Ethylene

What is ethylene considered and what is it produced in response to?

Stress/senescence hormone; produced in response to drought

How was ethylene discovered in the 1800s?

Coal gas used in street lighting; nearby trees exhibited premature yellowing and death of leaves

What did Neljubow discover in 1901?

Placed pea seedlings in dark; one group exposed to illuminating gas from lamps

What are ethylene's chemical properties?

C2H4; gas; lighter than air; easily oxidized completely to CO2; flammable

What discoveries followed regarding ethylene?

1910: fungus-damaged oranges shipped with bananas caused ripening/rot; fungus producing ethylene; 1917: Sarah Doubt discovered exogenous ethylene stimulates leaf and flower abscission; 1934: Gane discovered apple fruits synthesize ethylene; 1935: scientists suggest ethylene is plant hormone responsible for fruit ripening and abscission; gaseous hormone concept initially controversial

How is ethylene synthesized?

Synthesized from amino acid methionine; rate limiting step is synthesis of precursor (1-aminocyclopropane-1-carboxylic acid; ACC); requires oxygen; most other plant hormones positively or negatively influence ethylene synthesis

How is ethylene transported and regulated?

Not transported in plant; immediate precursor (ACC) can be transported

What is ethylene's autocatalytic property?

Promotes its own synthesis (autocatalytic); once produced

What does ethylene do regarding senescence?

Promotes senescence (cell and organ death): leaf senescence

What does ethylene do regarding abscission?

Promotes abscission (shedding of leaves

What is the mechanism of leaf abscission?

Due to separation between cell layers near base of petiole in abscission zone; cells closest to stem produce suberin incorporated into cell for protective function

What is epinasty and how does it occur?

Ethylene promotes epinasty - upperside of petiole grows faster than lower side; leaves turn downwards; flooded roots create anoxic conditions; anoxia induces ACC synthesis; ACC transported up xylem to leaf; converted to ethylene where O2 present; upperside of petiole grows faster; epinasty is sign soil is flooded

What does ethylene do for root hair formation?

Promotes root hair formation (with auxin); ethylene overproducing mutants produce many more root hairs (ectopic); ethylene biosynthesis or perception mutants don't produce normal number of root hairs

What is ethylene's role in wound healing?

Promotes wound healing in multiple ways; pruning induces ethylene formation; tyloses form in xylem cells below cut (plasma membrane pushes into vessel member from adjacent parenchyma cell); tyloses may prevent pathogen invasion; can also promote lignin and suberin synthesis

What does ethylene do for disease resistance?

Promotes increase in disease resistance; can involve complicated hormonal pathway with jasmonic acid and salicylic acid

What does ethylene do for radial expansion?

Promotes radial expansion in cells/organs; influences cortical microtubule orientation; Ein mutants (don't perceive ethylene) have elongated hypocotyls; ctr1 mutants (act as if constantly sensing ethylene) have fat hypocotyls

What is the structure of ethylene receptor?

Ethylene Receptor (ETR1) related to bacterial two-component histidine kinases; has histidine kinase domain (HK) and receiver domain (R); also interacts with CTR1 protein (serine/threonine kinase); CTR1 kinase activity important in signaling

When is CTR1 kinase active?

Activated when ethylene is NOT bound to receptor; repressed when ethylene is bound to receptor

How does ethylene signaling work in absence of ethylene?

CTR1 is active and phosphorylates EIN2 protein; phosphorylation inhibits EIN2 cleavage and movement into nucleus; without EIN2 peptide in nucleus

How does ethylene signaling work in presence of ethylene?

Ethylene binds to receptor; CTR1 is inactivated; EIN2 is NOT phosphorylated and is cleaved by protease; C-terminal end (contains nuclear localization signal) released to cytosol; protein fragment moves to nucleus; EIN2 fragment suppresses activity of SCF complex that degrades EIN3 transcription factor; EIN3 promotes transcription of ethylene response genes