Plant hormones and signalling

Which type of factors have more of an impact on crop yield?

Abiotic factors

What are the 3 different types of stresses a plant could face?

Abiotic - External/exogenous condition/cue (non-living: chemical/physical/environmental)

Biotic - External/exogenous condition/cue (living: different organisms)

Edaphic (abiotic) - Soil conditions as they relate to supporting plant life

which stages of the life cycle are more effected by stress:

Juvenile vegetative and reproductive

4 types of tolerance (against stress) mechanisms plants possess:

• Physiological mechanism - for tissues/organs (e.g. may create more suberin - more waxy)

• Biochemical - protection or reduction of photosynthesis

• Molecular levels - compatible osmolytes in the cytosol

• Hormonal - priming gene regulatory networks to cause chemical shifts

Modulators that manage signal processing:

Kinases (phosphorylation), miRNAs (gene silencing), and the Proteasome (protein degradation)

Plant stresses usually occur in _________.

Tandem - 2 at once

What is the growth-defence trade-off?

When resources like light or soil nutrients are scarce, plants cannot maximize both functions simultaneously.

What are JAZ and DELLA?

Key regulatory proteins in plants that interact to balance growth and defence - puts the brakes on growth ensuring that the limited energy is spent on defence

DELLAs (gibberellin signalling)

JAZs (jasmonate signaling)

Steps a plant takes when it faces multiple stressors simultaneously:

Molecules like Jasmonic Acid (JA) or Abscisic Acid (ABA) increase. These trigger proteins like JAZ and DELLA to inhibit growth-promoting pathways.

Instead of investing in "Growth" the plant reallocates energy toward "Defence" and "Cell wall modification".

Transcription factors associated with defence are activated to change gene expression, while growth-related factors are suppressed.

What kind of response is produced when 2 stresses effect a plant at once:

The plant doesn't just add the two responses together. It develops "Unique Responses" specifically for that combination

2 primary strategies to develop resistant plants:

traditional breeding and genetic engineering

What does the traditional breeding process involve for producing resistant plants:

Involves Quantitative Trait Locus (QTL) analysis to map which traits relate to specific loci.

Repeated backcrosses are used to ensure the resistance trait is integrated into a high-yielding "elite" line without losing desirable agricultural qualities.

It is highly time-consuming because it depends on biological generation times and compatible species.

What is Genetic engineering?

Involves transforming a desirable trait into an elite crop

Problems concerning GM:

Intellectual problems

Expense of biosafety regulation

Public acceptance

Must consider compatible species - as you simply insert a gene into your chosen individual

Positive of GM over traditional breeding:

• Much faster

• Can insert genes from pretty much any organism

• Very high precision

Approaches used to increase tolerance in plants

Starts by screening "germplasm variability" to find plants that naturally handle stress better

Identify the specific markers or genes that impart adaptation

Depending on whether the desired trait is found in a compatible or incompatible species, different methods are used:

Compatible Species: Marker-Assisted Backcross Breeding - Crossing an "elite" line with a "donor" that possesses the stress-tolerant trait.

Incompatible Species: Genetic Transformation - When traits cannot be bred naturally, they are inserted directly

OR genome editing - this method corrects or modifies existing sequences in the elite gene

What is transgenesis and cisgenesis?

Transgenesis: Inserting genes from unrelated species to create trans-GMOs.

Cisgenesis: Inserting genes from related wild species to create cis-GMOs

What is CRISPR/Cas9 used for?

Genome editing

What are forwards and reverse genetics?

Forward genetics: identify the genes

Reverse genetics: identify the phenotype

General strategies to increase crop yields:

• Establishing symbioses

• Incorporating genes from other species

• Overexpression of key lines

  • Osmoprotectants + Aquaporins

  • Hormones

• Knockouts - using CRISPR - a gene that gets in the way of a plant dealing with stress

• Pyramiding of traits - involves combining multiple resistance genes into a single elite line

2 hormones involved in wounding response?

Jasmonic acid and Salicylic acid

Hormones that control growth at tip and root of the plant?

