Fast Plants Do the Leaves Continue to Lengthen
Wisconsin Fast PlantTM : Brassica rapa rapid cycling (RCBr)
What is it?
The Wisconsin Fast PlantTM is a trademark name for a species of plant called Brassica rapa. Dr. Paul H. Williams, plant pathologist at the University of Wisconsin at Madison, spent 15 years developing rapid-cycling versions of six species of Brassica. Selective breeding enabled Dr. Williams to produce Wisconsin Fast Plants from the tremendous resource of Brassicas all over the world (both wild and domesticated). Bred for their small size and rapid life cycle (35 to 40 days, seed-to-seed), Wisconsin Fast Plants can reproduce at high densities under florescent lighting and are now being used for research and classroom teaching in schools and universities all over the globe. Many scientists are using Wisconsin Fast Plants as model plants for research in genetics, molecular biology, plant breeding, cell biology, and physiology.
Taxonomy: What does it look like?
Species in the genus Brassica belong to the mustard family, or Brassicaceae (also known as Cruciferae, so named because they have four-petaled flowers each resembling a cross or crucifix [crux: Latin, cross]). Higher up the taxonomic ladder (taxis: Greek, arrangement or order), the brassicas are part of the order Papaverales in the subclass Dicotyledeonae (flowering plants with two cotyledons and netted leaf venation). All flowering plants are in the class Angiospermae (Table 2). Angiosperms are in the subdivision Spermatophyta (the seed-bearing plants), in the plant division Tracheophyta (the presence of vascular tissue; trachia: Latin, artery).
The naming of Brassica species has been in a state of confusion for more than a hundred years. Great diversity in the forms of brassicas exist, even within a single species. For instance, within B. rapa itself, many subspecies (so-called cultivar groups) have been described.(Figure 1).
| a | b | c | d | e | f |
| 1/6 | 1/10 | 1/10 | 1/8 | 1/20 | 1/2 |
Figure 1. Forms of Brassica rapa representing various cultivar groups: (a) B. rapa, turnip group; (b) B. rapa, Chinese cabbage group; (c) B. rapa, pak choi group; (d) B. rapa, saichin group; (e) B. rapa, turnip rape group; and (f) B. rapa, rapid-cycling.
TABLE 1. Classification of Brassica rapa
KINGDOM - Planta
plants have cell walls and chlorophyll
other kingdoms are: Monera (bacteria), Protista (protozoans), Fungi, and Animalia
DIVISION - Tracheophyta
vascular plants
SUBDIVISION - Spermatophyta
seed plants
CLASS - Angiospermae
flowering plants
SUBCLASS - Dicotyledeonae (dicots)
two cotyledons, branching veins in leaves
ORDER - Papaverales special anatomy of fruit and embryo contains several families
FAMILY - Brassicaceae or Cruciferae (e.g., mustards and cabbages)
4 petals, 4 sepals, 6 stamens, ovary of two carpels
contains 375 genera and 3200 species
GENUS - Brassica
pod a silique, embryos conduplicate
SPECIES - B. rapa or B. campestris (B. rapa now preferred terminology)
chromosome number 2n = 20
subspecific groups
SUBSPECIFIC or CULTIVAR GROUPS - chinensis (pak choi), pekinensis (Chinese cabbage),
rapifera (turnip), oleifera (turnip rape), etc.
Scientists are still unsure as to whether to give subspecies names to each of the forms of B. rapa. Because the cultivars (cultivated varieties) of various forms can be crossed so easily, many intermediate forms are being produced by plant breeders. Rather than designating each form as a subspecies, many scientists are in favor of categorizing cultivars into broad cultivar groups. Thus, cultivars appearing more like Chinese cabbage than any other form are grouped in the B. rapa, Chinese cabbage group; turnip-like types are in the B. rapa, turnip group; oilseed types are in the B. rapa, turnip rape group; and so forth. Within each cultivar group are many cultivars. A cultivar is given a name by plant breeders and seed companies to differentiate distinctly different cultivars. Cultivar designations can be names or code numbers. The proper designation for a turnip commonly grown in the United States would be as follows: genus, Brassica; species, B. rapa; cultivar group, turnip; cultivar, Purple Top White Globe.
