2. General Stress Responses

Question 1

Structure of the lecture

Answer 1

1. What is stress?
2. Acclimation versus adaptation
3. Phases of a stress response
4. General responses: ROS
5. General responses: Ca2+ waves
6. General responses: SnRK1

Question 2

What is stress?

1

Answer 2

A condition that deviates from optimal environmental conditions, and can be either biotic or abiotic. Stress reduces crop yields by up to 50%

Question 3

What is acclimation?

2.1

Answer 3

An individual, non-heritable, reversible response that is generated by a local stress

Question 4

What is adaptation?

2.2

Answer 4

An irreversible, heritable population-level response that is caused by natural selection

Question 5

How can we model stress responses?

2.3

Answer 5

Stress response curves

Question 6

What are the different types of stress response curves?

2.4

Answer 6

1. **Essential environmental factors **(based on optimal range with supra- and sub- optimal conditions)
2. Non-essential environmental factors. These can either be immediate or tolerant

Question 7

What are the different phases of a plant stress response?

3.1

Answer 7

1. Alert
2. Acclimation
3. Maintenance
4. Recovery or Exhaustion

Question 8

What is stress priming?

3.2

Answer 8

If plants are ‘pre-exposed’ to a stress, then they may be able to prepare a response in advance. This can occur through memory genes or epigenetic factors

Question 9

What are ROS (definition)?

4.1

Answer 9

ROS are the accumulation of toxic chemicals when oxygen reacts with spare electrons. They can lead to protein, lipid, DNA and cellular damage under stressed conditions, but can be helpful when the environment is optimal.

Question 10

What is the ROS Pathway?

4.2

Answer 10

1. Stomata close in response to a stress (e.g., heat/drought etc.,)
2. No CO2 uptake can occur
3. No CO2 present for Calvin cycle, thus slowing CO2 assimilation rate
4. Low NADPH recycling rate in photosynthesis
5. Low NADP will be available for electrons
6. Electrons will bond with spare, unused O2, creating ROS

Question 11

Where do ROS occur?

4.3

Answer 11

ROS are produced in several cellular compartments. This includes mitochondria, peroxisomes, chloroplasts and the cytosol

Question 12

How do ROS cause damage under stress?

4.4

Answer 12

ROS lead to oxidative damage under stressed conditions. This means that ROS accumulate to toxic levels. Ths can lead to damage to proteins, lipids and DNA, as well as cellular dysfunction. This can only be reduced by antioxidants (enzymatic or non-enzymatic).

Question 13

How do ROS help under optimal environmental conditions?

4.5

Answer 13

ROS remain at a basal level. They have important roles in signalling processes, defense responses and tolerance induction

Question 14

Argument that ROS are a double-edged sword?

4.6

Answer 14

ROS perform very important functions in plants, despite causing damage in stressed conditions. There is a careful balancing act within plants to ensure that ROS are not causing excess damage, and that they are helping with their intended purposes.

Question 15

What are Ca2+ waves?

5.1

Answer 15

Stress can trigger the release of cytosolic Ca2+ waves with different stressors on different organelles releasing different Ca2+ signals. These waves propagate around the plant, and act like a warning response to stress.

Question 16

How are Ca2+ waves released?

5.2

Answer 16

1. Ca2+ accumulates in the cytosol from internal and external sources
2. Different organelles release different Ca2+ signatures, that induce different metabolic, transcriptional and signalling responses. These include CaM, CML, CDPKs, CBLs and CIPKs
3. Different signatures have different durations, amplitudes and oscillations

Question 17

How do Ca2+ waves combine with ROS (RICR)?

5.3

Answer 17

RICR: ROS-Induced Calcium Waves. These induce long-distance signalling.

This leads to long-distance signalling waves

1. ROS production via NADPH oxidases
2. Activation of Ca2+ channels
3. Signalling through Calcium-Dependent Kinases
4. More NADPH production
5. More ROS production

Question 18

How do Ca2+ waves auto-propagate (CICR)?

5.4

Answer 18

Auto-propagation forms a long-distance signal that acts as a warning signal

CICR: Calcium-Induced Calcium Release

1. Influx of Ca2+ into the cytosol
2. Activation of vacuolar TPCI Ca2+ channels
3. This causes an intracellular Ca2+ wave

Question 19

What is SnRK1?

6.1

Answer 19

SnRK1 is a protein that is produced by a low-energy state in a plant (i.e., shading, drought etc.,). It is important for inducing a survival response through its inhibition of TOR.

SnRK1 leads to the inhibition of plant growth, nutrient mobiliation, stress responses, autophagy and transcriptional reprogramming

Question 20

How is SnRK1 produced?

6.2

Answer 20

1. The plant enters a low-energy state (i.e., through shading, drought, heat etc.,)
2. This leads to a decline in energy, that causes the up-regulation of molecules like SNF1, AMPK & SnRK1
3. SnRK1 inhibits the Target of Rapamycin (TOR) complex, leading to the inhibition of plant growth

Question 21

What is the direct action of SnRK1on the plant?

6.3

Answer 21

SnRK1 engages in a trade-off with TOR. SnRK1 aims to promote survival over growth. This includes the inhibition of growth, nutrient mobilisation, stress responses, the induction of autophagy and transcriptinal reprogramming.

Question 22

What is the direct action of TOR on the plant?

6.4

Answer 22

TOR engages in a trade-off with SnRK1. TOR aims to promote growth and inhibit SnRK1 through engaging with T6P sugars. TOR aims to promote developmental progression, suppress autophagy, induce rRNA transcription and induce translation.