B10

Question 1

What are plant responses?

Answer 1

Evolved mechanisms that allow plants to respond to environmental signals in a way that allows them to maximise growth and survival

Question 2

What are the phases in plant growth and development?

Answer 2

Seed dormancy
Germination
Seedling growth, differentiation, maturation
Flowering
Seed and fruit production
Clonal growth
Seasonal growth

Question 3

What are types of plant growth regulators?

Answer 3

Auxins, gibberelins, cytokinins, abscisic acid, ethylene, brassinosteroids.

Question 4

What are some important properties of plant growth regulators?

Answer 4

Growth promotion and/or inhibition. Act in combination or antagonistically. Transported in different ways, plant growth regulators rather than hormones

Question 5

What happens in the signal transduction pathway?

Answer 5

Hormones bind to proteins associated with membranes of the cells they will affect. E.g. extra cellular signalling molecule activates a membrane receptor, that in turn alters intracellular molecules. This creates a response.

Question 6

What are the main effects of auxin and how is it transported?

Answer 6

Cell enlargement and tissue differentiation. Polar transport from apex to base in parenchyma surrounding vascular bundles, approx 1cm per hour, specific auxin transport proteins.

Question 7

What is the mechanism of auxin transport?

Answer 7

Basipetal transport, transport remains polar if shoot/root turned upside down, faster than diffusion, requires energy and carrier proteins, use of arabidopsis mutants very important in elucidation.

Question 8

What are some auxin effects?

Answer 8

Apical dominance, tropisms, tissue differentiation.

Question 9

What happens in apical dominance?

Answer 9

Auxin inhibits auxiliary bud growth, auxiliary bud dormant until apical top is removed. Lateral branches are formed

Question 10

What are tropisms?

Answer 10

Plant growth responses which may be positive or negative

Question 11

What are the tropisms in roots and stems?

Answer 11

Roots + gravitropic, stems - gravitropic. The stems of many plants are usually positively phototropic.

Question 12

What happens with positive phototropism?

Answer 12

Blue light is detected by protein-flavin complex, and auxin accumulates on shaded side.

Question 13

What is a diffusable growth hormone?

Answer 13

Auxin, can pass through gelatin but not mica.

Question 14

How does phototropism work?

Answer 14

Light has to be perceived at the tip. Growth response has to be elicited - in the elongation zone.

Question 15

What is phototropin?

Answer 15

Blue light receptor, it is a flavoprotein

Question 16

What is the curvature response?

Answer 16

Cholodny-went theory - redistribution of auxin with greater conc on shaded side. Auxins cause acidification of cell wall - loosening of wall matrix (expansins) - turgor pressure results in cell elongation

Question 17

What do auxins promote and how?

Answer 17

Tissue differentiation, auxin transported by polar transport mechanism to area of wound. Cells in pith differentiate into new vascular tissue and connect with vascular tissue above and below wound.

Question 18

What is the signal transduction pathway in auxins?

Answer 18

In the absence of auxin, the auxin response gene is repressed by AUX/IAA protein. Auxin receptor is a ubiquitin E3 ligase. Auxin-ubiquitin E3 ligase complex binds to AUX/IAA protein which becomes ubiquitanated. Ubiquinated protein is tagged for proteolysis via the 26S proteasome. Auxin response gene is turned on.

Question 19

What are the main effects of gibberelins and how are they transported?

Answer 19

Cell division and enlargement, seed germination, environmental effects, maturation, flowering initiation, fruit formation. Transportation is nonpolar bidirectional in phloem.

Question 20

What effect do gibberelins stimulate on dwarf plants

Answer 20

Internode elongation

Question 21

What can cause dwarf mutants?

Answer 21

Mutations in GA biosynthesis pathway genes. Mutations in genes that control response to GA.

Question 22

What is the role of gibberelins in germination?

Answer 22

Many seeds require light or cold periods to break dormancy. GA promoted alpha amylase activity in barley seeds.

Question 23

What happens in the gibberellin signal transduction pathway?

