lecture 12 - plant defence Flashcards
What are some examples of parasitic plants?
Parasitic plants have evolved independently about 12 or 13 times - there are more than 4000 species of parasitic plants
Cuscuta or dodder exploits the volatile emissions to preferentially select the identify the host plant
Volatile chemical cues guide host location and host selection by parasitic plants.
Cuscuta pentagona seedlings exhibit directed growth toward nearby tomato plants (Lycopersicon esculentum) and toward extracted tomato-plant volatiles presented in the absence of other cues. Impatiens (Impatiens wallerana) and wheat plants (Triticum aestivum) also elicit directed growth. Moreover, seedlings can distinguish tomato and wheat volatiles and preferentially grow toward the former. Several individual compounds from tomato and wheat elicit directed growth by C. pentagona, whereas one compound from wheat is repellent. These findings provide compelling evidence that volatiles mediate important ecological interactions among plant species.
The largest flower in the world - Rafflesia - is parasitic
Rafflesia lives inside the host plant and only emerges to flower
Parasitism has probably evolved five times in the Mistletoe family
Describe how plants can develop disease
Plants have an innate immunity
- but some pathogens can avoid this defence system
In most case the innate immunity is successful
Epidermis - thick cell walls and waxy cuticle
viruses and bacteria can gain access via wounds
stomata can also be entry point
secrete chemicals that keep stomata open - secrete enzymes that soften or digest the cell walls
viruses (single strand RNA) move through plasmodesmata and can enter the phloem - fungi can grow along cell walls, and like bacteria, can enter the xylem
Some crops are very susceptible to disease - bananas and grapes
Describe plant immunity
Virulent pathogens are able to overcome the host’s defences and lead to disease
Avirulent pathogens damage only a small part of the plant because the host is able to contain the infection
Plants have an innate immune system that allows them to detect pathogens and mount an appropriate response. Protein receptors on or in plant cells bind to molecules produced by pathogens and recognize them as foreign. This binding causes a conformational change in the receptor that triggers a cascade of reactions that enhance the plant’s ability to resist infection.
The plant immune system has two parts: one basal and one specific. The basal branch of the plant immune system consists of receptors located on the plasma membrane. These receptors recognize molecules (i.e., flagellin, chitin) generated by a broad class of pathogens.
Pattern-recognition receptors (PRRs) are found in both plants and animals, and they enable the detection of microorganism- associated molecular patterns (MAMPs). In plants, PRRs are membrane-bound receptor-like kinases (RLKs) or receptor-like proteins (RLPs).
There are fundamental differences between the immune systems of plants and those of animals. Plants lack an adaptive immune system or specialized cells of the immune response. Instead, plants rely entirely on innate immune responses and on the ability of each individual cell to recognize and mount resistance responses to pathogenic invaders (viruses, bacteria, fungi, oomycetes and nematodes). A key feature of the plant immune system is the presence of two classes of receptors for the perception of non-self that can trigger potent resistance responses: the first class enables the recognition of pathogen-associated molecular patterns (also called microbe-associated molecular patterns (MAMPs)) that are conserved among species of a microbial group1; the second class permits the detection of polymorphic strain-specific pathogen effectors (effector- triggered immunity)
Plants lack specialized mobile immune cells. Instead, every plant cell is thought to be capable of launching an effective immune response. So how do plants achieve specific, self-tolerant immunity and establish immune memory? Recent developments point towards a multilayered plant innate immune system comprised of self-surveillance, systemic signalling and chromosomal changes that together establish effective immunity.
What is pattern-triggered immunity?
A basal type of immunity conferred by the recognition of conserved microorganism-associated molecular patterns by specific transmembrane receptors that protect hosts against non-specialised pathogens.
Microbe-associated molecular patterns (MAMPs)
Molecular signatures typical of whole classes of microbes, and their recognition plays a key role in innate immunity
Endogenous elicitors are similarly recognised as damage-associated molecular patterns (DAMPs)
Corresponding pattern recognition receptors (PRRs) in plants.
