Chapter 17. Phytochrome and Light Control of Plant Development

1. Chapter 17. Phytochrome and Light Control of Plant Development

2. Localization of Phytochrome in tissues and cells

3. Phytochrome can be detected in tisuses spectrophotometrically In dark grown plants, phytochrome has been detected. In green tissue, it is difficult to detect phytochrome because its color is masked by chlorophyll Etiolated seedling the highest phytochrome levels are usually found in meristemic regions or vicinity. (Fig. 17.6)

4. Phytochrome is differentially expressed in different tissues Type1 Phy: PhyA In dark grown seedling, highest amount in apical hook and root tips In light grown seedling, same pattern but less expression (may be degraded) Type II: PhyB, PhyC, PhyD, PhyE Less active than the Type1, muchlower levels than PhyA different pattern from Type1

5. Characteristics of phytochrome induced whole-cell responses

6. Characteristics of Phytochrome-induced responses • 1. Rapid biochemical events • 2. Slower morphological changes

7. Phytochrome Responses Vary in Lag Time and Escape Time • Morphological response to the photoactivation of phytochrome may be observed after lag time(the time between stimulation and an observed response - a few minute to several weeks). • Rapid responses: • movement of organelles, • 2) volume changes, • 3) some growth responses • Developmental changes by light • A decrease in the rate of stem elongation • The beginning of apical-hook straightening • The initiation of the synthesis of pigments

8. Major step of lag time • < 8 min, red-light inhibition of stem elongation rate • 3 hrs, PhyA was disappeared and PhyB contributes • Several weeks, induction of flowering

9. Escape from photoreversibility A Model to explain: Phytochrome controlled morphological responses are the results of a step-by-step sequence of linked biochemical reactions in the responding cells.  each of these sequences has a point of no return

10. Phytochrome Responses Can Be Distinguished by the Amount of Light Required • Fluence : the amount of light • Units for fluence • : moles of quanta per square meter (mol m-2) • - Irradiance: fluence rate of light • - Units of irradiance • : moles of quanta per square meter per second (mol m-2s-1)

11. Three major categories of phytochome responses based on the amount of light required • Very low fluence responses (VLFRs) • Low fluence responses (LFRs) • High irradiance responses (HIRs)

12. Three types of phytochrome responses

13. Very Low Fluence Responses Are Nonphotoreversible • VLFRs • initiated by fluences as low as 1.0 nmol m-2 • saturate at about 50 nmol m-2. • example1) In dark-grown oat seedling, stimulate the growth of • coleoptile and inhibit the growth of the mesocotyl • example2) Arabidopsis can be induced to germinate with red light • Far red light can not reverse VLFRs • * The minute amount of light needed to induce VLFRs convert • * After far-red, 3% of the Phy remains as Pfr form

14. Low-Fluence Responses Are Phoporeversible • LFRs… • cannot be initiated until the fluence reaches 1,0 μmol m-2 • saturated at 1000μmol m-2. • include most of the red/far-red photoreversible reponses. Table 17-1.Typical photoreversible responses

15. LFR action spectra 660 nm 720 nm

16. Law Of Reciprocity Recoprocal relationship between fluence rate and time VLFRs and LFRs obey this law Total fluence is a function of two factors: fluence rate and irradiation time - a brief pulse of red light if the light is sufficient bright - very dim light if the irradiation time is long enough)

17. High-irradiance Responses Are Proportional To The Irradiation and The Duration • HIRs… • require prolonged or continuous exposure to light of relatively high irradiance • is proportional to the irradiation within certain range • saturate at much higher fluences than LFRs • at high irradiance light, the effect is not reversible with far-red light (at low fluences, the effect is reversible with far-red light)

18. Lists of Some HIRs (High-irradiance Responses )