Plant Reproduction - Ch 38
Asexual and Sexual Reproduction
Flowers
Pollination and Fertilization
Seeds
Plant Development
Many Plants can reproduce
vegetatively. This asexual
reproduction produces genetic
clones.
• Specialized roots
• Specialized stems
– Beach grass example
• Specialized leaves
Aspen Clones
Sexual Reproduction Produces
Genetically Diverse Offspring
• Review: sexual reproduction involves
production of haploid gametes, which
then fuse to form a zygote.
• Animal gametes don’t do much.
• Fern gametes, in contrast, do a lot!
• Flowering plants - in between…
Alternation of generations
• Multicellular haploid and diploid
stages take turns producing each
other
Sporophyte
Gametophyte
Fig. 29.6
Gametophyte-sporophyte variations
Fig. 30.1
Angiosperm life cycle
Fig. 38.1
Flowers are the reproductive
organs of sporophytes
• 4 whorls:
– Sepals
– Petals
– Stamens (male; pollen
here)
• Filaments
• anthers
– Carpel (female; egg
here)
• Stigma
• Style
• ovary
Monoecious vs. Dioecious Plants
• Monoecious (“one house”) - both male and
female roles in the same plant.
• Includes plants with bisexual (“perfect”)
and unisexual (“imperfect”) flowers
Dioecious Plants
• Diecious (“two houses”) - male and
female roles on different plants
– Sagittaria - in roadside ditches
Pollination is the first step
in fertilization
• Fertilization is indirect in plants.
• Mechanisms of pollination: how does
the pollen get to the stigma?
• Adaptations for pollination.
Coevolution.
Catapulting Pollen
• Edwards et al. (2005) “A recordbreaking pollen catapult” Nature
435: 164
• See also
http://www.ou.edu/cas/botany-micro/ben/ben194.html
Selfing vs. Crossing
• Self-fertilization vs. cross-fertilization
– 20% of plants are selfing - an evolutionary dead
end?
• How to avoid selfing:
– Dioeciousness
– Self rejection - self-incompatibility genes
• Analogy to animal immune system - self-recognition
– Structural and temporal adaptations in flowers
Self incompatibility
• There are various mechanisms of rejecting
the pollen grain (RNAses, aquaporins…)
Formation of gametophytes
– (we need a bigger picture)
Male gametophyte
• Microspores
Diploid
Haploid
Pollen
Fig. 38.5
Fig. 38.4a
Female
gametophyte
• Megaspores
Fig. 38.4b
Gametophytes, continued
• Just remember this:
• Male forms haploid tube cell nucleus
and generative cell (will form two
sperm cells) within pollen grain
• Female forms haploid egg and two
polar nuclei within embryo sac
• (three haploid cells each, to simplify)
The sequence of events
leading to fertilization
• Pollination
• Growth of pollen tube
• Sperm cells (2) travel down tube to
ovary
• Fertilization
Double Fertilization
• What is “double fertilization?”
• One egg + one sperm cell = zygote
– Zygote is diploid, and develops into mature
sporophyte plant
• Two polar bodies + other sperm cell =
endosperm
– Endosperm is triploid (!) and forms nutritive
tissue for the embryo
Seed formation
• After fertilization, ovule develops into
a seed and ovary develops into a fruit
Seeds
• Double Fertilization creates the zygote
and the endosperm
• The zygote divides to form an embryo
• The endosperm divides and grows, storing
nutrients for the embryo (oils, proteins,
starch)
• In some dicots, the nutrients are
transferred from the endosperm to the
cotyledons during seed formation.
Seed
formation
• Seed coat
• Endosperm
– Nutritive tissue
– Cotyledon(s)
– Seed leaves
• Hypocotyl & Radicle
– Embryonic root
• Epicotyl & Plumule
– Shoot tip
(Sketch)
Seed Dormancy and
Germination
• Dehydration during final stages of
seed formation
• Dormancy, seed banks, signals for
germination
Germination
Humans and plant reproduction
• We’ve taken advantage of plants
ability to reproduce asexually
• Cuttings (or fragments) from plants
are used to produce MANY plants
with certain desired characteristics
• At one end of a cutting is a mass of
dividing, undifferentiated cells
called a callus
• A callus forms adventitious roots and
eventually differentiates into all
parts of a plant
Carrot callus
Plant biotechnology
• Using plants in new
ways to help people
– Long history
• Today considered to
be using genetically
modified (GM)
organisms in
agriculture and
industry
From teosinte
to maize by
artificial
selection
– Very contentious
Fig. 38.19
Modern biotechnology
• Today we’ve moved beyond artificial selection of
closely related species or varieties of a single species
• Now we can transfer genes among very distantly
related species through genetic engineering
• Transgenic organisms have been genetically
engineered to express a foreign gene
Bt corn
• Genes from the bacterium
Bacillus thuringiensis are
inserted into corn plants
• The genes code for a protein
(Bt toxin) that kills insect pests
(especially the European corn
borer)
• Using these transgenic corn
plants reduces the need for
pesticides, which saves money
and reduces the environmental
impacts associated with
chemicals
Bt corn vs. monarch butterflies
• A 1999 study in Nature found high doses of pollen from
Bt corn could negatively impact (including kill) monarch
larvae feeding on milkweed that had been dusted with
the pollen
• Later published studies have found that the
concentrations in the study were unrealistically high,
and that there is likely little threat to monarchs at
normal levels of pollen
What’s the big deal?
• We’ve been modifying species through selective
breeding for thousands of years
• What’s the problem with modifying species
directly through their genes?