Etiolation

The stems of plants raised in the dark elongate much more rapidly than normal, a phenomenon called etiolation. It is a mechanism that increases the probability of the plant reaching the light.

Once light shines on it,

• the cotyledons spread apart
• the primary leaves
  • grow to full size
  • turn green
• the plant begins to produce internodes of normal size.

The image shows seedlings of the common garden bean grown in the light (left) and in darkness (right). The pale color of the dark-grown plant is caused by the lack of chlorophyll. When the food reserves of its seed are used up, the seedling will die (unless placed in the light).

Each seedling has three nodes (the bottom ones where the cotyledons were attached), but the internodes are greatly elongated in the dark-grown seedling.

This response is NOT dependent on photosynthesis, because light too dim to be useful in photosynthesis nevertheless halts etiolation. Exposure to dim

• red light (660 nm) or
• far red (730 nm; just beyond the visible — to us — region of the electromagnetic spectrum), or
• blue light

The red/far red effect is mediated by phytochrome.

The effect of blue light is mediated by cryptochrome.

How does phytochrome work?

The story is beginning to unfold.

• When sunlight (660 nm) converts P_R into P_FR, the P_FR moves from the cytoplasm into the nucleus.
• There it binds to a protein called PIF3 ("phytochrome-interacting factor 3").
• PIF3 is a helix-loop-helix protein as are many transcription factors.
• The complex of the two binds to and turns on promoters containing the sequence

  CACGTG
  GTGCAC

• These promoters are found in genes that themselves encode other transcription factors.
• These other transcription factors, in turn, initiate transcription of a variety of genes that are expressed when the plant is exposed to light.
• Exposure to far red light converts the P_FR back to P_R which
  • dissociates from PIF3 and
  • returns to the cytoplasm.

The Shade-Avoidance Effect

Natural sunlight contains more red (660 nm) than far red (735 nm). However, chlorophyll absorbs 660 nm light strongly, so that the light that filters through a canopy of foliage is enriched in far red light. This is sufficient to prevent etiolation in germinating seedlings, but after these have greened up, they will

• begin to elongate rapidly until the ratio of red to far red improves;
• speed up their normal time of flowering.

The shade-avoidance effect helps ensure that the plant reaches enough sunlight to be able to carry on photosynthesis (and reproduces even if it cannot).

This effect is also mediated by phytochrome.