Plant Responses to Signals IV

PhotomorphogenesisCircadian Rhythms

Gravitropism

http://sunflower.bio.indiana.edu/~rhangart/plantsinmotion.html

• Action Spectrum,

– graph of the magnitude of a biological response to light,

– as a function of wavelength.

Action Spectra

Germination

Stem elongation(inhibition)

Molecular Switch…looking for a photoreceptor,

Germinationhypothesis

…look for a photo-reversible pigment.

Absorption vs. Action Spectra…looking for a photoreceptor,

Germination

Phytochromephotoreceptor molecule

dimer

redlight

FRlight

Phytochromephotoreceptor molecule

Quantity,Time,Quality.

See Fig. 38.5

Phytochrome Location

Phtyochrome is a cytosolic protein.

Phytochrome…has multiple functions,

• Seed Germination,

• Flowering time (photoperiodism),

• Entraining (setting) the biological clock,

• End of day,

• Stem elongation,

• Leaf Expansion,

• Pigment synthesis.

Photoperiodism…flowering times,

Long-day plants,

…night breaks induce flowering.

Short-day plants,

…night breaks inhibit flowering.

See Fig 38.3, 4

Phytochrome…photoperiodism,

…use photoreversibility to establish phytochrome function. Fig 38.3

Phytochrome

absorbtion spectra,

…the wavelengths absorbed by specific pigments.

Germination

Stem elongation(inhibition)

not phytochrome

Cryptochromesblue light photoreceptors (I),…evolved from a light dependent DNA repair enzyme,

...across phylogeny, these proteins have been used for many functions,

• ranging from blue-light-dependent development in plants,

• blue-light-mediated phase shifting of the circadian clock in insects,

• to a core circadian clock component in mammals.

Stem elongation(inhibition)

Phototropins…mediate phototropism,

…contribute to stem, root and leaf movements in response to directional information,

• to maximize light gathering capacity,

• and to minimize light damage at high irradiances.

Phototropism action spectrum

blue light photoreceptors II

Concept Map

Phytochrome

Action Spectra, Absorption spectra

Photoperiodism

Photomorphogenesis

Cryptochrome

Phototropin

PhotoreversibleFunctions

Signal transduction

Circadian Rhythms• Relating to, or exhibiting

approximately 24-hour periodicity,

– circa around + dies day.

• Internal Biochemical Oscillators,

– found in all eukaryotes,

– eubacteria as well. sleep movementsFig 39.21

Circadian Rhythms

<- Period (24h) ->

amplitude

Entrainment

Circadian processes continue even if light (or

dark) is continuous...

…amplitude and period entrainment is continuous,

- allows fine control.

…of response,

~ 480 of 8,000 (tested) genes are under circadian control,

• ~1,500 (estimated) Arabidopsis genes, or ~6% follow circadian cycles of expression.

Photosynthesis genes...

Secondary metabolism (wood, defense).

Gravitropism

… the gravity directed growth processes that direct root and shoot orientation during a plants life-cycle,

roots,

…are positively gravitropic.

shoots,

…are negatively gravitropic.

– about 1.7%, or roughly 500 genes, are transcribed in Arabidopsis when it is re-oriented 90o.

Gravitropic Set Point

0o

90o

180o

Plant organs orient themselves to the gravity vector.

Starch Statolith Hypothesis

Re-orientation of heavy starch grains signals gravity vector.

http://www.biosci.ohio-state.edu/~plantbio/Sacklab/timelapse.html

Fig 39.25

Moss

Possible Essay Questions

• Describe the process and evolutionary advantage of de-etiolation in plants. Include in your discussion the function of phytochrome, second messengers, and two possible mechanisms of response. Also include the functions of some proteins and hormones produced during de-etiolation.

• Auxin plays many roles within a plant. One of it's chief functions is to stimulate cell expansion. Describe it's journey from where it is synthesized to where it is perceived, and the response.

• Angiosperm species account for more than three-quarters of all plant life on Earth. What characteristics/adaptations enable this plant type to be successful (structurally, at the cell and tissue level, reproductive advantages, etc.)?

• Describe the "relationship" that forms between rhizobium bacteria and their legume hosts. How does it benefit either party? Are there any relative drawbacks to the symbiosis? How can you (physically) tell if a plant has been infected with rhizobium (at macro and micro levels)?

• Several hormones act to counterbalance each other in plant physiology. Select two such hormones, describe the effects they have individually on an organism, and explain how they balance each other's effects on a plant.