Phytochrome

Question 1

phytomorphogenesis

Answer 1

• the process of changing in response to light (especially with respect to
  development)
• most common in seeds and seedlings
• Includes elongation apical-hook straightening, the formation of organs, and the
  initiation of pigment synthesis

Question 2

what can phytomorphogenesis be a response to?

Answer 2

• light intensity in terms of the total amount of light (fleunce) or the rate of light interception (irradiance), and growth rate etiolation
• Can also be a response to light quality (wavelength)—e.g., phototropism, sun/shade plasticity,
  germination

Question 3

what results from phytomorphogenesis?

Answer 3

• the transduction of an endogenous signal (hormone) in

response an exogenous light signal—light signals are detected by photoreceptors

Question 4

what are the three categories of photoreceptors?

Answer 4

• phototropins
• zeitlupe
• cryptochromes

Question 5

phototropins

Answer 5

absorb blue light mediating movement responses

Question 6

zeitlupe

Answer 6

• ZTL

- German for “slow”, play a role in day length perception and circadian rhythms

Question 7

cryptochromes

Answer 7

promote most responses to light (aka photomorphogenesis)
• phytochromes are an important category of cryptochromes that
respond specifically to red and far-red light very strongly

Question 8

phytochrome

Answer 8

a protein pigment that absorbs red and far-red light most strongly, but also blue and UVA light

Question 9

photoreversible photomorphogenesis

Answer 9

• how phytochromes work
• red light in the 650-680 nm causes a response
• far-red light in the 710-740 nm causes antagonistic response
• first seen in lettuce germination (stimulated by red light but inhibited by far red light)

Question 10

what is the structure of a native phytochrome?

Answer 10

• soluble protein occurring as a dimer of two subunits
• Each subunit consists of a light absorbing pigment molecule called a chromophore and a
  polypeptide chain called the apoprotein
• Phytochrome is a light-regulated protein kinase which first autophosphorylates, and
  then activates other proteins

Question 11

photoreversibility

Answer 11

• phytochrome can interconvert between forms
• in dark-grown or etiolated plants, phytochrome is present in a red light-absorbing form called Pr
• the form is converted by red light to far light absorbing form called Pfr

Question 12

Pfr

Answer 12

the physiologically active form (action spectra of processes best correlate with Pfr
concentration, rather than Pr) of the phytochrome

Question 13

how does Pfr work?

Answer 13

• In the light Pfr pools up in cells and physically moves to the cell nucleus, where it regulates gene
  expression to activated photomorphogenesis
• These changes in gene expression are mediated by phytochrome interacting factors or PIFs

Question 14

PIFs

Answer 14

• phytochrome interacting factors
• act as negative regulators of photomorphogenesis – most of these are
  constitutive activators of genes that are expressed in the dark.
• In the light Pfr pools up and binds to the PIFs promoting their degradation and
  blocking transcription of skotopmorphogenesis genes (for growing in the dark)

Question 15

what do PIFs do during etiolation?

Answer 15

During etiolation PIFs can act as precursors of some light-induced genes, allowing
expression of photomorphogenesis genes

Question 16

what is the reversibility of phytochrome responses sensitive to?

Answer 16

• fluence (the total amount of light)
• very low fluence - Pfr is not reversible
• low fluence - Pfr is reversible
• high fluence - Pfr is not reversible

Question 17

low red

Answer 17

far-red ratio causes elongation in sun plants

• sun-adapted plants show this response is more strong than shade-adapted plants
• domestic plants have been bred to lose this response so they can be grown in high densities without etiolating

Question 18

nyctinastic

Answer 18

• phytochrome also regulates leaf movements

- opening during the day and closing at night