynthesis. Electron transfer reactions require the highly ordered environment provided by the inner membrane. The Calvin cycle (light-independent reactions) are aqueous biochemical reactions which occur in the stroma and the inter-membrane space is needed to generate the pH gradient that is important for photophosphorylation (ATP production).

B.  Problem: Photosynthesis requires efficient light harvesting.
    Leaves are perfect solar collectors. These organs are broad and flat to allow for efficient light harvest. The leaves are broad to maximize surface area for light harvest and they are thin since light cannot penetrate too deeply into the leaf (the amount of light decreases exponentially with distance). As an aside, although the majority of light is absorbed near the leaf surface, in some situations, plant tissues can act like fiber optic cables to funnel some light deeply into the plant body.

    Even within the thin leaf, most chloroplasts are found in the upper layer of cells, the palisade layer or palisade mesophyll, which is a tissue layer just beneath the upper epidermis of the leaf. This makes "sense" since these cells will be receiving the greatest amount of light of any region in the leaf. This is an example of specialization at the tissue and cellular level.

C.  Problem: Photosynthesis requires an apparatus for gas exchange.
   Leaves also serve as a means to exchange photosynthetic gases (take up carbon dioxide and get rid of oxygen) with the environment. Leaves have pores in the surface (stomates) that regulate the entry/exit of gases and prevent the loss of excessive water.

    The spongy layer (or spongy mesophyll) of the leaf acts like a "lung" increasing the internal surface area and provides for more rapid diffusion within the leaf. Note again that leaves are thin - this avoids the need for lungs or other type of pump to move gases. Since diffusion rates are inversely related to distance, simple diffusion can account for gas movements into/out of a leaf. An added advantage of having large leaves for light harvest is that they provide lots of surface area for absorption of carbon dioxide.

    Note again the specialization of the leaf at the organ, tissue, and cellular levels for gas exchange.

D.  Problem: Thin leaves, required for light absorption and gas exchange, need support.
   This problem was solved by the evolution of the cell wall which provided for the support of thin structures without the need (or potential) for internal support structures.

E.  Problem: Photosynthesis requires a water supply
   With the exception of the algae and aquatic plants, plants obtain water through the roots from soil. Essentially the roots "mine" the soil for water. Thus, photosynthesis and the transition to a terrestrial environment stimulated the evolution of a root system to obtain water (specialization at the organ level). And, it required the evolution of specialized transport tissue (xylem) to move the water from the roots to the leaves.

F.  Problem: Photosynthesis requires a mechanism to transport end products throughout the plant.
   Once carbohydrate is produced during photosynthesis there must be a mechanism to transport it to other locations throughout the plant. The evolution of vascular tissue, specifically phloem, permitted movement of photosynthate from leaves to roots, fruits and other tissues where required.

IV. Consequences of Autotrophic Nutrition - Motility is no longer required; Or possible.
    One of the main reasons for motility is to obtain food. Since the nutrients required by plants are "omnipotent" there was never an evolutionary pressure for "motility". Let’s quickly compare the nutrients used by plants / animals:

Conclusion: plants must be adapted for harvesting dilute nutrients that occur everywhere, whereas animals are adapted for searching out and trapping widely dispersed, concentrated packets of food.

Supportive Evidence: If this is true, then we expect that animals with a nutrient source like a plant should have similar features to a plant. Check out corals, sea fans, and hydra. These are all non-motile animals that occur in aquatic environments which enables them to "feed like a plant" - food is essentially brought to them via water currents. Thus, they never had any pressure for motility and they have very similar lifestyles/forms as plants.

    In addition, note that motility is really not possible for terrestrial plants. Once plants evolved roots it precluded movement. These evolutionary "choices" are closely connected.

    However, being stationary has its own problems/consequences.

V. Consequences of a Stationary Lifestyle - The need to exploit a limited volume of the environment for
resources.
    
The Problem: a fixed (stationary) organism must be able to continually obtain nutrients without using them up. Plants face the additional problem that their nutrients are "dilute". Thus, plants must be designed for collecting dilute nutrients in the environment. Plants have several solutions to this "problem":

A. Plants are dendritic. 
    In other words, the basic shape of the plant body is dendritic - which means "tree-like" or "filamentous". The advantage of this shape is that it provides a large surface-to-volume (s/v) ratio which enables a plant to exploit a large area of the environment. In contrast, animals are more compact (spherical) to minimize their s/v ratio. Among other things, this is an advantage for motility. Surface-to-volume ratios are very important in many areas of biology.

B. Plants have indeterminate growth.
    This is the process by which a plant continues to grow and get larger throughout its life cycle. The advantage of this is that it allows the plant, especially roots, to grow into new areas. In contrast, determinate growth is where an organism or part reaches a certain size and then stops growing. This is characteristic of animals and some plant parts (i.e., leaves, fruits).

