Samenvatting van het tweede gedeelte van de cursus 'Plantenontwikkeling en omgeving' (Plant Development and Environment) aan de UU, gegeven door Kaisa Kajala.
Framework of plant development and intro to roots and EvoDevo
College 12-03-2019 Kaisa Kajala
Understand the four principles (the four Ps) of development that will be the framework for the
coming lectures.
Plant development is the process by which a single cell becomes a complex organism. Different
processes are involved in this: cell division, growth, differentiation, death and migration.
The 4P’s of plant development
- Polarity (polarisation of the apical/basal and radial side)
o How is an axis established and maintained?
o How is its orientation determined? Can it change?
- Positioning (signals for determining cell type)
o How does a cell or organ know where it is?
o How is fate determined in a spatial context?
- Patterning (for example epidermal cells and stomata)
o How do patterns emerge in development?
o What drives patterns and what are their purposes?
- Plasticity (interactions with the environment)
o How does interaction with the environment affect plant development?
o How does plasticity underlie the unique nature of plant development?
Describe the development of the root and the different root cell types and why the root makes a
good model for developmental processes.
A root can be monocot, like rice and maize, or dicot, where there is secondary growth. To understand
the process of morphogenesis in roots, both the developmental history of cell files in the root and the
ontogeny of the initial cells must be known. The morphologies, numbers and spatial relationships of
the cells must be determined. The morphology means the occurrence/traits of the cell.
Longitudinal root anatomy
- Meristematic zone: short, dividing cells at the tip of the root. This is where new cellular
material in the root always comes from.
- Elongation zone: higher up and longer cells
- Maturation zone: longest cells, final length is determined by hormonal balance
Radial root anatomy
Root cells are radially oriented in rings, so there is radial symmetry. The longitudinal temporal axis
makes that the root is a controllable system for developmental studies.
- Rotational symmetry
- Invariant position
- Little variation in cell numbers
Structure and function of root cell types
Epidermis Protection – skin of the root
Root hairs Uptake – larger surface
Cortex Protection against (a)biotic stresses and aerenchyma for O2 transport
Endodermis Regulating water/nutrients inside the root – barrier cell types
Pericycle Giving rise to lateral roots
,Vascular cell types in the root
Xylem Upward water transport – strong cells with ligine to cope with high pressure
Phloem Downward transport of sugars from leave (source) to roots (sink) – transport cells for
transport and companion cells for loading
Procambium For young roots to grow thicker via providing/dividing more cells for the xylem and
phloem - meristematic tissue
Columella Protecting and sensing gravity via high amounts of starch – located in the root cap
Lateral root cap Protection - can be lost in case of mechanical stress
Quiescent centre cells do not divide. They send
signals to surrounding cells to keep these from
differentiating, because the surrounding cells may
not differentiate.
Compare and contrast root and shoot cell types and how their form links to their function
Structure and function of shoot cell types
Epidermis Protection – skin of the leaf and no need to increase surface area
Mesophyll Photosynthesis – large cells inside the epidermis with the chloroplasts in them
Bundle sheath Cells with a function in the stress and defence response
Vasculature Xylem, phloem and procambium
Trichomes Sensing touch, regulating the air flow and the microclimate (cooling/heating) and
metabolic factory – sticking out
Guard cells Opening and closing of stomata – special structure
Consider the development in an evolutionary context
https://www.youtube.com/watch?v=ydqReeTV_vk
Remember: Ectopic = an organ in another place then where it’s supposed to be (eg eye on ass).
Thomas Henry Huxley 1825-1895: Studied the anatomy of fossils of animals and plants to reveal
evolutionary relationships of groups.
Barbara McClinktock 1902-1992: Maize geneticist and Nobel prize winner who discovered genetic
regulation (not all genes are on all the time).
Sean B Carroll 1960-?: How evolution of regulatory elements leads to the evolution of body parts and
patterns.
Review Augstein and Carlsbecker is important background reading!
, Cell type specification
College 14-03-2019 Kaisa Kajala
Cells acquire their identity via transcription factors. Different transcription factors in different
locations cause a different function because they can activate or inhibit promotors.
Cell fate is position dependent and not lineage dependent. Their position is what matters and not their
lineage, because cells that cross the clonal boundary appeared to switch fate according to positional
cues.
Cortex-Endodermis Initial (CEI)
This has to do with the Cortex-Endodermis Initial cell (CEI). There is extrinsic asymmetrical cell division
because the daughter cells have a different fates and there is a signal from outside. First there is an
anticlinal division where the cell doubles itself in the long direction. Then there is a periclinal division
where another layer is added in the radial direction and the cortex and endodermis cell are formed.
Cortical daughter cells are required for CEI periclinal division and to specify the fate of cortex and
endodermis daughters. The CEI is not necessary for the function because another cell can come in and
take over the function/signal. But destroying the cells above stops the signal which means the CEI
won’t happen and the cells won’t get their function. This means daughters above send specific signals
to the initial cells. Information guiding cell fate specification is directed towards the tip through an
individual cell file.
Effects of ablating 1 or 2 quiescence centre (QC) cells
After killing 1 QC cell there is columella differentiation in the initial cell and differentiation of the CEI.
Columella have starch statolites that can sense gravity, so starch can be used to identify which cells
have columella. When a QC cell is ablated the starch gets closer to the QC.
QC is required for keeping CEI from happening too early. When there is no QC the initial differentiates
earlier: the anticlinal division goes missing and the periclinal division happens. There is a balance
between short-range signals that reinforce cell fate decisions and inhibit differentiation. Daughters
send specific signal specific cell fates to the initials and the QC keeps the initials undifferentiated so
that they remain cells that produce more cells.
> QC inhibits CEI from periclinal division and columella from initiation starch
> If this happens too early you loose the initial (meristem cells)
Cell fate specification: mobile transcription factors as determinants of fate
Specification of cortex and endodermis: short root
To discover what molecular pathways regulate the specification of cortex and endodermis, root
mutant screens can be useful. With something (eg gamma radiation) initiates a mutation and
afterwards you look for the right mutants, in this case root growth/short root mutant.
In the short root mutant the cellular layer was affected. In the wild type all the layers are present, but
in the short root mutant the endodermis cell layer was missing (epidermis and cortex present).
Next they found out the layer did have cortex identity. The reason for there being only one ground
tissue layer instead of endodermis AND cortex is that there is no division of CEI. Also QC divides more
than it should and the mutated gene (SHR) encodes a transcription factor of the GRAS family.
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