Transcript tissue origins - The Virtual Plant

Applied Plant Anatomy:
Part 1:
The origin of cells, tissues and systems in plants
outline
Where do cells come from?
What controls their formation?
What controls their organization?
tissue origins
meristems
mother cells
procambium
sto
Go!
p
Go!
stop
protophloem
dermal
fundamental
vascular
stop
Go!
Go!
stop
protoxylem
stop
Go!
metaphloem
protoderm
stop
Go!
metaxylem
epidermis
parenchyma
collenchyma
sclerenchyma
The shoot & root apex
The dermal tissue system arises in the shoot
and root apical meristems
The protoderm produces epidermal
cells
similar layers are
formed in roots
differentiation
differentiation starts in meristematic
cells, and new cells which are
formed in specialized layers, such
as the procambium shown at left.
Here, you can see a strand of
protophloem differentiating. These
cells are not functional yet.
Question: Why does the protophloem
differentiate before the protoxylem?
Question: Why do cells enlarge,
what is the optimum size for cells
Do all cells have the same optima?
systems
cell and tissue systems
protective
epidermis, periderm
filling
parenchyma, collenchyma, sclerenchyma
support
collenchyma, sclerenchyma
conductive
phloem, xylem
meristematic
procambium, cambium. periderm
functional
storage, synthesis transport reproductive
the dermal system
the epidermis may become replaced by a new
protective layer, called the periderm. This layer is also
responsible for gas exchange, through structures
called lenticels
Specialized cells
Plants are composed of many different cell types – all
have specific functions and size, shape, wall structure and
function, will be determined by position in the root, stem or
leaf. Specialization does not equate to complexity.
For example, parenchyma cells in the
cortex of roots and shoots, may well have
a storage function.
OR even bigger:-
Remember;.. gas exchange
requires the presence of
stomata
in leaves, gas exchange is
facilitated if intercellular spaces
are present, as in these
aerenchyma cells in the Canna
leaf.
specialized cells
Gymnosperms contain modified cells
which form ducts that contain resins
and terpenoids
all leaves contain
parenchyma cells which
are specialized for
photosynthesis, called
mesophyll cells
supporting tissue 1
supporting tissue takes on many forms
and can be simple or complex.
supporting tissue 1
waterlilly petiole
3
2
pea root
Xylem in 1-4
provides support to
roots, stems &
leaves
1
pea stem
New Zealand flax
extensive fibre caps support vascular
tissue & leaf
5
4
young Pelargonium stem
supporting tissue 2 -- collenchyma
in stems,
angular
lamellar
distribution varies (a) at corners in angular stems
OR:-
the shape in cross section varies usually angular or lamellar
it forms an ring, under the epidermis [outer cortex]; or it
occurs mixed between other tissues, or as an inner cortical
band
collenchyma - facts
supporting tissue 4
supports vascular bundles in leaves
distribution varies (a) at corners in angular stems
(see collenchyma).
it forms an ring, under the epidermis [outer cortex]; or it
occurs mixed between other tissues, or as an inner cortical
band
sclerenchyma
- facts
transport systems 1
Vascular tissue is always
arranged into vascular
bundles in stems and
leaves.
In stems, xylem is normally
inside of (endarch to) the
phloem is described as being
outside of (exarch to) the
xylem. In a minority of
families, phloem occurs on
both sides of the xylem.
These are bicollateral
vascular bundles.
phloem
xylem
collateral
bicollateral
phloem
xylem
transport systems 2
in roots
and an equivalent number of
phloem poles or strands
monocotyledonous roots
contain many (more than 6-7
xylem strands.
The xylem – cell organization
protoxylem
Protoxylem forms in
regions where rapid
cell elongation is still
ongoing. As such,
secondary thickening
is limited, to
accommodate
stretching.
xylem development – juvenile to mature
xylem differentiation involves a
number of critical steps – during each
degradation of content occurs
simultaneously with formation and
synthesis of new, secondary cell walls.