C. Plants have an architectural design  
    In other words, the plant body is constructed like a building, modular. It is built of a limited number of units, each of which is relatively independent of the others that are united into a single structure. Thus, just like a building is made of rooms, the leaves, stems and roots of a plant are analogous to a rooms in the building. Each room is somewhat independent, yet they all function together to make an integrated whole. You can seal off a room in a building, or remove a leaf or fruit, with little harm to the overall integrity of the structure. This is critical for plants to be able to add or remove parts (leaves, stems, flowers, fruits) as necessary. One conclusion is that plants are not limited by size, and this gives them the ability to colonize and exploit new areas for resources.

    In contrast, an animal has a mechanical design. In other words, animals are built more like a machine, made of numerous, different parts that function together. The parts are highly integrated. Parts cannot be added or removed without reducing the efficiency of the operation of the whole. As a result, animals are limited by size.

    Further, plants are not a static shape - plants constantly changing by adding/loosing parts.  In contrast, animals don’t change their basic shape.

D. Plants have a well developed ability to reproduce asexually. 
    This can be viewed as a quick and energetically inexpensive way to expand the influence of the parent into a new location. 

E. Plants (may) exhibit heterophylly.
    Heterophylly is the fancy way of referring to a plant with leaves of different shapes. For example, in aquatic plants the aerial leaves are entire but the submerged leaves are dissected. Leaves in the sun tend to be smaller and thicker than shade leaves.

F. Plants can forage. 
    The growth patterns of plants, especially lianas and plants with stolons (runners), are similar to the foraging tactics of animals. A brief overview of the anatomy of a clonal plant like Glechoma hederacea (ground ivy): parent plant, stolon, ramet (individual of a clone).

    Let’s make some predictions concerning clonal growth of ground ivy. In favorable conditions (i.e., lots of nutrients) clonal plants (& vines) will:

1. branch more - to take advantage of the resource and to reduce the possibility of growing away from the resource.
2. have shorter internodes (reason as above).
3. produce more ramets/clones (to take advantage of the resource).
4. have larger leaves (as #1).

    Test your predictions by examining the following data:

Foraging in Ground Ivy (Glechoma hederacea).  Data from Hutchins and Slade; Plants Today Jan-Feb, 1988.
Treatment	Stolon Branches	Internode length (cm)	Ramets/clone
hi light/hi nutrients	37	6.4	110
hi light/low nutrients	22	7.0	62
low light/hi nutrients	5	10	23

 

Data from Tooley - Journal of Biological Education 23: 263 (1989)
Light Intensity (lux)	Petiole Length (% initial)	leaf number (% initial)	Stolon length (cm)	Leaf surface area (% initial)
400	28	30	18	7.8
2000	132	219	13.3	122.7

    Another example of foraging: Ray (1975) found that Syngonium vines were of two types: (1) Long stem/small leaves - the traveling form; and (2) short stem/large leaves - the feeding form. Under what conditions do you predict to find each of the two forms?

    Thus, plant growth is essentially analogous to animal behavior. One of the first to express this idea was Arber (1950; The Natural Philosophy of Plant Form. Cambridge) who said: "Among plants, form may be held to include something corresponding to behavior in the zoological field...for most though but not for all plants the only available forms of action are either growth, or ascending of parts, both of which involve a change in the size and form of the organism."

VI. Consequences of a Stationary Lifestyle - Positioning in the environment.

The Problem: a non-motile organism is unable to move to a more favorable location to carry out its vital functions. Thus, it has at least three major problems to contend with:

A. Environmental Positioning/Location - or, getting started in the right spot. Obviously a motile organism can move to a favorable location, but a plant is stuck in one spot once the seed germinates. For most plants getting started in the right place is a matter of luck.  They produce lots of offspring with little parental care of offspring and very few survive - think oak tree and acorns.  However, there are a few "tricks" that seeds use to help increase the odds that they germinate in a favorable environment:

1.  Light.
    Some seeds, like certain varieties of lettuce, require light for germination. This is a mechanism to insure that they germinate on the soil surface. It's no surprise that a garden develops a healthy crop of weeds after the soil is turned - it brings light-sensitive seeds to the surface. Light sensitive seeds are usually small and without much stored food. Thus, it is important that they begin to photosynthesize soon after germination.

    Action spectra for this response show that red light (ca. 660 nm) triggers seed germination and that treatment with far-red light (ca. 730 nm) inhibit/prevent germination. A typical experiment would yield the following results:

Lettuce Seed (var. Grand Rapids) Germination in Response to Red and Far-red light
Treatment	Germination?
dark	no
light	yes
red light	yes
far-red	no
red, then far-red	no
red, then far-red, then red	yes

    Note that the seeds are responding to the last "flavor" of light to which they are exposed. Essentially the response is "reversible" much like using a switch to turn on a light bulb.

