Plant phys exam 2

When do new plant cell walls arise?

during cytokinesis

What is the position of a new plant cell wall determined by?

pre-prophase band of microtubules

Define pro-prophase

before the mitotic spindle forms but after the interphase microtubules have broken down (unique structure to plants)

• band is completely broken down at the end of prophase… remembers is orientation during cytokinesis

How is the plane of cell division determined in animal cells?

determined by the position of the mitotic spindle (perpendicular)

How do new plant cell walls arise?

• - cell plate= developing cell wall

• - cell plate formed inside to outside in late anaphase/early telophase

• cell plate organized by the phragmoplast

What is the phragmoplast?

• unique to plant cells

• microtubules, ER, vesicles

What forms the plasmodesma?

• tubular channels of cytoplasm and ER

• connect adjacent plant cells

• provides cytoplasmic continuity

Ho w do adjacent cells communicate?

• transcription factors

• gene transcripts (mRNAs)

• hormones

• proteins

Plant cell wall composition

• cellulose

• hemicellulose

• pectin

• proteins

some also have- waxes/suberin/cutin and lignin

VARIABLE

What system is a key player in cell wall formation?

/

endomembrane system

• proteins, hemicellulose/pectins, and cellulose

What is the middle lamella?

• outermost layer of plant cell wall

• made of carbohydrates= pectin

• glue adjacent plant cells together

• synthesized/ secreted during cell plate formation

What is the primary cell wall?

• synthesized after middle lamella

• made of carbohydrates and proteins

• thin and flexible (rigid AND extensible)

Explain the components of the primary cell wall

• cellulose: structural backbone (linear polymer); H+ bonds to form polymers to form higher order= cellulose microfibrils

What is the benefit of H+ bonding within cellulose polymers?

• produces a highly ordered structure and high tensile strength, and resistant to chemical attack

How are cellulose microfibrils formed?

ring structure with 6 particles (particle rosette)

• each particle synthesized in ER, processed through the golgi and delivered to and incorporated into plasma membrane

In plants, how many cellulose synthases enzymes are predicted to be present in each rosette?

3 cellulose synthase enzymes

What happens when cellulose microfibrils form?

1- polysaccharides (hemicelluloses/pectin) are synthesized in the GOLGI by membrane bound enzymes

2- transported to cell wall to be exocytosis

3- form a meshwork around the cellulose microfibrils

Describe hemicelluloses and pectins

• composed of many different carbohydrates

• each cell types has a unique combination

• large complex polymers

Characteristics of hemicellulose

• Beta 1,4 sugar backbone with many short side chains 

• diverse and named for sugars present

• bind VERY tightly to cell wall

• H bonds to microfibrils

• hemicellulose- spaces apart microfibrils;   resists compression forces

• hydrophilic= help resist compression forces

What is the significance of the 1,4 sugar backbone in hemicelluloses?

• produces a very flat/linear polymer (ribbon-like)

• side branches prevent aggregation with each other

What are the characteristics of pectin?

• hydrated polymers

• resist compression forces, provide charged surface for cell-cell adhesion

• diverse but lots of acidic sugars

Carboxyl group characteristics at pH variations

• pH of 7= COO- form is predominate/unprotonated (charged)… less H+ = more interactions; creates a stiffer gel around the other wall components

• pH of 5.5= protonated; more H+= less interactions; salt bridges are released… helps loosen wall for expansion

What happens when cellulose microfibrils form?

• proteins are also added to cell wall

• synthesized in ER/modified in Golgi

• transported to wall in vesicles and exocytosed

What are the functional proteins found in cell walls?

• oxidative enzymes- peroxidases AND expansins

• hydrolytic enzymes- pectinases/cellulases/hemicellulases AND chitinases

What do the oxidative enzymes do?

• peroxidase- strengthens, rigidifies cell walls

• expansins- requires low pH, catalyzes cell wall growth/expansion

What do the hydrolytic enzymes do?

• pectinases/cellulases/hemicellulases- weaken wall… allows expansion to occur

• chitinases- defense against insects… weakens exoskeleton of fungi/insects

What is the purpose of structural proteins in plant cell walls?

