IB biology: topic 9: growth in plants

what are meristems?

tissues in a plant consisting of undifferentiated cells capable of indeterminate growth

what do meristem tissues mean for propagation of plants?

meristematic tissue can allow plants to regrow structures or even form entirely new plants - totipotent.

what are the different meristems?

apical meristems (shoot and root tips) and lateral meristems (cambium).

describe apical meristems

apical meristems occur at shoot and root tips and are responsible for primary growth (i.e. plant lengthening).

apical meristems give rise to new leaves and flowers.

describe lateral meristems

lateral meristems occur at the cambium and are responsible for secondary growth (i.e. plant widening / thickening).

responsible for the production of bark.

how does growth at apical meristems occur?

cell enlargement and repeated cell division (mitosis and cytokinesis).

how are a variety of stem tissues and structures grown in plants?

the differentiation of the dividing meristem gives rise to a variety of stem tissues and structures – including leaves and flowers.

what are nodes? what is their role in growth of plants?

in the stem, growth occurs in sections called nodes – with the remaining meristem tissue forming an inactive axillary bud.

what is the the role of axillary (lateral) buds in the growth of plants?

these axillary (lateral) buds have the potential to form new branching shoots, complete with leaves and flowers.

what are IAAs?

group of plant hormones involved in shoot and root growth - AUXINS

where is auxin released from?

the shoot apex

how do auxins promote plant growth?

auxins cause change to the pattern of gene expression with a plant’s cells, causing cell elongation and division.

what is the role of auxin efflux pumps?

set up concentration gradients within tissues – changing the distribution of auxin within the plant.

• auxin efflux pumps can change position within the membrane (due to fluidity).

what is the role of auxin in the shoots?

• increases the flexibility of the cell wall.

• stimulates cell elongation and thus high concentrations of auxin promote growth (cells become larger).

outline HOW auxin promotes growth in the shoots

• Auxin activates a proton pump in the plasma membrane which causes the secretion of H⁺ ions into the cell wall.

• The resultant decrease in pH causes cellulose fibres within the cell wall to loosen (by breaking the bonds between them).

• Additionally, auxin upregulates expression of expansins, which similarly increases the elasticity of the cell wall.

• With the cell wall now more flexible, an influx of water (to be stored in the vacuole) causes the cell to increase in size.

what is the role of auxin in the roots?

auxin inhibits cell elongation and thus high concentrations of auxin limit growth (cells become relatively smaller).

what is apical dominance?

the production of auxins in the shoot apical meristem INHIBITS growth in lateral (axillary) buds.

what is the purpose of apical dominance?

apical dominance ensures that a plant will use its energy to grow up towards the light in order to outcompete other plants.

what factor affects the strength of apical dominance? how does this affect axillary bud growth?

the strength of the inhibitory effect of apical dominance depends on concentration, as the stem grow higher, auxin concentration nearer the bottom of the stem decreases, promoting growth of axillary buds further down the stem.

what are tropisms?

the growth or turning movement of an plant in response to a directional external stimulus.

what is phototropism?

a growth movement in response to a unidirectional light source.

what is gravitropism/geotropism?

a growth movement in response to gravitational forces.

what hormone controls phototropism and gravitropism?

auxin

how does auxin control gravitropism?

auxin will accumulate on the lower side of the plant in response to the force of gravity.

how does auxin control phototropism?

light receptors (phototropins) trigger the redistribution of auxin to the dark side of the plant.

auxin - phototropism - shoots

dark side of the shoot elongates and shoots grow towards the light - positive phototropism

auxin - gravitropism - shoots

lower side of the shoot elongates and roots grow away from the ground - negative gravitropism

auxin - phototropism - roots

dark side of the root becomes shorter and the roots grow away from the light - negative phototropism

auxin - gravitropism - roots

lower side of the root becomes shorter and the roots turn downwards into the earth - positive gravitropism

what is micropropagation?

a technique used to produce large numbers of identical plants (clones) from a selected stock plant.

vegetative propagation vs microproporgation

vegetative propagation - when a plant cutting is used to reproduce asexually in the native environment.

micropropagation - when plant tissues are cultured in the laboratory (in vitro) in order to reproduce asexually.

what is an explant?

the tissue sample

how is microproporgation done?

• specific plant tissue is selected from a stock plant and sterilised.

• the explant is grown on a sterile nutrient agar gel, treated with growth hormones (e.g. auxins) to stimulate shoot and root development.

• The growing shoots can be continuously divided and separated to form new samples (multiplication phase).

• Once the root and shoot are developed, the cloned plant can be transferred to soil.

what are some of the uses of micropropagation?

• rapid bulking up of new varieties.

• production of virus-free strains of existing varieties.

• propagation of orchids and other rare species.

micropropagation: outline the rapid bulking of new varieties of plant

• desirable stock plants can be cloned via micropropagation.

• more reliable than selective breeding - new plants are genetically identical to the stock plant.

• used to rapidly produce large quantities of plants created via genetic modification.

micropropagation: outline the production of virus-free strains of plant

• viruses typically spread through infected plants via the vascular tissue – which meristems do not contain.

• propagating plants from the non-infected meristems allows for the rapid reproduction of virus-free plant strains.

micropropagation: outline the propagation of rare species

• micropropagation is commonly used to increase numbers of rare or endangered plant species.

• it is also used to increase numbers of species that are difficult to breed sexually (e.g. orchids).

• it may also be used to increase numbers of plant species that are commercially in demand.