Week 14 L5: Plant Development 3 - Embryo Development Flashcards
Do plants have an axis when they germinate?
YES, have a sense of polarity
Are meristems formed early in development of the embryo?
yes, it can be seen between globular and heart that meristems are becoming visible
What forms of the meristem show polarity in the embryo?
SAM and root meristem
What are the stages of embryogenesis?
globular , heart , torpedo , mature.
What is the relationship between embryo and endosperm development?
develop in tandem
What is the endosperm like when the seed is mature?
very little endosperm left, seed been taken up by the embryo.
What is a transient endosperm?
endosperm has virtually disappeared by germination. When the seed is mature there is very little endosperm left
What are the different types of endosperm?
transitive and persistent
What is germination?
the development of a plant from a seed or spore after a period of dormancy.
What plants have transitive endosperm?
arabidopsis
What type of plant have persistent endosperm?
barley grain (and other cereal grains)
What does a persistent endosperm mean?
it supports the germinating seedling
Why would cereals want a persistent endosperm?
It has a large source of calories.
In sweetcorn what is the largest source of calories in the endosperm?
starch
What is it about grains which make them a good food source?
double fertilisation and persistent endosperm.
What is syncytial development?
nuclei in the endosperm are just dividing. increase number of nuclei very quickly.
development of early endosperm
What happens in double fertilisation?
process of double fertilization occurs, since one of the nuclei of the pollen grain fuses with the two polar nuclei of the embryo sac to form a cell with three nuclei and the other fuses with the nucleus of the ovum.
What are the stages of endosperm development in embryogenesis?
free nuclear development
cellularization
cellular growth
What are the filial tissues?
endosperm and embryo
are apoplasmically separated from the maternal tissue
What maternal tissue delivers nutrients to the developing endosperm and embryo?
nucellar projection
and nucellar epidermis
What receives the nutrients from the nucellar projection?
modified aleurone (also called transfer cells)
What 2 structures are important to deliver nutrients into the embryo?
nucellar projection
modified aleurone
What works coincidently with maternal tissue degradation?
grain filling
What happens 10 days after pollination?
cavity and programmed cell death of the nucellar projection
What promotes maternal tissue degradation?
B-sister genes
What are the B-sister genes?
closely related to B-class MADS-box
What happens if the maternal tissue does not degrade?
no endosperm development
What is dormancy?
Generally an adaptive trait ensuring
that germination occurs in a suitable
environment at a suitable time
What is a dormant seed?
doesn’t germinate under normal
growth conditions.
Why have dormancy?
The conditions may be optimal for an adult but not for a seedling. seedlings need more water than adults.
Seedlings are laying down cuticle, don’t have mature cuticle. sensitive part of their life. Not a lot of leaves etc.
Allows plant to wait of the optimal environment to germinate.
• Water –seedlings much more sensitive to
drought than adult plants.
• Time for dispersal
• Light
• Sufficient to generate enough photosynthate
to survive.
• Avoid germination in shady places, e.g; canopy cover
What is photosynthate?
a sugar or other substance made by photosynthesis.
Why don’t we want a seed to prematurely germinate?
as the seed will use all the starch in the endosperm
Defective barley.
e.g. premature spouting of corm
What factors can cause dormancy to break?
physical
chemical
physiological
What are the physical aspects leading to dormancy to break?
Needs breaking to germinate (freezing, high
temp, digestive tract, drying) as otherwise water
can’t get in.
• Can be one cycle or multiple lasting months to
years.
What are chemical aspects leading to dormancy to break?
Inhibitors present in the seed coat – need to be
washed out before germination can occur.
What are the physiological aspects leading to dormancy breaking?
Hormone antagonism
• Abscisic acid (ABA) vs Giberellins (GA)
What hormone promotes dormancy?
aba
When will a mature seed undergo a secondary dormancy?
Potentially if hydration occurs and a physical barrier is overcome.
But this isn’t enough to germinate so a secondary dormancy occurs.
GA levels rise slightly but are overridden by ABA levels increasing.
insurance policy, seed is wet but unsure, needs more barriers overcome.
What is a seedling called when it undergoes a second dormancy?
Imbided
What is an imbided seed?
e.g. hydration and physical barrier overcome.
Seed swells, increase GA levels but ABA shoot up.
What is a breaking dormancy signal?
many siganls
light, hydration, cold, chemical suppression lost etc.
What happens when the seedling breaks the dormancy signals? GA/ABA
GA increases and aba DECREASES. But also the sensitivity to ABA decreases and the sensitivity to GA increases
The seedling is committed to germination.
GA is produced by the seed after imbibition but the seed isn’t sensitive
to GA.
• Light (and cold or other stimuli) promote increased production of GA
and increased sensitivity to GA by repressing ABA synthesis and
promoting ABA catabolism.
• Eventually GA production causes inhibition of ABA synthesis and
promotes degradation of ABA by upregulating ABA catabolism genes.
What is hormonal antagonism?
influencing the levels and sensitivity of each there
ABA/GA
What is an example of a cereal that has been bred to have reduced dormancy?
wheat
What is an advantage to crops having reduced dormancy?
Immediate synchronous germination straight after planting
What is a disadvantage of crops having reduced dormancy?
• Rain fall on ripe crops can
induce pre-harvest
germination
• Germination in storage. wet conditions can cause it to pregerminate
What kind of germination is wanted in malt?
Vigorous and synchronous germination important for
malting, release sugars for fermentation.
GA induces production of an enzyme amylase. amylase breaks starch to sugar. This sugar is what the yeast will go on and ferment and give alcohol.
When do malters want to have vigorous germination of malt?
sugar release stage
They want enough sugar release but want to kill that seed so it doesn’t use the sugar and grow.
