Lecture 3: Hormones and Tropisms

Question 1

Genotype

Answer 1

genetic makeup/unique DNA sequence of an organism

Question 2

Phenotype

Answer 2

often visible/presentable characteristics of an organism, affected by genotype
(e.g. weight, physics, eye colour, behaviour)

Question 3

Growth

Answer 3

1. Zygote 2. Division and Enlargement 3. Individual Plant

Question 4

Plant morphogenesis

Answer 4

Biological process in which the plant assumes its specific form during development.
-Encompasses its external shape and internal organization
-Genotype: mutations of the DNA sequence can affect the phenotype

Question 5

Impact of environment on plants

Answer 5

Differences in environment affects growth, shape and size of the plant

Question 6

Differentiation (Hormones and Tropisms)

Answer 6

Increase in complexity occurs as some cells become different from one another (i.e. the process of meristem becoming primary tissue).

-Development+morphogenesis occurs at the level of organs (entire plant) and is possible through hormones.

Question 7

Adaptation

Answer 7

capacity to detect environmental factors, and to alter the development+morphogenesis to ensure survival and reproduction

Question 8

Strong selective pressure from environment (adaptation)

Answer 8

-Gravity (one directional guideline for growth)
-Light (sunny+dark side, day length)
-Temperature (day/season)
-Moisture/humidity
-Pests
-Changes in microenvironments (e.g. soil/canopy)

Question 9

Communication and coordination

Answer 9

Stimulus triggers the need of modification
Communication form includes:
-Perception
-Transduction
-Response

Question 10

Hormones

Answer 10

organic chemicals produced (messengers) in one part of a plant and transported to other plants, where they initiate a response.

-Act at low concentrations
-Are released into general circulation (xylem+phloem) and aren’t carried specifically to the target
-Receptor molecules at the site of response, then triggers a response as the hormones bind to them

Question 11

Hormone auxin

Answer 11

Indoleacetic acid (IAA)
-Synthesis from tryptophan (aromatic amino acid): in the shoot apical meristem
-Transports downwards through phloem and polar transport

Polar auxin transport: through protein channels in the plasma membrane
Phloem transport: hormones flow with fluids in a mass flow

Question 12

What mechanisms is auxin involved in?

Answer 12

1. Cell elongation:
   -Binds to auxin receptors just below the stem apical meristem in cells of young internodes.
   -Elongated cells (grown, mature cells) lack auxin receptors
2. Apical dominance:
   -Apically produced auxin induces dormancy in lateral buds
   -Each shoot tip gets one active apical meristem: apical dominant
   -Growth of stem apical meristem from an axillary bud causes the concentration of auxin around the axillary bud to decrease (if concentration drops below threshold, inhibits growth and lateral bud becomes active to grow out)
3. Differentiation of vascular tissues

Question 13

Gibberellins

Answer 13

Out of 125: GA3, gibberellin acid, GA2, GA19 are most active

Mechanisms:
1.) Stimulates elongation (similar to auxin)
-higher GA concentration leads to larger growth

2.) First chemical signal after inhibition
-The way water enters the seed
-GA is the signal that water entered the seed cell: GA+H2O can breakdown starch
-Starch is transformed into glucose, allowing for cell respiration

Question 14

Ethylene

Answer 14

-Gaseous plant hormone
-Relevant in fruit development: climacteric fruits
-Works with a positive feedback system (when ethylene production starts it doesn’t stop)

-Fruit development & maturation:
–ethylene production peaks with fruit ripens
–decreases when fruit reaches senescence (deteriorating, stagnant)

Question 15

Tropisms

Answer 15

growth response involving bending/curving of a plant towards/away from an external stimulus that determines the direction of movement.
-Positive: towards stimulus
-Negative: away form stimulus

Question 16

Phototropism

Answer 16

growth in which light is the determining factor
-uses auxin
-results in elongation of the cells (shaded side of tip)

Question 17

Role of pigments in sensing light

Answer 17

-Redistribution of auxin via a photoreceptor
-Signal happens through phototropic
-Response triggered by UV+blue light (400-500nm: wavelengths don’t overlap w/light absorbed in chlorophyll)

Question 18

Photoreceptor

Answer 18

Pigment-containing protein that converts light into a biochemical response

Question 19

Gravitropism

Answer 19

-Primary roots of plants are positively gravitropic
-Shoots of main axis are negatively gravitropic

-Starch-statolith hypothesis: amyloplasts play the role of gravity sensors (statoliths)
–Gravity-sensing cells in which statoliths occur are statocytes: statocytes become displaced because they follow the direction of gravity

–Role of auxin: stimulates growth/elongation of root
Asymmetric growth on bottom (towards gravity): less growth because IAA>threshold
More growth on top (away from gravity) IAA<threshold

Question 20

Photoperiodism

Answer 20

Critical length of the day for the initiation of flowering
-Critical length: generally 12-14 hours
-Short day plants: plants that flower when day length<critical length
-Long day plants: plants that flower when day length>critical length
-Neutral day plants: bloom under any length of day, require minimum amount of light for normal growth.

-Extremely precise+efficient for plants but can be affected by slight changes too (too little daylight=no flowering, latitude can affect certain plants)
Other responses to day length: germination/dormancy of seeds/buds, tuber production of tubers

Question 21

Phytochrome

Answer 21

light-sensitive pigment

-Pred or Pr: absorbs red light (660-670nm)

-Pfar-red or Pfr: active form that absorbs far-red light (710nm)

Normal/inactive form of Pr synthesises into active form Pfr when hit by red light.

Pfr results in a biological response (like destruction or breakdown of products)

Question 22

Ways to revert back to Pr

Answer 22

1. Hitting Pfr with far-red light
   2.Dark reversion: waiting for it to degrade to its inactive form

Question 23

Plants and day length

Answer 23

Plants keep track of day length by measuring length of darkness.

-Short-day plants/long-night plants: don’t flower if exposed to red light at night

-Long-day plants/short-night plantS: flower even if exposed to red-light at night

Initiation of flowering can be inhibited if plants are exposed to red light during the night.

Question 24

Flowering under natural daylight

Answer 24

Light contains both red and far red wavelengths
-Needs to be a photo equilibrium: rates of conversion of two phytochrome is equal

-Proportion of each type of photoreceptor molecule is constant