• rigidify/strengthen cell walls

• vary in abundance, by cell type, maturation state of cell

What materials forms bonds within the plant wall?

polysaccharides and proteins (high lysine content)

Plant cell wall structure

Secondary cell wall characteristics

• produced last/internally and after cell elongation/expansion

• primarily composed of cellulose, hemicelluloses and lignin

• very thick, rigid

Orientations of cell wall

• provides MAXIMUM strength to cell wall

Lignin characteristics

• synthesized from 3 different precursors in cytosol (H, G, or S lignin)

• transported into cell wall via ABC transporters (hydrophobic substances)

How do cellulose microfibrils and lignin impact the secondary cell wall?

impart great strength against gravity, rigidity, and hydrophobicity

• physical block to pathogens (attack/wounding by pathogens0

• chemically resistant to breakdown

• undigestible- not nutritional value

How do animal cells drive changes in cell shape/movement?

they use the cytoskeleton- microfilaments and myosin

How do plant cells drive changes in cell shape/movement?

by loosening cell wall & turgor pressure to change shape/expand

What is the acid growth hypothesis?

a mechanism for cell wall loosening and expansion

• auxin promotes elongation of shoots/roots

  • auxin rapidly stimulates elongation in shoots

  • maximal growth in 30-60mins… suggests transcription of genes might play a role

• most cell elongation occurs close to meristem

What is coleoptile?

• in monocots; hollow cylindrical sheath

• surrounds/protects young leaves during germination

What does the apical hook do during germination?

protects apical meristem in dicots

• suggests acidification is needed in elongation

What are the roles of Auxin in cell expansion/growth?

• promotes H+ efflux from cell

• prevents H+ pumps from being endocytosed into cytoplasm

• promotes transcription of new H+ pumps/K+ channels/ expansin

How does auxin promote transcription?

promotes degradation of the repressor Aux/IAA… removal allows transcriptional activator to dimerize/promote transcription

How does auxin mediate Aux/IAA protein degradation?

Auxin promotes ubiquitin (UB) mediated protein degradation

• ubiquitin is attached to a protein marking it for degradation in the cytoplasm

What is required for ubiquitin mediated degradation?

• 3 enzymes: E1, E2, E3 enzymes (activating, conjugating, ligase

• proteins must be at least 4 UB molecules to be targeted/degraded

• most variable enzyme is E3… moves to target (several different kinds)

What are the names for the proteins associated with auxin degradation?

• S = Skp1 (ASK1-arabidopsis skip1)

• C = cul1 protein

• F = F-box protein… conserved region (very diverse)

What mediates cell wall loosening?

expansin protein… breaks bonds between cellulose microfibrils and hemicelluloses

What happens during the cessation of growth/expansion?

• decrease acidification of cell wall

• increase the # linkages within wall

• form secondary call wall and add lignin (irreversible)

What is cell shape/direction of cell expansion determined by?

orientation of cellulose microfibrils

• growth is perpendicular to orientation of the cellulose microfibrils

** newest cellulose microfibrils determine direction of growth

Tip growth is an example of…

polarized growth and cell expansion

• all growth occurs near the tip of the cell… loosening of cell wall is localized to tip

What determines orientation of cellulose microfibrils?

cortical microtubules… just under plasma membrane

• its orientation parallels the cellulose microfibril orientation

What is the role of the cortical microtubular?

• guidance tracts (propelled through plasma membrane)

• directly associated with microtubules (motor protein walks along)

Cell expansion/elongation depends on # of interacting factors:

• Position within plant and plant body (pressure forces by other cells)

• Cell type and genetics- epidermal vs vessel 

• environmental influences- light vs dark

• hormonal influences in plants

What do plant hormones do for plants?

• important for efficient communication between cells, tissues, and organs

• synthesized in one area of plant- transported to another area to invoke response (typically vascular tissues)

Synthesis and transport of Auxins

• synthesized in actively dividing cells (meristems)

• transported in parenchyma cells in xylem tissues

• transport is ‘polarized’ (directional)

What are two forms of auxin?

• protonated (IAAH)  neutral

• unprotonated (IAA-)  charged… leaves through transport proteins

** pH determines ratio of forms

PIN proteins are…

efflux carriers

• # of different PINS in plants, distributed among different tissues

How does auxin promote formation of lateral adventitious roots

• Auxin moves down root functioning as morphogen

• At specific concentration the outermost layer of the vascular tissues (pericycle) begin dividing

How does auxin promote root hair formation

Auxin is redirected back towards shoot in epidermal/cortical cells

• requires specific concentration

• regulates gene transcription

How does auxin promote patterning during embryogenesis

• cotyledon (embryonic leaves) formation during embryogenesis

• PIN1 is relocalized to create changes in auxin flow

How does auxin promotes vascular tissue differentiation

• xylem differentiation during secondary growth

How does auxin determines position of new floral primordia in reproductive meristems

• auxin is localized to where new floral primordia arise

• once initiated-auxin concentrations decrease

• primordia eventually produce flowers

How are adventitious roots form by auxin?