Auxins and cytokinins

Hormone that controls end of life of a plant:

Ethylene

What are phytohormones?

Signalling molecules produced by plants to regulate almost every aspect of growth, development and environmental stress responses.

2 types of signalling control in plants:

1-to-1 (1 chemical to 1 receptor): steroid-like (hydrophobic - can pass through membrane); tight regulation

Phosphorylation cascades (type of kinase signalling approach): amplified signalling, cause an all-or-nothing response, one group goes on to activate a bigger group, which activates a bigger group

What is happening in a plant cell prior to a stress signal?

Baseline level of hormone synthesis occurring within the plant.

Programmed Development balances against environmental inputs to regulate how synthesis occurs.

How are hormones regulated?

Environmental input and programmed development control hormone synthesis

These hormones are transported to a site of activation and received by a receptor

The signal is transduced resulting the response (a gene being up or down regulated)

Once the threshold is reached the signal will no longer be transduced to produce a response

What is response termination in a plant?

The process of turning off a physiological response and returning the plant to its basal (resting) state

What are the 4 primary mechanisms of response termination?

Compartmentalization: Stunting signalling molecules by moving them into vacuoles.

Conjugation: Adding chemical motifs to hormones to make them less bioactive.

Catabolism: Breaking down the hormones completely.

Efflux: Pushing hormones out of the cells, often into the roots.

What do plants use photoreceptors for?

To detect light quality, quantity, and direction'

Plants have 5 groups of photoreceptors that detect light at different wavelengths

Hormones that work antagonistically to control seed dormancy:

Abscisic acid and GA

If don’t have anything holding GA back we would have continual uncontrolled seed germination

How could the down regulation of development genes when there is environmental stress provide a fitness effect?

as the energy available in the plant is finite, and the energy is used for defence response rather than development

Constitutive defence in plants:

• Some structural components: e.g., cutin (wax)

• Chemical: basal levels of protective compound

4 ways a plant can sense pathogens?

PAMPs: Pathogen associated molecular patterns

PRR: Pathogen recognition receptors (either external on plasma membrane or internal)

R proteins: resistance proteins upregulate primarily salicylic acid (SA)

Effectors: inhibit plant defence responses and this impacts growth and development.

Once the pathogen has cleared from the cell, how does the plant revert to development and growth?

the plant must use the termination methods to clear the defence-related hormones

What happens if the defence response is not stopped after the pathogen has cleared?

it can have serious fitness costs

When do JA and SA get induced?

Jasmonic acid gets upregulated when you have munching insects

Salicylic acid is a much stronger response, for when insects are damaging the vascular tissue -  phloem-sucking

How can plants inform other plants in the vicinity about an infection?

SA + JA can produce volatile compounds to prime other individuals - chemical communication

 

What is a mechanism that plants use to protect themselves against herbivory?

Constitutive production of toxic secondary metabolites

What pathway converts JA into its bioactive form?

Jasmonoyl-isoleucine (JA-Ile) pathway

What does wounding upregulate in a plant?

DAMPs - Damage associated molecular patterns

These are anything broken up from the cell

What can happen if a cell detects Ca2+ floating freely within the cell?

It can trigger a stress response - as it should be bound to the cell surface

What do DAMPs bind to?

PRRs - pattern recognition receptors

What other genes aside from JA get upregulated when insects infect a plant?

Genes of anti-digestive proteins such as protease inhibitors which inhibit proteolytic enzymes found in the gut of insects. As a result, insects suffer reduced growth rates.

What is the systemic acquired response?

a whole-plant immune response triggered by localized pathogen infection, providing long-lasting, broad-spectrum resistance

What is required for systemic acquired response:

Transport of JA (in the form of systemin) in phloem is required

Also need production of volatile methyl jasmonate

Function of methyl jasmonate?

It is a volatile derivative that mediates long-distance signalling and inter-plant communication, often acting as a signal for wounding or defence activation

What is the precursor of JA?

Linolenic acid

Jasmonic acid biosynthesis:

In chloroplasts:

Membrane lipids are processed to form linolenic acid and transported in vesicles to the plasma membrane.

