Chapter 38 Plant Reproduction and Biotechnology Fig. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
The life cycles of angiosperms and other plants are characterized by an alternation of generations
Life cycle in Angiosperms • Sporophyte is the dominant generation in angiosperms. • Sporophyte of angiosperms also developed a sporophyte flower, a reproductive structure that is unique to angiosperms.
The structure of flower • Flowers are typically composed of four whorls of highly modified leaves called floral organs. • From outside to inside, these four whorls of floral organs are: sepal, petal, stamen, and carpel.
The gametophytes • The stamen and carpels of flowers contain the sporangia, the structures where first the spores and then the gametophytes develop. • Male gametophytes are pollen grains; female gametophytes are embryo sacs (egg-producing structures).
Pollination • When pollen released from anthers and carried by wind or animals land on a stigma, pollination can occur. • Each pollen grain produces a structure called a pollen tube, which grows down into the ovary via the style and discharges sperm into the embryo sac, resulting in fertilization of the egg.
Complete flower • Plants like Trillium bear flowers with all four organs are called complete flower or “perfect” flower. Their flowers are bisexual flower.
Incomplete flower • Plants like Maize or Sagittaria produce flowers with one floral organ eliminated – usually stamen or carpel – are called incomplete flower. Their flowers are sometimes called “imperfect” flower or unisexual flower.
Unisexual flower - monoecious • Unisexual flowers are called staminate or carpellate, depending on which set of reproductive organs is present. • If staminate and carpellate are located on the same individual plant, then that plant species is said to be monoecious. • Example: Maize
Unisexual flower: dioecious • A dioecious plant has staminate flowers and carpellate flowers on separate plants.
Inflorescence • Some plants like Lupines have clusters of flowers called inflorescences.
Composite inflorescence and ray flower • Plants like sunflowers have composite inflorescence in the central disk consists of tiny complete flowers. The “petals” of sunflower is actually imperfect flowers called ray flowers.
Development of male gametophyte • Within the sporangia (pollen sacs) of an anther are numerous diploid cells called microsporocytes. • Each microsporocyte undergoes meiosis, forming four haploid microspores.
Development of male gametophyte • A microspore divides once by mitosis and produces two cells, a generative cell and a tube cell. The generative cell will eventually produce sperm. The tube cell, which encloses the generative cell, will produce the pollen tube, a structure essential for sperm delivery to the egg.
Development of male gametophyte • The two-structure is encased in a thick, resistant wall that becomes sculptured into an elaborate pattern unique to the particular plant species. • These two cells and their wall constitute a pollen grain.
Development of male gametophyte • A pollen grain becomes a mature male gametophyte when the generative cell divides by mitosis to form two sperm cells. In most species, this process occurs after the pollen grain lands on the stigma of a carpel and the pollen tube begins to form.
Development of female gametophyte • Ovules, each containing a single sporangium, form within the chambers of the ovary. • One cell in the sporangium of each ovule, the megasporocyte, grows and then goes through meiosis, producing four haploid megaspores.
Development of female gametophyte • In many angiosperms, only one of the megaspores survives. This megaspore continues to grow, and its nucleus divides by mitosis three times, resulting in one large cell with eight haploid nuclei.
Development of female gametophyte • Membranes then partition this mass into a multicellular female gametophyte – the embryo sac.
Development of female gametophyte • 1 egg cell • 2 synergids flank the egg cell and function in attraction and guidance of the pollen tube. • 3 antipodal cells (function unknown). • 2 polar nuclei, share the cytoplasm of the large central cell of the embryo sac.
Pollination and prevention of self-fertilization • Pollination brings the male and female gametophytes together. • Some flowers self-fertilize (selfing), but the majority of angiosperms have mechanisms that make it difficult or impossible for a flower to fertilize itself.
Prevention of self-fertilization • In some plants with bisexual flowers, the stamens and carpels mature at different times or are structurally arranged in such a way that it is unlikely that an animal pollinator could transfer pollen from the anthers to the stigma of the same flower
Self-incompatibility is the most common anti-selfing mechanism • Self-incompatibility is referring to the ability of a plant to reject its own pollen and the pollen of closely related individuals.