One of the best characterised MAMP/PRR pairs are flagellin/FLS2.
flg22 - flagellin is a protein present in the flagella of bacteria
recognises highly conserved molecules from a broad class of pathogens
Upon binding of flg22, FLS2 changes its conformation, allowing protein-protein interaction between the extracellular domains of FLS2 and BAK1. This interaction brings the intracellular protein kinase domains of FLS2 and BAK1 in close proximity and initiates signalling, e.g., by transphosphorylation.
Describe how the second branch of the plant immune system works
The second branch of the plant immune system allows plants to resist specific pathogens
It consists of receptors located inside the cell as opposed to the plasma membrane (basal immunity).
These receptors, called R proteins, are expressed by different R genes
Pathogens produce proteins called AVR proteins that enter into plant cells and facilitate infection. Each R protein recognizes a specific AVR protein.
In the absence of a matching R protein, the AVR protein blocks the plant’s basal resistance, allowing the pathogen to infect the cell
When a R protein binds with an AVR protein, it prevents the AVR protein from blocking the plant’s basal resistance and directly activates defensive genes.
Because this branch of the immune system depends on interactions between specific plant and pathogen genes, it is commonly called the gene-for-gene model of plant immunity.
Describe the hypersensitive response
Once a pathogen has been detected, plants protect themselves by:
Reinforcing their natural barriers, strengthening their cell walls, closing stomata, and plugging their xylem
Producing an antimicrobial compound
Launching a hypersensitive response
A hypersensitive response causes uninfected cells surrounding the site of infection to rapidly produce reactive oxygen species, which causes cells to die. The dead cells form a barrier of dead tissue that prevents the spread of biotrophic pathogens and slows the growth of necrotrophic ones.
What is systemic acquired resistance?
Plants can acquire immunity to pathogens after being exposed to the pathogen in another part of the plant. This form of immunity, called systemic acquired resistance, allows them to resist further attack by the same pathogen.
Systemic acquired resistance was proven by an experiment carried out by plant pathologist A.F. Ross. He infected tobacco plants with TMV (tobacco mosaic virus). The leaf infected turned a mottled yellow. One week later, he exposed another leaf on the same plant to the virus. This second leaf showed no visible signs of infection.
This result indicates that a signal has been transmitted from the originally infected leaf to the undamaged parts of the plant, and that the transmitted signal subsequently triggers the development of an immune response that protects the plant from further infection.
Describe viral defence in plants
Most plant viruses have genomes of single stranded RNA (ssRNA) - RNA genome is replicated in the host forming double stranded RNA (dsRNA)
Plants have evolved to detect dsRNA and recognise it as foreign
Enzymes cleave the dsRNA into pieces of 21 to 24 nucleotides forming fragments called small interfering RNA or siRNA
siRNA fragments then help target complementary sequence (the viral genome) - siRNAs can spread and the plant acquires immunity
Describe the targeted response to viruses
Plants have evolved responses to viral infections: hypersensitive response and a targeted response to viruses.
The targeted response: Most plant viruses have genomes made of single-stranded RNA molecules (ssRNA).
1. The virus injects its RNA genome into the host cell.
2. During the replication of the viral genome, double-stranded RNA molecules (dsRNA) are formed.
3. The production of double-stranded viral RNA opens the viral genome to counterattack by its host plant. Since plants do not generally make dsRNA, the replicating viral genomes are identified as foreign. Enzymes produced by the plant cell cleave the dsRNA molecules into small pieces of 21 to 24 nucleotides, forming fragments called small interfering RNA (siRNA).
4-5. These fragments enable the plant to target and destroy ssRNA molecules that have a complementary sequence, the viral genome.
When a plant cell is attacked by a virus, it can acquire immunity against specific viruses. This is a result of the siRNA molecules spreading. These molecules move through the plasmodesmata, enter the phloem, and spread throughout the plant.
Describe and give an example of how some pathogenic bacteria alter their host’s biology by inserting their own genes into the host’s genome
One pathogen that does this is Rhizobium radiobacter.
Virulent bacteria enter the plant through a wound, and move through the plant’s cell walls, propelled by flagella.
A section of the bacterial Ti plasmid is inserted into the plant’s genome
Ti genes cause the host cells to divide to form a tumour and to produce compounds the bacteria can metabolise.