Further enzymatic cyclization steps form OPDA which is transported via a channel protein, called JASSY into the peroxisomes.

In peroxisomes:

Further enzymatic modifications to the intermediate cyclized chemical structure, produce JA which is released into the cytosol, when it will be conjugated to isoleucine, rendering it bioactive.

Where does JA biosynthesis take place:

Begins in the chloroplasts then in peroxisomes

Enzyme that conjugates the joining of JA to isoleucine?

JAR1

What does it mean that JA has steroid action

JA is cyclosized so functions in a way that is structurally and mechanically similar to steroid hormones found in animals

What must happen to JA-Ile in order to facilitate binding to the JAZ repressor protein?

It must bind with SCF^COI1

What is SCF^COI1

It is an E3 ligase essential for jasmonate signalling, acting as a receptor that binds the hormone JA-Ile to trigger the degradation of JAZ repressor proteins

How is SCF^COI1 named?

named by the F-box protein: CORONATINE-INSENSITIVE1

How does SCF^COI1 cause the JAZ repressor to be degraded?

Causes a conformational change which induces a vast amount of ubiquitin binding causing degradation

What happens after the JA-SCF^COI1 complex has removed JAZ?

The MYC2 transcription factor is no-longer repressed so is able to induce jasmonate-responsive gene expression

Why kind of control is JA signalling under? + why?

circadian rhythm control - JA gets turned on at a certain time every day - as insects only feed at a certain time of day due to being cold blooded

What are the implications if a plant is out of its JA circadian rhythm?

They are highly susceptible to herbivory + wounding

How do we turn off the JA pathway?

JA conjugation to a carboxyl group - this is reversible and can be converted back if needed again

JA sequestration - into the ER and vacuoles

Conjugation followed by sequestration

Catabolism (degradation) - oxidation of JA-Ile - permanently removes it

Conversion to JA to methyl jasmonate - changes the chemical structure so it no longer triggers the internal signal processing hub + MeJA is volatile, meaning it can evaporate and leave the plant's tissues entirely

What is salicylic acid involved in?

• Senescence trigger

• Role in response to abiotic stress with GAs

• Wound response

What is hyper sensitive response?

Rapid, localized programmed cell death mechanism triggered at the site of pathogen infection to halt the spread of infection

How is systemic acquired resistance induced

A pathogen binds to the plasma membrane, inducing a Signal Transduction Pathway (STP).

Induces a hypersensitive response. Before dying, these cells release antimicrobial molecules like phytoalexins.

Dying cells release Salicylic Acid, which is transported throughout the entire plant.

In healthy cells, the transported SA induces another STP that produces antimicrobial molecules. This makes the entire plant resistant to further infection - known as systemic acquired resistance.

What gets upregulated during SA signalling?

PR Proteins (pathogenesis resistance): to help carry the defence signal throughout the plant.

What effect does the presence of SA have on NPR?

SA reduces NPR

What is NPR1?

It serves as a master regulator for Pathogenesis-related (PR) genes - interacting with TFs

What controls NPR1 activity?

It is dictated by the redox state of the cell, which is influenced by the concentration of Salicylic Acid.

How a change in the cells redox state effects the regulation of NPR1:

1. The Oxidized State (Inactive)

When SA levels are low, the plant is in a basal state:

• Oligomerization: In an oxidized state, NPR1 proteins group together into large complexes called oligomers.

• Inhibition: In this grouped, oligomeric form, NPR1 remains in the cytosol and cannot bind to DNA.

2. The Reduced State (Active)

An increase in SA levels changes the chemical environment of the cell to favour reducing conditions:

• Monomerization: Under these reducing conditions in the cytosol, the NPR1 oligomer breaks apart into individual reduced NPR1 monomers.

• Nuclear Entry: These monomers are then able to enter the nucleus.

• Transcription Factor Binding: Once in the nucleus, the reduced NPR1 monomer binds to TGA transcription factors

  • This allows for the transcription of the PR-1 gene

What happens to an NPR1 monomer that has formed accidently or is left over from a previous infection?

it must be phosphorylated to be recognized by the plant's protein-clearing machinery.

