Plant cell signalling Flashcards
Describe animal v plant development?
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
How is development determined through animals and plants?
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
How are plants not inactive in comparison to animals?
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
Describe the pathway of plant cell signalling?
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
What are some plant cell signalling responses?
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)
Describe plant hormones?
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
Give an summary of plant hormones?
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
Give some types of plant hormones?
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
Describe jasmonates?
Jasmonates are fatty acid derived signal molecules analogous to prostaglandins in animal cells
Lipid dervied - OPC:6-CoA
What are some biological roles of jasmonates? Example?
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
Describe jasmonate biosynthesis?
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)
Describe the early steps of JA biosynthesis?
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
Describe the later stages of JA biosynthesis?
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
How does OPDA get into the peroxisome?
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
Describe JA conjugation?
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
What is the role of JA - evidence from mutants?
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
What are jasmonate response mutants?
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)
What can coi1 associate with?
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
Describe the ubiquitin conjugation pathway?
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
How do jasmonates turn genes on?
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
Describe auxins?
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
What are some types of auxins?
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
What are auxins synthesised from?
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
How is activity of auxin controlled?
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
What are the types of auxin transport?
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
Descibe auxin movement?
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
How can auxins be visualised?
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
How do auxins carry out different functions?
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
Describe how auxins regulate tropisms?
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
How do the locations of PIN proteins change?
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
How are auxins involved in gene regulation?
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
Give a summary of regulation of gene expression methods?
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