Answer 23

Inhibited by a repressor, binding to receptor, transcription activated. Repressor has two domains, one binds GA-receptor complex, the other binds to response gene. Mutation in DELLA domain results in no response to GA. Mutation in GRAS domain results in growth response in the abstened of added GA.

Question 24

What are the main effects of cytokinins and how are they transported?

Answer 24

Cell division, germination, retards senescence, root growth and differentiation, overcomes bud dormancy. Transportation is long distance in xylem

Question 25

What happens when cytokines interact with other plant regulators?

Answer 25

Cytokinins and auxin interaction in tissue culture, high cytokinin ratio = shoots, high auxin ratio =roots

Question 26

What are other cytokinin effects?

Answer 26

Promote cell expansion in dicot cotyledons, promote chloroplast development, delay leaf senescence, promote movements of nutrients.

Question 27

What happens with cytokinins and signal transduction pathway?

Answer 27

Cytokinins receptors are transmembrane protein kinases, binding of cytokinin to the receptor protein initiates a phosphorylation cascade. Leading to transcription of genes.

Question 28

What happens with cytokinins and signal transduction pathway? Steps

Answer 28

Cytokinin binds to a receptor, activates histidine kinase activity - P transferred to a protein. Phosphorylated protein moves to the nucleus, transfers P. Gene transcription begins in the nucleus

Question 29

What are the main effects, stress responses and transportation of abscisic acid?

Answer 29

Main effects are that it is a growth inhibitor, seed and bud dormancy, embryo development, senescence. Stress response is stomatal closure. Transportation is root to shoot in xylem, shoot to root in phloem.

Question 30

What happens in dormancy in abscisic acid?

Answer 30

ABA levels increase during early seed development, promotes production of seed storage proteins, prevents premature germination. Breaking dormancy corresponds with decrease in ABA in seeds. Bud dormancy also under ABA control. ABA effects overcome by GA.

Question 31

What is the drought stress response in ABA?

Answer 31

Stressed plants contain much higher amounts of ABA than non-stressed plants. Roots respond to drought stress by increased synthesis of ABA. ABA transported from root to shoot in xylem. ABA reaches leaves and induces stomatal closure. ABA promotes efflux of K+ out of guard cells

Question 32

What are the main effects and transportation of ethylene?

Answer 32

Fruit ripening, leaf and flower senescence, abscission, root hair development, stress responses, inhibition of cell expansion - triple response. Transportation involves diffusion.

Question 33

What is ethylene triple response?

Answer 33

Exposure of seedlings to ethylene in the dark results in, short hypocotyl, thickened hypocotyl, horizontal growth. In absences of ethylene, seeds exhibit etiolation.

Question 34

What is the relationship between ethylene triple response and arabidopsis mutants?

Answer 34

Seedlings grown in the dark with atmosphere containing ethylene. Arabidopsis gene ETR1 codes for ethylene receptor protein.

Question 35

What is the relationship between ethylene and leaf abscission?

Answer 35

Over life span of leaf auxin level decrease whilst ethylene level increases. Synthesis of enzymes that hydrolyze the cell wall polysaccharides, resulting in cell separation and leaf abscission.

Question 36

What happens with protein kinases and phosphatases in signal transduction pathways?

Answer 36

Nearly all biological signalling systems involve protein phosphorylation and de-phosphorylation in cascade systems. Signal transduction pathways in plants are still incompletely understood. Experiments with arabidopsis mutants has advanced our understanding of these processes enormously.

Question 37

What happens with vernalisation and stratification?

Answer 37

Plants that have to endure unfavourable winter usually become dormant in autumn and initiate new growth in spring. In temperate zones, many seeds require cold exposure to break their dormancy. Similarly many buds require cold period before they initiate new growth. These processes are under hormonal control. Abscisic acid (ABA) is implicated in dormancy.

Question 38

What happens in seed germination?

Answer 38

Seed contains a desiccated, dormant embryo. Must absorb water and obtain oxygen. Impervious coat may impose dormancy. Physiological dormancy imposed by hormones. In temperate regions, a cold period may be required. In deserts, seeds remain dormant during the dry season and may require inhibitors to be leached out by rain. Some seeds require light to stimulate germination. A cold period is often required to break bud dormancy or promote flowering.