Fruit Ripening and Ethylene

HORT 301: Plant PhysiologyDecember 9th, 2009

Dr. Michael Van Oosten

Skotomorphogenesis

Seed germination

Genes and enzymes

Embryo and Seed development

Plant life cycle

Photomorphogenesis

Photoreceptors

Phytochrome

Cells and cell growth

Phytochrome: regulation oflight responses

Photosynthesis: light reaction

Photosynthesis: carbon fixationPhotorespiration

Fruit ripening

RespirationPrimary metabolism

Secondary metabolismNitrogen fixation

Mineral nutritionWater transport

Phloem translocation

Abiotic stress

Biotic stress

Flowering

Fertilization and embryogenesis

Seed development

What Defines a Fruit?

• A fruit is a ripened ovary

• A seed is a ripened ovule

Fruit forms in higher angiosperms

The Ripening Process

• Process of becoming edible

• Fruit becomes sweeter – (accumulation of sugars)

• Fruit becomes softer – (more palatable)

• Fruit becomes less green– (accumulation of pigments/decrease

of chlorophyll)• Fruit generally becomes more

acidic

Fruit ripening

•Partial digestion of cell walls and middle lamella

•Degradation of chlorophyll and starch

•Synthesis of anthocyanins and carotinoids

•Respiration of organic acids

Respiration and Ripening

• Normally when a tissue reaches maturity, respiration rates drop off

• Climacteric fruits show a rise in respiration during onset of ripening– Apples, Bananas, Aavocados

• Climacteric ripening is triggered by Ethylene

• Nonclimacteric do have a ethylene/respiration rise– Oranges, Grapes, Strawberries

Ethylene during ripening process

Changes in the Cell Wall

• Much of the cell wall is degraded

• Expansins are produced to “loosen” cell wall

• Middle lamella can be selectively degrade to allow cells to become “unglued” from each other

Abscission

Triple response

Pathogen defense

Root hairformation

Fruit ripening/Senescence

Elongationinhibiation

Increased Radial growth

Ethylene effects

Ethylene: it’s a gas!!

Biologically active at less than 0.1ppm

Transported as ACC

Synthesized in ripening fruit and senescing tissues

Induced by auxin, draught, wounding, cold, stress,fruit ripening, senescence, pathogen attack

1864 illuminating gas powered street lights defoliate trees1901 Russian Dimitry Neljubov identifies ethylene as phytohormone1917 Doubt identifies ethylene as defoliant1934 ethylene biosynthesis in plants detected1935 ethylene is proposed as the “ripening hormone”

Ethylene: it’s a gas!!

C C

H

H

H

H

Biologically active at less than 0.1ppm

Transported as ACC

Synthesized in ripening fruit and senescing tissues

Induced by auxin, draught, wounding, cold, stress,fruit ripening, senescence, pathogen attack

Ethylene biosynthesis from methionine

H2C CH2 Ethylene

O2

ACC-Oxidase

Methionine

S-Adenosyl-Methionine

1-Aminocyclopropan- 1-carbonic acid

NH3

COO-

H2C

H2CC

+

(ACC)

(SAM)

ACC-Synthase

SAM Synthetase

Ethylene mutations in tomato

Antisense-Inhibition of ACC-Oxidase stops flower senescence

Ethylene as a fruit ripening hormone

And Ethylene the defoliant….

Abscission zone at base of leaf at the where it joints the stem

leaf

stem

Ethylene induces abscission

Auxin prevents abscission

However: unphysiological auxin concentrations have herbicide effects (agent orange)

Genetic epistasis

Phenotype of first gene is masked by phenotype of a second gene

ETR1, EIN4, ETR2

EIN2, EIN3, EIN5

CTR1

etr1-3 ctr1-1ein4-1 ctr1-1etr2-1 ctr1-1ein2-1 ctr1-1ein3-1 ctr1-1ein5-1 ctr1-1

constitutive triple responseconstitutive triple responseconstitutive triple response ethylene insensitive ethylene insensitive ethylene insensitive

Ethylene Mutants

• Ethylene insensitive (EIN)

• Ethylene resistant (ETR)

• Constitutive Triple Response (CTR)

• Ethylene Response Sensor (ERS)

• Ethylene Overproducer (ETO)

Ethylene: triple response

Triple response:- thickening of hypocotyl, radial growth- reduction of cell elongation in hypocotyl and root- exaggerated curvature of apical hook, reduced geotropism

Understanding the hormone:Searching for ethylene mutants

(CTR1)

ctr1

Constitutive tripleresponse

air

Wild-typectr1

air ethyleneethylene

Recessive loss-of-function ctr1 mutations:- Constitutive activation of ethylene response- Ethylene induced genes are always “on”- Constitutive triple response

CTR1 leads to inhibition of ethylene response in absense of ethylene

Searching for ethylene mutants

etr1

ctr1

Ethylene resistant (exogenous ethylene added)

Constitutive tripleResponse (no exogenous ethylene)

Air

Ethylene

Wild-type

Understanding the hormone:Searching for ethylene mutants

(ETR1)

Ethylene resistant

Air Ethylene

Air Ethylene

Wild-typeEtr1

etr1 Etr (ethylene resistant)Ein (ethylene insensitive)

Ethylene receptors

Structure of ethylene receptors

GMP binding

Understanding the hormone:Searching for ethylene mutants (ETO1)

eto1

Ethylene overproducer

- Ethylene overproducer show same phenotype as ctr1

- ctr1 not reversible by inhibitors of ethylene biosynthesis

- eto phenotype is reversed by ethylene synthesis inhibitors

ETO1 has de-regulated ethylene biosynthesis

Air

Ethylene

Wild-type

Horticultural Question

• Why does blocking ethylene perception work much better than blocking ethylene production within the plant?

Ethylene and Horticulture• 1-Methylcyclopropene 

– Binds tightly to ethylene receptor and blocks activation

• Marketed as:– EthylBloc (Flowers– SmartFresh

• Apple• Kiwi• Tomatoes• Bananas• Plums• Avocados• Melons

– INVINSA (drought & heat stress)• Corn• Soybeans• Cotton• Sunflower