Download Lec-3 Cell differentiation, Senescence

Plant Physiology
Cell differentiation and Senescence
Cell differentiation
Differentiation:
both ways
Totipotency: the ability of a single cell to divide and
produce all the differentiated cells in an organism
Tracheary Elements: Vessel elements+Tracheids
•facilitate water and solute movement
•Construct secondary wall composed of cellulose and
lignin
Cells of adult plants remain totipotent:
cloning a carrot
Regenerated adult plant…
1 mm3 fragments (“explants”)
from adult root…
Induce with hormones to initiate shoot
and root formation e.g. Auxin, Cytokinin,
Ethylene
Culture “embroid” in liquid culture,
then agar…
Culture explants in liquid culture medium…
Cells “dedifferentiate” and begin to divide,
forming “callus” tissue…
Move to soil…
Culturing Plant Tissue - the
steps
• Establishment of the
explant in a culture
medium. The medium
sustains the plant cells and
encourages cell division. It
can be solid or liquid
• Each plant species has
particular medium
requirements that must be
established by trial and
error
Culture Medium constituents
• Inorganic salt formulations
• Source of carbohydrate
• Vitamins
• Water
• Plant hormones - auxins, cytokinins, GA’s
• Solidifying agents
Media Composition
Macronutrients
(required content in the plant - 0.1% or % per
dry weight) - C, H, O, P, K, N, S, Ca, Mg
Micronutrients (requirement - ppm/dry
weight) - Fe, Mn, Zn, Cu, B, Cl, Mo, Na, Se and
Si are essential for some plants
• Two Hormones Affect Plant Differentiation:
– Auxin: Stimulates Root Development-cell enlargement
– Cytokinin: Stimulates Shoot Development-cell division
• Generally, the ratio of these two hormones can
determine plant development:
– ↑ Auxin ↓Cytokinin = Root Development
– ↑ Cytokinin ↓Auxin = Shoot Development
– Auxin = Cytokinin = Callus Development
Plant Tissue Culture
Applications
Micropropagation
A single explant can be multiplied into several
thousand plants in less than a year - this allows
fast commercial propagation of new cultivars
e.g. Strawberry, Orchids
Developmental Pathways
Transcription Factors
•1500 in Arabidopsis
MADS box TFs
•30 in Arabidopsis
•Developmental events in roots, leaves, flower, ovule and fruit
Homeobox TFs
•Contain Homeodomain
•KNOTTED1 (KN1) involved in shoot apical meristem maintenance
Cell Signaling
Different proteins, hormones interact to control plant
development
Protein Kinases
Mitogen Activated Protein Kinases
•Protein Phosphorylation
•Arabidopsis MPK4
Arabidopsis
60 MKKKs
10 MKKs
Receptor Kinases
•Contain receptor domain CLAVATA-mutations
increased size of vegetative shoot apical and
floral meristems
•Receptor Ligand
•Small proteins or peptides that activates the
receptor kinases
20 MAPKs
Leaves in fall tell a story about life and death
Plant Senescence and Organ Abscission
Concept
• Senescence: A program in which the function of
organ or whole plant naturally declines to death.
This is a genetically programmed essential phase of
the growth and development in plant.
• Abscission: Specific cells in petiole form an
abscission layer, thus facilitating the senescent
organ to separate from the plant
Types of plant senescence
• (1) Overall Senescence
• Senescence occurs in
whole plant body, such as
annuals which senesces
to death after flower and
setting.
• (2)Top Senescence
• The part aboveground dies with the end
of growth season but the part
underground is alive for several years.
• Perennial weeds , corm and bulb—
—lily.
In summer
In winter
• (3) Deciduous senescence
• The leaf falls in specious
season, in summer or
winter.
• Deciduous
• trees
• (4)Progressive senescence
• Senescence only occurs in older organ or
tissue. New organ or tissue develops
while old ones are senescing.
• Green trees。
(A) Development of Arabidopsis thaliana
plants is shown at various times after
germination. Photographs show plants at 14,
21, 37, and 53 days after germination ( left to
right). Note the yellowing of shoots of the 53day-old plant. (B) Age-related changes in
rosette leaves of Arabidopsis 7, 9, and 11 days
after leaf expansion had ceased. Note the
progressive yellowing of leaves, beginning
farthest from the main veins
Senescence in Arabidopsis thaliana.
Arabidopsis mutants affected in senescence
Arabidopsis leaf senescence
Rice leaf senescence
Abscission zone at base of
leaf at the where it joints
the stem
leaf
stem
Auxin prevents abscission
Senescence Regulation
SDG
Senescence down regulated genes
include chlorophyll a/b-binding protein
gene (CAB), Rubisco small subunit gene (SSU).
SAG (up-regulated during leaf senescence)
Class I SAG
Senescence associated genes
expressed only during senescence (senescence-specific).
Class II SAG
Senescence associated genes
have basal level of expression
during early leaf development, but
expression increases during senescence
Basal expression (SAG II)
Leaf develops to full expansion
Senescence progression
Onset (SAG I)
Leaf Senescence in Arabidopsis
Class I – genes that control
developmental aging
Class II – genes that control both
senescence and other growth
processes
Class III- genes that control
senescence in response to
environmental factors
Class IV- regulatory genes that
up-regulate senescence activity
AND down regulate cellular
maintenance activities
Class V- genes that control
degradation of senescence
regulatory factors
Programmed cell death is a specialized type of senescence
Process whereby individual cells activate an intrinsic senescence program
= Programmed Cell Death (PCD)
In animals, PCD may be initiated by specific signals (errors in DNA
replication during division)
- involves expression of a characteristic set of genes, resulting in
cell death
- accompanied by morphological and biochemical changes
(apoptosis, Greek: “falling off”)
- during apoptosis, cell nucleus condenses and DNA fragments in
a specific pattern
Programmed cell death is a specialized type of senescence
PCD in plants, less well characterized
- but similar histological changes as in animals observed
- PCD occurs during differentiation of xylem tracheary elements,
during which nuclei and chromatin degrade and cytoplasm
disappears → activation of genes encoding nucleases and
proteases
- protection against pathogenic organisms
- infection by pathogen causes plant cells to quickly accumulate high
concentrations of toxic phenolic compounds and die (it’s not quite as
simple) → dead cells form small circular island of cell death (necrotic
lesion)
- necrotic lesions isolate and prevent infection from spreading to
surrounding healthy tissues by surrounding the pathogen with a toxic
and nutritionally depleted environment (hypersensitive response)
Further Readings
• Growth and Development, Plant Physiology by
Taiz and Zeiger