9.3 Growth in Plants
Undifferentiated cells in the meristems of plants allow indeterminate growth.
Can grow throughout their lives when conditions permit it.
Mitosis and cell division in the shoot apex provide cells needed for extension of the stem and development of leaves
Plant hormones control growth in the shoot apex
Plant shoots respond to the environment by tropisms
Meristems: Tissues in a plant consisting of undifferentiated cells capable of indeterminate growth.
Auxin efflux pumps can set up concentration gradients of auxin in plant tissue
Auxin influences cell growth rates by changing the pattern of gene expression
Apical meristems: Tissues at the tips of roots/shoots that are responsible for primary growth (vertical).
Lateral meristems: Tissue responsible for secondary growth (widening of plant).
Shoot apical meristems
Gives rise to leaves, buds, and other above-ground structures
Dormant meristems in the axillary buds -- grow into new shoots or branches
Chemical messengers that carry information from one cell to another.
Only very low concentrations are needed to trigger a change in the plant.
Plants sense environmental conditions and respond by changing the number of cells produced by the meristem and altering the pattern of growth and specialisation to produce
Transmitted from one part of the plant to another in the sap of the phloem.
Produced in the shoot apical meristem.
Auxins inhibit growth in the axillary buds (horizontal growth)
Auxins found in the apices of shoots and roots
Apical dominance - Grow vertically upwards to trap more light
In the shoots = stimulate growth; in the roots = inhibit growth
The further down auxin travels from the apical meristem, the lower its concentration --> Axillary buds grow.
When light is equally bright, auxin moves symmetrically downwards, pumped through specialised protein pumps
Phototropism - Response to light
When there is a difference in brightness, auxin efflux carriers increase on the internal lateral side of the membrane
Positive phototropism (Shoots) = Grow towards the source of light.
Overhead sunlight: Auxin diffuses evenly down the stem --> Shoot grows vertically upwards toward the light.
Auxin gathers in the shaded side of the shoot and establishes a concentration gradient.
Uneven sunlight: Auxin pumps redistribute auxin to it accumulates on the shaded side --> Cell elongation on the shaded side --> Shoot bends towards the light.
The shaded side of the shoot experiences more cell elongation, bending towards the light
Auxin alters gene expression --> stimulates transcription of genes that produce proton pumps to bind to receptors
Gravitropism - Response to gravity
The proton pumps pump protons (hydrogen ions) out of the cytoplasm into the cell wall
Positive gravitropism (Roots) = Roots grow into the soil.
Allows absorption of water and necessary minerals to support the plant.
A higher H+ concentration makes the cell wall more acidic, which breaks bonds between cellulose fibres (by 1. disrupting hydrogen bonding; 2. activating pH-dependent expansin proteins that sever cellulose connections)
Reduced number of bonds between cellulose microfibrils that make it more flexible
An influx of water makes the cell increase in size
Cellulose fibres can slide apart as they are pushed by turgor pressure inside of the cell --> more flexible
In a root placed horizontally, light from above and gravity will both cause redistribution of auxin so it accumulates on the lower shaded side.