young leaf shoot tip axillary bud node internode
TRANSCRIPT
Cell expansion plays a major role in growthRoot cells expand their volume 50 times by expanding lengthwise but not widthwise
In roots, cell expansion plays a major role in growthTwo competing plant hormones determine the
direction of cell expansion:GA (gibberellic acid) promotes growth along the lengthEthylene promotes growth along the width
Auxin and Cytokinin control shoot and root growth
High levels of Cytokinin and low levels of Auxin promote shoot development (stems with leaves)
High levels of Auxin and low levels of Cytokinin promote root development
The Miller-Skoog Experiment: Cloning
1. Place single cell on medium with high levels of Cytokinin and low levels of Auxin to promote shoot development (stems with leaves)
2. Place shoots on medium with high levels of Auxin and low levels of Cytokinin to promote root development at the base of the shoot
3. Transfer rooted shoots to soil and grow plants to maturity
Even at this one cell stage the embryo reveals polarity.The first cell division is asymmetric, producing a small
apical cell and a larger basal cell.
The apical cell will later give rise to the entire“embryo proper”. The basal cell will give rise to a small
umbilical cord-like structure called the suspensor.
The small apical cell divides several times to generate the globular embryo. All cells of this embryo appear
morphologically similar.
Gerd Jurgens searched for embryo pattern mutants.
1. Soak seeds in a mutagen
2. Grow plants to maturity
These plants would be carriers of mutations (m/+)
3. When these carriers self-fertilize, the resulting
embryos would be: +/+, m/+, m/m
Mutants similar to gap mutants in flies, lacking regions of the embryo, including the apical structures, the stem (hypocotyl) and root, were identified
L1 and L2 cells divide anticlinally:perpendicular to the surface
These divisions contribute to surface growth without increasing the number of cell layers
Shoot Apical Meristem
-stem cells
The central zone is maintained as a pool of undifferentiated stem cells.
Shoot Apical Meristem
-stem cells -peripheral zone
As cell divisions occur in the central zone, the resultingcells are pushed into the peripheral zone where they areincorporated into organ primordia.
Shoot Apical Meristem
-stem cells -peripheral zone
The central zone cells will give rise to all of the above-ground organs of the mature plant.
Shoot Apical Meristem
-stem cells -peripheral zone
How is the stem cell population maintainedthroughout the life of the plant?
Shoot Apical Meristem
-stem cells -peripheral zone
A feedback loop between organ initiation and the stem cell population regulates the size of the meristem.
WUSCHEL and SHOOTMERISTEMLESS mutants fail to develop a shoot apical meristem.
Genes Controlling Meristem Development
Normal heart-stage embryo
WUS or STM mutantembryo
CLAVATA1 and CLAVATA3 mutants developa greatly enlarged shoot apical meristem.
Genes Controlling Meristem Development
Normal heart-stage embryo
WUS or STM mutantembryo
CLV1 or CLV3 mutantembryo
• STM and WUS are required to form and maintain the stem cell population
• CLV1 and CLV3 are required to prevent the over-proliferation of the undifferentiated stem cell population
Genes Controlling Meristem Development
• The greatly enlarged meristems that form in clv mutants are largely suppressed when the activity of STM is reduced (for example, in stm/+ plants).
Genetic Interactions between STM and CLV
• The greatly enlarged meristems that form in clv mutants are largely suppressed when the activity of STM is reduced (for example, in stm/+ plants).
• Similarly, the loss of shoot meristems in stm mutants is restored in plants that have reduced CLV activity (for example in clv/+ plants).
Genetic Interactions between STM and CLV
• STM and WUS are homeobox genes and encode proteins that function as transcription factors
• CLV1 encodes a receptor protein
• CLV3 encodes a small protein that functions as a signaling molecule that binds to the CLV1 receptor
Genes Controlling Meristem Development
CLV / WUS Interactions
CLV3 expression is lost in WUS mutants. Therefore, WUS activates CLV3 expression.
CLV / WUS Interactions
The expression domain of WUS is greatly enlarged in CLV1 and CLV3 mutants.
CLV3 binds to and activates the CLV1 receptor, which then represses WUS expression.
CLV / WUS Interactions
WUS is part of an “organizing center” that promotes stem cell proliferation in overlying cells.