Plant Physiology -- 04/29/09

Lecture 4-8

Last lecture

• rest of gibberellins
• cell walls & growth

Today's lecture:

• last bit of cell walls
• cytokinins (hormone #3)
• start ethylene

What happens to the pressure when the cells grow?

• 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

So far we have seen three hormones associated with growing tissue, auxins, gibberellins (GA), cytokinins ("cell division"). They are associated with generally good growing conditions and the resultant good growth.

Today we will look at the last two hormones, ethylene and abscisic acid. These are interesting because they are most often synthesized when times get hard. They usually allow the plant to better tolerate stressful conditions.

Note that all hormones can cause selected inhibition of growth at some concentration, but the last two are much more likely to.

How was the activity of ethylene discovered?

• old saying: one bad apple ruins barrel (ripe fruit produces ethylene)
• oranges couldn't be shipped with bananas
• people lit bonfires to synchronize blooming in fruit crops
  • Puerto Rican pineapples
  • Phillippine mangoes
• lighting gas in greenhouses promoted fruit development
• gas street lights promoted leaf senescence
• 1934 discovery: ethylene as a product of plants