Chapter 17.

Phytochrome and Light Control of Plant Development

Localization of Phytochrome in tissues and cells

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)

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, much lower levels than PhyA

different pattern from Type1

Characteristics of phytochrome induced whole-cell responses

Characteristics of Phytochrome-induced responses

1. Rapid biochemical events

2. Slower morphological changes

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: 1) movement of organelles, 2) volume changes, 3) some growth responses

Developmental changes by light 1. A decrease in the rate of stem elongation2. The beginning of apical-hook straightening3. The initiation of the synthesis of pigments

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

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

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-2 s-1)

Three major categories of phytochome responses based on the amount of light required

1. Very low fluence responses (VLFRs)

2. Low fluence responses (LFRs)

3. High irradiance responses (HIRs)

Three types of phytochrome responses

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

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

660 nm

720 nm

LFR action spectra

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)

Law Of Reciprocity

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)

Lists of Some HIRs (High-irradiance Responses )