auxins + giberellins

What do gibberellins do and so what do they determine about a plant

• stem elongation

• Height of a plant

• Also involved in germination of seeds

What is the tallness in pea plants determined by

• a gene with two alleles - Le and le

• Homozygous recessive with always be dwarf

• Le (dominant) results in regular, tall plant

What does the dominant Le allele in pea plants regulate

The synthesis of the final enzyme (3-beta-hydroxylase) in a pathway that produces an active form of gibberellin - GA1

• GA1 or active gibberellin stimulates cell division and cell elongation in stems, causing them to elongate.

Why does the recessive le allele in pea plants give rise to dwarf plants.

• substitution mutation in gene gives rise to recessive allele

• Changes alanine to threonine in primary structure of 3-beta-hydroxylase’s active site, change in tertiary structure so non-functional enzyme.

• Cannot convert inactive gibberellin to active form

How can dwarf plants be made to grow taller

Artificially applying active gibberellin to make its stem elongate

Give the structure of a dormant seed (containing an embryo) (3)

• embryo surrounded by endosperm

• Around endosperm in the aleurone layer

• Seed coat/testa covers whole seed

What is the role of the endosperm

• surrounds embryo

• Energy store of starch

What is the role of the aleurone layer

• surrounds outer edge of the endosperm

• Rich in protein

How is germination brought about (4)

• absorption of water at start of germination causes the embryo to start producing gibberellins

• Gibberellin diffuses to aleurone layer and cells start to synthesis amylase

• Amylase hydrolyses starch in endosperm to soluble maltose

• Maltose converted to glucose, transported to embryo, providing respiratory substrate allowing germination and growth

What is the role of DELLA protein and how

• DELLA proteins inhibit germination or growth by regulating genes that are involved in synthesis of amylase or growth

• They do this by binding to transcription factors, eg PIF, preventing them from binding to the promoter and thus preventing transcription

How do gibberellins work

• By causing the destruction of DELLA proteins

• With DELLA protein destroyed, the transcription factors (PIF) can bind to DNA promoter allowing transcription of mRNA

What are growth hormones called

Plant growth regulators

What are plant growth regulators responsible for

Communication between different parts of the plant

Where are plant growth regulators produced

In tissues

What are the different means by which plant growth regulators move around the plant

• directly from cell to cell by diffusion or active transport

• Carried along with the contents of the phloem

How do plant growth regulators work

They bind to receptors on cell surface membrane or in the nucleus and initiate a signalling cascade.

What do auxins do

Causes cell elongation

What is the most common auxin

IAA (indole 3-acetic acid)

Where is auxin made

In the growing tips - meristems - of shoots and roots, where the cells are dividing.

What are the 3 stages in which growth occurs

• cell division via mitosis

• Cell elongation by absorption of water

• Cell differentiation

How does auxin work to cause elongation (7)

• Auxin binds to receptors in the cell surface membrane and activates proton pumps which pump H+ into the cell wall

• The acidification of the cell walls loosens the H-bonds between the cellulose microfibrils

• Expansins (proteins) also loosen the linkages between cellulose microfibrils and the surrounding substances.

• Potassium ion channels are stimulated to open, K+ diffuses into cytoplasm, increasing potassium ion concentration within the cytoplasm.

• Decreases water potential so water moves by osmosis into cell via aquaporins.

• Cells become more turgid and can elongate as a result of loosened bonds in cell wall.

• Disrupted bonds now reform and the wall becomes stable again.

Explain the role of auxin in cell elongation [8] {exam example}