Plant Physiology -- 04/27/09

Lecture 4-7

Last lecture

• quiz
• started gibberellins

Today's lecture:

• rest of gibberellins
• cell walls & growth
• start next hormone

How does GA work?

• makes cell walls looser
  • only true for some plants
  • does not involve H+ efflux (not acid growth)
  • lag before growth starts (transcription)
  • enzymes break down cell wall polysaccharides?
• causes aleurone layers to transcribe "-amylase gene
• breaks down storage materials in cells (not just seeds)
• increases cell division in shoot apex (but not the main promoter)

Cell walls

• location & role
• structure: parts & variations
• synthesis
• mechanical properties
• growth of cell walls
• walls and signals

location -- around each cell

• different from most animal cells
• fungi have walls, too
• bacteria have heftier, less permeable walls

What cell walls do for the plant

• allow turgor pressure to form in cells so they get rigid (no bones in lettuce)
• allow materials to flow apoplastically
• control shape by directing growth
• make plant body out of pretty "cheap" ingredients: CHO

What they don’t do

• keep chemicals out of cells (very little selectivity)
• separate one cell completely from another (plasmodesmata)

Importance to humans

• paper (from wood), fibers (from surface cells [cotton], phloem fibers [linen])
• fiber in diets
• soil organic matter

Structure: layers of wall

• membrane
• primary wall
• secondary wall (multiple layers)
• thicknesses vary
• middle lamella (pectins) glues to next wall

Primary wall as a composite material

• polymer strings & matrix
• like fiberglass (polymer strings & plastic binder)
• polymer--cellulose microfibrils (bundles of bundles of cellulose)
• matrix
  • hemicelluloses (not relatives of celluloses) bind cellulose fibrils
  • pectin glues (calcium makes them harder)
  • proteins (structural, enzymatic)

Secondary walls

• don’t always form
• often thick (wood)
• inside the primary walls (formed later)
• still have cellulose as primary material
• matrix -- usually have lignin
  • polymer that acts as strengthening glue
  • breaking down lignin is the big problem in paper-making
• many other materials in matrix

Synthesis of cellulose for walls

• synthesis of cellulose from sugars by enzyme floating in cell membrane
• cellulose microfibril formed on the outside
  • like building a railroad
  • new microfibril is formed in place; construction equipment moves on
  • follow "tracks" of microtubules on the inside (how?)
  • UDP-glucose (we met making starch) is building block

Synthesis of matrix

• hemicelluloses & pectins
• synthesized in golgi
• delivered in vesicles to cell membrane
• fuse with developing cellulose microfibrils

Mechanical properties of walls (lots of biophysicists in this field)

• how much they stretch when pulled
  • elasticity
  • plasticity
• how tested (elasticity & plasticity)
• importance in growth, cacti, leaf movements

Growth of cell walls

• how cells can grow
  • orientation of microfibrils (they don't stretch)
  • dictates direction of growth (to a beach ball, sausage, etc.)
  • mirrors orientation of microtubules underneath
  •  turgor pressure is what actually pushes the cell bigger
• walls must relax (loosen) to grow
• two ways to loosen walls
  • enzymes already out there in the walls (auxin works on this)
    • auxin turns on proton pumps
    • acidifies walls (low pH)
    • some enzymes in walls are active at low pH
    • wall loosening enzymes
    • wall stretches under the influence of turgor pressure (plastic)
    • called acid growth
  • different enzymes: excreted by cells
    • excreted enzymes loosen walls
    • GA (and to a limited extent other hormones) do this

Will any turgor pressure do?

