Plant signalling 1 Flashcards
Describe this process below
stimulus–>receptor–> signal transduction–> gene expression–>response
poke a plant (stimulus) cell stretch mediated calcium channels will allow Ca2+ to flow into the cell from the apoplast which is then detected by calcium dependent kinases and they can then phosphorylate something which causes a cascade and amplification. Calcium becomes information which is moved through the signal transduction cascade and this causes changes in gene expression. The final things that at the end of the cascade that are phosphorylated tend to be transcription factors and these then allow for RNA to be made and eventually proteins to do jobs in the cells
Stimulus
Ca2+ movement across a membrane
Binding by Ca2+-binding proteins
Phosphorylation of target proteins, e.g. transcription factors
Changes in gene expression
Physiological response e.g. stomatal closure, cell death
Responses to environmental stimuli involves complex signalling give an example of signalling intermediates
Combined stresses may be deconvolved by the plant cell using a combination of receptors and signalling intermediates
such as;
Ca2+
ROS
Second messengers in plant cell signalling
Give more examples of second messenger and what they do
Is this activation reversible?
It is important for plant cells to sense and respond to environmental information
Information in cells is encoded and passed on via second messengers
Examples include cyclic nucleotides (cAMP and cGMP), Inositol triphosphate (IP3), reactive oxygen species (ROS) and Calcium ions (Ca2+). Second messengers cause the AMPLIFICATION of signals
For example: cAMP will activate protein kinases, which in turn will phosphorylate target proteins which rely on phosphorylation for their activity
This activation switch is reversible and dependent on [cAMP]
Calcium is a ubiquitous second messenger in plant cell signalling.
What induces changes in calcium influx in the cytosol and give examples of would lead to this induce by these molecules?
Many environmental stimuli induce transient changes in cytosolic free Calcium concentration, usually written as Delta[Ca2+]cyt
*Interactions with ROS, ABA, CO2
*Cold, light, salt, drought, touch, wounding all induce Delta[Ca2+]cyt
There are many Ca2+ interacting proteins. E.g. Calcium Dependent Protein Kinases (CDPK) and Calmodulin (CaM)
Why Calcium?
Plant cells evolved in the ocean, where [Ca2+] is high
Ca2+ is quite insoluble - cytotoxic
Cells evolved proteins to bind Ca2+ and move it across a membrane and against a concentration gradient
These proteins could be co-opted for signalling use
What happens when a plant cell is stimulated?
Upon stimulation, calcium is mobilized from extracellular and intracellular stores
when ions are moving into the cytosol. This could be from outside the cell wall as well as from organelles =intracellular (influx)
When they are moving into organelles=extracellular efflux
Calcium signatures what does calcium do during a response
Cold shock response for example
Characteristic shape for a given stimulus
Refractive period
Mechanism is re-set allowing plant to retain sensitivity
plants respond in a very dynamic way and this response is proportional to the extent of the signal
Calcium binding proteins are activated upon binding calcium
Calcium dependent protein kinases (CDPK) are a large group of calcium binding proteins which “decode” an increase in calcium concentration and pass that signal on by phosphorylating a target protein when activated.
Local elevations in calcium can have big effects!
Root hair growth is co-ordinated by rhythmic oscillations in cytosolic free calcium concentrations
There are many examples;
Stomatal closure
Pollen tube growth
Circadian oscillations
NOD factor signalling
Reactive Oxygen Species in vivo
What do they include?
What can generate/produce ROS?
What do ROS cause?
Reactive Oxygen Species include: superoxide radical (O2*-), hydrogen peroxide (H2O2), hydroxyl radical (OH∙), singlet oxygen (1O2)
Normal cellular processes generate ROS
Many stimuli induce the production of ROS such as salt, drought, heat, cold, wounding etc.
ROS cause oxidative damage to cells
What can plants do to limit ROS during photosynthesis how what do the photosystems do?
ROS are produced during photosynthesis, so plants minimise photooxidative damage by moving the chloroplasts in the cell to decrease exposure of high light.
PHOT1 and PHOT2 mediate the accumulation response and PHOT2 controls the avoidance response.
What does ROS do that could affect protein activities?
ROS causes the oxidation of cysteine residues to make disulfide bridges which causes conformational changes in proteins and thereby may alter their ability to interact with partners.
(also cross-linking in cell walls etc)
When is hydrogen Peroxide produced and what does it do?
Hydrogen peroxide is produced by aerobic metabolism – chloroplasts / mitochondria
Important 2nd messenger (chemical stability c.f. other ROS)
both Plants and animals
Role in response to multiple abiotic stresses
e.g. high light, blue light – PHOT2 mediated HIR)
Developmental signal
e.g. root hairs, leaf senescence
Crosstalk with Ca2+ / pH (e.g. NADPH oxidases have EF-hands)
Hormonal response
ABA induced stomatal closure
Transcriptional regulator
What does H2O2 do for stomata and what ameliorates this response?
H2O2 causes stomatal closure and the antioxidant ascorbic acid (ABA) ameliorates this response. ABA induces H2O2 production in guard cells
What is DCFH-DA
(dihydrofluoroscein diacetate Non-fluorescent) which is a green dye that can monitor the exogenous ABA-induced production of H2O2 in guard cells.
