end

Applied Plant Anatomy:

Part 1:The origin of cells, tissues and systems in plants

outline

Where do cells come from (origins)?

What controls their formation?

What controls their organization?

1. tissue origins

meristemsmeristems

mother cellsmother cells

dermal fundamental vascular

protodermprotoderm

epidermisepidermis

parenchymaparenchyma

collenchymacollenchyma

sclerenchymasclerenchyma

stopstop

stopstop

stopstop

stopstop

procambiumprocambium

protophloemprotophloem

protoxylemprotoxylem

stostopp

metaphloemmetaphloem

metaxylemmetaxylem

stopstop

Go!Go!

Go!Go!

Go!Go!

Go!Go!

Go!Go!

Go!Go!

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 cellsDo all cells have the same optima?

systems

cell and tissue systems

filling

protective

support

conductive

meristematic

functional

epidermis, periderm

parenchyma, collenchyma, sclerenchyma

collenchyma, sclerenchyma

phloem, xylem

procambium, cambium. periderm

storage, synthesis transport reproductive

the dermal system

the epidermis may become replaced by a new protective layer, called the periderm. This layer is also . This layer is also responsible for gas exchange, through structures responsible for gas exchange, through structures called 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.

in leaves, gas exchange is facilitated if intercellular spaces are present, as in these aerenchyma cells in the Canna leaf.

Remember;.. gas exchange requires the presence of stomata

OR even bigger:-

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

pea stempea stem

waterlilly petiolewaterlilly petiolepea rootpea root

young young Pelargonium Pelargonium stemstemNew Zealand flaxNew Zealand flax

extensive fibre capsfibre caps support vascular tissue & leaf

11

2233

4455

XylemXylem in 1-4 provides support to roots, stems & leaves

supporting tissue 2 -- collenchyma

the shape in cross section varies - usually angular or lamellar

angular

lamellar

distribution varies (a) at corners in angular stems

in stems,

OR:-

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

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

supports vascular bundles in leaves

sclerenchyma - facts

transport systems 1

phloemphloem

xylemxylem

bicollateralbicollateral

collateralcollateral

Vascular tissue is always arranged into vascular vascular bundlesbundles in stems and leaves.

In stems, xylemxylem is normally inside of (endarch toendarch to) the phloemphloem is described as being outside of (exarch toexarch to) the xylem. In a minority of families, phloem occurs on both sides of the xylem. These are bicollateral bicollateral vascular bundles.

xylemxylem

phloemphloem

transport systems 2

in roots

monocotyledonous roots contain many (more than 6-7 xylem strands.

and an equivalent number of phloem poles or strands

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 – phloem thus differentiates first

• 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

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

from Esau: Anatomy of seed plants

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.

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

Xylem in dicots and monocots contain vessel members (V); tracheids (T); fibres (F) and xylem parenchyma elements

XT

F

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

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.

vascular tissue in a pine needle

phloem

transfusion tissue

evolution of sieve tubes from Esau Anatomy of Seed Plants

CC

STsieve plate

phloem in melon petiolephloem in melon petiole

transport systems - the phloem 1

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 TWST are not associated with 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 not 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 platessieve platessieve plates maintain cell integrity. Keep structures an proteins within cells, in place. The also have an important regulatory function

phloem is the principal carbohydrate transport channel – this channel is controlled

moving metabolitesmoving metabolites

The phloem is protected from damage (sudden pressure change) through the formation of callose on sieve plates and sieve areas

sieve areas

sieve plate

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

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-to-volume 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

Origins: Control and regulation through genes

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.

All differentiation is under gene control.

A great deal of work has been done using Arabidopsis

3. Misexpression of the knox genes, KNAT1 or KNAT2, in Arabidopsis produces a variety of phenotypes, including lobed leaves and ectopic stipules2 and meristems in the sinus, the region between lobes.

1. Genes which affect early stages of vascular patterning, prior to provascular network formation, may promote differentiation along wide pathways rather than narrow canals, because of failure to establish efficient channels for auxin flow.

For example:

----------------------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

2ectopic recombination refers to recombination between sites, usually containing identical or similar sequences, at different locations in the genome, ... i.e., unusual distribution

Complex issues, simple solutions?

PIN genes: Encode components of auxin efflux carriers. Two promoters

Applied plant anatomy

Next: Part 2:

the root-stem-leaf continuum Intro Anatomy 1