Lecture 41 Comparative Cell Biology

Front 1

All plant organs can undergo senescence:

Back 1

- flower senescence

- senescence of fruit

- whole aerial part of plant in annuals: monocarpic senescence

- sequential leaves in perennial herbaceous plants

Front 2

Senescence involved remobilisation of nutrients:

Back 2

- breakdown of macromolecules (hydrolytic enzymes)

- the building blocks are transported back into other parts of the plant for growth and storage

e.g. chloroplast proteins are major nitrogen and phosphorus resources in green tissues (RubisCo - N, plastid ribosomes - P)

Front 3

The life span of leafs vary:

Back 3

1) cotyledons: short lifespan 1-2 weeks

2) arabidopsis leaves = life of the planst ~ weeks

3) bristlecone pine needles can last up to 40 yrs

Front 4

Life span of petals varies greatly as well

Back 4

- hemerocallis - 24 h

- Phaleonopsis - months

Front 5

How is senescence regulated?

Back 5

- Ethylene

- peak concentration is reached right before visible signs of senescence

Front 6

What are the stages of flower development and senescence?

Back 6

- closed bud cell (division/expansion) = Nutrient sink

- Open flower, PCD beginning in mesophyl = nutrient sink

- Wilting flower, PCD is widespread = nutrient source

- Petals and other organs are abscised = abscission zones

Front 7

Loss of internal cell integrity:

Back 7

- mesophyll cells die first

- electrolyte leakage increases when cells die

Front 8

What hormones regulate senescence in plants?

Back 8

- ABA - abscisic acid

- ROS - reactive oxygen species

- JA - jasmonic acid

- SA - salicylic acid

Front 9

How is senescence regulated in plants?

Back 9

1) stress or developmental signal

2) hormonal signal

3) hormonal receptor

4) MAP kinase cascade

5) Network of transcription factors

6) gene activation for remobilisation and cell death

Front 10

Cell death in response to abiotic factors:

Back 10

1) heat

2) ozone

3) UV

Front 11

What happens in response to heat?

Back 11

- cytological response

- chromosome condensation, cytoplasm shrinkage;

e.g. heat shock in tobacco; increase in ROS, DNA laddering, gene expression changes

Front 12

What happens in response to ozone?

Back 12

- ethylene and salicylic acid stimulate ozone-induced cell death;

- DNA fragmentation, increase of ROS, early release of cyt C

- activation of proteases

- chromatin condensation

- blebbing of plasma membrane

- up regulation of genes

Front 13

What happens in response to UV?

Back 13

- In arabidopsis - induced death is dose and light dependent

- DNA laddering

- changes in nuclear morphology

- caspase inhibitors or expression of DAD1 blocks PCD

Front 14

Cell death in response to biotic factors (2)

Back 14

- self incompatible pollination

- responses to pathogens

Front 15

How does cell death in the pollen tube takes place?

Back 15

- Ca2+ signalling

- cytoskeletal changes

- ROS and NO involved

- caspase activity required

Front 16

How does the cell death in response to pathogens take place (hypersensitive response)?

Back 16

- restricts growth of pathogen (seen as lesions)

- Receptors recognise pathogen signals

- oxidative burst and ion fluxes

- intracellular signalling is triggered

- activation of defence responses

- activation of PCD

- activation of cell protection mechanisms to regulate the PCD