In the he final stages, the cytoplasmic
content (nuclei, organelles etc.) are
broken down, and flushed out to be
recycled. The end product is a series
of cells fit for rapid transport of water
and water-soluble products.
from Esau: Anatomy of seed plants
the xylem – structural changes in development
Diagrams and micrographs from Esau: Plant Anatomy,
the xylem - an overview
although most of the cells of the xylem are dead at maturity
(vessels, tracheids, fire-tracheids and fibers), xylem
parenchyma cells are alive and contain cytoplasm a nuclei
and organelles
Diagrams and micrographs from Esau: Plant Anatomy,
the xylem 3 cells and tissue
F
X
T
Xylem in dicots and monocots
contain vessel members (V);
tracheids (T); fibres (F) and
xylem parenchyma elements
In gymnosperms, vessels are absent
Diagrams and micrographs from Esau: Plant Anatomy,
The xylem – transporting water
moving water longitudinally
as well as laterally, requires
apertures within cell walls.
The xylem contain=s a
variety of apertures called
pits, which facilitate water
movement, and minimize
potential damage caused by
embolisms (cavitation of the
water column under high
negative water potential).
Diagrams and micrographs from Esau: Plant Anatomy,
the xylem 4 – safer transport
Perforation plates in vessels are
important structures, that will retard,
or trap air bubbles which are formed
during embolisms. Embolisms will,
unless trapped, cause complete loss
of functionality of the file of xylem
vessels in which the bubbles occur.
unsafe
transport systems - the phloem 1
CC
sieve plate
ST
phloem in melon petiole
evolution of sieve tubes from Esau Anatomy of Seed Plants
phloem tissue in angiosperms
contains sieve tube members, joined
end to end to form sieve tubes; as
well as companion cells and phloem
parenchyma cells.
in gymnosperms, the phloem comprises
sieve cells, albuminous cells and
parenchyma cells. It is also associated
with transfusion tissue. This is a more
primitive system than the angiosperm
one.
phloem
transfusion tissue
vascular tissue in a pine
needle
transport systems -the phloem in detail
light microscopy is only a starting point to
understanding the structure of cells and
tissues. The image to the right is a
Transmission electron micrograph, which
demonstrates the level of detail and
power of TEM!
in the monocotyledon leaf, the phloem is made up of
parenchyma cells, sieve tubes (ST) and companion
cells (CC). Two types of sieve tubes occur - thick(TWST) and thin-walled (ST) ones. TWST are not
associated with CC.
transport systems - the phloem electron microscopy
in leaves, the sieve tubes are
always narrower in diameter that
associated parenchymatous
elements, including the companion
cells. This is because most phloem
loading is an active process, either
mediated by osmotic potential
alone, or in combination with
sucrose transporters, which are
involved in loading the companion
cell sieve tube complex.
moving carbohydrates -- the phloem
phloem is a complex tissue. Moving
metabolites requires very specialized cells,
called sieve tubes (in angiosperms) which, at
maturity, do not have a vacuole, and do nto
have a nucleus! Their end walls are perforate
and the cells, joined end to end by these
walls, form sieve tubes, through which
assimilates move from a source (of the
assimilate) to the sink (where they are used).
Diagrams and micrographs from Esau: Plant Anatomy,
sieve plates
sieve plates maintain cell integrity.
Keep structures an proteins within
cells, in place. The also have an
important regulatory function
moving metabolites
sieve areas
The phloem is protected
from damage (sudden
pressure change) through
the formation of callose
on sieve plates and
sieve areas
sieve plate
phloem is the principal
carbohydrate transport channel –
this channel is controlled
electron microscopy
sieve tubes contain
plastids. These sieve type
plastids have prominent
protein bodies in them,
with unknown function.
Sieve tubes are relatively
uncluttered with a clear
lumen. Companion cells
are associated nucleate
cells
Origins: Control and regulation through genes
All differentiation is under gene control.
A great deal of work has been done using Arabidopsis
For example:
1. Genes affecting early stages of vascular patterning, prior to
provascular network formation, may promote differentiation along
wide pathways rather than narrow canals, because of failures to
establish efficient canals of auxin flow.
2. It is known that Knotted1-like homeobox1 (knox) genes are
expressed in very specific patterns within shoot meristems and
these genes play an important role in meristem maintenance. In
plants, MADS box genes are most well known.
3. Misexpression of the knox genes, KNAT1 or KNAT2, in
Arabidopsis produces a variety of phenotypes, including lobed
leaves and ectopic stipules and meristems in the sinus, the
region between lobes.
