Plant Growth and Development II
"It is at the edge of a petal that love awaits.”
...William Carlos Williams
apical/basal, axial
Patterns of
Development
SAM
embryogenesis
Zygote
Embryo
RAM
primary
growth
SAM
? ? ?
primary
growth
Cotyledons
Hypocotyl
Leaf
Primordia
Cell
Differentiation
Stem
Tissues
?
1o Growth
2o Growth
Structure/Function
RAM
Root
Tissues
1o Growth
?
2o Growth
Today
(continue)
• How do plant organs, tissues and cells develop?
– Examine Plant Growth,
• primary growth,
• secondary growth,
• cell elongation
– How is Plant Cell Differentiation Studied,
• discovering the process by which a cell acquires metabolic,
structural and functional properties.
Lateral
Meristems...
…provide for secondary growth by producing
secondary vascular tissue and periderm (secondary
dermal tissue).
Secondary Growth of Stems
• Two Lateral Meristems,
– Vascular cambium; produces secondary vascular
tissue,
– Cork cambium; produces tissue (periderm) that
replaces the epidermis,
• Secondary phloem and periderm comprise bark.
Vascular Cambium
Fig. 35.20
Secondary Growth
Year 1
Fig. 35.21
Lateral Meristem Cells
Fusiform initials: meristematic
cells that give rise to xylem and
phloem.
Ray initials: meristematic cells
that give rise to (primarily)
parenchyma cells that serve as
radial connections.
Tangential Section
Secondary Growth
Year 2
Fig. 35.21
Secondary Growth
Fig. 35.21
Secondary Growth
Assignment
Be able to construct a tree from a seedling using these meristems,
- at the tissue level.
Growth / Differentiation
• Growth,
• the irreversible increase in size that (in plants)
almost always results from both cell division and
cell enlargement,
• Differentiation,
• the process by which a cell acquires metabolic,
structural and functional properties distinct from
those of its progenitor.
Cell Division / Cell Walls / Cell Growth
Fig. 12.8
Fig. 35.10c
Plane of Division
Fig 35.28
Fig 35.27
Plant Cell Walls
Cell Morphology
Water Relations
Bulk Flow
Plant Morphology
Cell Morphology
Biochemistry
Mechanical and Structural
Pathogen Defense
Cellulose / Cell Walls
Fig. 5.8
Cell Wall Synthesis
Fig 35.29
Microtubules (pp. 127, Fig. 7.21)
Cell Expansion
Biased Microfibril
Distribution allows for
directional growth.
Secondary Walls:
More ordered, restricts general
enlargement, often lignified
(wood).
Primary Walls:
Less ordered, allows general enlargement.
Turgor: water potential is
lowered in the cell, allowing
water uptake. The force of the
water pressure drives cell
expansion.
Acid-Growth
Hypothesis
1. Plasma Membrane H+-ATPases acidify the apoplast (cell wall).
2. Cell wall loosening enzymes are activated.
3. Electrochemical gradient drives solutes into the cell,
- lowers osmotic potential, H2O?
4. Vacuolar ATPase provides membrane potential
for transport of solutes into the vacuole, etc. etc.
ATP hydrolases (ATPases)
Developmental Biology
Modern approach is driven primarily by the study
of genetics,
– primarily through the study of mutants, organisms
blocked in specific developmental pathways,
– Model Organisms.
Model Organisms
•
•
•
•
•
•
Ease of cultivation,
Rapid Reproduction,
Small size,
Fecund (large brood size),
Mutants are available and easy to identify,
Scientifically relevant (ecologically, organ system,
etc.)
• Extant Literature, co-ordinated research emphasis.
60 - 70 % similarity in all eukaryotes.
flowering
plants
QuickTime™ and a
Photo - JPEG decompressor
are needed to see this picture.
Arabisopsis thaliana
Thale cress/Mouse Ear Cress
•
Arabidopsis is a plant belonging to the Mustard family, Cruciferae.
Arabidopsis' agronomic value is as a Model Organism,
•
•
•
•
•
•
weedy: world-wide distribution and easily grown in the lab.
self-fertilizing: it is easy to generate and maintain genetic stocks.
lifecycle: about 42 days at 200 C and continuous light.
fecundity: up to 50,000 seeds per plant.
mutable: yes, lots of ways.
literature: 9718 journal articles (PubMed)
– - ~ 1000 devoted labs.
•
Arabidopsis is THE plant model organism with over 7000 full-time scientists
devoted to understanding the growth and development of this organism, and
the extension of this knowledge to other plants and organisms.
Arabisopsis thaliana
+ 26,000 Genes
Genetics: analysis of
mutant phenotypes,
Reverse Genetics:
analysis of mutant
genotypes,
Genomics: use of
DNA sequence to all
aspects of plant
growth, development,
evolution, ecology...
FASS gene: not cloned
Cell Growth
fass Mutant: cortical
microtubules do not organize.
Pattern Formation
lacks apical-basal axis
GNOM: guanine nucleotide
exchange factor
Homeotic Gene
KNOTTED gene expression
results in the differentiation of
cells into vasculature.
Shoot Development Begins at the Shoot Apical Meristem
Maintenance of the Meristem
•
CLAVATA and WUSCHEL protein interactions
constitute a tightly regulated control mechanism to
maintain and delimit the meristem,
•
wuschel Mutant = Meristem Disappears
•
clavata Mutant = Enlarges Meristem
•
WUSCHEL Gene = Maintains the Meristem
•
CLAVATA Mutant = Delimits the Meristem
Clavata’s Molecular Mechanism
•
clavata 1, 2 and 3 mutants have
identical phenotypes of enlarged
meristems
•
CLAVATA3 protein acts as a signal
molecule on the two-component
clavata receptor, constituted by
CLAVATA 1 and 2
•
The clavata receptor is a leucine rich
repeat (lrr) serine/threonine kinase
receptor.
Clavata Phenotypes
Clavata phenotype
mutant
wt phenotype
CLAVATA turned off
during development
results in determinant
growth, I.e. a leaf.
Transport
Friday
Quiz: Through Chapter 36, 748 - 754