    Phytochrome is the pigment receptor for this response and other photo-reversible responses in plants. Phytochrome is a blue green pigment and it exists in two form: Red form (Pr; absorbs red light at about 660 nm) and Far Red form (Pfr; absorbs far-red light at about 730 nm). Pr is converted to Pfr when exposed to red light. Pfr is converted back to Pr when exposed to far-red light. Since the plant only makes phytochrome in the red form, initially all phytochrome is in the Pr form until the plant is exposed to light. The reversibility is one of the characteristics of a phytochrome effect. The last wavelength will affect the germination response.

    Pfr is the "active" form of phytochrome that will either induce or inhibit a response. In this case, the phytochrome induces germination presumably be activating a number of processes in the seed including amylase production.

2.  Ethylene.
    Some seeds require ethylene to germinate. This naturally occurring plant hormone is produced by plants and soil microbes. Once the ethylene concentration reaches a critical level it induces the seeds to germinate. This happens if the seed is buried. These seeds are usually larger than light sensitive ones. One advantage of being buried is that the seeds will be more likely to be in a moist, humid environment.

B. Axis orientation - once a seed germinates in a favorable environment it must determine which way is up/down to insure that the roots grow down and shoots up. Thus, gravitropism is a very important physiological response characteristic of all plants.

    Gravitropism is still not completely understood, but we are learning more. Recent shuttle flights have helped expand our knowledge of this phenomenon because they provide an opportunity to study the process in the microgravity of space. Shoots are negatively gravitropic while roots are positively gravitropic. The cells on the upper side of the root elongate faster than those on the lower side. For the stem, it is just the opposite. The receptor in the root is located in the cap. It may be starch grains, that settle out to show which way is up. The movement of these heavy bodies (called statoliths) causes a redistribution of plant hormones, like IAA, so that there is more on the lower side of the stem and root. The increased concentration stimulates shoot elongation but inhibits root elongation.

C. Fine Tuning - Even non-motile organisms need to "fine-tune" their position in the environment. Thus plants have a variety of mechanisms that enable them to optimize their position in the environment including:

1. Phototropism
       Growth toward light, maximize light reception. The receptor for the response is the shoot tip. A yellow pigment is the specific receptor and the blue light is the stimulus. Light is absorbed by the receptor pigment. This causes IAA to be transported from the lighted side of the stem toward the darker side. This stimulates elongation on the side away from the light;
   
2. Skototropism
       Growth of vines (e.g., Monstera) toward a darkened region of the environment. Mechanism by which some tropical vines find a support to grow up (Ray, 1975);
   
3. Thigmomorphogenesis
       Response to touch in which the plant is shorter with thicker stems - prevents plants from getting too spindly and reduces risk of breaking in wind;
   
4. Solar tracking
       Flowers follow the movement of the sun, keeps pollen dry, maximize photosynthesis;
   

5. Leaf mosaics
       Pattern of leaves which minimizes overlapping (i.e., ivy on a building); and
   

6. Apical dominance
       The Christmas tree shape exhibited by some plants. In other words, the apical bud controls the development of the lateral buds resulting in a plant with a Christmas tree shape. The function is to prevent the plant from becoming too top-heavy and to maximize light exposure to all leaves. IAA is partly responsible. It is produced at the tip and moves toward the base of the plant. The concentration decreases as it proceeds from the shoot towards the root. The buds are presumably inhibited by high concentrations of IAA. Although there is evidence to support this theory, other data suggest that cytokinins, another group of plant hormones, are also involved. Cytokinins help direct the transport of sugars and it is suggested that the lateral buds don't develop because cytokinins at the apex prevent nutrients from getting to the lateral buds to develop. Finally, there is some evidence that an inhibitory hormone may also be involved.
   

7. Etiolation
       The response of plants to growth in the dark or with reduce light. Etiolated plants are typically yellow, with an apical hook (dicots) or unopened coleoptile, have elongated internodes (stems) with unfolded leaves and the stems are thinner. These features can be considered ways of conserving energy until conditions improve (i.e., light). Phytochrome is also important in this response.
   

8. Habitat selection
       Rhizomatous plants like western ragweed (Ambrosia psilostachya) preferentially colonize non-saline soil - example of habitat selection (Salzman, 1985). The growth rate is higher in saline soil than non-saline - "moving away from the salt" increasing the likelihood of finding new habitat.
    

VII. Consequences of a Stationary Lifestyle - the need to predict environmental changes and activities.

Problem: Plants respond to changes in their environment by growth and developmental changes. Since these take time, a plant must "know" or "predict" when the environment will change and prepare for the change. Indeterminate growth is important here - it provides plants with the ability to change developmentally through the life cycle. Some examples of this phenomenon include: preparing for winter (by forming buds in summer), photoperiodism (timing flowering so the appropriate pollinator is available and that the seeds have enough time to develop before winter); circadian rhythms (various types in response to day/night), and nyctinasty (sleep movements).

