Phototropism & Geotropism - controlling cell growth

Controlling growth by elongation

what is auxin ?

• Indole-3-acetic acid (IAA), which is an auxin, is a specific growth factor found in plants

• IAA is synthesised in the growing tips of roots and shoots (i.e. in the meristems, where cells are dividing)

• IAA coordinates phototropisms in plants by controlling growth by elongation

• IAA molecules are synthesised in the meristem and pass down the stem to stimulate elongation growth

• The IAA molecules activate proteins in the cell wall known as expansins, which loosen the bonds between cellulose microfibrils, making cell walls more flexible

• The cell can then elongate

phototropic mechanism

• Phototropism affects where ?

• shoots and the top of stems

what does the con.c of IAA determine?

• The concentration of IAA determines the rate of cell elongation within the region of elongation

• If the concentration of IAA is not uniform on either side of a root or shoot then uneven growth can occur

what is positive phototropism ?

• When the shoots grow towards the light it is known as positive phototropism

  • It is described as positive because growth occurs towards the stimulus

Auxins in shoots

• higher concentrations of IAA results in a greater rate of cell elongation

  • Experiments have shown that IAA moves from the illuminated side of a shoot to the shaded side

  • The higher concentration of IAA on the shaded side of the shoot causes a faster rate of cell elongation

  • This causes the shoot to bend towards the light

Geotropism in plant shoots and roots

Gravity affects both plant shoots and roots, but in different ways

Positive and negative

When shoots grow away from gravity it is known as negative geotropism

• • Gravity modifies the distribution of IAA so that it accumulates on the lower side of the shoot

  • As seen in the phototropic response, IAA increases the rate of growth in shoots, causing the shoot to grow upwards

• When roots grow towards gravity it is known as positive geotropism

• In roots, higher concentrations of IAA results in a lower rate of cell elongation

• The IAA that accumulates at the lower side of the root inhibits cell elongation

• As a result, the lower side grows at a slower rate than the upper side of the root

• This causes the root to bend downwards