Plant responses to internal and external signals

Chapter 39

I) Plant responses to hormones*hormones are the major way plants can have a long term

response to stimuli.

A) detection of first hormone1) experiments by Darwin and Darwin

2) experiment by Boysen-Jenson

B) Auxins1) any hormone that causes elongation and growth

a) mainly found in apical meristem of shoot or root tip

b) moves by active transport to dark side of plant. Those cells elongate and plant grows toward light( or inhibit growth in root)

c) can be added to cuttings to promote adventitous root formation

C) Cytokinins1) made in the roots2) promotes cell division, root growth, and germination

3) in equal amounts auxins and cytokinins cause cell growth but not differentiation

4) if cytokinin levels rise, cells turn into shoot buds, but if auxin levels rise roots form

D) Gibberellins1) Found in meristems of apical buds and roots, young leaves, embryo

2) seed/bud germination, stem and leaf growth

3) must be present for a fruit to set

4) when water is absorbed by seed gibberellins are released to break dormancy

E) Abscisic Acid(ABA)1) high levels of ABA in developing seeds prevent germination

2) if leaf begins to wilt from loss of water, ABA buildup closes stomata

F) Ethylene **1) auxin levels fall, and ethylene triggers leaves to fall off in the fall

2) Allow fruit to ripena) acts as positive

feedbackb) “one bad apple

can spoil the whole bunch”

3) triple responsea) if seed germinates under something the mechanical stress on the tip triggers ethylene production, which:

a1) slows growtha2) thickens stema3) causes it to

bend

II) Plant responses to light

1) photomorphogenesis: effects of light on plant morphology

2) action spectrum: graph that relates response to wavelenth

A) Blue-light photoreceptors

1) cryptochromes: pigments that inhibit hypocotyl elongation

2) phototropin: pigment that allows phototropism

B) Phytochromes and seed germination

1) phytochrome: a protein with a non-protein section that responds to light

2) some seeds germinate when provided with red light

a) phytochrome response is reversible and is determined by the last flash of light received.

b) Pr(P660) recognizes red light

Pfr(P730) recognizes far red

c) as Pr is hit by red light, it changes to Pfr. Seeds in the dark have just Pr, but the appearance of Pfr triggers germination.

C) Shade avoidance

1) in full light amount of Pr and Pfr is equal.

2) in shady areas, more red light is absorbed during photosynthesis from the canopy, so the presence of more far red causes a shift to the production of Pr

3) the increase of Pr triggers growth of the stem to try and reach sunlight so the tree isn’t shaded out.

III) Biological clocks and circadian rhythms.

1) circadian rhythm: an internal “clock” that keep time for an organism

A) Light sets the clock

1) Pr is made by plants, and is converted to Pfr during the day, which is the active form. a) a flash of red light during the night resets the clock and shortens the night for the plant

2) Pr accumulates during the night as the plant makes it and it isn’t converted to Pfr because there is no red light.

3) this is what triggers flowering.a) long-day plants

a1) flash of red during the night “shortens” the night and can trigger flowering

a2) if you flash red-far red, the initial red flash is cancelled out.

b) Short day plantsb1) are also affected this way

B) Photoperiodism

1) the response of plants to length of day and night

2) Control of floweringa) short-day plant: will only flower if day is shorter than a critical time( 14 hours, for example)

a1) can also say they flower when day length is decreasing

b) long-day: flower if day is longer than a critical period(14 hours for example)b11) flower when day length is increasing

c) Day neutral: flower plants reaches a certain point of maturity, regardless of day length

d) all of these trigger the hormone florigen in leaves, which then travels to shoot tips to make the flower

e) night length is what controls this

IV) Responses to things other than light

A) Response to gravity1) gravitropism

a) gravity causes statoliths to settle at bottom of horizontal root. Auxins then congregate here and inhibit growth, so opposite side grows faster to turn root

b) if stem is horizontal, same thing happen, but the auxins stimulate growth to turn stem up towards sun.

2) thigmotropisma) respond to touch to grow around an object