Phototropism and auxin

Experiments

Many conducted using coleoptiles (a sheath that surrounds the young growing shoot of grass plants)

Darwin (1880)

  • Discovered that removing the tip of a coleoptile stopped the phototrophic response to a unidirectional light source (light coming from one side) from occurring

  • To ensure this was not simply due to the wounding caused to the plant, he covered the tip of a coleoptile with an opaque cover or ‘cap’ instead, to block out the light.

  • Also stopped the phototrophic response from occurring, showing that the tip of the coleoptile was responsible for detecting light.

Boysen-Jensen (1913)

  • Found that if he replaced the cut tip back on top of the coleoptile and inserted a gelatine block as a barrier in between, the phototrophic response was restored

  • Shows that stimulus for the growth was a chemical (hormone) which was able to travel through the gelatine block

  • Then inserted a mica (impermeable to chemicals) barrier halfway through the coleoptile just below the tip, first on the lit side then the shaded side.

  • When inserted into the lit side, phototropic response occurred

  • When inserted into shaded side, didn’t occur

  • Confirmed that stimulus for growth was a chemical (hormone) and showed that is was produced at the tip, before travelling down the coleoptile on the side opposite to the stimulus

  • Also showed that stimulus acted by causing growth on the shaded side (rather than inhibiting growth on lit side)

Paál (1919)

  • Cut tip of coleoptile and then replaced it off-centre in the dark

  • Side of coleoptile that tip was placed on grew more than the other side, causing coleoptile to curve (similar to phototropic response)

  • Showed that in the light the phototropic response was caused by a hormone diffusing through the plant tissue and stimulating the growth of the tissue

Went (1926)

  • Placed cut tip of a coleoptile on a gelatine block, allowing hormones from tip to diffuse into block

  • Placed on coleoptile, off centre and in dark

  • Side of coleoptile that the block was placed on grew more than the other side, causing coleoptile to curve

  • The greater the concentration of hormone present in the block, the more the coleoptile curved

How it works

  • Auxins exert their influence in an unknown way, possibly by somehow attracting nutrients to the matrix

  • As auxins move down the other parts of the plant, they stimulate elongation of cells just behind the apical meristem, but inhibit the growth of the lateral pubs

  • Apical dominance is a classical example of one [art of a plant controlling another, via the influence of a growth substance

  • This is called CORRELATION

  • The degree of dominance or the apical bud is, however, very variable between species.

  • More auxin = more cell elongation

  • Auxin increases the stretchiness of the cell wall by promoting active transport of H+ ions into the cell wall (energy provided by ATPase)

  • H+ ions decrease pH which wall-loosening enzymes (expansins) work best at

  • Expansins break bonds within the cellulose. Simultaneously, H+ ions disrupt hydrogen bonds within the cellulose. Both lead to less rigid cell walls that can expand and stretch