Plant Physiology -- 04/24/09

Lecture 4-6

Last lecture

• finished auxin
• signal transduction in lab

Today's lecture:

• gibberellins, the next hormone
• cell walls & growth

How were the gibberellins discovered?

• Japanese noticed that some  rice grew so tall it fell over
  • called "lodging"
  • caused by fungal infection
  • lose the crop from disease, difficulty in harvesting
• looked for a compound associated with the fungal infection
• found the fungus made a substance that caused stem elongation
• war (and publication in Japanese) meant West didn’t find out

What was this compound?

• GA--gibberellic acid
• composed of isoprene units
• many closely related gibberellins (50+)
• active and inactive forms, easily interconverted
• some glucosylated (conjugated) forms, probably inactive

What do the gibberellins do, and how are they different from auxins?

• generally growth promoter (like auxins)
• promote cell elongation (like auxins)
• GA more effect in intact plants (auxins more effect in excised tissue)
• both important in live plants

Where are the gibberellins made, and how are they transported?

• made in young leaves (like auxin)
• made in young roots, but don’t promote growth there
• transport not polar (auxin is preferentially one way)
• transport not well understood

How do GA problems show up in plants?

• some do not make GA (or make little)
  • dwarf corn grows tall when GA applied
  • GA doesn't affect normal plants
• some mutants cannot use GA normally
• both groups are genetic dwarfs
• we want dwarf plants (wheat, sorghum)

How is GA involved in growth of rosette plants?

• rosette plants have basal leaves
• cabbage, broccoli, beets, radishes
• rapid stem elongation (internodes): bolting
• often seen in second year in biennials
• controlled by normal (internal) GA
• can be induced by applying GA

How is GA involved in the flowering of rosette plants?

• differentiation of shoot into flower parts (when small)
• requires long day or cold
• can be induced by applying GA
• bolting and flowering are separate processes

How is GA involved in dormancy?

• many plants have dormant period in nongrowing season
• even when general conditions are OK, still won't grow
  • seeds without right conditions = quiescent (as opposed to dormant)
• environmental signal ends dormancy
  • some seeds require cold to germinate
  • some buds require particular day length to grow
  • some seeds require red light to germinate
  • sometimes interactions between signals
• GA can be substituted for all of these

How is GA involved in seed germination?

• seeds imbibe water
• may require light to go to next step
• GA produced in embryo
• GA diffuses through wet seed to layer under seed coat
• aleurone layer (also embryo) produces amylase and other enzymes
• enzymes diffuse to endosperm, where mobilize starch, etc.
• aleurone layer only in monocots

What commercial uses do we have for GA?

• increase celery size (but don't last)
• increase size of seedless grapes (without: many seedless grapes tiny)
• increase malting rate in beer production
• prevent senescence in navel oranges
• increase sugar cane growth (want stem for harvest)

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 concrete (sand & cement)
• 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