Session 2: Plant Hormones and Reproduction

Question 1

What are meristems?

Answer 1

A region of cells capable of mitotic division and indeterminate growth in plants. There are two types: apical (causes primary growth, occurs at shoots and root, produces new leaves and flowers) and lateral (causes secondary growth, occurs at the cambium - produces bark on trees). Meristematic cells are totipotent (can give rise to all the cells of the adult plant).

Question 2

What are the plant Hormones?

Answer 2

Growth promoters: auxins, gibberellins, cytokinins. And Growth inhibitors: ethylene gas, abscisic acid. Some hormones can act as both.

Question 3

What do auxins do?

Answer 3

Auxins carry out multiple roles regarding plant development:
◻ Plant growth (tropisms)
◻ Flowering
◻ Fruit development

Auxins stimulate genes in cells associated with plant growth that promote cell elongation and increasing the rate of cell division.

Question 4

What are tropisms?

Answer 4

Tropisms are the growth of a plant toward or away from a stimulus, including: Phototropism: in response to light, Gravitropism/geotropism: in response to gravity, Thigmotropism: in response to touch. In general, tropisms involve cell elongation (or suppression of cell elongation) on one side of a plant, causing the plant to grow in a particular direction

Question 5

How do auxins regulate plant growth?

Answer 5

Auxins are produced by the tip of a shoot or root (i.e. apical meristems). Auxin efflux pumps can set up concentration gradients within tissues – changing the distribution of auxin within the plant. These pumps can control the direction of plant growth by determining which regions of plant tissue have high auxin levels. Auxin efflux pumps can change position within the membrane (due to fluidity) and be activated by various factors.

Question 6

In more detail, how do auxins make a plant grow?

Answer 6

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 and become longer (elongation)

Question 7

Describe Phototropism.

Answer 7

Under dark conditions auxin moves evenly down the stem. If the tip is exposed to light on one side, light receptors (phototropins) trigger the redistribution of auxin to the dark side of the plant. This prompts that side to grow more (the cells elongate), bending the tip towards the light source.

Question 8

Describe Gravitropism.

Answer 8

Auxin will accumulate on the lower side of the plant in response to the force of gravity. Auxin’s mechanism of action is different in shoots and roots as different gene pathways are activated in each tissue. In shoots, auxins are more concentrated on the lower side of the stem, causing the cells there to elongate. In roots, auxin concentration on the lower side of the root suppresses cell elongation.

Question 9

Describe apical growth.

Answer 9

The apical meristems give rise to primary growth (lengthening) and occurs at the tips of the roots and shoots. Growth at these regions is due to a combination of cell enlargement and repeated cell division (mitosis and cytokinesis). Differentiation of the dividing meristem gives rise to a variety of stem tissues and structures – including leaves and flowers. In the stem, growth occurs in sections called nodes - with the remaining meristem tissue forming an inactive axillary bud (which has the potential to form
new branching shoots, complete with leaves and flowers)

Question 10

What is apical dominance?

Answer 10

Apical dominance ensures that a plant will use its energy to grow up towards the light in order to outcompete other plants. As shoots grow further from buds, inhibition of lateral buds is diminished, allowing spread of lateral growth.

Question 11

What are adventitious roots?

Answer 11

Adventitious roots are those growing out of places where roots don’t normally grow. Auxins stimulate root growth on the end of a cutting.

Question 12

What do gibberellins do?

Answer 12

Promote cell elongation in the internodes of plants. Stimulates growth of the ovary wall into a fruit. Stimulates seed germination and release of food reserves in seeds.

Question 13

How does flowering occur?

Answer 13

Auxins and gibberellins help create changes in gene expression in the shoot apex to cause the meristem to produce flowers instead of leaves.

Question 14

What do cytokinins do?

Answer 14

Promotes cell division (cytokinesis) in meristems and ensures roots and shoots grow at equal rates. Promotes secondary growth (thickening) and help to control the rate of branching by a plant. Cytokinins are also involved in stimulating the growth of fruit and seed development

Question 15

What is ethylene?

Answer 15

A gas which acts as a plant hormone and stimulates maturation and ageing (senescence). It is responsible for the ripening of certain fruit (auxins and gibberellins promote fruit growth but inhibit ripening). It also contributes to the loss of leaves (abscission) and the death of flowers

Question 16

What is abscisic acid?

Answer 16

Abscisic acid (ABA) principally functions to inhibit plant growth and development. It promotes the death of leaves (abscission) and is responsible for seed dormancy (inhibits gibberellins). It generally initiates stress responses in plants (like winter dormancy in deciduous plants). Abscisic acid controls the closing of stomata and hence regulates water loss in plants.

Question 17

Describe circadian rhythms in plants.

Answer 17

The plants’ biological clock is set to a 21-27 hour cycle. Some plants close their flowers or leaves at night. This prevents loss of heat and frost damage. Photosynthesis, auxin production and cell division also rise and fall with the circadian rhythm.

