Week 9 Flashcards
Shoot apical meristem
give rise to new tissue; at tip of each stem
Upward growth
initiates leaves and produces new meristems which allow plants to branch
As new cells are added, cells further away stop dividing
meristem identity genes
controlled by chemical signals from cells at very tip of stem; help cells maintain ability to divide
stem elongation
in the internodes located immediately beneath the shoot apical meristem
In an elongating internode, cells grow more in length than width because of orientation of cellulose molecules which are primarily wrapped around the cell perpendicular to the axis of the stem rather than parallel with it
Because cellulose is very strong, cell expands more in length
Large central vacuole forms in cell elongation- most growth is mainly due to uptake of water
Moving away from shoot tip, cells reach final size and differentiate
branches- spore dispersing vascular plants
shoot apical meristem divides into two, giving rise to two stems, each with its own shoot apical meristem
branches- seed plants
branches grow out of axillary buds, which are meristems that form at the base of each lead
procambial cells
When a developing leaf is still very small, files of cells within the lead begin to elongate, forming discrete strands of procambial cells that extend from near the tip of the leaf to the mature vascular tissues within the stem
Procambial cells ultimately give rise to xylem and phloem
florigen
Flowering triggered by florigen, a protein produced in leaves and transported through the phloem to apical meristems and axillary buds
auxin influences placement of new lead primordia
Auxin is synthesized in rapidly dividing cells within the shoot meristem and is transported in the outermost cell layer toward the shoot tip.
High levels of auxin in the outermost layer of cells trigger the growth of new leaf primordia.
The movement of auxin into leaf primordia depletes auxin from surrounding regions and thus prevents new leaf primordia from forming nearby.
As the shoot continues to grow, already formed leaf primordia become displaced from the shoot tip and spread farther and farther apart.
This allows auxin moving upward in the outermost cell layer to accumulate to levels needed to trigger new leaf primordia close to the shoot tip.
auxin transport guides the formation of vascular bundles that connect the leaf to the stem
As auxin is transported into leaf primordia, it promotes cell expansion and the young leaves grow.
However, auxin does not remain in the developing leaf.
Instead, auxin that has accumulated at the tip of the young leaf is transported back toward the stem, but this time through cells in the interior of the leaf.
As auxin moves through these cells, it causes the cells both to elongate and to produce more of the membrane transport proteins needed for auxin to exit the cell.
The cells that initially, by chance, have the highest auxin levels become highly efficient at transporting auxin toward the stem.
This, in turn, creates distinct “channels” through which auxin drains from the leaf.
These continuous strands of elongate procambial cells eventually develop into veins and vascular bundles containing mature xylem and phloem.
polar transport of auxin
coordinated movement of auxin across many cells in a single direction; requires energy input
Depends on difference in pH between cell wall and cytoplasm
For auxin to leave a cell, an auxin-specific plasma membrane transport protein called PIN is needed
gibberellic acid
Produced naturally in plants, particularly in growing regions like developing leaves and elongating stems
Increase internode elongation by reducing the force need to cause cell walls to expand
cytokinins
Produced in plant meristems
Promote shoot growth by increasing the number of dividing cells in the apical meristem and, when applied directly to axillary buds, can stimulate them to develop into branches
apical dominance
Removing the shoot tip stimulates the outgrowth of axillary buds along that branch or stem, suggesting that the shoot apical meristem somehow suppresses the growth of axillary buds
This suppression is referred to as apical dominance
Apical dominance prevents branches from being formed too close to the shoot tip, yet new branches can still be formed if the shoot meristem is damaged.
Results from chemical signals from the shoot tip like auxin
Auxin suppresses growth of axillary buds by inhibiting synthesis of cytokinins
strigolactone
made in roots and transported upward in the xylem; inhibits the outgrowth of auxillary buds
primary vs secondary growth
Primary growth: increase in length made possible by apical meristems
Secondary growth: increase in diameter by lateral meristems
lateral meristems
source of new cells for plant growth in diameter
surround the stem not tip
become larger over time
vascular and cork cambium
vascular cambium
source of new xylem and phloem
Those inside of it become secondary xylem
Those outside become secondary phloem
cork cambium
plants with secondary growth have a second lateral meristem called the cork cambium, which renews and maintains a protective outer layer
as the plant diameter increases
epidermis formed during primary growth eventually ruptures
fibres
function is support; thick walls and almost no lumen
vessel elements
extremely wide; function is water transport
tropism
bending or turning of an organism in response to an external signal such as light or gravity
positively phototropic
bend to light; plant stems
negatively gavitropic
grow against force of gravity; also plant stems
plant roots
positively gravitropic and negatively phototropic
phytochrome
photoreceptor that switches back and forth between two stable forms depending on its exposure to light
allows dormant seeds to detect the presence of plants overhead
Independent of whether there is light or not, Pfr converts back into inactive Pr form
removing apical meristem shoot tip
stimulates outgrowth of axillary buds along that branch or stem
active xylem
sapwood layer
inactive xylem
hardwood layer
gymnosperm xylem
tracheid
angiosperm xylem
fibres and vessels
root apical meristems
have root cap
does not produce any lateral organs like leaves
root hairs only formed after elongation stops
root elongation and vascular developments are coordinated
auxin moves down root apical meristem
as it passes through elongation zone, triggers formation of procambial cells that become phloem and xylem
root branching
new root apical meristem develop from pericycle
auxin in roots
decreases elongation
gravity sensing cells in root cap contain
statoliths
plants in shade of others
grow taller and branch less
water is scarce
roots elongate more and branch less
produce abscisic acid
exposure to winds
shorter and stronger stems
increase in ethylene