• auxin collects/concentrates at base of stem… promotes root formation

  • adventitious= forms in unusual place (produced for structural support)

How is auxin involved in apical dominance

Inhibits growth of the youngest axillary buds

How does auxin mediate tropic curvature

• requires asymmetric cell elongation across the organ

• Auxin promotes cell elongation in shoots but inhibits cell elongation in roots

** tropism- positive and negative

Gravitropism

roots- positive gravitropic response (towards gravity)

shoots- negative gravitropic response (away from gravity)

What is the starch statolith hypothesis?

• perception of gravity is due to the movement of statoliths(heavy, starch storing plastids) in statocytes (gravity sensing cells)

• change is statolith position triggers gravitropic response (starts with signal cascade)

Auxin is required for… 

cell division (with cytokinin)

** can’t do cell cycle without

Auxin promotes…

fruit formation (in part by stimulating cell division and cell expansion)

How does auxin regulate so many processes during development?

• partly by degrading repressor proteins and regulating differential gene expression

  • family of 22 ARF’s

  • 24 different AUX/IAA proteins interact with the ARF’s

• development is regulated by differential gene expression

Describe cytokinin

Discovered in search for factors that promote cell division

• kinetin was first identified cytokinin (breakdown product of autoclaved herring sperm DNA

• ones that are naturally occurring are derivatives of adenine

What is a major site of cytokinin synthesis?

the root of plant

• many tissues can synthesize cytokinin (ovules, immature seeds)

What are cytokinin functions?

• help regulate cell cycle along with auxin

• help regulate shoot and root formation in callus along with auxin

  • callus- ball of undifferentiated cells

• causes ‘greening’ of plant

• delay leaf senescence (death process)

What is cytokinin required for?

normal activity of SAM and RAM

• promotes cell division in shoot

• repress cell division activity of the root

Hoe does cytokinin support apical dominance?

keeping cytokinin concentration low by degrading biosynthesis

• IPT is the first enzyme required for cytokinin synthesis

Cytokinin receptors…

plasma membrane/ER

What are ARRs?

Arabidopsis Response Regulators

• Type-B ARR: positive regulators of CK response (positive)

• type-A ARR: negative regulator of CK response (negative)

  • shuts down transcription pathway

How were gibberellins discovered?

Foolish seedling disease- severe mutation caused plants to fall over into water and die

• japan late 1890s

gibberellin characteristics

• 4 ringed structures (biosynthesis starts in plastid and ends in cytosol)

• numbered in order of discovery (many intermediates)

• made is various tissues (plastids associated)

• transport primarily through phloem

What is the the Role Of Gibberellins In Plant Development?

1. Seed germination-discuss specifics later in semester

2. Promotes phase changes: juvenile to adult morphology

• GA is especially important in leaf morphology changes (shape/arrangement)

3. Promotes stem elongation and flowering in some plants

• flowering requires long day to grow

4. pollen development/complete development in seeds/fruit

• not enough GA= seed development is effected

What is bolting?

• growth of flowering stock

• induced by gibberellin

• stimulates internode elongation

Describe the molecular Mechanism for GA responses

DELLA: repressor protein

PIF3/4: Transcription Factors for GA mediated growth and development

SCF complex (SCFSLY1)

Abscission means…

the shedding of various parts of an organism

Abscisic acid

ABA promotes synthesis of the plant growth hormone ethylene

• ethylene is actually responsible for abscission

• ABA plays an important role in bud dormancy

• all cell with a plastid can synthesize ABA… transported in xylem

Abscisic acid functions

Primary function is as a growth inhibitor

• negative regulator of growth (dormancy in seeds & buds)

2. Stress hormone

• response to drying, cold temps, salt change

3. Can respond to some pathogens and promote plant defenses

Abscisic acid can regulate…

gene transcription

ABA Receptor Kinase (SnRK2):

• phosphorylates itself and ABA transcription factors (ABF)

• ABFs bind to promoter elements & initiate transcription of ABA inducible genes

Abscisic acid can mediate…

ion channel activity

• guard cell closing

ABA Synthesized in root, transported in transpiration stream

• Enters guard cells

• SnRK2 kinase can phosphorylate and activates Ion channels that lead to guard cell closure (and prevent water loss)

• water follows ion/osmotic gradient

What features are unique to guard cell?

1. Guard Cells are anchored to each other at their “ends”

2. Thickened Walls that face the pore

3. Radial Micellation of cellulose microfibrils (provide rigidity)

guard cells are attached to each other at their tip and have reinforced walls lining the pore