Light and plant development

Photomorphogenesis - a change in plant development induced by specific kinds of light

and not dependent on photosynthesis.

Photomorphogenesis involves special photoreceptors thatinitiate developmental changes.

Photoreceptors transduce information in the environmentinto appropriate developmental patterns.

Information examples • position in a layered plant canopy • seed depth in soil• presence of competitors• approach of sunrise• day length

Two main photoreceptors

1. Phytochrome (Chapter 17) - a protein pigment that absorbs red and far-red light and interconverts between two forms.

involved with many and varied responses

2. Blue-light photoreceptors (Chapter 18)

guard cell responsesphototropisms

Phytochrome

Detects red and far-red lightProvides information the environmentAnswers questions for plants:

Am I in the light?Do I have competitors?Is it time to flower?

Many phytochrome responses are reversible

Absorption of red light will cause a response that is reversed by far red light.

Red ≈ 650 - 680 nmFar red ≈ 710 - 740 nm

“photoreversible”

"But seeds are invisible. They sleep deep in the heart of the earth's darkness, until some one among them is seized with the desire to awaken." —Antoine de Saint Exupery, The Little Prince (Harcourt, Brace & World Inc.)

The classic phytochrome system - lettuce seed germination

Dark = very little germination

Brief red light causes germination

second

Red followed by far-red = no germination

first

secondfirst third

Red then far-red then red again = germination

secondfirst third fourth

Red then far-red then red then far-redPhotoreversibility

Lettuce seed germination

Red and far-red have opposing effectsEach can reverse the effect of the other

How does this work?

1. Two antagonistic receptors?

2. One receptor with two forms?

Lettuce seed germination

Red and far-red have opposing effectsEach can reverse the effect of the other

How does this work?

1. Two antagonistic receptors

2. One receptor with two forms?

Two forms of phytochrome

“Pr” is red form, peak absorption in red

“Pfr” is far red form, peak absorption in far-red

Pr PfrRed light

Far-red light

This is the basis of the photoreversible responses

Red light converts Pr to PfrFar red light converts Pfr to Pr

Pr

Pfr

Pr PfrRed light

Far-red light

Physiologically active form of phytochrome

germination

De-etiolation

Chlorophyllproduction

Phytochrome location

In meristematic regions of etiolated seedlings,areas of active cell division and expansion.

Pfr PrRed light

etiolatedde-etiolated

General effect of Pfr is to reduce internode elongation.Sun plants tend to be sensitive to amount of Pfr, while

shade plants are often insensitive.

Shaded environmentshave lower red light to far-red light ratio.

What does this do toPfr/Ptotal

ratio?

Slow vs. rapid phytochrome responses

Slow RapidMorphological responses Biochemical responses

Effects on gene Effects on ion fluxesexpression and turgor

Phytochrome responses vary in timing.

Nyctinastic leaf movements involve a rapid phytochrome response.

Day Night

Folding and opening of leaflets involves changes in turgorof two sets of ”motor” cells, the turgor changes being drivenby fluxes of K+ and Cl-. Changes in the PMF also involved.

Fig. 17.14

Opening and closing of leaflets can be entrainedas a circadian rhythm.

Phytochrome is involved with effects on leaf movement.

1. Red light followed by darkness causes leaflets to close, and this effect can be reversed by far-red light.

Example of a rapid response based on an immediatebiochemical change.

2. Red/far red treatments influence gene expression of light harvesting proteins, which in turn alters leaf movement responses.

Example of a slower response requiring a change in gene expression.

The mechanism of phytochrome-mediated leaf movements

Phytochrome (Pfr) regulates H+ pumps and K+ channelsof motor cells.

Fig. 17.14

Phytochrome and plant competition: how do plantsdetect the presence of neighbors that compete

for sunlight?Web Essay 17.2

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

With increased density of

neighbors, the R/FR ratio perceived by stems decreases

because of increased FR

reflection from leaves and stems.

Plant neighbors?

Far red reflected from other plants.

Red absorbed

Far red enriched = neighbors

R/F

R r

ati

o

Leaf area index, m2 m-2

i

With increased density of neighbors, the R/FR ratio perceived by stems decreases

A low R/FR ratio allows internode elongation,a favorable response to potential light competition

by neighbors

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Pr PfrRed light

Far-red light

Physiologically active form of phytochrome

De-etiolationetiolation

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

The effect of FR reflection by neighboring plants can be simulated using mirrors that selectively reflect Red

or Far Red light. As for reflection by plants, FR reflection increases

internode elongation. Recall that this is because little ofthe Pfr form is then present to inhibit internode elongation

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Filtering out the FR light received by the stem reduces internode elongation at high densities of neighboring plants.

