Microbiology- L14

how does leghemoglobin solve the problem of creating anaerobic conditions in the root nodule symbiosis

It maintains a strict control over the free oxygen, bound to unbound in the ratio of 10,000:1. This is sufficient oxygen for bacteria to respire, but too little to inhibit nitrogenase.

What kind of plants form symbioses with rhizobia

Legumes- genera include species of alfalfa, clover, peas, beans and lentil

what mycobionts are involved in lichens

Mostly belong to Ascomycota. Around half known members of this group are believed to be lichenized - ca. 13,500 spp. Some lichens are associated with basidiomycetes - such as Omphalina.

what are the phycobionts that can be involved in lichens

Mostly belonging to Chlorophyta - Trebouxia, Coccomyxa being commonest spp. Cyanobacteria (doesn't just have to be algae) such as Nostoc and Scytonema

where do you normally find lichen species

They are dominant cover species in Arctic tundra, on or just above the rocky shore tide-line and in some rocky deserts and lava flows.

What are the charactaeristics of Rhozbia

They are gram-negative rods, aerobic, motile, produce excessive amounts of extracellular slime

What are lichens

They are symbiotic assocaitions between fungi and algae. They colonise substartaes that are not ameanable to other organisms- they grown on rock, bark, man made structures (asbestos roofs, statues)

Air-dried lichens have a water content of.......

<1.5%

What are zooanthellae

Algae symbiotic with corals; give corals their colour. Corals are invertebrate animals. Algae are dinflagellates, mostly of the species symbiodinium.

what happens when corals are stressed

Animals under stress lose their zooxanthellae and become bleached (lose their colour too).

How is leghemoglobin to similar to Hb

Because it binds oxygen

Why are legumes a preferred choice for crops

Because they form symbioses with rhizobia, they have a high protein content and don't need fertiliser

How is LHb synthesised

It is synthesised co-operatively by both parties- globin by plant and heme by the bacterium

the lichen cladonia has what species as its photobiont

Cladonia always associated with Trebouxia.

what are the algae contained in within the coral

Contained in vacuoles within cells of coral, yet not digested.

What are the thallus layers in a typical lichen species

Cortex- outermost layer of the thallus composed of compacted fungal cells On the underside of the thallus can form rootlet like rhizines which help attach lichen to its substrate Medulla-elongate central cells composed of mycrobiont (pseudoparenchyma) Photobiont layer- a thin zone immediately beneath the cortex in which photobionts are concentrated

Who introduced the term symbiosis?

De Bary

Why is it so hard to fix nitrogen?

Dinitrogen has an extremely strong covalent bond It is extremely difficult to separate atoms of N2, so that it can be used in the biosynthesis of essential molecules (proteins and nucleic acids) Energetically costly for the bacteria to fix, equivalent to 100g of glucose for the fixation of 1g of Nitrogen Enzyme responsible is called nitrogenase- it is inhibited by oxygen

What are the 2 types of mycorrhaizae that we are going to look at

Ectomycorrhizae and endomicorhizae

what have symbionts got to do in order to establish a relationship with the host plant?

For symbionts to establish a relationship with a "host plant" they a) have to suppress the plants native defence responses (secrete effectors - which counter normal host defences) b) establish a nutritional relationship with the host (involving modification of specialised host-plant interface).

how do arbuscular mycorrhizal association form with trees and shrubs (with regard to forming the exchange surface)

Form arbuscles - coralloid ingrowths into root cells.

Describe the key points of mycorrhizal associations

Fungi which grow in association with roots- to mutual benefit of both partners Fungi gain nutrients Host plant showed improved efficiency of nutrient uptake-particulalry phosphate Nearly every plant has a mycorrhizal fungu- which are divided into a number of groups There is a huge literature on these-play key roles in improving agriculture production and bioremediation of polluted soils

how do ectomycorrhizal association form with trees and shrubs (with regard to forming the exchange surface)

Fungus mycelium envelops roots and forms a sheath around them. Extends between cells forming a structure know as the Hartig net. This forms an outer covering of hypahe called the mantle- this is external to the plant root- and the innner hartig net which is the site of exchange. Fungus scavenges scarce nitrogen and phosphorus, which is translocated into plant.

Give exmaples of positive symbioses involving microbes

Mycorrhizas and lichens- both involve intimated association between a photosynthetic and non-photosynthetic organism. In all cases the photosynthetic organism provides fixed carbon

Give examples of symbioses involving microbes and state what they have in common

Mycorrhizas, Nitrogen fixing root nodules, Lichens, Zooxanthellae in corals They all involve intimated association between a photosynthetic and non-photosynthetic organism. In all cases the photosynthetic organism provides fixed carbon

what are the three types of symbiosis

One partner gains at expense of the other - i.e. they are parasites (negative). Neither partner benefits (neutral). Both partners gain from the deal (positive). It is this last mutually beneficial arrangement that mostly gets applied to SYMBIOSIS these days.

the lichen peltigera is always associated with what photobionts

Peltigera with Coccomyxa as primary photobiont and Nostoc as secondary photobiont.

what do algae do for animals involved in corals

Responsible for 20% or more of carbon requirements of animals.

why are root nodules pink?