Life Cycle
Brassica rapa of the rapid cycling group (RCBr) possess an average life cycle of approximately 35 to 40 days when grown under continuous cool white fluorescent light. They were developed from brassicas with a normal 6 to 12 month life cycle by continuous genetic selection. Selection criteria included isolating populations exhibiting: 1) minimum time from planting to flowering; 2) rapid seed maturation; 3) absence of seed dormancy; 4) small plant size; 5) high female fertility; 6) uniformity of flower maturation.
These rapid-cycling plants facilitate plant breeding research and the teaching of biology and genetics. During one generation, RCBr can be used to teach basic biological concepts like diversity, interaction with the environment, adaptation, genetic continuity, homeostasis, and evolution. With regard to genetics, more than 50 traits under simple genetic control have been incorporated into RCBr.
RCBr plants should produce flowers 14 days after planting and be about 13 cm tall. Fertilization occurs within 24 hours of pollination and pods visibly swell 3 to 5 days after pollination. Plants can be dried 20 days after the last pollination, and by day 40 to 42, seeds can be harvested and a new cycle begun.
After proper planting of seeds in a quad (a four-sectioned, styrofoam planting unit) under the proper growing conditions, the following events in the life cycle should occur.
Days 1 to 3
The radicle (embryonic root) should emerge from the seed on day 1. This is easily observed by germinating seeds on moist filter paper in a petri dish. By day 3, seedlings emerge from the potting mix. Two cotyledons (seed leaves) appear and the hypocotyl (embryonic stem) begins to extend upward. Chlorophyll and purple anthocyanin pigments are readily apparent.
Days 4 to 9
True leaves develop by day 5 and the cotyledons continue to enlarge. By day 8, flower buds appear in the growing tip of the plant.
Days 10 to 12
The stem elongates between the nodes (points of leaf attachment). The leaves and flower buds continue to enlarge. As the stem elongates, the flower buds are raised to a height well above the leaves.
Days 12 to 17
Flower buds open and reveal flower structure. The pedicel, receptacle, sepals, petals, stamens (anthers and filaments), pistil (stigma, style, and ovary), and the nectaries can be identified. Pollination should be initiated. Cross-pollinate for 3 to 4 days (e.g., pollinate on days 13, 15, and 17). Pollen is viable for 4 to 5 days, and stigmas remain receptive to pollen for 2 to 3 days after flower opening. On day 17, pinch or snip off the remaining unopened flower buds and side shoots; continue to do this until day 35. Pruning directs the plant's food into developing embryos that result from pollination of the first flowers on the plant.
Days 18 to 22
Petals drop from the flowers, and pods elongate and swell. Endosperm and embryo development in the seeds has begun and will continue until day 34 to 36. The stages in embryo development can be observed by removing pods from the plant at different times, opening the pod to expose the ovules, and opening the ovules to expose the embryo. The embryo is surrounded by endospenn, a fine granular liquid that provides nutrients.
Days 23 to 36
The embryo development is complete, and seeds are formed with seed coats from the integuments. The ovary walls and related structures have developed into the large pod (silique), and the pod begins to dry. On day 36, plants are removed from the water source and the ripening process continues. As the seeds ripen, the pods turn yellow, the embryo dehybrates, and the seed coat turns brown.
Days 36 to 40
Plants are allowed to dry. On day 40, pods are removed from the dried plants. If the plants are brittle, pods can be rolled between the finger and thumb, and the seeds can be harvested. The cycle is complete.
Figure 2. The Life Cycle of Rapid-cycling Brassica rapa
Bees and Brassica
Symbiosis is the close association of two or more dissimilar organisms. Such associations can be beneficial to both organisms (mutualistic) or detrimental to one (parasitic). Symbiotic relationships among species occur frequently in nature. When the two or more species in a symbiosis evolve reciprocally, in response to each other, they are said to coevolve. Under close examination each symbiosis stands out as an example of the miraculous complexity which has evolved in our everyday world. The coevolution of brassicas and bees, each dependent upon the other for survival, is such a relationship.
