An Introduction to Plant Development

1. An Introduction to Plant Development 23

2. Key Concepts • In sharp contrast to animals, plants develop continuously, do not commit cells to gamete production until late in development, and produce gametes by mitosis in haploid cells. • In flowering plants, double fertilization results in the production of a zygote and a nutritive tissue that supports embryogenesis. • Embryogenesis results in the formation of the major body axes and three types of embryonic tissue.

3. Key Concepts • Vegetative development is the function of meristems, in which cell division occurs throughout life, producing cells that go on to differentiate. • When the function of a meristem shifts from vegetative to reproductive development, key regulatory transcription factors are activated and control the position and identity of floral organs.

4. Introduction • Unlike many animals, plants continue to grow and develop throughout their lives, whether that life lasts two weeks or thousands of years. • In addition, most plant cells retain the ability to de-differentiate, or begin producing proteins typical of another type of cell. • The small flowering plant Arabidopsis thaliana is used as a model organism throughout this chapter. It is relatively easy to grow, produces large numbers of offspring, and completes its entire life cycle in six weeks.

5. The Life Cycle of a Flowering Plant • The life cycle of a flowering plant begins with gametogenesis, gamete formation. • In flowering plants, fertilization occurs when sperm and egg combine in a womb-like ovule inside the protective female reproductive structure of a flower. • Development continues inside the ovule with embryogenesis. • In many plants embryogenesis ends with the maturation of the ovule into a seed, which contains the dormant embryo and a supply of nutrients and is surrounded by a protective coat.

6. The Flowering Plant Life Cycle • When conditions are favorable, the seed undergoes germination, resuming growth to form a seedling. • The seedling undergoes organogenesis, becoming an adult plant with vegetative (nonreproductive) organs. • The three vegetative organs are leaves, roots, and stems. • Later, cells in the stem are converted to reproductive structures, producing flowers. • Gametogenesis occurs in these flowers, starting the cycle again.

8. Gametogenesis • One of the most dramatic differences between plant and animal development is gametogenesis. • In plants, sperm and egg cells are produced from haploid cells via mitosis, not from diploid cells by meiosis as in animals. • Because the haploid, multicellular structures called pollen grains and embryo sacs alternate with a diploid, multicellular plant as one generation gives rise to the next, this type of life cycle is called alternation of generations.

9. Sperm Formation in Flowering Plants • In male reproductive organs, diploid cells undergo meiosis to form four haploid cells. • These cells then undergo mitosis to form pollen grains. • One of the haploid cells within the pollen grain will undergo mitosis to produce two sperm cells.

10. Egg Formation in Flowering Plants • In the female ovule, a diploid cell divides by meiosis, producing four daughter cells. • Only one cell survives; the other three undergo a programmed death. • The surviving cell divides by mitosis several times to produce a tiny, multicellular structure called the embryo sac. • Inside the embryo sac, a haploid cell differentiates into an egg. • The ovule is housed at the bottom of the carpel, the female reproductive structure. The top of the carpel is the stigma.

14. Pollination • Pollen grains are carried by wind, water, or animal to a mature flower, where pollination occurs. • During pollination, pollen grain surface proteins interact with stigma surface proteins. • Interactions are specific, preventing cross-species fertilization and, often, self-fertilization. • Following a successful interaction, a pollen tube begins to grow and extend down toward the egg cells. • Pollen tube growth is guided by signals released from the egg at the base of the carpel.

18. Double Fertilization • When the pollen tube reaches the base of the carpel, the two sperm cells move down the pollen tube, through the ovule wall, and into the embryo sac, which contains the egg cell and a maternal cell with two haploid nuclei. • One sperm nucleus fuses with the egg to form the diploid zygote, while the other sperm nucleus fuses with the maternal cell to form a triploid (3n) cell. This event is known as double fertilization.

19. Endosperm • The triploid cell divides repeatedly to form a nutritive tissue called endosperm. This tissue (like the yolk in animal eggs) stores nutrients inside the seed for embryonic development, seed germination, and early seedling growth.

23. Embryogenesis • In flowering plants, embryogenesis takes place inside the ovule as the seed matures. • Embryogenesis produces a tiny, simplified plant.

24. What Happens during Embryogenesis • After fertilization, the zygote divides asymmetrically, producing a large basal cell and a small apical cell. • The basal (bottom) cell gives rise to the suspensor, which anchors the embryo as it develops. • The apical (top) cell gives rise to the mature embryo. • The asymmetries in the basal and apical cells help establish the apical-basal axis (top and bottom) of the plant.