Self-incompatibility • When a pollen grain from an anther happens to land on a stigma of a flower on the same plant, a biochemical block prevents the pollen from completing its development and fertilizing an egg.
Self-incompatibility • Recognition of “self” pollen is based on genes for self-incompatibility, called S-genes. If a pollen grain and the carpel’s stigma upon which it lands have matching alleles at the S-locus, then the pollen grain fails to initiate or complete formation of a pollen tube, and thus no fertilization occurs.
Mechanisms of self-incompatibility • Different plant families have different mechanisms for blocking pollen tube growth. • Some plants produce RNases that will destruct RNA of the rudimentary pollen tube.
Possible mechanism of sporophytic self-incompatibility aquaporins? This happens in plants of mustard family
Pollination and fertilization • After pollen germinated, the tip of the pollen tube enters the ovary (possibly by chemical attractant [Ca2+]) through the micropyle, and discharges its two sperm within the embryo sac.
Double fertilization • After sperms discharged, one sperm fertilizes the egg to form the zygote. The other sperm combines with the two polar nuclei to form a triploid (3n) nucleus in the center of the large central cell of the embryo sac. This large cell will give rise to the endosperm, a food-storing tissue of the seed.
Double fertilization is a distinctive feature of the angiosperm life cycle • The union of two sperm cells with different nuclei of the embryo sac is termed double fertilization. • This process is almost unique to angiosperms, sharing with only a few gymnosperms. It is to ensure that the endosperm will develop only in ovules so nutrients will not be wasted.
After double fertilization • Increase in the cytoplasmic Ca2+ levels of the egg. • Establishment of a block to polyspermy (fertilization of an egg by more than one sperm cell). • Then endosperm development occurs, which is always precedes embryo development.
Endosperm development • After double fertilization, the triploid nucleus of the ovule’s central cell divides, forming a multinucleate “supercell” having a milky consistency. This liquid mass becomes multicellular when cytokinesis partitions the cytoplasm by forming membranes between the nuclei. Eventually, these cells produce cell walls and endosperm becomes solid. • In many dicots, the food reserves of the endosperm are completely exported to the cotyledons (seed leaves) before the seed completes it development so the mature seed lacks endosperm.
Embryo development • The first mitotic division of the zygote is transverse, splitting the fertilized egg into a basal cell and a terminal cell. • The terminal cell eventually give rise to most of the embryo.
Embryo development • The basal cell continues to divide transversely, producing a thread of cells called the suspensor, which anchors the embryo to its parent. • The suspensor also functions in the transfer of nutrients to the embryo from the parent plant (or endosperm).
Embryo development • The terminal cell divides several times and form a spherical proembryo attached to the suspensor. • The cotyledons begin to form as bumps on the proembryo. • Dicots look heart-shaped at this stage; monocots develop only one cotyledon.
Embryo development • Development of plant embryo must establish two features: the root-shoot axis (meristems at opposite ends) and a radial pattern of protoderm (dermal), ground meristem (ground tissue) and procambium (vascular tissue).
Embryo development • The last stage of embryo development involves dehydration. The seed dehydrates until its water content is only about 5~15% of its weight.
Structure of the mature seed Above cotyledon Shoot tip & leaves Below cotyledon Embryonic root
Structure of the mature seed Specilized cotyledon to absorb nutrients from endosperm Covers the young shoot Covers the young root
Fruit development • Pollination tirggers hormonal changes that cause the ovary to begin its transformation into a fruit. • The wall of the ovary becomes the pericarp (thickened wall of the fruit).
Seed dormancy • Seed dormancy increases the chances that germination will occur at a time and place most advantageous to the seedling. • Many different factors can “break” the dormancy of seed.
Germination • For seeds to germinate, imbibition (the uptake of water due to the low water potential) must occur first. • Imbibition causes seed to expand and rupture its coat and also triggers metabolic changes in the embryo.
Germination • After imbibition, the embryo releases hormones called gibberellins (GA) as signals to aleurone.
Germination • Aleurone responds by synthesizing and secreting digestive enzymes (for example, a-amylase) that hydrolyze stored foods in the endosperm, producing small, soluble molecules. • The first organ to emerge from the germinating seed is the radicle (embryonic root).
Germination • Then the shoot tip must break through the soil surface.