Symbiosis Initiation and Consequences in Mycorrhizae

Question 1

What 3 aspects are involved in the disease triangle?

Answer 1

Severity of the environment
Pathogen: virulence, abundance
Host susceptibility

Question 2

What is non-host resistance?

Answer 2

Most plants are resistant to more pathogens, most of the time

Resistance occurring in all genotypes of a plant species to all genotypes of a pathogen species

Question 3

What is basal resistance (innate immunity)?

Answer 3

Most plants have the ability to limit the effects of any infection; the basal level of resistance is based on the ability to recognize non-self cues

Defense that plant species mount against unadapted microbial intruders

Question 4

What are PAMPs and what recognize them?

Answer 4

Pathogen-associated molecular patterns

Can come from a wide range of microbial molecules, including glycans and lipopolysaccharides

They are recognized by PRRs

Question 5

What do PRRs do?

Answer 5

They lead to pattern-triggered immunity - phytoalexins, and other secondary metabolites, proteinase inhibitors

Question 6

What is the first level of innate host immunity?

Answer 6

Pattern-triggered immunity

Question 7

What do pathogens do?

Answer 7

Deploy effector proteins

Question 8

What are effectors?

Answer 8

Any secreted regulatory molecules

Question 9

What are effectors often matched by?

Answer 9

Host receptor-like proteins encoded by R (resistance) genes

Question 10

What do R genes usually contain?

Answer 10

A nucleotide-binding domain and a leucine-rich repeat

Question 11

What does the secretion of effectors that suppress the plant’s basal immunity cause?

Answer 11

Effector-triggered susceptibility

Question 12

What is a second-level immune response by the host?

Answer 12

Effector-triggered immunity
More robust than PTI

Question 13

What are some examples of ETI?

Answer 13

Secretion of phytohormones and oxidative substances

Deposition of callose

This causes a hypersensitive response

Question 14

What does the hypersensitive response do?

Answer 14

Restricts pathogen spread/ingress

Question 15

What is Rhizobium?

Answer 15

A nitrogen-fixing, non-photosynthetic bacterium

Question 16

How does Rhizobia get there?

Answer 16

Historically they were thought to penetrate root cells by cell wall degradation

Similarly, AMF was assumed to open root apoplast with cell wall-degrading enzyme

Question 17

How does rhizobial symbiosis initiate?

Answer 17

Two membrane-bound receptors bind nod factors, interact with SYMRK receptor kinase

SYMRK activates the synthesis of mevalonate

Mevalonate transport into the nucleus triggers calcium spiking

Calcium-dependent kinase activates gene expression for symbiosis

Question 18

How are AM and EM fungi different?

Answer 18

Em fungi stay in the apoplastic space while AM fungi penetrate the root hair cells

Question 19

What is mRNA an indication of?

Answer 19

Gene expression

Question 20

What are MiSSPs?

Answer 20

Mycorrhiza-induced small secreted proteins

8-28% of symbiosis-induced genes encode candidate secreted effectors - these are called MiSSPs

Question 21

What does the affector affect?

Answer 21

Alters the physiological status of the plant host such that symbiosis is favored

Question 22

How do MiSSPs work?

Answer 22

When jasmonic acid accumulates, a protein called JAZ is degraded

MiSSP7 prevents JAZ degradation which suppresses the plant immune response

Question 23

What does JAZ do?

Answer 23

It keeps specific transcription factors downregulated
When it’s degraded, they can activate, enabling stress responses

Question 24

What source of carbon do fungi most often use?

Answer 24

Monosaccharides such as glucose and fructose

Question 25

Are monosaccharides usually freely available?

Answer 25

Nope

Question 26

What are many fungi uniquely equipped to do?

Answer 26

Cleave monosaccharides out of polysaccharides

Question 27

What are the 3 main cellulases?

Answer 27

Endoglucanases
Cellobiohydrolases
Beta-glucosidases

Question 28

What are endoglucanases?

Answer 28

Endo-acting enzymes which break up cellulose into progressively smaller fragments
Size = 15-60 kDa

Question 29

What are cellobiohydrolases?

Answer 29

Exo-acting which reduce cellulose to cellobiose disaccharides
Size = 50-60 kDa

Question 30

What are beta-glucosidases?

Answer 30

Cleaves cellobiose into two glucose monomers

Question 31

What is lignocellulose?

Answer 31

It is embedded in lignin, a hydrophobic, phenolic biopolymer

Question 32

How much cellulose does wood contain?

Answer 32

40-45%

Question 33

What is selective delignification in white rot fungi?

Answer 33

They remove the lignin first and mostly leave the cellulose exposed also for use by other organisms

Question 34

What are CAZymes?

Answer 34

Glycoside hydrolases which cause the hydrolysis of glycosidic bonds

Question 35

What are absent in AM fungi?

Answer 35

Important glycoside hydrolases