• like a stiff balloon -- a little pressure won't blow it up
• get to a certain point -- yield threshold -- and the balloon expand
• cell walls have yield thresholds
• amount of pressure above the yield threshold is what counts
• wall loosening enzymes lower the yield threshold

• when walls relaxed (loosen), cell gets bigger, turgor pressure drops
• so water potential drops
• so they take on more water, raising the turgor pressure partly back up
• cells also take on extra solute, and water follows
• this way they can keep on growing

Walls are also involved with defense signals

• plants come under attack from microbes all the time
• if they know soon enough, they maybe can resist
  • make something that harms the microbe or its chemicals
  • have cells be hypersensitive and die, depriving the microbe
• one way a plant knows it is under attack is that disassembled pieces of cell wall material are floating around

Walls and defense

• pectinases from fungus partly degrade plant cell wall pectin-- plant responds
• cellulases and chitinases from the plant partly degrade fungus cell wall
• plant uses both to decide to make defense compounds (information)
• defense can be local or systemic
• tobacco hornworm system
  • example of SAR, systemic acquired resistance
  • hornworm chews on leaf (actually needs chewing)
  • signal goes to rest of leaves (up)
  • salicylic acid made and distributed
  • signals plant to mount defense
  • salicylic acid is one of a few systemic signals 
• shrimp shells and tomato fungus
  • tomato plant gets fungus that attacks leaves
  • fungal walls are made of chitin polymer; so are shrimp shells
  • ground up shrimp cells induce SAR

Another hormone was discovered with an  experiment using aged herring sperm...maybe a Friday afternoon idea

What have cytokinins got to do with aged herring sperm?

• tissue culture for a long time (growing cells on medium)
• coconut milk was a favorite additive (worked)
• many unknown compounds, tried substituting other things
• added some aged herring sperm
• it worked!

Another hormone was discovered.

• tested for it by bioassay--promote cell division (cyto....kinin)
• worked for different tissues
  • tobacco pith
  • carrot phloem
  • soybean stems
• standard hormone, generally present in plants

What do the cytokinins look like?

• substituted adenine compounds (adenine plus extra arm)
• side chains composed of isoprene units
• type of compound found in tRNA's (herring sperm)
• different types exist, can be interconverted
• synthetic versions exist (K = kinetin)
• some bacteria (etc.) make cytokinins & cause disease

How are cytokinins synthesized and transported?

• made in shoot tips, young leaves (divide & growth areas)
• in roots, transported in xylem
• most shoot cytokinin stays put – acts locally

What do cytokinins do?

• promote cell division, organ form, chlorophyll synthesis, chloroplast development
• necessary for tissue culture
• promote some protein synthesis
• affect Ca concentration (signal transduction)
• increase cell expansion (also IAA, GA)

How is cytokinin action related to auxin concentration?

• depends on ratio to other hormones (esp. to auxins)
• sometimes adds to auxin effects
• sometimes opposes auxin action (OH)
• ratios count, not just amounts
• everything depend on species, tissue type, age, etc.
• this is how we can get so many specific effects from so few hormones

How are cytokinins involved in delaying senescence?

• senescence is the programmed shutdown of tissues (and plants)
• application of cytokinins to bean leaf prevents senescence
• promote chloroplast development, chlorophyll synthesis
• affect transport
  • increase nutrient sink (import) activities
  • postpone source (export)

What do cytokinins do in other tissues?

• in cotyledons
  • promote sugar synthesis (from starch) and protein
  • make wall loosening enzymes, excrete them (non-acid growth)
• lateral bud development in dicots (witches' broom, other disease)

How are cytokinins from the roots important?

• no roots, increased senescence
  • cut flower problem
  • can substitute exogenous cytokinins
  • roots may partly control plant senescence
• small rooting volume leads to smaller shoots
• many plants experience small rooting zones
  • desert, flooded, permafrost, drip irrigation
  • potential for water deficit in plant (dry, flooded, cold, disease)
  • plants need to balance shoot growth with root growth
• how can plants tell they have limited rooting volumes?
  • new, growing root tips produce cytokinin
  • lots of healthy roots, lots of cytokinins
  • limited roots leads to limited shoot growth
• things are kept in balance