Plant Biology BasicsPlant responses to the Environment

PhototropismPhotoperiodism

Responses to Pathogen

Learning Objectives

• Plants have cellular receptors that allow them to detect and respond to external stimuli• Plants produce hormones to coordinate physiological responses• Phototropism and Auxins• Photoperiodism and phytochromes• Plant immune responses: MAMP receptors

Signal transduction pathways link signal reception to response

• Plants have cellular receptors that detect changes in their environment.• For a stimulus to elicit

a response, certain cells must have an appropriate receptor.• Stimulation of the

receptor initiates a specific signal transduction pathway.

REVIEW: Signal Transduction PathwaysCELLWALL CYTOPLASM

Reception Transduction Response

Relay proteins and

second messengers

Activationof cellularresponses

Hormone orenvironmental stimulus

Receptor

Plasma membrane

1 2 3

Observation: Plants grow towards lightQuestion: What part of the plant responds to light?

RESULTS

Light

Tipremoved

Tip covered by opaquecap

Tip covered by trans-parentcap

Site ofcurvature covered by opaque shield

CONCLUSION

• Any response resulting in curvature of organs toward or away from a stimulus is called a tropism = a growth response.• Phototropism=

growth towards light Phototropic response only when tip is illuminated,

Only the tip responds to light.

RESULTS

Light

Tip separatedby gelatin(permeable)

Tip separatedby mica(impermeable)

• Phototropic Response is blocked by an impermeable barrier, but not by a permeable barrier. • Conclusion: Hormones produced

in the tip of the plant diffuse downward and coordinate the response in other parts of the organism.• Any response resulting in

curvature of organs toward or away from a stimulus is called a tropism = a growth response.

How does the tip of the plant cause curving of the plant stem?

Auxin is produced by the tip and moves down the shaded side of the growing stem

The stem bends towards the light because Auxin causes cell elongation on the shade side.

Excised tip placedon agar cube

RESULTS

Growth-promotingchemical diffusesinto agar cube

Agar cubewith chemicalstimulates growth

Offset cubescause curvature

Control(agar cubelacking chemical) has no effectControl

How does Auxin coordinate growth on the shade side of the plant in response to light?

• Auxins are plat hormones that promote elongation of stems1. Auxins are produced and released on the side of the plant in the shade.2. Auxin activates proton pumps in the plasma membrane to move H+ into

the cell wall.3. The acidic pH in the cell wall activates enzymes that cut the carbohydrate

cell wall.4. Turgor pressure on the cell wall causes the cell to elongate. 5. The shade side elongates and the plant bends towards the light.

Cell elongation in response to auxin: acid growth hypothesis

Cell wall becomes more acidic.

2

1 Auxin increases proton pump activity.

Enzymes cut carbohydrate cell wall3

4

5

CELL WALL Cleaving allowsturgor pressure to

push cell wall outward.

CYTOPLASM

Plasma membrane

H2O

CellwallPlasma

membrane

Nucleus Cytoplasm

Vacuole

Cell can elongate.

Photoperiodism: how do plants detect hours of light and dark? •Plants have two types of light receptors•Blue-light photoreceptors: control stomatal opening,

and phototropism.•Phytochromes: control flowering and seed

germination.

Structure of a phytochrome

Two identical subunits

Chromophore

Photoreceptor activity

Kinase activity

Phytochrome conversion marks sunrise and sunset, providing a biological clock with environmental cues for plants.

Synthesis

Pr

Far-redlight

Slow conversionin darkness(some plants)

Enzymaticdestruction

Responses:seed germination,control offlowering, etc.

Pfr

Red light

Phytochromes exist in two photoreversible states, with conversion of Pr to Pfr triggering many developmental responses.

Photoperiodism and Responses to Seasons

• Photoperiod, the relative lengths of night and day, is the environmental stimulus plants use most often to detect the time of year.• Photoperiodism is a physiological response to photoperiod.• Some processes, including flowering in many species, require a certain

photoperiod.• Plants that flower when a light period is shorter than a critical length are

called short-day plants.• Plants that flower when a light period is longer than a certain number of

hours are called long-day plants.• Flowering in day-neutral plants is controlled by plant maturity, not

photoperiod.

Photoperiodic control of flowering24 hours

Light

Criticaldark period

Flashof light

Darkness

(a) Short-day (long-night) plant

Flashof light

(b) Long-day (short-night) plant

Critical Night Length•Flowering and other responses to photoperiod are actually controlled by night length, not day length.•Short-day plants are governed by whether the critical night length sets a minimum number of hours of darkness.•Long-day plants are governed by whether the critical night length sets a maximum number of hours of darkness.

• Red light can interrupt the nighttime portion of the photoperiod.• Action spectra and photoreversibility experiments show that

phytochrome is the pigment that receives red light.

Synthesis

Pr

Far-redlight

Slow conversionin darkness(some plants)

Enzymaticdestruction

Responses:seed germination,control offlowering, etc.

Pfr

Red light

Reversible effects of red and far-red light on photoperiodic response.

24 hours

R

RFR

RFRR

RFRRFR

Critical dark periodShort-day

(long-night) plant

Long-day(short-night)

plant

Plants use molecular recognition systems with systemic responses• Plants have receptors for

MAMPs similar to Toll like receptors in vertebrates.• Plant MAMP receptors can

recognize fungi, bacterial and viral specific proteins. • Infection triggers chemical

responses that destroy infected and adjacent cells, thus localizing the effects.