Plant cell signalling

Question 1

Describe animal v plant development?

Answer 1

Animal
Develops from single celled zygote
Cell division and differentiation
Extensive cell migration occurs
Most cell types terminally differentiated
Germ line segregated early in development

Plant
Develop from a single celled zygote
Cell division and differentiation
No cell migration only expansion
Meristems retain stem cell character throughout life of adult plant
Meaning they can divide and differentiate
They’re found in the shoot apical, axillary meristems (normally inactive) and in the lateral roots

Question 2

How is development determined through animals and plants?

Answer 2

Animal development is predominantly genetically determined
Animals interact with their environment by behaviour

Plant development is genetically determined but is also plastic
Plants respond to their environment by adaptation at the biochemical, physiological and developmental level

Question 3

How are plants not inactive in comparison to animals?

Answer 3

An animal will run away or hide if it thinks it is going to be eaten
A plant may produce poisonous or unpleasant tasting compounds to deter herbivores, or
It may simply regrow the parts that are eaten

Animals have a brain that acts as a central processor
Plants perceive a wide range of environmental inputs and respond by generating diverse signal molecules that integrate their response without a central processor

Question 4

Describe the pathway of plant cell signalling?

Answer 4

Despite animal/plant differences they use many of the same types of molecular components:
Stimulus
Membrane or intracellularly localised receptors (perception)
Second messengers e.g. kinase cascades lipid molecules, pH, Ca2+ (transduction)
Modulation of activity of transcription factors, cytoskeleton, enzymes (transduction)
Response

Question 5

What are some plant cell signalling responses?

Answer 5

They can occur on different time scales
Milliseconds - Venus fly trap closing (electrochemical response)
Minutes - reorientation of growth in response to gravity/light
Hours - production of defence compounds (gene expression changes)
Days/months/years - long term change in growth (gene expression changes)

Question 6

Describe plant hormones?

Answer 6

They co-ordinate many processes such as: metabolism, growth, defence responses and development
Can act cell autonomously i.e. in the same cell that produces them
Can be transported to a distant site of action to act on different cells
Each hormone has differing actions on different aspects of plant growth
The balance and interactions between hormones shape plant development and responses

Plant hormones & their signalling pathways interact in complex ways to regulate growth and development in response to genetic and environmental factors

Question 7

Give an summary of plant hormones?

Answer 7

Small organic molecules and also peptides
Produced throughout the plant
Both local and distant targets
Effects vary depending on interaction with other hormones
Locally regulated

Question 8

Give some types of plant hormones?

Answer 8

Classic plant hormones - Auxins, cytokinins, gibberellins, abscisic acid and ethylene

Other signalling molecules - Polyamines, jasmonates, salicylic acid, brassinosteroids, strigolactones, florigens, phytochrome, nitric oxide, hydrogen peroxide and peptides e.g. systemin, defensin

Question 9

Describe jasmonates?

Answer 9

Jasmonates are fatty acid derived signal molecules analogous to prostaglandins in animal cells
Lipid dervied - OPC:6-CoA

Question 10

What are some biological roles of jasmonates? Example?

Answer 10

Normal
Carbon partitioning
Mechanotransduction
Senescence
Reproductive development

Stress responses
Biotic- insects, microbes
Abiotic- UV, ozone etc.

Example - Feeding insects wound plant; Insect derived elicitors detected
JA (and derivatives) produced
Many changes in gene expression including:
Up regulation of proteinase inhibitors = blocks insect digestion
Production of volatiles = prime defences of nearby plants and attract parasitoids

Question 11

Describe jasmonate biosynthesis?

Answer 11

Begins in the chloroplast with cleavage of membrane lipid α-linolenic acid (18:3) from chloroplast phospholipids
Continues in the peroxisome where the β-oxidation pathway produces jasmonic acid (JA)

Question 12

Describe the early steps of JA biosynthesis?

Answer 12

In the cholorplasts
Phospholipids are cleaved from the membrane by phospholipase
This releases a fatty acid - a-linolenic acid
13-LOX converts this to 13(S)-HPOT
Aline oxide synthase (AOS) then forms allene oxide
Aline oxide cyclase (AOC) forms an intermediate - cis-(+)-OPDA

Cis-OPDA is released from the cholorplasts - we don’t know how (it is also a signalling molecule in it’s own right)
This is transported to the peroxisome

Question 13

Describe the later stages of JA biosynthesis?

Answer 13

In the peroxisome
Cis-OPDA is acted on by OPR3 to produce OPC:8
It then undergoes 3 rounds of b-oxidation - losing 2 carbons at a time
Arriving at Jasmonic acid

Question 14

How does OPDA get into the peroxisome?

Answer 14

Two methods:
Comatose - ABC transporter protein in peroxisome membrane (ATP dependent - active transport)

Ion trapping - pH of peroxisome lumen is higher than the cytosol (8.2 and 7.2)
OPDA is a weak acid and hydrophobic molecule = it can partition across the membrane in its protonated form

Question 15

Describe JA conjugation?

Answer 15

Converting into other moleules
Occurs in the cytoplasm (likely)

Can be converted into 7-iso-JA as well as:
Methyl-JA - volatile
Other JACs - conjugated to other amino acids
JA-Ile - conjugated to isoleucine
Coronatine - bacterial toxin mimics JA-Ile
Other derivatives are AKA oxylipins

Question 16

What is the role of JA - evidence from mutants?

Answer 16

Jasmonate synthesis mutants e.g. opr3, fad 3,7,8 triple mutant (can’t make α-linolenic acid)

Therefore if JA deficient:
Male sterile
Susceptible to disease
Rescued by application of JA

Question 17

What are jasmonate response mutants?

