lec 29- organ formation from shoot apical meristem Flashcards
what does the shoot apical meristem produce during vegetative growth?
leaves
what does the shoot apical meristem produce during reproductive growth?
flowers
what is the leaf primordium?
leaves first arise as swellings around the outer edge of the meristem
as the shoot grows, what happens at the meristem?
leaves grow at the meristem in regular intervals with particular spacing and arrangement
what Is the arrangement of organs around the meristem called?
phyllotaxy
what are the steps to organ production during vegetative growth?
-first indication of leaf formation is the leaf primordium
-the protrusions form when cell divisions in a localized area increase, Extension of the protrusions is regulated by controlling the directions in which cells expand
-in spiral arrangement, new leaf primordia form at a fixed angle from the previous
how are the positions of the leaf primordia determined?
by the lateral inhibition model
explain the lateral inhibition model with respect to leaf primordia?
-each primordium generates an inhibitory field that blocks organ formation near it
-the next primordium emerges in a space furthest away from all of the other primordia
-as the plant grows, existing organs move away from the meristem, allowing for the inhibition are the new location to lower so a new leaf primordia can grow
what is the evidence that supports the lateral inhibition model?
-surgical removal of a primordium causes a shift in the formation of the next primordium towards the vacant space
-this is caused by there no longer being an inhibitory field in that area that can prevent leaf primordium growth
does auxin regulate organ formation and distribution around meristems?
yes
what evidence is there of auxin regulating organ formation and distribution around meristems?
-many studies have shown that auxin is transported directionally towards sites where primordia will form
-PIN1 auxin export proteins are located on the side of cells where the new organ is initiating, they pump auxin towards the convergence points
-the areas that have been depleted with auxin become inhibitory zones
-since the position of the new primordia correlates with the auxin maxima, it is thought that auxin acts as an activator of organ initiation, while the depleted auxin areas prevent other organ initiation near it
what is evidence of auxin helping organ inititation seen in mutants?
-mutants that lack the PIN1 export proteins are unable to produce lateral organs along the stem, only a few misshapen leaves
-when auxin is applied to a spot on the side of a pin1 mutant meristem, a lateral organ is induced
what causes vegetative meristem to switch to producing flowers
via a response to extrinsic and intrinsic cues
explain the first steps for the switch from vegetative meristem to reproductive meristem:
-vegetative meristem is converted into an inflorescence meristem (IM), the IM then forms floral meristems (FMs) on its flanks which each produce a single flower
-inflorescence meristems are indeterminate, cell division produces new cells for the stem and new floral meristems
-floral meristems are determinate because they only produce flowers
why does the cauliflower not have flowers?
-because they carry a mutation in a gene that causes the inflorescence meristem to continuously generate replicas of itself on its flanks, these meristems fail to make the transition
-they can occasionally flower in cooler temperatures
what causes vegetative meristem to turn into inflorescence meristem?
-environmental (extrinsic) and developmental (intrinsic) signals
-5 pathways: Gibberellin, autonomous, vernalization, thermosensory, and photoperiod
-three transcription factors in the pathways: 1) FLC and SVP 2) AGL4 and SOC1 3) FT and FD
explain how the transcription factors cause switch from vegetative meristem to inflorescence meristem:
-during vegetative growth, FLC/SVP inhibits AGL4/SOC1 and FT/FD, preventing them from promoting flowering
-some sensory pathways either inhibit FLC/SVP or promote AGL4/SOC1 and FT/FD
-sensory pathway signals are low during vegetative growth but but when increased to higher levels, FLC/SVP becomes low enough to the point where it cant inhibit
-when AGL4/SOC1 and FT/FD reach high levels, the proteins promote the transition from vegetative meristem to inflorescence meristem by activating the transcription of meristem identity genes (LEAFY and AP1)
how does climate change affect reproduction in plants?
-changes in the climate affect the sensory environmental cues of some species, causing them to experience changes in flowering timing
-this affects reproduction because if flowering occurs at wrong time, embryos may experience environmental conditions they cant tolerate, can also cause mismatches in different species (e.g. presence of pollinators)
how does day length control flowering?
-day length is sensed by the leaves, when the period of continuous light becomes long enough, it causes a diffusible flower inducing signal to be produced and sent through the phloem to the shoot meristem
-when the signal reaches the shoot meristem, it induces the switch from vegetative to floral meristem
how is the flower inducing gene produced?
-one of the genes is a transcription factor known as CONSTANS (CO), its expression oscillates on a 24-hour cycle with the highest levels in the afternoon
-in short days, the CO proteins is degraded in the dark
-in long days, expression of the protein still increases at the same point in the 24-hour cycle, but now it stay lighter for longer in the late afternoon
-because the increase in CO expressions begins in the afternoon when there is still light, there is enough of it to induce flowering
explain how CONSTANS molecularly induces the change from vegetative meristem to inflorescence meristem:
-CONSTANS induces expression of FT
-when CONSTANS is high enough, FT accumulates in leaves and travels through the phloem to the shoot apex
-in the shoot apex, FT interacts with the transcription factor FLOWERING LOCUS D (FD), the two proteins form a complex that activates expression of floral meristem identity genes and the meristem is converted