    In contrast, animals typically respond to their environment behaviorally. Since they are motile, they can "move" to a favorable position. In fact, because they are motile they need a nervous system to respond to the environment. Plants don’t need a nervous system since they are constrained to respond to the environment by growth and developmental changes; hence they never had a pressure to evolve a nervous system.

    Flowering is an excellent example of this phenomenon. Two important signals for flowering are temperature and daylength. Some plants need to be cold treated in order to germinate. This process is called vernalization and is the reason why farmers plant winter wheat in the late summer/early fall.

    Daylength or photoperiod is another excellent flowering signal and is important in most species. This is a "better" signal than temperature because it is more predictable. Some plants flower when the days are long and the nights are short (Long Day plants) while others prefer short days and long nights (Short Day plants). Some plants are insensitive to daylength (day neutral plants). There are also various combinations of long short day plants and short, long day plants. Surprisingly, experiments in which the day or night is interrupted have shown that plants are primarily responding to the length of the night. Thus, long day plants can more appropriately be called "short night" plants and short day plants should be called "long night" plants.

    The receptor for this phenomenon is in the leaves and appears to be phytochrome. Plants appear to measure the relative amounts of Pr/Pfr. Long day plants will flower when the ratio of Pr/Pfr is low whereas this would stimulate short day plants. Phytochrome presumably triggers the production of appropriate flowering hormones that cause the meristems to produce flowers. Although various experiments (i.e., grafting) show that hormones are involved in this process, no hormone has been unequivocally been shown to be involved. Florigen is the name given to the putative hormone involved in flowering.

VIII. Consequences of a Stationary Lifestyle - need to protect themselves from physical and biological dangers in the environment.

Problem: a non-motile organism cannot flee when conditions get tough. It must "fight" it out. Both the physical environment and biological environment threatens the well being of plants.

A. Physical dangers - wind, water (flood), drought, cold (winter) are among the physical dangers that a plant faces. In general, plants cope with these (at least the predictable ones like winter and drought during summer) by dormancy, senescence and even death.

B. Biological dangers - predators (=herbivores) and competitors (=other plants). Plants have: (1) anatomical weapons (thorns, hairs, thick cuticle); or (2) chemical weapons - produce toxic, unpalatable chemicals. These can be inducible (produced in response to attack) or constitutive (always present; Karban & Myers. 1989. Ann Rev Systemat. Ecol 20:331); Allelopathy is chemical warfare between plants. Phytoalexins are chemicals produced by plants to resist microbial infection; (3) mimicry - "tricking" predators. For example, lithops in S.African deserts look like pebbles - are stone mimics. Alsueosmia is a non-toxic New Zealand plant that looks very similar to Wintera pseudocolorata (a toxic species). Check out the article by Barrett in Scientific American. Also, the Private Lives of Plants video series (Alcuin Library) has some great examples of plant defense mechanisms.

IX. Consequences of a Stationary Lifestyle - reproduction

Problem: A non-motile organism cannot seek a mate (for gamete transfer) or easily disperse offspring. Plants solve the gamete transfer problem by relying on various pollination vectors. Fruits/seed dispersal mechanisms help disperse offspring. Check out the Private Lives of Plants video series available in the Alcuin Library for some great footage of dispersal and pollination mechanisms.
 

X. Additional Consequences: The consequences of the wall

Problem: As we mentioned, plants evolved cell walls as a means of support. However, by surrounding their cells with a rigid box, this imposed certain limitations. These include:

1. Growth is restricted to meristematic regions.
        In contrast, growth in animals occurs throughout the body. Meristems are areas of active cell division in plants. The typical pattern for a plant is that the meristem produces new cells which then enlarge and finally differentiate (become specialized). There are two major types of meristems: (a) Primary - responsible for growth in length and were originally present in the embryo. The apical meristems, found at root and shoot tips, are examples. Check out a diagram of the apical meristem of roots and shoots; (b) Secondary - responsible for growth in girth. These are derived from the primary (apical) meristems. These are several secondary meristems including vascular cambium (produces xylem and phloem), cork cambium (produces cork) and the pericycle (found in stele, gives rise to the lateral roots)
   
2. Since plants are built from rigid structures, morphogenesis occurs by way of cell addition, not cell movement (as is found in development of animals)
   
3. Plants exhibit indeterminate growth (vs. determinate for animals)
   
4. Plants grow by the progressive accumulation of similar units (i.e., architectural design); whereas animals have a fixed shape that enlarges (from Adrian Bell, 1986)
   
5. Since each cell is walled off from neighboring cells, plants need an effect means of cell-to-cell communication. Plants accomplish this through plasmodesmata - cytoplasmic connections, hormonal regulation, and some electrical signals.

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