Auxin

How does auxin cause responses in target cells?

• Auxin binds to a complementary receptor protein in the cell surface membrane

• This triggers production of an intracellular second messenger

• The second messenger activates transcription factors

• Transcription factors cause changes in the transcription and translation of specific genes

• The changes in the proteins produced alters the behaviour of the cell, resulting in elongation, cell division or differentiation

Outline the role of auxin in phototropism

• Auxin is produced by meristem tissue

• Light causes it to accumulate on the shaded side

• It causes cells on the

  shaded side to elongate

• This results in the shoot

  bending towards the light

• Explain the role of auxin in phototropism

• Auxin binds to a cell surface receptor protein

• This triggers intracellular signaling causing activation of proton pumps

• Protons are transported from the cytoplasm, and into the cell wall

• Proteins called expansins are activated by the lower pH (optimum pH)

• They break down the crosslinks in cellulose and hemicellulose, weakening the cell wall

• When water enters the cells, the turgor pressure causes the cell to burst

Explain how phytochromes act as photoreceptors

• Phytochromes exist in two isomeric forms, Pr and Pfr

• Pr absorbs red light and is converted into Pfr

• Pfr can absorb far-red light and is converted into Pr

• Longer days, Shorter nights, means Pfr accumulates

• The different levels of Pr and Pfris how plant cells can detect the length of the day

• Pfr is the active form and can activate proteins and genes for certain plant responses

Describe how phytochromes control cellular processes

• Pr and Pfr have different tertiary structures

• The Pfrform is able to activate other proteins in the cell

• These proteins may now act as transcription factors

• Specific genes will be transcribed and translated to produce new proteins

• These new proteins may cause response such as flowering, greening, seed germination and stem