EFB530 Plant Physiology

Ethylene

The hormones described so far, auxin, gibberellin, and cytokinin all tend to promote growth and/or cell division

• ethylene and abscisic acid (ABA) can be thought of as inhibiting growth or promoting the final stages of development, including fruit ripening & senescence

When coal gas was used in street lights, those trees nearby displayed premature leaf drop and senescence

Also - exhaust from gas lamps in the lab would cause the triple effect on dark-grown seedlings

• the gas ethylene was discovered to be the active agent

ethylene (C₂H₄) is a very simple molecule, gas, lighter than air

• can diffuse through the air spaces of a plant
• acts locally - is not transmitted over long-distances through vasculature
• can dissolve in water, but has better solubility in lipid phase

Pathway of ethylene biosynthesis:

methionine -> s-adenosyl methionine (SAM) -> ACC -> ethylene

• the enzyme ACC synthase catalyzes SAM -> ACC reaction & is usually limiting
• the enzyme ACC oxidase (also called ethylene forming enzyme (EFE)) catalyzes ACC -> ethylene, this step requires oxygen
• addition of ACC often results in ethylene production, indicating that ACC oxidase is not limiting
• ACC was discovered to be the precursor of ethylene by Shang Fa Yang (1979)
• SAM -> ACC step can be inhibited by AVG
• ACC -> ethylene step is inhibited under anaerobic conditions (ACC accumulates)

Ethylene can be synthesized throughout the plant - where there is ACC synthase and ACC oxidase

• highest levels are in ripening fruit, senescing tissues, wounded tissue

The gene for ACC synthase is induced by ethylene, therefore - once it is initially induced, the ethylene it produces then feeds back and further induces ACC synthase gene expression - this is how one can detect rapid, large spikes in ethylene synthesis

• ethylene produced by one plant can diffuse to another plant at a different developmental stage and prematurely induce the expression of ACC synthase - leading to ethylene production

1) ethylene promotes fruit ripening and flower senescence

• an increase in expression of ACC synthase causes an increase in ethylene
• ethylene accelerates the process of fruit ripening, which includes cell wall breakdown, production of pigments, can include a burst of respiration (climacteric) and sweetening
• applied ethylene (as ethephon) can promote ripening of fruit before or after harvest
• control of ethylene is important in post-harvest storage of fruit and flowers; can use low O₂, low temp, ethylene scavengers (also controls respiration)
• can repress the expression of ACC synthase gene in tomato fruit and get delayed ripening

2) ethylene promotes leaf and fruit abscission

• ethylene (NOT abscisic acid) is the primary trigger inducing leaf and fruit abscission = breakdown of cells in a specific region of stem / petiole for shedding
• only those target cells in the abscission zone perceive ethylene and respond by producing cellulase and other cell wall-degrading enzymes
• auxin produced in the leaves normally represses ethylene production in the abscission zone

3) ethylene causes the "triple response" of etiolated seedlings

• inhibition of elongation and stem thickening
• enhanced apical hook curvature
• horizontal growth (diageotropic = 90 degrees turned relative to gravity)

this may be a response associated with the seedling growing into an obstacle

4) ethylene causes epinasty in flooded tomato plants

• flooding tomato roots causes the leaves to bend down
• the anaerobic environment of the roots inhibits the reaction ACC -> ethylene, causing an accumulation of ACC
• ACC is carried up the xylem to the leaves, then is converted to ethylene
• ethylene causes a redistribution of auxin, resulting in differential growth on the upper and lower sides of the petiole

5) ethylene promotes formation of female flowers in cucurbits

• more female flowers = more fruit
• mechanism is unknown

the gene for the ethylene receptor protein has been cloned  from Arabidopsis and tomato = the first hormone receptor identified in a plant

• based on mutants that were insensitive to application of ethylene (ethylene resistant=ETR in Arabidopsis, never ripe=NR in tomato)
• the receptor is a membrane protein that forms a dimer and binds ethylene in the membrane phase
• receptor also has a protein kinase domain, which is activated when ethylene binds, autophosphorylating the receptor itself
• when the receptor is phosphorylated, it is then active to activate the next protein in the signal transduction chain - it is a type of two component signalling system
• this next component was identified in a mutant that ALWAYS shows the triple response, even in the absence of ethylene called consitutive triple response or CTR, this protein is also a protein kinase,
• after that ethylene can regulate transcription and activate gene expression

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