Plant Responses to Signals IV

Photomorphogenesis

Circadian RhythmsGravitropism

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

Signal Transductiongeneral

Fig. 39.2

General Signal Transduction

Signals,

- hormone, - light, - temperature, - gravity, - etc.

Receptors,

- G-protein linked, - enzyme linked, - ion channel, - etc.

Second messengers,

- kinase cascades, - calcium concentration, - etc.

Responses,

- gene expression (+/-), - membrane dynamics, - metabolism - cytoskeleton - etc.

ein,

…blocks pathway.

ethylene,

…or ctr mutant,

no ethylene

…no triple response.

active

inactive

induces transcription,erf: ethylene response factor.

?no transcription

• Light is used by plants as a signal, as well as an energy source,

– quantity,

– quality (wavelength),

– direction,

– duration.

Photomorphogenesis

• Germination (+/-)

• Stem length (-)

• Leaf expansion (+)

• Flowering (+/-)

• Phototropism (+/-)

• Stomatal opening (+)

• Chloroplast development (+)

• Pigment synthesis (+)

• and more...

• 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,

Germination

hypothesis

…look for a photo-reversible pigment.

Fig. 39.18

Phytochromephotoreceptor molecule

dimer

Pfr

redlight

FRlight

Pr

Fig. 39.19

Pfr

Phytochromephotoreceptor molecule

Pr

Quantity,Time,Quality.

Fig. 39.20

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.

Fig 39.22

Phytochrome…photoperiodism,

…use photoreversibility to establish phytochrome function. Fig 39.22

Phytochrome

absorbtion spectra,

…the wavelengths absorbed by specific pigments.

Germination

Stem elongation(inhibition)

not phytochrome

Cryptochromes

blue 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,

blue light photoreceptors II,

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

…also contributes to the alignment of chloroplasts within mesophyll cells,

• to maximize light gathering capacity,

• and to minimize light damage

at high irradiances. Phototropism action spectrum

Concept Map

Phytochrome

Action Spectra

Photoperiodism

Photomorphogenesis

Cryptochrome

Phototropin

PhotoreversibleFunctions

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

Final

• All material since Lecture 11 (Reproduction),

– lecture, book and other assigned readings (i.e.

• Review: 5 pm Monday, (will post room on WEB),

• Final, here in this room…

– Tuesday 11/9, 10:30 – 12:30.

Assigned Essays

• Explain the importance of auxin in plants. How it is signaled, what are some of its function, where is it made, how is it transported? Give an example how it interacts with other hormones.

• Why would a plant want to prevent self pollination? Discuss two mechanisms used by angiosperms to avoid self fertilization.

• Give examples of heterospory and homospory and explain the differences between the two, mentioning the evolutionary significance.

• Describe how phase changes are used by developmental biologists to uncover biological processes.

• Describe how plants use light and hormones to influence the germination of the seed.

• What is phytochrome?

Assigned Essays

• Explain the importance of auxin in plants. How it is signaled, what are some of its function, where is it made, how is it transported? Give an example how it interacts with other hormones.

• Why would a plant want to prevent self pollination? Discuss two mechanisms used by angiosperms to avoid self fertilization.

• Give examples of heterospory and homospory and explain the differences between the two, mentioning the evolutionary significance.

• Describe how phase changes are used by developmental biologists to uncover biological processes.

• Describe how plants use light and hormones to influence the germination of the seed.

• What is phytochrome?