Lecture #6: Reproduction and Development in Plants
I. THE FLOWER (4 whorls of modified leaves) (the angiosperm's sex
organs) (Fig 38.2)
1. Sepals,
usually green, enclose and protect a floral bud.
2. Petals,
usually bright, advertise the flower to insects, birds, and other pollinators.
3. Stamens
consist of a filament stalk and a terminal anther:
a. filaments
raise the anther out of the flower.
b. anthers
produce the pollen grains that contain male gametes, the sperm nuclei.
4. Carpels
(pistils) consist of the stigma, style, and ovary:
a. stigma
serves as the landing platform for pollen. (self-incompatibility)
b. style
raises the stigma to the wind and the pollinators.
c. ovary
in a more protected area at the base of the flower, contains one or more
ovules, which are multicellular structures with eggs.
(Monoecious angiosperms have staminate and
carpellate flowers on the same plant.)
(Dioecious angiosperms have staminate and
carpellate flowers on separate plants.)
II. AN OVERVIEW
OF THE ANGIOSPERM LIFE CYCLE (Fig 38.1)
(phyton = "plant" in Greek)
1. The sporophyte
is a diploid plant that produces haploid spores by meiosis.
2. The gametophyte
is the haploid generation arising from a spore by mitosis and eventually
producing haploid gametes by mitosis.
III. GAMETOPHYTE
DEVELOPMENT
A. DEVELOPMENT OF
POLLEN (Fig 38.4)
1. A pollen
grain is an immature male gametophyte.
2. In sporangial
chambers of anther, diploid microsporocytes form 4 haploid microspores.
3. Each microspore
divides once to give rise to a generative nucleus and a tube nucleus.
4. About the time
of pollen release, generative nucleus divides to form 2 sperm nuclei.
5. When pollen
grain germinates, tube nucleus leads the way down the style to ovary.
B. DEVELOPMENT OF
EMBRYO SAC (Fig 38.4)
1. The sporangium
of an ovule contains a diploid megasporocyte to form 4 megaspores.
2. One megaspore
grows and divides 3X to form 8 nuclei in a single cell.
3. This large
cell transforms into a 7-celled "embryo sac", the female
gametophyte.
4. At the micropyler
end, two synergid cells flank the egg cell.
5. At the
opposite end, three antipodal cells form.
6. The two
"polar" nuclei share the cytoplasm of the large central cell.
IV. SELF-INCOMPATIBILITY
OF SOME FLOWERS (monoecious
angiosperms)
1. Recognition of
"self" pollen is based on S-genes (for self-incompatibility).
2. As many as 50
different alleles can occur at the S-locus.
3. If the haploid
S-allele of pollen grain matches either of the two S-alleles of the stigma, the
grain will not germinate. (Fig 38.7)
4. Pollen ®
gives chemical signal (S) ® (S) reacts with receptor (R) in cell wall of stigma cell
® activated receptor (Ra) reacts with membrane
protein kinase (K) ® causing activation of signal transduction pathway (STP) ®
which generates effector protein (E) ® and (E) blocks formation of
pollen tube. (Fig. 38.8)
V. GROWTH OF THE
POLLEN TUBE AND DOUBLE FERTILIZATION (Fig. 38.9)
1. A chemical
attractant (Ca++???) draws the tip of the pollen tube thru
micropile.
2. The sperm
nuclei are discharged into the embryo sac.
3. One sperm
nucleus fuses with the egg cell and one with the large central cell.
4. This large triploid
cell undergoes mitosis to form the multicellular endosperm.
5. The
nutrient-rich endosperm serves as the "placenta" for the embryo.
6. Sometimes, the
endosperm nutrient is transferred to the cotyledons.
VI. DEVELOPMENT
OF THE EMBRYO (Fig 38.10)
1. 1st mitotic
division (cleavage) is transverse, forming a terminal cell (animal pole) and
basal cell (vegetal pole).
2. The terminal
cell goes on to form the embryo.
3. Dicots form
two embryonic leaves called cotyledons.
4. Meristematic
tissue above the cotyledons is embryonic shoot (stem) (epicotyl).
5. Meristematic
tissue below the cotyledons is embryonic root (radicle) (hypocotyl).
V. SEED
GERMINATION
1. The embryo and
its food supply become surrounded by a seed coat. (Fig. 38.11)
2. Oftentimes, a
fruit (ripened ovary) forms around the seeds.
3. The dormant
state increases the chances of germination at a good time and place.
a. seeds of
desert plants may require substantial rainfall.
b. seeds of
forest plants may require extreme heat of a fire.
c. seeds of
northern plants may require extended cold exposure.
d. small seeds
(e.g., lettuce seeds) may require light (near soil surface).
e. seeds of
water plants may require abrasions over a rocky stream.
f. other seeds
may require chemical attack by an animal GI tract.
4. Examples of germination:
(Fig 38.14)
a. a bean (water
promotes activation of a-amylase) (hypocotyl hook)
b. a pea
(epicotyl hook)
c. a kernel (in
monocots, the coleoptile leads the way) (Fig 38.13)