Flowers, Fruits, and Seeds Flashcards
how can floral meristems be distinguished from vegetative meristems?
by their large size
what is the transition to reproductive development marked by?
an increase in the frequency of cell divisions in the central zone of the shoot apical meristem
primary inflorescence meristem
- when reproduction is initiated, the vegetative meristem is transformed in the primary inflorescence meristem
- bears cauline leaves and flowers
secondary inflorescence meristems
- the axillary buds of the cauline leaves
inflorescence meristem vs flowers
- inflorescence meristem has indeterminate growth
- flowers have determinate growth
what are the four different types of organs in floral meristems?
- sepals
- petals
- stamen
- carpels in concentric rings called whorls
initiation of carpels
The initiation of the carpels consumes all of the meristematic cells in the apical dome
what are the three types of genes that regulate floral identity?
- meristem identity genes
- floral organ identity genes
- cadastral genes
- These are groups of genes from which individual genes continue to be discovered via “loss of
function mutants”
meristem identity genes
encode transcription factors that induce organ identity
- Must be active for the immature primordia at the shoot or inflorescence apical
meristem to become a floral meristem
floral organ identity genes
encode transcription factors that directly control floral
identity
- Homeotic genes that act as major developmental switches to activate the
genetic programing for a specific structure
cadastral genes
encode transcription factors that act as spatial regulators of the floral
organ identity genes by setting boundaries for their expression
- from French word cadastre, meaning ‘register of property’
ABC model
Floral organ identity genes determine floral organ patterning according to an elegant system, as
predicted by the ABC model proposed in 1991
how is each whorl determined by the three gene activities?
- Type A activity alone specifies sepals
- Type A activity with type B activity specifies petals
- Type B activity with type C activity specifies stamens
- Type C activity alone specifies carpels
type A
Type A activity controls organ identity in the first and second whorl
Type B
Type B activity controls organ identity in the second and third whorls
Type C
Type C activity controls organ identity in the third and fourth whorls
what is a loss in type A?
loss of type A activity results in the formation of carpels instead of sepals in the first whorl
and of stamens instead of petals in the second whorl
what is a loss in type B?
Loss of type B activity results in the formation of sepals instead of petals in the second whorl
and of carpels instead of stamens in the third whorl
what is a loss in type C?
Loss of type C activity results in the formation of petals instead of stamens in the third whorl,
and the formation of a new flower instead of carpels into the fourth whorl
what is a loss in type A, B, and C?
- Loss of A, B, and C activity results in the formation of a pseudoflower in which all floral organs are replaces with green leaf-like structures - Since AB and C genes were identified another class of floral homeotic genes, class E genes (D genes are required for ovule formation).
when do annual plants produce flowers?
- within a few weeks after germinating
when do perennial plants grow flowers?
- example: forest trees
- may grow 20+ yeas before producing flowers
- Thus size or age of a plant is an internal factor controlling the switch to reproductive
development
autonomous regulation
plants flower entirely according to internal cues
obligate response
plants that exhibit an absolute requirement for environmental cues in order to flower
facultative response
flowering is promoted by environmental cues but will eventually occur in the
absence of such cues
what is the phase changes , development stages, and trajectory reproduction in higher plants?
- juvenile phase
- adult vegetative phase (ability to form reproductive structures)
- adult reproductive phase
how is the transition from juvenile to adult frequently displayed?
- changes in vegetative characteristics such as leaf morphology, phyllotaxy, and thorniness
how can the transition from juvenile to adult be seen in a single leaf?
- The juvenile pinnately-compound leaf is replaces by adult phyllodes
(flattened petiole) and transition leaves can be observed (juvenile at
base adult at tip)
combinatorial model
- shoot development can be described as a series of
independently regulated, overlapping programs, which modulate the expression of a common
set of processes - Thus, different shoots on a given plant may be in different phase sand different areas of
a given shoot can be in different phases, and different areas of a given leaf can be in
different phases
where does developmental phase changes occur first?
- tends to occur first at the base of the shoot
in rapidly flowering herbacious species, how long does the juvenile phase last?
- may only last a few days
in woody species, how long does the juvenile phase last?
- more prolonged juvenile phase, can last for decades
what happens once a meristem has switched to the adult phase?
only adult vegetative structures are
produced culminating in floral evocation
what are the two stages in floral evocation?
- competence
- determination
- A bud is competent if it is able to flower when given the appropriate signal, which leads
to determination
circadian rhythms
- some flowers can open during the day and close at night
- organisms can sense and respond to the time of day
- attuned to light/dark cycles
endogenous oscillator
- internal pacemaker
- exists such that the cycles can continue
in the absence of external cues, in continuous light or darkness
what are the three parameters for circadian rhythm?
period (time between comparable points in the cycle,
e.g., peaks); phase (recognizable features, e.g., peaks for troughs), and amplitudes (distance
between peaks and troughs)
what happens to circadian rhythm in constant light or darkness?
- becomes free-running and depart from an exact 24 hour period?
photoperiodism
he ability of an organism to detect day length, to allow an event to occur at
a particular time of year, i.e., to show a seasonal response
short day plants (SDPs)
flower only in short days, or flowering is accelerated by short days
- flowing SDPs occurs only when day length is less than a critical day length (or night length
exceeds a critical night length)
- wait till the end of summer or fall (sometimes winter) to flower
long day plants (LDPs)
flower only in long days, of their flowering is accelerated by long days
- Flowering in LDPs is promoted when the day length exceeds a critical day length (or night length
is shorter than a critical night length)
- will wait till spring or early summer to flower
day neutral plants
species that will flower under any photoperiodic condition
where is the site of perception of the photoperiodic signal?
the leaf
- The treatment of a single leaf of a SDP with short photoperiod is enough to trigger flowering in that plant
how do plants measure day length?
- by measuring the length of the night?
- Giving a short flash of light i.e., a “night break” during a long night cancels the effect of the long-night, and disrupts flowering in SDPs and triggers flowering in LDPs
what is the primary photoreceptor in periodism?
- phytochrome
- If a night break is given as red light, it has the effect of disrupting SPD flowering and triggering LDP flowering, which can be reversed by far-red light
Vernalization
- the process whereby flowering is promoted by a cold treatment given to a fully hydrated seed or to a growing plant
- Without the cold treatment, plants that require vernalization show delayed flowering or remain vegetative
- some plants must reach a minimal size before they become sensitive to low temperature
- effective temp range is from below freezing to about 10 degrees
- occurs primarily in the shoot apical meristem
when are winter annuals sensitive to vernalization?
respond to low temperature very early in their life cycle
florigen
- grafting studies revealed there was a transmissible floral stimulus
- experiment: grafting experiments showing that noninduced
receptor plants were stimulated to flower by being joined to a leaf or shoot from a photoperiodically induced donor plant - is self-propagating
- macromolecules and specific mRNAs for given genes would travel to the phloem into the shoot apical meristem via plasmodesmata
