8.1.8 Phytochrome

• Flowering in plants is controlled by the stimulus of night length
  • Nights are shorter during the spring and summer and longer in the autumn and winter
  • Some plants flower when nights are short and some flower when nights are long
• When the nights reach a certain length, genes that control flowering may be switched on or off, leading to the activation or inhibition of flowering
  • Genes that are switched on are expressed, leading to production of the polypeptides for which they code, while genes that are switched off are not expressed, so the polypeptides for which they code are not produced
• The length of night can be detected by a plant because it determines the quantities of different forms of a pigment called phytochrome in the leaf

Phytochrome

• The phytochrome pigment exists in two forms
  • P_R is the inactive form of phytochrome, it absorbs light from the red part of the spectrum (wavelength 660 nm)
  • P_FR is the active form of phytochrome, it absorbs light from the far red part of the spectrum (wavelength 730 nm)
• Absorption of different wavelengths of light causes a reversible conversion between the P_R and P_FR forms of phytochrome
• • When P_R absorbs red light (660 nm) it is converted into P_FR
  • When P_FR absorbs far red light (730 nm) it is converted back into P_R 
  • In the absence of red light, the unstable P_FR gradually converts back into P_R
• During the day levels of P_FR rise
  • Sunlight contains more wavelengths at 660 nm than 730 so the conversion from P_R to P_FR occurs more rapidly in the daytime than the conversion from P_FR to P_R
• During the night levels of P_R rise
  • Red light wavelengths are not available in the darkness and P_FR converts slowly back to P_R

PR is converted to PFR in a reversible reaction which controls flowering

E.g. long day plants

• Long day plants flower when the nights are short e.g. in summer
  • When nights are short, the day length is longer, hence the term 'long day plants'
• In long day plants high levels of the active form of phytochrome activate flowering
• Flowering occurs due to the following process
  • Days are long so P_R is converted to P_FR at a greater rate than P_FR is converted to P_R
  • The active form of phytochrome, P_FR, is present at high levels
  • High levels of P_FR activate flowering 
    • P_FR activates expression of genes that stimulate flowering
    • The active gene is transcribed and translated 
    • The resulting protein causes flowers to be produced rather than stems and leaves