Photomorphogenesis: 1) phytochrome-mediated development

1. Photomorphogenesis: 1) phytochrome-mediated development phytochromes - absorb red and far-red light (appear blue) may: trigger seed germination cause deetiolation of seedlings inhibit flowering

2. deetiolation: characteristics of etiolated seedlings events during deetiolation

3. phytochrome can exist in two forms: Pr and Pfr characteristics of Pr characteristics of Pfr - phytochrome is synthesized as Pr - Pfr undergoes dark reversion to Pfr

4. selective degradation - ubiquitination - degradation Pfr is ubiquitinated; selectively degraded Pr is not ubiquitinated; not selectively degraded relative amount of Pr vs. Pfr is influenced by many factors

5. Additional factors: a) Pr does absorb some far-red light - plant in far-red light will have about 3% Pfr (97% Pr) b) Pfr does absorb some red light, - plant in red light will have about 85% Pfr (15% Pr)

6. - sunlight usually acts like a red light source - however, proportion of red vs. far-red wavelengths varies widely with time, and with placement of the plant - in some cases, sunlight may act like a far-red light source

7. Seed germination and seedling growth under a leaf canopy 1) seed germination for many species, germination will not occur under a deep canopy 2) seedling growth deetiolation will not occur under a deep canopy

8. three categories of phytochrome-mediated responses: 1) low fluence responses - occur at fluence of 1 – 1000 mmole of photons/m2 - law of reciprocity - are photoreversible

9. 2) very low fluence responses - occur at fluence as low as 0.0001 mmole of photons/m2 - responses are not photoreversible 3) high irradiance responses - require high fluence rate - are not photoreversible

10. Phytochrome and circadian rhythms: circadian rhythms - include many plant processes (oxygen evolution, stomatal opening, etc.) - the period of a rhythm

11. also called endogenous rhythms 1) light is responsible for entrainment of circadian rhythms - entrainment resets the period 2) light can directly influence some rhythms - blue or red light may trigger some processes

12. nyctinasty = sleep movements - leaves fold vertically at night; unfold during the day special structure at the base of the leaf (or leaflet) - the pulvinus motor cells undergo cyclical changes in turgor pressure - ventral motor cells dorsal motor cells

13. during the day, K+ and Cl- enter the ventral motor cells and exit the dorsal motor cells; at night, the opposite occurs

14. mechanisms of action of phytochrome: signal transduction standard signal transduction mechanisms 1) external stimulus causes opening of membrane channels

15. 2) G-protein systems - external stimulus activates a membrane-bound G-protein - G-protein activates another protein, e.g.: a) guanylylate cyclase - cyclic GMP - protein kinases - gene regulatory proteins - effects on gene transcription b) phospholipase C - catalyzes the breakdown of a specific phospholipid - products cause - opening of calcium channels - activation of another protein kinase - effects on gene expression

16. information known about phytochrome mechanism of action: 1) can have rapid effects on ion movements and membrane potential 2) has effects on gene expression

17. Photomorphogenesis: 2) cryptochrome-mediated development cryptochrome is a blue-light photoreceptor - the three finger absorption pattern and blue-light responses

18. cryptochrome-mediated photomorphogenetic changes - many are similar to those mediated by phytochrome, but occur more quickly

19. other blue-light photoreceptors, and effects: A. Phototropism and phototropins phototropism positive phototropism - most studies carried out on coleoptile tips early studies by Darwins: 1) light perception occurs at the tip of shoot or coleoptile 2) unequal growth occurs on the dark side vs. the lighted side of the shoot

20. - light absorption at tip produces signal sent downward into lower portions, causing unequal growth - signal is a chemical signal

21. auxin (stimulates cell elongation) - synthesized at shoot tips and transported down - equal distribution produces straight growth - during phototropism, - light is absorbed by phototropin - phototropin is a protein kinase, activated by light absorption - influences distribution/transport of auxin

23. B. Stomatal movements and zeaxanthin - previously discussed stomates open in response to blue light zeaxanthin - blue-light photoreceptor for this response hypothesis: - zeaxanthin absorbs blue light; triggers protein kinase signal - activates H+-ATPase in guard cell membrane

24. Photoperiodism - plant responds to the length of dark vs. light in each daily cycle - synchronization with seasonal changes can include control of: seed germination stem elongation anthocyanin synthesis flowering flowering is most well-studied of these phenomena

25. photoperiodic control of flowering: a) plants that flower in early fall b) plants that flower in early spring ** synchronous flowering and cross-fertilization

26. plants respond specifically to night length; not to day length response occurs depends on night length as compared to critical night length (CNL) 1) short day plants (SDP) response induced when night length is > CNL - often late summer or fall-flowering plants

27. 2) long day plants (LDP) response inhibited when night length > CNL (flowering occurs when night length < CNL) - often spring or early summer-flowering plants

28. other variations: 3) day neutral plants 4) long-short day plants 5) short-long day plants

29. ** a SDP and a LDP could flower in the same photoperiod since the CNL varies with species inductive cycle - some species will respond following a single inductive cycle; some require several inductive cycles ** response can be absolute or facultative

30. experimental evidence of CNL 1) photoperiods of more than, or less than, 24 hours e.g., use SDP that flowers in a photoperiod of 10 hr. L: 14 hr. D subject plant to 10 hr. L: 10 hr. D photoperiod - results? subject plant to 14 hr. L: 14 hr. D photoperiod - results?

31. use LDP that flowers in photoperiod of 13 hr. L: 11 hr. D subject to 13 hr. L: 13 hr. D photoperiod - results? subject to 11 hr. L: 11 hr. D photoperiod - results?

32. 2) night break experiments brief exposure to light during the dark period e.g., SDP that flowers in a photoperiod with 9 hr. L: 15 hr. D - interrupt "night" after 6 hours - results? LDP that does not flower in photoperiod of 12 hr. L: 12 hr. D - interrupt "night" after 5 hours - results?

33. ** "day breaks" have no affect on photoperiodic response

34. photoreceptor appears to be phytochrome - flash of red light causes a "night break" effect; far-red does not - a flash of red, followed by far-red, cancels red light effect

35. detection of light in photoperiodism occurs in leaves photoperiodic induction grafting of induced leaves - induction produces chemical stimulus(mRNA?); - transported to shoot meristem through phloem - at shoot meristem, causes conversion to floral meristem

36. cold-induced promotion of flowering = vernalization - of imbibed seeds (in annuals), or of growing plants (in biennials) - requires temperatures of slightly < 0 C to ~ 10 C - usually requires several weeks of exposure chemical flowering stimulus - grafting of vernalized leaf

37. - grafting can carry chemical flowering stimulus (florigen) between different species