25. What Happens during Embryogenesis • The radial axis (inside and outside) of the plant is established next, when the embryo is in its globular stage. • Once the apical-basal and radial axes are established, the vegetative organs begin to take shape. • The initial leaves, called cotyledons, are connected to the root by the stem-like hypocotyl. • The cotyledons and hypocotyl make up the shoot, which will become the aboveground portion of the plant body. • The root forms the belowground portion.

26. What Happens during Embryogenesis • Groups of cells called the shoot apical meristem (SAM) and rootapical meristem (RAM) form next. • A meristem consists of undifferentiated cells that divide repeatedly, with some daughter cells becoming specialized cells. • Meristematic tissues produce cells in this way throughout the plant’s life. • Unlike animals, plant growth and development take place without cell migration. • Plant embryonic structures take shape because cell divisions occur in precise orientations; the resulting cells exhibit differential growth.

30. What Happens during Embryogenesis • In addition to establishing the two body axes, early development in Arabidopsis produces three embryonic tissues. • The epidermis is the outer protective covering. • Inside the epidermis lies the ground tissue, the mass of cells that may later differentiate into specialized cells for photosynthesis, food storage, and other functions. • The vascular tissue in the center of the plant will differentiate into specialized cells that transport food and water between the root and shoot.

32. Which Genes and Proteins Set Up Body Axes? • To identify genes involved in establishing the body axes, Arabidopsis mutants with misshapen bodies were studied. The researchers focused on the development of the apical-basal axis. • They found that a gene they called MONOPTEROS is critical in setting up the apical-basal axis. • The MONOPTEROS gene codes for the MONOPTEROS protein, a transcription factor.

34. Auxin’s Role in Establishing the Apical-Basal Axis • Auxin is a cell-to-cell signal molecule that is produced in the shoot apical meristem and transported toward the basal parts of the embryo. • The concentration of auxin along the apical-basal axis of the plant forms a concentration gradient that provides positional information. • The auxin signal is part of a regulatory cascade that triggers production of MONOPTEROS and other regulatory transcription factors specific to cells in the developing hypocotyl and roots, setting up the apical-basal axis.

35. Vegetative Development • Plants cannot move to a different location when their environment proves unsuitable. • Instead, they adjust to changing environmental conditions through continuous growth and development of roots, stems, and leaves. • This constant adjustment is possible because of the meristems that are located at the tips of shoots and roots.

36. Meristems Cause Continuous Growth and Development • Once embryonic development is complete, further plant body development is driven by the meristems. • Shoot apical meristems exist at the tips of shoots, while root apical meristems are found at the tips of roots. • These meristems allow the plant to grow in any direction, both above- and belowground. • Within each meristem, the rate and direction of cell growth are dictated by cell-cell signals produced in response to environmental cues.

40. Which Genes and Proteins Determine Leaf Shape? • Initiation of leaf development depends on the concentration of auxin in parts of the shoot apical meristem, as well as other cell-cell signals. • Three leaf axes form: • Proximal-distal • Lateral • Upper-lower (adaxial–abaxial)

42. Which Genes and Proteins Determine Leaf Shape? • Researchers found that a gene they called PHANTASTICA (PHAN)is critical in setting up the upper-lower axis of leaves. • PHAN’s protein product is a regulatory transcription factor that triggers the expression of genes that cause cells to form the upper surface of leaves and suppresses transcription of genes required for forming the lower leaf surface. • Changes in PHAN expression may underlie at least some of the evolutionary changes in leaf shape.

44. Reproductive Development • Unlike animals, plants do not have germ cells that are set aside early in development. • Flowering and gametogenesis occur when a shoot apical meristem converts from vegetative development to reproductive development.

45. The Floral Meristem and the Flower • A floral meristem is a modified shoot apical meristem that produces reproductive organ-containing flowers. • The floral meristem produces four whorls of organs: • Sepals • Petals • Stamens • Carpels • All of these organs are modified leaves.

46. Floral Organs • Sepals are found on the outside of the flower and provide it with protection. • Inside the sepals are petals, which enclose the reproductive organs and may be colored to attract pollinators. • Inside the petals are stamens,the pollen-producing organs. • In the middle are the carpels containing the egg-producing ovules.

50. Which Genes Control the Development of Flowers? • Several types of mutant flowering plants are homeotic mutants in which one kind of floral organ is replaced by another. • Elliot Meyerowitz and colleagues found that homeotic mutants in Arabidopsis flowers can be divided into three general classes. • Each type of mutant lacks the elements normally found in two of the four whorls.