How does the cell ensure that NPR1 does not get reduced accidentally and switch the pathway on?

NPR is constantly cycling with binding partners

What are NPR3 + NPR4?

Plant receptors (paralogs of NPR1) that bind salicylic acid (SA) to negatively regulate plant immunity.

Unlike NPR1, which activates defence, NPR3/NPR4 act as repressors of SA-responsive genes in the absence of pathogens

How does the cell ensure just the right amount of signalling of SA?

Balances the reduction of NPR: it will bind preferentially to NPR4 and the rest will go on to bind to TGA2, causing the right amount of signalling

Produces the right amount of protective molecules

What targets NPR1 for degradation in the nucleus?

A ubiquitin molecule being attached to it - targets the protein for destruction by the proteasome

Which cells does ubiquitin binding of NPR1 take place?

In uninfected cells, when a stray NPR1 monomer enters the cell

In primary infected cells to control the number of NPR1 molecules activating PR-1 genes and plays a role in hyper sensitive response

What happens at the site of primary infection vs. the distal site?

P: plant cell death + hypersensitive response

D: PR proteins expressed - resistance to secondary infection

What happens when too much SA is present in a cell’s nucleus?

Modulates the binding affinity of NPR1, favouring its interaction with NPR3 and Cul3.

This specific binding triggers ubiquitination and subsequent degradation of NPR1

As NPR1 is a key component of the survival pathway, its rapid destruction in these specific cells favours the hypersensitive response (HR)

Function of MORC1?

MORC1 stabilizes NPR1, allowing it to bind successfully with the TGA2 transcription factor

pathway of MORC1 with and without a pathogen:

In the absence of a pathogen or Salicylic Acid:

Unphosphorylated MORC1 is targeted for degradation by the 26S proteasome.

NPR1 remains in its inactive, oligomeric form in the cytoplasm.

The transcription factor TGA2 remains inactive in the nucleus, and the plant focuses on normal growth.

When a pathogen (or phloem-sucking insect) attacks, it triggers a signalling cascade:

Pathogen detection causes calcium channels to open - Ca2+ floods into cytoplasm

This calcium influx activates the kinase CPK5, which then phosphorylates MORC1.

MORC1 is no longer degraded by the proteasome. Instead, it becomes stabilized and moves into the nucleus.

NPR1 also enters the nucleus.

What happens to SA when we no longer need it?

Can break it down (catabolism) into pyruvate - to contribute to carbohydrate production in the mitochondria

How can phytohormones be used in real world applications?

These tend to squash the production of reactive oxygen species and this increases your yield

How do SA and JA work together?

Antagonistically - both used for wounding but in different scenarios

What must you remember when applying synthetic SAs to crops?

There is an optimal level of SAs - too much and the plant grows equally badly as too little

What would you spray JA onto in the field?

Seeds

What are the 2 secondary messengers?

Reactive oxygen species and Ca2+

What are the quickest systemic signalling methods:

ROS and Calcium

Why phytohormone is quicker at responding to munching insects?

Jasmonic acid is much quicker than SA

What happens when resources are diverted into defensive genes?

This will have fitness costs

What do giberellic acids and auxin promote?

Growth

What is triggered when there is is wounding via herbivory:

This triggers the cleavage of the precursor protein prosystemin.

Which is processed into systemin

Systemin binds to a specific receptor on the plasma membrane of a cell.

This activates a signalling cascade (via MAP kinase) that stimulates phospholipase.

Phospholipase acts on the plasma membrane phospholipids to release linolenic acid.

Linolenic acid results in the production of jasmonic acid, another mobile signal.

What happens when HAMPs/DAMPs are proteolytically cleaved?

Systemin trigger signalling cascades and upregulation of JA

What is methyl jasmonate essential for?

Plant-plant interactions - informing other plants in the vicinity of the attack

It attracts natural enemies to eat the wounding insects and allows other plants to prime their defence response

Effect of proteinase inhibitors on munching insects?

They go into the haemolymph and prevents amino acid formation and causes the over production of digestive enzymes - slows the herbivore development and stops them eating