9. Microbial Associations with Plants

Question 1

what are legumes? 6

Answer 1

1. 1300M years ago, Eukaryotas evolved from microbes and co-evolved with bacteria
2. Legumes are a plant family that evolved to grow with the microbes to gain nitrogen
3. 15000 species in the leguminous family, eg. peas, beans, clover, trees in tropics
4. specialised organs on their roots called nodules, and bacteria called rhizobia live inside them
5. live in nodule itself or in soil but can only fix N2 in the nodule
6. host specificity

Question 2

what are the types of legumes? 4

Answer 2

1. peanuts, chickpeas, anything with a pod
2. clover grows in nitrogen poor areas
3. leguminous trees grow in tropics, they have pods on them
4. eg. on pea pant, nodule grows out the side of the root as host for n fixing bacteria

Question 3

how are nodules formed on leguminous plants? 6

Answer 3

1. root hairs have receptors for attachment of rhizobial bacteria (rhcadhesin-mediated)
2. NOD factors excreted by bacteria lead to curling of the root hairs
3. rhizobia invade and form infection thread, which penetrates down into tissue
4. thread invades basal cells of plant roots. Infection causes cells to divide and form nodules
5. rhizobia multiply inside nodule and turn into n fixing state - bacteroids
6. forms symbiotic relationship, plants provide sugars and a safe place to grow for the bacteria, which give nitrogen in return

Question 4

what genes are responsible for the nodules that grow in leguminous plants? 2

Answer 4

1. two sets- transcribed in opposite directions

2. 12-16 genes involved in making a plant form a nodule

Question 5

how to rhizobia rif nitrogen? 6

Answer 5

1. photosynthesis creates sugars, which are transformed to organic acids eg. citric, acetic, which turn into succinate, malate and furnarate, which are converted into pyruvate
2. pyruvate leads to nitrogenase, which is very sensitive to oxygen and should be protected
3. e- produced utilised to fix nitrogen and ammonia, which can be used to create amino acids
4. the three intermediate compounds also contribute to the krebs/citric acid cycle, then electrons go down the electron transport chain
5. the, leghaemoglobin binds to oxygen to transport oxygen for use in the electron transport chain
6. there can be no free oxygen in nodule, so this allows nitrogenase to continue to function

Question 6

describe the symbiotic relationship between leguminous plants and bacteria. 3

Answer 6

1. mutual - plant and bacteria contribute to infection process
2. plant gains nitrogen and bacteria gains anaerobic environment for nitrogen fixation and carbon for energy
3. physiologically, bacteria are organelle like.

Question 7

what is crop rotation? 6

Answer 7

1. nitrogen fixing was only discovered 40 years ago, but crop rotation has been used since the 1730s
2. field was left fallow for a year, but later introduced leguminous crops during fallow year as part of four field crop rotation system eg. legumes and turnips for animal feed
3. led to a greater cereal yield
4. more food, more animals and more manure for fertiliser
5. greatly reduced workforce and contributed to increased population in citites
6. factor in agricultural/industrial revolution, as greater yields meant more people looked for city work

Question 8

what are mycorrhizal associations? 5

Answer 8

1. fungal associations with plant roots
2. 95% of all vascular plants have these
3. two main types
4. ectomycorrhiza are mainly between basidiomycetes and tress
5. endomycorrhiza are mainly between zygomycetes and grasses

Question 9

describe ectomycorrhiza. 6

Answer 9

1. Sheath of mycelium penetrates between root cortex cells
2. Cortex and mycelium hyphae form harting net
3. Phosphate and other inorganic nutrients are generally limited, and fungi have enzymes that solubilise phosphate for them
4. hard to tell where root ends and hyphae begins, fungus massively extends root system
5. trees provide fungus with sugars from photosynthesis, and without fungus trees can’t even grow
6. host specificity varies, each species has 3-6 potential fungi

Question 10

how does spore dispersal work and what is the wood wide web? 4

Answer 10

1. rootlet of the tree moves into mycorrhiza fungus
2. grows out into toadstool for spore dispersal
3. mycorrhiza can interconnect different trees
4. research is focusing on the role hyphae play in tree-tree communication

Question 11

What are endomycorrhiza? 6

Answer 11

1. penetrate root cells of plants, grow and branch inside plant cells in structures called arbuscles
2. cell reacts and kills mycorrhizal fungus within 4-15 days and new ones formed
3. endomycorrhiza are most common, found in 2/3 of all land plants
4. finely branched clusters of hyphae giving a large surface area for nutrient exchange
5. fungus solubilises phosphate and ghets sugars from plant
6. fungi produce inter/intra-cellular vesicles in roots containing sugars in the form of lipids for storage

Question 12

describe the evolution of endomycorrhiza. 4

Answer 12

1. distantly related to other zygomycetes
2. earliest associations with plant roots
3. diverged early, origin about 353-462million years ago
4. coincides with colonization of land by plants and fossil evidence suggests pivotal role in colonization

Question 13

describe mycorrhizal associations with orchids. 6

Answer 13

1. 22-26K species, 2nd largest family of flowering plants. 2x as many as birds, 4x as many as mammals (species wise)
2. every species associates with one endomycorrhiza, intense host specificity
3. orchid seeds are very small with little storage starch, enough for germination but not to grow a whole seedling
4. all orchids lack chlorophyll initially and rely on sugars from mycorrhiza in early stages.
5. some lack chlorophyll as adults eg. ghost orchid.
6. some orchids could be classed as parasites, as the fungus doesn’t seem to benefit

Question 14

what is a peloton? 6

Answer 14

1. when cells are invaded, hyphae form coils called peletons
2. peleton increases surface area between orchid and fungi
3. each peleton has a life span of a few days before it degenerates and is digested by the orchid cell, which sees it as an enemy
4. this is unstable and infection can have three outsomes within one population. The best is that germination and mycorrhizal formation occur
5. Parasitisation - too many cells are invaded and the orchid dies
6. The orchid acts too strongly and rejects the infection, so dies

Question 15

What is the structure of a lichen? 5

Answer 15

1. Mycobiont (fungus) and photobiont (algae), which is the photosynthetic partner. Obvious, visible part.
2. Each can grow independently but form lichen upon coming together.
3. Have 3 layers - top layer is the cortex
4. Algal layer is in the middle, where photosynthesis occurs
5. Medulla, the bottom layer, is the fungal hyphae which often form attachment to the surface

Question 16

Describe the lifestyle of a lichen. 6

Answer 16

1. rarely free living but can be separated in the lab
2. long living - alpine lichens live 4000+ years
3. slow growing, most only grow 2mm p.a max
4. hyphae penetrate cells to form internal haustoria
5. alga provides sugars and fungus protects alga from dessication
6. primary coloniser

Question 17

What sort of conditions do lichen live in? 5

Answer 17

1. lichens are tolerant of environmental extremes
2. they are pioneer coloniszers on rock faces, tree bark and rooves
3. only organism that can grow on bare rock surfaces
4. when it dies, its broken down into soil. then, seed dispersal by air means new plants can grow there.
5. despite this, lichens are very sensitive to air pollution and are used as indicators.