Auxin and differential growth in roots and shoots

Homeostasis and response • Plant hormones (biology only)

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Why can very high auxin concentrations kill some plants when used as a weedkiller?

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Excess auxin disrupts normal growth regulation, causing uncontrolled growth in some tissues and growth inhibition in others, which can lead to plant death .

Key concepts

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Auxin: definition and site of production

Auxin is a plant hormone that regulates cell elongation and directional growth. Meristematic regions near shoot and root tips act as primary sites of auxin production. Auxin molecules diffuse away from the tip into adjacent tissues and move asymmetrically in response to environmental signals such as light or gravity .

Auxin movement and experiments that show diffusion

Auxin moves from the tip down through tissues by diffusion and polar transport. Barrier experiments demonstrate that auxin passes through permeable media but is blocked by impermeable materials. A permeable gelatine layer allows bending to occur, while an impermeable mica layer prevents bending on the blocked side, showing that auxin diffuses down the shaded side of a shoot to create a gradient .

Mechanism in shoots (phototropism) - cause → effect

Light applied from one side causes auxin to accumulate on the shaded side of the shoot. Cause: asymmetric light reception in the shoot tip redirects auxin down the dark side. Effect: higher auxin concentration on the shaded side increases cell elongation there, so cells on the shaded side lengthen faster than those on the light side. Resulting curvature causes the shoot to bend toward the light, a response called positive phototropism .

Mechanism in roots (gravitropism) - cause → effect

Gravity detection in the root tip leads to relocation of auxin toward the lower side when a root is horizontal. Cause: auxin concentration increases on the lower side of a horizontally oriented root. Effect: high auxin concentration in roots inhibits cell elongation on the lower side, while cells on the upper side elongate relatively more. Resulting curvature causes the root tip to bend downwards, producing positive gravitropism .

Concentration dependence and limiting factors

Auxin effects depend on concentration and tissue type. Low to moderate auxin concentrations promote cell elongation in shoot cells, but the same or higher concentrations inhibit elongation in root cells. External manipulation of auxin concentration changes growth angle and rate; for example, increasing applied auxin in experiments produces larger curvature in shoots until inhibitory levels are reached. Limiting factors include auxin concentration, tissue sensitivity, presence of transport pathways, light intensity and gravity vector, all of which modify the final growth response .

Key notes

Important points to keep in mind

Auxin is produced mainly in shoot and root tips and moves through tissues by diffusion and polar transport .

Higher auxin concentrations promote cell elongation in shoots but often inhibit elongation in roots.

Light causes auxin to accumulate on the shaded side of a shoot; shaded-side elongation causes bending toward light .

Gravity causes auxin to accumulate on the lower side of a root; inhibited lower-side growth causes the root to curve downwards.

Permeable vs impermeable barrier experiments show that auxin diffusion from the tip is required for curvature responses .

Auxin effects depend on concentration and tissue sensitivity; too much auxin can inhibit growth or cause abnormal development .