Plant-Microbe Associations Flashcards
What are the main organisms taht have intracellular and extracellular associations with plants?
Intracellular: viruses, prokaryotes.
Extracellular: Prokaryotes, eukaryotes- fungi, oomygetes, nematodes, insects, plants.
All have evolved independently to colonise their host in some way.
What are some benefits and costs for a microbe that colonises a host?
Get nutrients they need, use the plant as shelter, help with survival.
Some microbes just need plant in order to grow, some need plant to grow and reproduce, some depend in it- these microbes may be missing some genes for things that they need to get from the plant.
What are some benefits and costs to a host that is colonised by microbes?
Microbes help the host to take up nutrients g. mycorrhizae, rhizobium for nitrogen fixation.
There is a cost to the plant if it is a pathogen- disease that plant must deal with.
Mycorrhizae compete with other microbes, so act as a microbial shield against pathogens.
How does Rhizobium work as a mutualistic symbiont?
Fix nitrogen in root nodules- process is very heavy in energy use.
Rhizobium enzyme nitrogenase fixes atmospheric nitrogen, converting it into ammonia (natural fertilizer).
Large amounts of energy required for this.
Nitrogenase only functions in anaerobic conditions.
This is created by the root enzyme Leghemoglobin, which ties up the oxygen.
Nitrogenase is an inefficient enzyme and needs a lot of energy.
How does mycorrhizae work as a mutual symbiont?
Plants often cannot take up enough water & minerals to sustain growth.
Mycorrhizae colonise roots where they help water & mineral uptake.
Two types: Ectomycorrhizae- wrapped around, outside root.
Endomycorrhizae- specuialised to colonise inside root.
Fungus obtains organic compounds (sugars, amino acids) from plants
Besides minerals and water, EMs may provide growth hormones & protection against pathogen attack.
Not beneficial to fungi if host dies, so fungi tries to keep plant alive and healthy.
How does Ectomycorrhizae work?
Wrap themselves around the root, increasing the area for uptake. Example: Pisolithus tinctorius with eucalypts.
Fungus forms a sheath around the root, with hyphae emanating through the soil, greatly increasing the surface area. Sheath forms a protective layer against other microbes.
Fungus penetrates between cells of the cortex (forming the Hartig Net) to facilitate nutrient exchange. Have access to roots, but grow around plant cells.
How to endomycorrhizae work?
Arbuscular mycorrhizae (e.g. Gigaspora gigantea).
Infect roots and penetrate root
cell walls, branching within the
cells and forming treelike
(arbuscular) structures that
provide nutrients.
Grow around cell, but arbuscles enter plant cells.
Structures are elaborate- enhance surface to volume ratio- increased transfer efficiency of nutrients.
Spore germinates in response to plant signals.
Fungal signals pass to plant root cells.
A hyphopodium forms to attach fungus to root cells.
Pre-penetration apparatus (PPA)-altered plant cytoplasm prior to penetration by fungal hyphae.
An arbuscle forms (branched hyphae) within root cells for nutrient exchange.
Exudates from fungus start cascade that allows fungi to grow into root.
What do plant pathogens do?
Detrimental association with living plants (parasitic)
viruses, bacteria, fungi, oomycetes, nematodes.
Up to 50% of crop losses in developing countries are due to disease (Food security).
What types of plant pathogens are there?
Facultative – pathogens can attack living plant cells but can also grow by themselves, e.g. on artificial medium.
Obligate – can only grow on their specific living host.
Biotrophic – feed on living plant tissue.
Necrotrophic – kill plant cells and then feed.
Hemibiotrophic – initially biotrophic and then become necrotrophic.
(eg. Obligate biotroph- must compromise host immune system, but needs host immune system to fend off other infections. These are usually very specialised to make the most of the host.
Necrotroph- kill host, not as specialised, deal with fending off competition later.)
How are viral genomes encoded?
Can be encoded by various molecules: dsDNA. ssDNA (circular). dsRNA. ssRNA (positive or negative strand).
Most plant viruses have small RNA genomes.
Viruses replicate using plant cell materials and machinery (intracellular).
Obligate- need components of host to replicate.
Describe the virus life cycle.
Viruses can enter cells by wounding.
Disassembly, replication and assembly of progeny viruses within the plant cell using host resources.
Cell to cell movement- MP/vRNA/Host factors can move between adjacent cells through a channel.
Can get systemic movement throughout the plant.
How do the bacteria Xanthomonas axonopodis pv. Citri and Ralstonia Solanacearum affect plants?
Xanth- causes citrus canker. Bacteria is gram negative. Causes fruits to drop early and severe yield losses. Hemi-biotroph. Destroys Florida oranges. No genetic resistance available.
Ralst- can infect >200 plant species. Colonises plant xylem system. Blocks water uptake, causes plant wilting, stunting and yellowing. Serious threat to potato in warmer climates.
How does the bacteria Agrobacterium tumefaciens affect plants?
Causes Crown gall disease.
Gram negative, biotroph, uses a type IV secretion system to transfer DNA to host plants.
Genes produce auxin-like compounds, which causes an overproduction, cells overdivide and a tumour forms.
Tumour can fall off as part of the reproductive cycle which releases bacteria to infect a new host.
Genetic engineering in nature.
How does the bacteria Pseudomonas syringae affect plants?
Bacteria is a hemi-biotroph.
Narrow host range.
Grows in between plant cells.
Initially biotrophic, it can suppress plant defences with a T3SS- delivers effector proteins inside living plant cells.
Then becomes a necrotroph, killing the plant cell and feeding off the contents.
Lesions caused by infection are full of bacteria.
How does the bacteria Erwinia (Pectobacterium carotovora) affect plants?
Necrotroph.
Wide host range.
Water and soil borne, but also insect transmitted.
T2SS secretes cell wall degrading enzymes (pectinases).