Responsible for the red pigment in functioning nodules.

What microbes form symbiotic relationships with plants

Rhizobium, Sinorhizobium, mesorhizobium and bradyrhizobium (these taxa are referred to as rhizobia)

In order to disperse both the algal and fungus together lichens propagate asexually. What are the two main types of asexual structure are involved?

Soredia - these are derived from the breakup of the upper cortex releasing granules which are composed of both hyphal fragments and algal symbionts. Isidia - these are finger-like projections from the upper thallus - include cortex, medulla and photobionts.

In the formation of the root nodule symbioses, the microbe induces plant to create a specific organ called a nodule (a nitrogen-fixing organ). Plant and microbe change their respective phenotype (appearance), including the production of compounds that neither make in the absence of the other. What, exactly, are the steps involved in the formation of this symbiosis.

Step 1: Rhizobia are attracted to the root Plant releases compounds called flavonoids that attracts the rhizobia bacteria (chemotaxis). Step 2: Rhizobia receive the plant signal and produce Nod factors that in turn stimulate cell division in the plant. Bacteria then firmly attach to plant root hairs using a protein called rhicadhesin - which induces the next phase of development. Step 3: Root hairs curl and rhizobia enter via invagination process. The interaction between bacterium and causes the root hairs to bend (curl), which leads to the development of an invaginated thread - which faciliates the invasion of rhizobia into the interior root tissues. Step 4/5: An infection thread forms allowing invasion of bacteria into root cortex into the root cortex. The bacterial cells rapidly divide forming new cell type - bacteroids which accumulate in modified host cells. Step 6: Plant cells continue to divide and become infected, forming the nodule. The cells containing the bacteroids continue to divide forming the characteristic pink spherical nodules on the roots.

What are the benefits of the plant to form symbioses with rhizobia

There bacteria are nitrogen fixing and this means that they will have improved growth, pigmentation and development

how is the exchange surface formed in lichens

There is a close physiological integration of carbon nutrition between the phycobiont and host fungus, which is achieved by the production of peg-like feeding structures known as haustoria. These tap into algal cells and absorb nutrients - particularly carbon.

what is unique about glomeromycota and their involvement in mycorrhizal association

These have the longest fossil history of any fungi - appearing in the roots of earliest rooted land plants (Ordivician 450 mya).

how do lichens regulate their water content

They absorb water from their surroundings (rain or humidity) - thus their water content varies passively with the surrounding environment. This behaviour is known as poikilohydric.

how do lichens reproduce sexually?

This only involves the fungus, which will reproduce sexually and then re-establish the symbiosis with another algal cell it finds. Since most lichens have ascomycote mycobionts - they produce sexual spores (ascospores) in asci. Ascospores are distinctive and also important in identification. Can be undivided or septate.

True or false. Lichens are very slow growing

True. This has lead to a field of study known as lichenometry.

put the layers in a lichen in order from upper to lower

cortex, photobiont layer, medulla and rhizines

what are arbuscual mycorrhizas

mainly found on herbaceous plants e.g beans, peas and grasses involve glomeromycota fungi play a role in phosphorus uptake from the soil.

What does the term symbiosis mean

meaning 'living together' in broadest sense to define microbe/plant/animal interactions.

what are the benefits to the secondary partner in the 4 symbioses we are discussing in this lecture

mycorrhiza- fungus unlimited carbon- transferred directly to the symbiont (no competition) root nodule- bacterium protected (competition free) living space- unlimited carbon and low oxygen environment lichen- alga competition free environment. mineral nutrients such as nitrogen and phosphorus, sheltered environment. coral- alga protected competition free environment, nutrients, organic nitrogen etc.

state the 2 parters involved in the following symbioses- mycorrhiza, root nodule, lichen and coral

mycorrhiza- plant and fungus root nodule- plant and bacterium lichen- alga and fungus coral- alga and animal

what are the benefits to the host in the 4 symbioses we are discussing in this lecture

mycrrohiza-plant nutrients- nitrogen and phosphorus root nodules- plant organic, usable nitrogen lichens- fungus carbon coral- animal carbon

what is the potential problem with the root nodule symbiosis and how is this problem overcome

plants produce oxygen, rhizobia are aerobic bacteria and require oxygen. The whole symbiosis is based on the rhizobial ability to fix N2 and to do this, the nitrogenase enzyme requires anaerobic conditions. Other bacteria that fix N2 (that do not enter into symbiosis) live in anaerobic conditions, others have specialized compartments that prevent the entry of O2 - but neither strategy is used in the root nodule. Leghemoglobin is the solution.

what are ectomycorrhizas

they form mainly with forest trees and shrubs, particularly in temperature and boreal regions involve the fungi basidiomycota and Ascomycota have a major role in nitrogen uptake from the soil