Unit 3 Chapter 22 Ethylene: The Gaseous Hormone BIOL3745 Plant Physiology Unit 3 Chapter 22 Ethylene: The Gaseous Hormone

process associated with leaf and flower senescence and abscission, Summary Ethylene regulates fruit ripening and process associated with leaf and flower senescence and abscission, root hair development and nodulation, seedling growth and hook opening Flowering in family Bromeliaceae

Figure 22.1 Triple response of etiolated pea seedlings 10 ppm ethylene untreated PP5e-Fig-22-01-0.jpg The treated seedlings show: radial swelling, inhibition of epicotyl elongation, and horizontal growth of epicotyl (diagravitropism)

22 In-Text Art, p. 650 Ethylene PP5e-ITA-22-p650-0.jpg

22 In-Text Art, p. 667 Ethephon releases ethylene slowly by a chemical reaction PP5e-ITA-22-p667-0.jpg

Figure 22.2 Ethylene biosynthetic pathway and the Yang cycle PP5e-Fig-22-02-0.jpg

Biosynthesis of ethylene The precursor for ethylene biosynthesis is methionine, which is converted sequentially to S-adenosylmethionine, ACC, and ethylene. ACC can be transported and thus can produce ethylene at a site distant from its synthesis. Two key enzymes: ACC synthase and ACC oxidase Ethylene biosynthesis is stimulated by environmental factors, other hormones (auxin), physical and chemical stimuli The biosynthesis and perception (action) of ethylene can be antagonized by inhibitors, some of them have commercial applications

Biosynthesis of ethylene ACC can be converted to a major conjugated form, N-malonylACC (MACC), and a minor conjugated form, 1-γ-L-glutamylamino cyclopropane-1-carboxylic acid (GACC). ACC deaminase can breakdown ACC to ammonia and α-ketobutyrate to regulate ethylene biosynthesis

Figure 22.3 ACC concentrations, ACC oxidase activity, and ethylene during ripening of apples PP5e-Fig-22-03-0.jpg

Figure 22.4 Two inhibitors that block ethylene binding to its receptor PP5e-Fig-22-04-0.jpg inactive Active

Figure 22.5 The triple response in Arabidopsis PP5e-Fig-22-05-0.jpg

Ethylene signal transduction pathways Two classes of mutants have been identified by experiments in which mutagenized Arabidopsis seeds were grown on an agar medium in presence or absence of ethylene for 3 days in the dark mutants fail to respond to ethylene: ethylene-resistant or insensitive mutants mutants that display the response even in absence of ethylene (constitutive mutants)

Grown in dark in ethylene Figure 22.6 Screen for the etr1 mutant of Arabidopsis the mutant is completely insensitive to ethylene PP5e-Fig-22-06-0.jpg Grown in dark in ethylene

Figure 22.9 Screen for Arabidopsis mutants that constitutively display the triple response (ctr1 mutant) PP5e-Fig-22-09-0.jpg

Figure 22.7 Schematic diagram of five ethylene receptor proteins and their functional domains PP5e-Fig-22-07-0.jpg The GAF domain is a conserved cGMP-binding domain. H: histidine; D: aspartate residue, both participate in phosphoralation

5 ethylene receptors are identified, all share at least two domains. The amino-terminal domain spans the membrane at least three times and contains the ethylene binding site. The carboxyl-terminal half of the ethylene receptors contains a domain homologous to histidine kinase catalytic domains They are all located on endoplasmic reticulum. ETR1 may also be localized on the Golgi apparatus. Ethylene binds to its receptor via a copper cofactor

Ethylene receptors Unbound ethylene receptors are negative regulators of the response pathway Binding of ethylene inactivates the receptors, allowing the response pathway to proceed ETR1 activates CTR1, a protein kinase that shuts off ethylene responses

Figure 22.8 Model for ethylene receptor action based on the phenotype of receptor mutants PP5e-Fig-22-08-0.jpg

Ethylene regulation of gene expression Ethylene affects the transcription of numerous genes via specific transcription factors Analysis of epistatic interactions revealed the sequence of action for genes ETR1, EIN2, EIN3, and CTR1

Figure 22.10 Model of ethylene signaling in Arabidopsis PP5e-Fig-22-10-0.jpg

Figure 22.11 Ethylene production and respiration in banana PP5e-Fig-22-11-0.jpg

Ethylene promotes the ripening of some fruits Climatic fruits: fruits ripen in response to ethylene exhibit a respiratory rise. Apples, bananas, avocados, tomatoes. Non-climatic fruits: do not exhibit the respiration and ethylene production rise. Citrus, grapes.

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Figure 22.12 Leaf epinasty (downward bending) in tomato PP5e-Fig-22-12-0.jpg

Figure 22.13 Amounts of ACC in the xylem sap and ethylene production in the petiole PP5e-Fig-22-13-0.jpg

Figure 22.15 Kinetics of effects of ethylene addition and removal on hypocotyl elongation PP5e-Fig-22-15-0.jpg

Figure 22.16 Promotion of root hair formation by ethylene in lettuce seedlings PP5e-Fig-22-16-0.jpg

Figure 22.17 Inhibition of flower senescence by inhibition of ethylene action PP5e-Fig-22-17-0.jpg

Figure 22.18 Formation of the abscission layer of jewelweed (Impatiens) PP5e-Fig-22-18-0.jpg

The trees are fumigated with 50 ppm ethylene for 3 days Figure 22.19 Effect of ethylene on abscission in birch (Betula pendula) Left: wild-type Right: etr1 transgenic The trees are fumigated with 50 ppm ethylene for 3 days PP5e-Fig-22-19-0.jpg

Figure 22.20 Schematic view of the roles of auxin and ethylene during leaf abscission PP5e-Fig-22-20-0.jpg

Developmental and physiological effects of ethylene Ethylene is involved in seeding germination, seedling growth, hypocotyle elongation, fruit ripening, leaf epinasty; influences cell expansion and orientation of the cellulose microfibrils in the cell wall; stimulates rapid internode or petiole elongation when plants are submerged. regulates flowering, sex determination, and defense responses in some species; stimulates root hair formation; promotes leaf and flower senescence and leaf abscission.