Plant Hormones and Signaling

Question 1

What is phototropism?

Answer 1

bending of growing seedlings towards the light

Question 2

What did Darwin and Darwin find about phototropism and how?

Answer 2

the tip of the emerging plant is what responds to light
4 groups were measured
tip removed: no bending
opaque tip: no bending
transparent tip: bending
shield over curvature: bending

Question 3

What did Boysen and Jenson find about phototropism and how?

Answer 3

the response of bending is induced chemically, something moves through the shoot (a hormone)
2 groups were measured
permeable gelatin roadblock: bending
impermeable mica roadblock: no bending

Question 4

What did Went find about phototropism?

Answer 4

the distribution of the chemical signal is responsible for bending
also discovered auxin, now known as indoleacetic acid (IAA)

Question 5

What does IAA cause?

Answer 5

growth of a seedling by inducing cell elongation; bending occurs because IAA concentrations are higher on the ‘dark’ side of the plant

Question 6

What does absolute concentration of auxin promote and inhibit?

Answer 6

promotes stem growth at moderate concentrations

inhibits stem growth at high concentrations

Question 7

How does relative concentrations affect plants?

Answer 7

apical dominance and lateral branching controlled by antagonistic effects of auxin and cytokinins

Question 8

How does gravitropism affect plants?

Answer 8

plants detect gravity
roots grow in positive gravity
shoots grow in negative gravity
detected by statoliths (plastids with dense starch grains)
not light dependent

Question 9

How does thigmotropism affect plants?

Answer 9

Plants respond to mechanical stimuli:
change in growth form (erect to prostrate in high winds)
direction of growth (vine tendrils)
short term movements (leaf collapse)

Question 10

How do plants respond to attacks?

Answer 10

release of toxins, repellants within attacked plant

release of chemical signals to other plants

Question 11

What are examples of cyclic environmental changes?

Answer 11

germination
leaf and stem growth phases
flowering and fruit and seed production
photosynthate production and storage in summer and winter dormancy

Question 12

What mechanism detects night length?

Answer 12

the phytochrome switch

Question 13

What is a phytochrome?

Answer 13

a dimer of two identical peptide chains that has a chromophore with two isomers
Pr (phytochrome red light)
Pfr (phytochrome far red light)