---------------------1
A DNA sequence within genes involved in the regulation of development, about 180 base pairs long. It encodes protein (the
homeodomain, which binds DNA http://en.wikipedia.org/wiki/Homeobox#Plants
More examples
In the Arabidopsis root meristem, initial cells undergo asymmetric
divisions to generate the cell lineages of the root. The scarecrow
mutation results in roots that are missing one cell layer owing to the
disruption of an asymmetric division that normally generates cortex
and endodermis. Cell 86: 432-433 Laurenzio et al. 1996
When differentiating cells leave the meristem field, they actively maintain
a pool of uncommitted cells in the SAM. This suggests the maintenance
of the meristem cells in an undifferentiated state - this is likely to be
shared by other plant species
Shoot apical meristems (SAMs) do not share their set of regulatory
factors with root apical meristems (RAMs), yet both adjust their cell
populations according to the same basic mechanisms, such as
intercellular signaling. In both SAMs and RAMs, these mechanisms
involve interactions between two groups of cell populations, the
pluripotent undifferentiated cells –in the organizing center of the
SAM and in the quiescent center (QC) in the RAM – and the
differentiating cells that will be incorporated into the plant body1.
1Nakajima
and P. Benfey, Signalling in and out: control of cell division and differentiation in the
shoot and root. Plant Cell 14 Suppl. (2002), pp. S265–S276.
Complex issues
PIN genes: Encode
components of auxin
efflux carriers. Two
promoters
More…zippers
it has been suggested that class III homeodomain leucine-zipper1
proteins (HD-Zip III) are involved in vascular development. However,
little is known about the mechanisms of spatial and temporal
organization in each vascular cell.
Arabidopsis inflorescence stems develop extraxylary fibers at specific
sites in interfascicular regions. The spatial specification of
interfascicular fiber differentiation is regulated by the
INTERFASCICULAR FIBERLESS1 (IFL1) gene because mutation of
that gene abolishes the formation of normal interfascicular fibers in
Arabidopsis stems.
A protein structural motif involved in protein-protein interactions in many eukaryotic
regulatory proteins (C/EBP prototype). Contain a repeat structure: Leu residues in
every seventh position, causes a large amount of DNA to loop out.
http://www.fhsu.edu/chemistry/twiese/glossary/biochemglossary.htm
Zipping vascular evolution
•
See http://www.nature.com/nrm/journal/v5/n5/full/nrm1364.html
•
Plant signals
Arabidopsis class III
homeodomain-leucine
zipper (HD-Zip III)
proteins play
overlapping, distinct,
and antagonistic roles
in key aspects of
development that
have evolved during
land plant evolution.
Figure 1 Model of how CLASS III HD-ZIP1 and KANADI activities pattern lateral organs
and vasculature. A centrally derived signal (red) activates CLASS III HD-ZIP genes,
whose activity is antagonistic with that of KANADI activity. Both KANADI and
MIR165/166 negatively regulate CLASS III HD-ZIP genes, (relationship between the two
is not presently known). In lateral organs, CLASS III HD-ZIP activity promotes
adaxial fates and KANADI activity promotes abaxial fates. In the vascular bundles,
interactions between the two gene classes pattern the arrangement of xylem and
phloem tissues. The vascular bundle shown is already differentiated, but the initial
patterning events likely occur just below the apical meristem where provascular cells are
being specified.
–––––––––––––––-–
1Class III homeodomain-leucine zipper proteins
Which comes first?
•
The provision of nutrient and water becomes
priority problems within the developing shoot or
root axis. As the axis elongates and the diameter
increases, so transport becomes more problematic
• Short-distance transport may be accommodated by:diffusion, provided there are adequate cell to cell connections
•or by transmembrane transport, either through cells or along
the cell wall free space interface
Size and shape; long is better for transport
Fig. 1. Changes in surface-to-volume ratios during cell expansion. When a cell (shown here as a cube),
doubles its dimensions via (a) isotropic expansion its volume increases 8-fold whereas its surface increases
only 4-fold and its surface-to-volume ratio is reduced from 6 to 3. (b) Anisotropic expansion of the same
volume, producing long thin cells, increases the surface area to a greater extent and improves the surface-tovolume ratio. (c) The original ratio of surface-to-volume can be maintained when cell expansion is followed by
cell division.
From: Kondorosi et al, Current Opinion in Plant Biology
Volume 3, Issue 6, 1 December 2000, Pages 488-492
Applied plant anatomy
Next: Part 2:
the root-stem-leaf continuum Intro Anatomy 1