4.VL Models 2

• a symbiosis between plants and members of an ancient phylum of fungi, the Glomales

• evolutionary old symbiosis (>400 Mio years)

• found on roots: herbaceous angiosperms, most trees, mosses, ferns

• not present on Cruciferae (Arabidopsis), Chenopodiaceace, Proteaceae

• Outside root

  • spores (multinucleate), filamentous hyphae

• Inside root

  • intercellular mycelium

  • “intracellular” arbuscule —> not really intracellular becuase surrounded by symbioses membrane

  • vesicles with reserves

1. Plant root produces strigolacetones (and other chemicals) —> initiates branching in fungal hyphae

2. Fungi produces a Myc factor —> identifies it as symbiont

3. The plant prepares to permit the fungi to penetrate its cells

• lipochito-oligosaccharides

• produced by fungus

• have a backbone of N-acetylglucosamine (chitin) sugar residues

• have a lipid moiety —> helps plant to recognize fungi as symbiont and not pathogen

• epidermal cells form a pre-penetrational apparatus (PPA)

• the plan assists activly via nucleus transport and ER the penetration of the fungus

• it comes to the building of a periarbuscular membranes with transporters to fasciliate nutrient exchange

  • phosphates and nitrogens from the fungus

  • sugars out of the plant cell to the fungus

With nitrogenase under useage of 16 ATP.

Triple bonded N2 is transformed to Amonia (2 NH3)

N2 + 8H+ + 8e− → 2 NH3 + H2

Nitrogenase is deactivated by oxygen

1. The plant root produces specific flavonoids that attract rhizobia

2. The flavanoids (luteolin) binds to the NodD transcription factor and binds to the nod gene promoter.

3. This lead to the transcription of the nod genes and products like Nod factor (N-acetylglucoseamine with lipid mooiety, similar to Myc factor)

4. It comes to root hair curling and the plant starts integrate bacteria in the curl

Nod factors of different bacteria vary greatly in their structure (length of chitin backbone, length of Acyl group, other modifications). The structure of the Nod factor determines the bacteria’s host range.

Specific legume hosts recognise specific symbionts through their Nod factors

Nod-factor receptors (NFR) are located on the root epidermis and root hairs, nod factors are located in bacterial cell wall. Upon recognition, the bacteria induced root hair curling, that envelops the bacteria and causes an infection thread. Deletion mutants of the NFR do not show root curling

The common SYM pathway (indicated in red) transduces information from the plasma membrane to the nucleus.

Signals from nodulating bacteria (Nod factors) and mycorrhizal fungi (Myc factors) are transduced through the SYM pathway

1. NFR1 and NFR5 recognise Nod-Factor, SYMRK (SYM receptor like kinase) has an unknown ligand. All 3 produce second messenger molecules.

2. Second messengers activate ion channels CASTOR and POLLUX, which cause Calcium spiking.

3. Ca2+ ion Oszillation causes conformational change of Calmodulin (CaM) which changes its affinity for other proteins

4. Activated calcium- and calmodulin- (CaM) dependent protein kinase (CCaMK) phosphorylates targets including CYCLOPS, a protein of unknown function, to elicit nodule formation

5. CYCLOPS activates Transcription factors, which induce genes for nodule development

6. NUP133 and NUP85 are nuclear pore complexes that transport transcription factors into the nucleus and RNAs out.

SYMRK, CASTOR, POLLUX, NUP85 and NUP133 are essential for calcium spiking and symbiosis success.

Bacteria are released from the infection thread

into the cytoplasm of the host cells, but remain surrounded by the peribacteroid membrane. Failure to form the PBM results in the activation of host defenses and/or the formation of ineffective nodules.

Infected root cells swell and cease dividing. Bacteria within the swollen cells change form to become endosymbiotic bacteroids, which begin to fix nitrogen.

• ATP is produced by oxidative phosphorylation —> this requires oxygen

• low oxygen environment is mainatined with an oxygen permeability barrier in the inner cortex

• a high affinity cytochrome oxidase (transfers e- to O2 and produces H2O) functions at low O2 concentrations

• leghemoglobin (= oxygen binding protein) buffers oxygen and delivers it to respiring symbiotic cells

  • leghemoglobin maintains a free oxygen concentration that is low enough to allow nitrogenase to functions but a high enough total oxygen concentration for aerobic respiration

• plant provides organic carbon (Malate) for use in production of ATP

• bacteroid fixes N2 to NH3 which is exported to the plant cell

• —> In bacteroid memebrane: respiratory chain for ATP production (oxidative phosphorylation) with O2 as electron acceptor

• toxic NH4+ is shuffled out of bacteroid & peribacteroid membrane into plant cell —> plant assimilates NH4+ into amino acids using glutamine synthase

• Plant provides amino acids to bacteia via AA transporters

• gram-negative soil bacterium, aerobic, rod-shaped

• (hemi) biotrophic pathogen

• has over 40 pathovars with distinct host range

  • infects e.g. Arabidopsis, bean, soybean, tomato

  • possible because of multiple effector proteins

P. syringae uses its hrp-pilius (=type III secretion system) to secrete effector molecules into the plant cell and suppress plant Defense (effector-triggered susceptibility)

On the other hand: effector molecules are often detected by so-called Resistance (R)-genes (effector-triggered immunity)

Multiple effector proteins determine the host specificity of the pathovar

The Pattern recognition receptor FLS2 is blocked by the effector AvrPto and can no longer recognise flg22

—> causes ETS