What is a flower? In our eyes it is something to enjoy. For bees and other nectar-gathering insects, it is a source of food. For the plant, flowers are vital organs of reproduction containing both male and female gametes. Within each Brassica flower the male and female parts are just millimeters apart so that when pollen from anthers falls onto the stigma, pollination may occur.
For many brassicas, however, the act of pollination does not insure fertilization and seed formation. Some brassica species contain special recognition compounds, glycoproteins, which are unique to each plant. These compounds enable the plant to recognize "self," causing the abortion of the plant's own pollen. The prevention of fertilization of "self" pollen is called self incompatibility In order for fertilization to occur pollen must travel from one brassica plant to the stigma of an entirely different brassica (cross-pollination). In this way brassicas ensure that their genes will be well mixed throughout the population.
The pollen itself is heavy and sticky--unable to be easily windborne. For brassica plants, bees are marvelously coevolved pollen transferring devices. Bees are members of the insect family Apidae, which are unique in that their bodies are covered with feather-like hairs. The bright yellow flower petals act as both beacon and landing pad for the bees, attracting them to the flower and guiding them to the nectaries. The bee drives its head deep into the flower to reach the sweet liquid (nectar) secreted by the nectaries and brushes against the anthers and stigma. Quantities of pollen are entrapped in its body hairs. As the bees work the brassica fields, moving from plant to plant, cross-pollination occurs and genetic information is widely transferred.
Bees depend on the flower for their survival. Sugars in the nectar provide carbohydrates to power flight and life activities. Pollen is the primary source of proteins, fats, vitamins, and minerals to build muscular, glandular, and skeletal tissues. The average colony of bees will collect 44 to 110 pounds of pollen in a season.
A worker bee foraging for pollen will hover momentarily over the flower as she uses her highly adapted legs for pollen collection. The foreleg is equipped with the antenna cleaner, a deep semi-circular notch with a row of small spines. This is quickly passed over the antenna. Using the large flat pollen brushes on the midlegs, the bee quickly brushes the sticky pollen from her head, thorax and forelegs. The pollen is transferred to pollen baskets by special adaptive features of the hind legs. First the pollen captured on the midleg brushes is raked off by the pollen combs onto the pollen press. This press is a deep notch located in the joint just below the pollen basket. Flexing the leg, the bee packs the pollen into the baskets which are enclosed spaces on the upper hindleg formed by a concave outer surface fringed with long curved hairs. When the baskets are filled, the worker bee returns to the hive with her supplies to feed the colony--nectar in her honey stomach, pollen in her baskets. In the process the continuation of a new generation of brassicas is ensured through her pollination activities.
Figure3. Brassica flower and it's pollinator, the honeybee.
Fertilization and Seed Delvelopment
After pollination, each compatible pollen grain adhering to the stigma sends a tube through the style. This tube carries two male gametes (sperm cells) to the ovule, where the egg and other cell nuclei are contained in the embryo sac. One sperm unites with the egg cell producing a zygote which becomes the embryo. The second sperm unites with the diploid fusion nucleus forming the triploid endosperm, the food source of the developing embryo. This process is known as double fertilization. Endosperm resulting from the union of one of the two male gametes with the fusion nucleus grows mainly as non-cellular tissue, filling the enlarging ovules. The embryo's development is then nourished by the endosperm. Within 2 to 3 days after fertilization, the pistil elongates and swells to become the seed pod. The sepals and petals wither and drop off, having completed their functions.
During days 17 to 35 in the Wisconsin Fast Plants growth cycle, the ovules develop into seeds. The embryos go through a series of developmental stages, called embryogenesis, and enlarge, filling the space occupied by the endosperrn. Through the development of seeds, the plant packages its new generation (embryos) to survive until favorable growth conditions promote germination. As the seed matures, the walls of each ovule develop into a protective seed coat and the entire ovary becomes a fruit (seed pod). Ovules mature into seeds in 20 days.
Figure 4. Growth, Development and Reproduction of rapid cycling Brassica rapa.
Source: http://www2.lv.psu.edu/jxm57/irp/brassica.html
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