Question 18

Describe photoperiodism.

Answer 18

A pigment called phytochrome detects the amount of daylight in each diurnal (day/night) cycle. It has two forms: Pr (biologically inactive) and Pfr (biologically active). Pr converts to Pfr in red light. Pfr converts back to Pr in far red light and darkness. Photoperiodism is involved in seed germination, leaf growth, flowering and dormancy. It is involved in flowering with short day plants and long day plants, and also other light initiated responses, such as germination and shoot growth.

Question 19

Describe development of the flower (e.g. what happens before flowering).

Answer 19

In order to flower, a plant must pass through several stages of development with several changes in gene expression occurring at the shoot meristem. The plant must first pass from an immature to a sexually mature stage. The apical meristem must change from a vegetative meristem to a floral meristem, The organs of the flower must grow and develop. Environmental cues perceived in the leaves are transmitted to the apical meristem by hormonal messengers to trigger development of flowers.

Question 20

What is the male part of the flower?

Answer 20

Called the stamen, composed of: anther (pollen producing organ) and filament (slender stalk supporting another - making it accessible to pollinators)

Question 21

What is the female part of the flower?

Answer 21

The pistil, composed of: stigma (sticky receptive tip that catches pollen), style (tube shaped connection between stigma and ovule), and ovule (structure containing female reproductive cells that develop into a seed)

Question 22

How can people induce or control flowering in plants?

Answer 22

By controlling the exposure of light. E.g. exposure during the night or covering of plants (long day vs short day)

Question 23

Describe pollination.

Answer 23

The transfer of pollen to the ovary in flowering plants
(angiosperms). Flowers are pollinated in three different ways (animal, wind, or water) and their structures differ accordingly. Of the animal pollinators, insects provide the greatest effectiveness of pollination as well as the most specialized pollination. Flowers attract insects with brightly coloured petals, smells (scent), and food such as nectar and pollen.

Question 24

Describe fertilisation.

Answer 24

Pollen grains are immature male gametophytes. Pollination is the actual transfer of the pollen from the stamens to the stigma. After landing on the sticky stigma, the pollen grain is able to complete development, germinating and growing a pollen tube that extends down to the ovary. The pollen tube enters the ovule through the micropyle, a small gap in the ovule. One sperm nucleus fuses with the egg to form the zygote. A second sperm nucleus fuses with the two polar nuclei within the embryo sac to produce the endosperm tissue (3N) (double fertilization). There are usually many ovules in an ovary, therefore many pollen grains (and fertilizations) are needed before the entire ovary can develop.

Question 25

Describe seed dispersal.

Answer 25

Plants have evolved many ways to ensure that their seeds are dispersed. This has given them opportunities to expand their range. In some cases the seed itself is the agent of dispersal, but often it is the fruit or an associated attached structure. The main agents of seed dispersal are wind, water, and animals (burs/seed hooks and fruits).

Question 26

Describe germination.

Answer 26

Germination is the beginning of growth following dormancy. A thick seed coat protects the seed during dormancy. Abscisic acid inhibits chemical reactions within the seed. Different plant species require different triggers for germination, to optimise the chance of the seedling surviving. In most species during winter, enzymes in the seeds degrade the abscisic acid. By spring the abscisic acid is gone and the seed can germinate. Some require other triggers: Light - only in seeds with thin seed coat (where there is a space in the forest canopy letting light through, indicating a space for a new plant to grow). Darkness - seeds that germinate underground

Question 27

How does a seed survive?

Answer 27

A seed is an entire reproductive unit, housing the embryonic plant in a state of dormancy. During the last stages of maturing, the seed dehydrates until its water content is only 5-15% of its weight. The embryo stops growing and remains dormant until the seed germinates. At germination, the seed takes up water and the food store is mobilized to provide the nutrients for plant growth and development. When a seed is exposed to the proper conditions, water and oxygen are taken in through the seed coat. The hormone gibberellin is produced and starts chemical reactions occurring. Hydrolytic enzymes start to break down starch reserves to release glucose for growth. This provides energy to synthesise new molecules –> growth.

Question 28

Once the seed is metabolically activated, germination proceeds according to the following stages:

Answer 28

The seed coat (testa) ruptures and the embryonic root (radicle) grows into the ground to extract key nutrients and minerals. The cotyledon emerges and produces the growing shoot’s first leaves. The growing plant can be divided into the epicotyl (embryonic shoot), hypocotyl (embryonic stem) and developing roots

Question 29

Describe asexual production.

Answer 29

Many plants reproduce both asexually and sexually. In asexual reproduction, a part of the parent plant is used to generate a new plant which is genetically identical. This process can natural or artificial. Examples of natural: runners, rhizomes, bulbs, tubers, suckers. Examples of artificial: grafting, cutting, micropropagation.