Answer 17

The coi1 mutant (coronatine insensitive) was isolated because it was resistant to the effects of coronatine and MeJA
Coronatine - bacterial metabolite that mimics the effect of jasmonates

coi1 mutants are also male sterile and susceptible to insect attack but can’t be rescued by JA application (they make JA but can’t respond to it)

Question 18

What can coi1 associate with?

Answer 18

F-box proteins
When COI1 was cloned it was found to encode an F box protein
F-box proteins function within the ubiquitin pathway for protein degradation

Ubiquitin is a 76 amino acid peptide and is added to proteins to mark them for degradation by a large protease complex called the proteosome

Question 19

Describe the ubiquitin conjugation pathway?

Answer 19

E1 enzymes activate Ub (1 or 2 per genome)
E2’s transfer ubiquitin to the target protein (a few per genome)
E3’s provide the specificity and may be single proteins or multi subunit (many per genome)
Leading to degradation by the 26S proteosome

COI1 is the F-BOX protein component of a multi subunit SCF type E3 ligase

Question 20

How do jasmonates turn genes on?

Answer 20

Unchallenged plant (low JA-Ile) 
JAZ proteins act as transcriptional repressors = JA response gene expression low

Herbivore-challenged plant (high JA-Ile)
JA-Ile promotes interaction between JAZ repressors and SCFCOI1 which is part of the ubiquitin mediated protein degradation pathway
JAZ proteins are degraded allowing expression of response genes

Question 21

Describe auxins?

Answer 21

Involved in many processes:
Tropic growth (stimulant growth)
Apical dominance 
Organogenesis
Pattern formation and polarity

Essential - no mutants that completely lack auxin are known
However, as with JA, mutants in auxin perception and response have been invaluable in working out the molecular basis of auxin action

Question 22

What are some types of auxins?

Answer 22

Natural 
Indole-3-acetic acid (IAA) - the major and most widespread auxin
Indole-3-butyric acid
4-Chloroindole-3-acetic acid
Phenylacetic acid

Synthetic
These are more stable and used experimentally
2,4-Dichlorophenoxyacetic acid
Naphthalene-1-acetic acid

Question 23

What are auxins synthesised from?

Answer 23

Tryptophan
There are a number of steps and different routes from tryptophan to form different auxins
Molecules can be interconverted
The pathways are regulated by development (cells signalling) and environmental factors

Question 24

How is activity of auxin controlled?

Answer 24

Controlled by synthesis/degradation
But also modulated by conjugation
Converted often to sugar esters - an inactive storage form
This will be re-converted when needed

Question 25

What are the types of auxin transport?

Answer 25

Fast non-polar transport in vascular system

Slow cell to cell polar transport:
Unique property of auxin
Give rise to concentration gradients that can shape cell responses
Auxin moves from cell to cell
Chemiosmotic driving force for movement

Question 26

Descibe auxin movement?

Answer 26

In a plant cell pH = 7 and outside the pH = 5.5
Auxin has a carboxylic acid group - meaning it can lose a proton (pKa 4.75)

IAA- has to be transported by the influx carrier but IAAH can diffuse through
The equilibrium is in favour of the anionic from - which are removed by the PIN efflux carrier

H+ gradient is maintained by H+ ATPase

Question 27

How can auxins be visualised?

Answer 27

Couple a promoter which responds to the presence of auxin with a reporter
Reporters e.g. GUS or GFP - will report the presence of auxin

Question 28

How do auxins carry out different functions?

Answer 28

Asymmetric distribution of influx and efflux carriers results in polar transport
Pin protein localisation and therefore auxin flows change in response to developmental or environmental stimulus

E.g. embryo development or regulation of tropisms

Question 29

Describe how auxins regulate tropisms?

Answer 29

Changes in PIN protein location and auxin concentration in response to gravistimulation (this is for the stem falling down)
Starch granules in the root cap cells sense gravity by sedimenting
PIN3 relocates and auxin accumulates on lower side of root
Results in asymmetric root growth = downward bending

Question 30

How do the locations of PIN proteins change?

Answer 30

PIN and AUX proteins are synthesised on ER and transported in vesicles to the plasma membrane
PINS & AUX are thought to constitutively cycle between plasma membrane and endosomes (internal membrane compartments)
Cycling and the site of vesicle fusion with the plasma membrane is regulated to change distribution of the PIN proteins
The delivery of PIN proteins to apical or basal membranes is regulated by PIN phosphorylation state

The levels of PIN protein on the membrane are dynamically regulated by controlling the rates of export and internalisation, recycling and degradation

Question 31

How are auxins involved in gene regulation?

Answer 31

Auxin responsive genes are turned on by Auxin through the degradation of AUX/IAA repressors

AUX/IAA binds to the ARF promotor = gene off
When AUX/IAA is degraded through the proteasome the ARF promotor isn’t repressed = gene on
E3 ligase helps recruit polyubiquitination

Question 32

Give a summary of regulation of gene expression methods?

Answer 32

A - Auxin binds to the F‐box TIR1 forming an active complex that targets the transcriptional repressors Aux and IAA for degradation

B - Jasmonate (JA) as an amino acid conjugate binds to COI1 leading to degradation of the JAZ family of transcriptional repressors

C - Gibberellic acid (GA) binds to GID1 leading to a conformational change allowing binding of DELLA, this complex is then detected by SCFSLY which directs the DELLA protein for degradation, releasing downstream transcription factors from repression