Ethylene

1. Ethylene

2. Discovery -- abnormal growth symptoms.

3. Detection by gas chromatography.

4. Role of endogenous ethylene • Maintenance of apical hook. • Flower senescence. • Fruit ripening. • Leaf abscission. • Floral initiation in Bromeliads.

5. Role of endogenous ethylene • Maintenance of apical hook.

6. Role of endogenous ethylene • Maintenance of apical hook. • Flower senescence.

7. Cell Enlargement Fruit size Cell Division Time • FRUIT GROWTH AND RIPENING • Fruit Growth • Pollination induces fruit growth • Pollen rich source of auxin. • Ovary growth produces hormones which sustain fruit growth: IAA or GA. • Fruits grow initially by cell division, later by cell enlargement.

8. Fruit ripening • Softening, color change, change in chemical content • Disappearance of organic acids • Solubilization of middle lamella • Solubilization of cell wall polysaccharides • Hydrolysis of starch and fats

9. Control of ripening in fruit itself • Ripening associated with respiratory climacteric The climacteric rise in respiration Ripening Initiation of ripening CO2 evolution Maturation Overripening Time • Low temp, high CO2, N2 delay ripening • Ripening a metabolically active process

10. Ethylene promotes the climacteric. • Natural climacteric results from fall in auxin and rise in ethylene production. Climacteric Ethylene Ripening Initiation of ripening CO2 evolution Maturation Over- ripening Time • Ethylene production starts just before the climacteric • If ethylene production inhibited or ethylene removed, fruits do not ripen.

11. SAM ACC Ethylene Action ACC Synthase ACC Oxidase Blocked by Silver ion (STS) Synthesis From methionine via S-adenosyl methionine (SAM) and 1-aminocyclopropane-1-carboxylic acid (ACC).

12. To ripen fruit have to be exposed and respond to naturally-produced ethylene. • Ethylene stimulates synthesis of those enzymes involved in ripening.

14. Ethylene also stimulates its own synthesis via the induction of ACC synthase transcription.

15. Ethylene application promotes fruit ripening commercially. • Ethylene application also by liquid Ethephon which at neutral pH degrades into ethylene and phosphate. Used in rubber tapping.

16. Ethylene action blocked by silver ion; e.g., Carnation flower senescence • Ethylene biosynthesis blocked in transgenic plants with antisense ACC synthase or antisense ACC oxidase (e.g., carnation flower senescence, tomato, papaya or banana fruit ripening)

17. C2H4 • Role of endogenous ethylene • Leaf abscission.

18. LEAF ABSCISSION AND SENESCENCE • Abscission • An overwintering mechanism. • Also used for dissemination of seeds. • Abscission layer of specialized cells. • Abscission promoted by SD and low temperatures.

19. IAA C2H4 • Auxin from leaf blade delays abscission. • Abscission promoted by ethylene, but only in aged leaves.

20. During natural abscission ethylene produced by abscission zone cells.

21. Stages of abscission in abscission zone cells: • All the following are promoted by ethylene • (1) Aging C2H4 Cellulase mRNA C2H4 cellulase enzyme Cellulase secretion C2H4 • (2) Inhibition of auxin transport from leaf • (3) synthesis of wall degrading enzymes, notably cellulase • (4) secretion of cellulase into the cell wall leads to wall degradation

22. (5) cells swell and push each other apart.

23. C2H4

24. LEAF ABSCISSION AND SENESCENCE

25. Ethylene Perception and Signal Transduction (as elucidated using Arabidopsis mutants) 1 cm Etiolated phenotype in darkness maintained by ethylene.

26. Ethylene related mutants in Arabidopsis • Ethylene insensitive - ein • Ethylene response sensor - ers • Constitutive response - ctr (respond as if ethylene is present in the absence of ethylene).

27. Ethylene Perception and Signal Transduction (as elucidated using Arabidopsis mutants) Receptor mutants are blind to ethylene: grow tall and open up Severe phenotype in ethylene atmosphere: stunted with accentuated hook.

28. Ethylene Signal Transduction • Mutations in receptor (etr) confer a lack of sensitivity to ethylene. • Mutations in the above reduce ethylene binding.

29. Transduction mutants show severe ethylene phenotype in the absence of ethylene.

30. A single mutation in the ethylene receptor is dominant as ethylene no longer interacts with the receptor and the receptor vigorously promotes the negative regulation action of CTR-1.

31. Loss-of-function mutations in negative regulator transduction chain components (such as ctr1) confer constitutive ethylene responses throughout development. Ethylene Signal Transduction • Binding of ethylene to the receptors turns off the inhibition brought about by CTR1.

32. Ethylene Signal Transduction Pathway • High level of EIN3 expression confers constitutive expression of the ethylene response at all stages of development. • Suggested to be transcription factor

33. The development of sensitivity to ethylene as fruits mature involves the appearance of ethylene receptors and/or other components of the ethylene signal transduction chain.

34. Phenotypes of wild-type tomato and plants with antisense for the ethylene receptor gene. Wild-type (WT) and antisense plants treated with 10 p.p.m. for 12 h. Fruit of the antisense plants with the flowers that do not senesce. Fruit at 35 days after anthesis from wild-type (WT) and antisense plants. (Tieman Plant Journal 2001)

35. A transgenic line of Petunia (right) overexpressing a dominant mutant form of etr1, the ethylene receptor. Flowers of wild-type (left) and etr1 transgenic lines (right) after an overnight treatment with 6 ppm ethylene. Direct From H. Klee 2001

36. Transgenic plants with reduced LeETR4 gene expression display multiple symptoms of extreme ethylene sensitivity, including severe epinasty, enhanced flower senescence, and accelerated fruit ripening. LeETR4 is a negative regulator of ethylene responses equivalent to CTR in Arabidopsis. Phenotypes of the LeETR4 (A) Epinasty of petioles and leaves of LeETR4 antisense plants. (C) Prematurely senescing flowers from LeETR4 antisense line. (Tieman PNAS 2000)

37. Abscisic Acid • Discovery in plant extracts promoting bud dormancy and cotton boll abscission. • (The discovery in relation to bud dormancy was probably in water-stressed tissue!) • Structure.

38. Role • Root signaling of soil water status leading to stomatal closure.

39. ABA increases: Stomata close • Stomatal closure. Wet Dry

40. Formation of storage proteins in seeds. • Seed dormancy (in some cases). Viviparous seeds lack ABA

42. Synthesis • From carotenoids. Carotenoids (violaxanthin) are synthesized from acetyl-CoA via glyceraldehyde phosphate (GAP). • Genes for some steps in pathway now isolated

43. Action • Activates genes for the production of storage proteins in seeds and the repression of enzymes of germination. • In guard cells ABA opens K+ channels (by several mechanisms) enabling K+ to flow out so closing stomata (covered earlier).