Under other plants?

Red absorbed

Far red reflected from other plants or transmitted.

Far red enriched = understory

Phytochrome and flowering.When is the right time to flower?

• Unreliable indicators of time of year– Temperature – Moisture – Light levels

• Reliable: length of day/night – Varies with season– Varies with latitude

• Detected by phytochrome

Sunlight

Mostly red

A little far red

Pr

Pfr

In sunlight

Pfr

PrPr

Pr

Pr

Pr

Pr

Pr

Pr

Pr

Pfr

Pfr

Pfr

Pfr

Pfr

PfrPfr

Pfr

Pfr

Pfr

Pfr

Pfr

Pfr

Pfr

Pfr

Pr

Pfr

Pfr

In sunlight most P gets converted to Pfr form.

Pr

Pfr

Start of night

Pfr

PrPr

Pr

Pr

Pr

Pr

Pr

Pr

Pr

Pfr

Pfr

Pfr

Pfr

Pfr

PfrPfr

Pfr

Pfr

Pfr

Pfr

Pfr

Pfr

Pfr

Pfr

Pr

Pfr

Pfr

Most P in Pfr form.

Pfr

In the dark

Pfr

PrPr

Pr

Pr

Pr

Pr

Pr

Pr

Pr

Pfr

Pfr

Pfr

Pfr

Pfr

PfrPfr

Pfr

Pfr

Pfr

Pfr

Pfr

Pfr

Pfr

PfrPr

Pfr

Pfr

Pfr form changes gradually to Pr form.

Pr

Pr

Pr

Pr

Pfr

After a short night

Pfr

PrPr

Pr

Pr

Pr

Pr

Pr

Pr

Pr

Pfr

Pfr

Pfr

Pfr

Pfr

PfrPfr

Pfr

Pfr

Pfr

Pfr

Pfr

Pfr

Pfr

PfrPr

Pfr

Pfr

Much Pfr still left.

Pr

Pr

Pr

Pr

Long day plant = Short night plants

• Needs short night to flower• Needs Pfr still present at end of night• Pfr promotes flowering for LDPs

Pfr

Later in the night

Pfr

PrPr

Pr

Pr

Pr

Pr

Pr

Pr

Pr

Pfr

Pfr

PfrPfr

Pfr

Pfr

Pfr

Pfr

Pfr

Pfr

Pfr

Pfr

Pfr

Pfr

Pfr

Pr

Pfr

Pfr

More Pfr changes to Pr.

Pr

Pr

Pr

Pr

Pfr

After a long night

Pfr

PrPr

Pr

Pr

Pr

Pr

Pr

Pr

Pr

Pfr

Pfr

Pfr

Pfr

Pfr

PfrPfr

Pfr

Pfr

Pfr

Pfr

Pfr

Pfr

Pfr

PfrPr

Pfr

Pfr

All the Pfr is gone.

Pr

Pr

Pr

Pr

Pr

Pr

Pr

Pr

Pr

Pr

PrPr

Pr

PrPr

Pr

Pr

Pr

Pfr

Day dawns

Pfr

PrPr

Pr

Pr

Pr

Pr

Pr

Pr

Pr

Pfr

Pfr

Pfr

Pfr

Pfr

PfrPfr

Pfr

Pfr

Pfr

Pfr

Pfr

Pfr

Pfr

Pfr

Pr

Pfr

Pfr

Most P gets converted to Pfr form again.

Short day plant = Long night plant

• Needs long night• Needs Pfr gone at end of night• Pfr inhibits flowering for SDPs

Can trick a SDP into not flowering with a brief flash of red light during the long night, this resets much of its phytochrome to P fr form.

LDP SDP

Long day: Pfr left at end of short night.

Pfr promotes flowering for LDPs.

Pfr inhibits flowering for SDPs.

Short day: Pfr gone at end of long night.

No Pfr to promote flowering for LDPs.

No Pfr to inhibit flowering for SDPs.

Waiting for the right time

• Plants grow leaves until it is time to flower• LDPs wait until the day is long enough

– Really night short enough– Some time before June 21

• SPDs wait until the day is short enough– Really night long enough– Some time after June 21

• Flower opening happens later

Day neutral plants

• Flower when mature enough• Maybe other environmental signals (temp?)• Day length (dark length) doesn’t matter