Symbiosis
Sym biosis
together
Symbionts:
Host:
life
the organisms involved
the larger organism, if there is one
Mutualism:
both symbionts benefit
Commensalism:
one symbiont receives benefit while
neither harming nor helping the other in
any significant way
Parasitim: one symbiont, called a parasite, benefits at the
expense of the other, usually a host
Anthropomorphism in biological definitions?
Examples
Ants and plants: mutualism with a large host
Epiphytic plants: commensalism with a large host
Parasitism: many species of aphids
Parasites need to be distinguished between cases
where a pathogen may lead to death and where
there may be a balance in the host-parasite
relationship. For example, there can be genetic
balances in virulence and resistance that operate
at the populations level. Myxomatosis and rabbits
Symbiosis in the origin of the eukaryotes
Two components
of the theory
1. The endomembrane
may have evolved by
infolding of the plasma
membrane of an ancestral
prokaryote
2. Serial endosymbiosis.
Mitochondria and
chloroplasts evolved from
prokaryotes that lived
symbiotically within some
ancestral eukaryotes.
Molecular evidence –
circular DNA in
chloroplasts and
mitochondria
Nitrogen fixing bacteria
In biological nitrogen fixation two moles of ammonia are produced
from one mole of nitrogen gas, using 16 moles of ATP and a supply
of electrons and protons (hydrogen ions):
N2 + 8H+ + 8e- + 16 ATP = 2NH3 + H2 + 16ADP + 16 Pi
This reaction is performed exclusively by prokaryotes (the bacteria
and related organisms), using an enzyme complex termed
nitrogenase. This enzyme consists of two proteins - an iron protein
and a molybdenum-iron protein.
A point of special interest is that the nitrogenase
enzyme complex is highly sensitive to oxygen
Nodule formation 1
Roots emit chemical
signals that attract
Rhizobium bacteria.
The bacteria emit
signals that stimulate
root hairs to elongate,
and to form an
infection thread by an
invagination of the
plasma membrane
Soya bean infection
with Rhizobium
The bacteria penetrate the root
cortex within the infection
thread. Plant cells start dividing
and vesicles containing the
bacteria, bacteriods, bud into
the cells from the branching
infection thread
Nodule formation 2
Nodule formation 3
Growth continues in the
affected regions of the cortex
and pericycle and these fuse
to form the nodule
Nodule formation 4
The nodule grows and
vascular tissue
connecting it to the
plant’s xylem and
phloem develops
Lens culinaris (lentil) root nodulation.
7 days after infection
12 days after infection
Root hairs
http://www.sunderland.ac.uk/~es0man/tem2.htm
7 days
12 days
Small numbers of bacteria
Large numbers of bacteria
TEM photomicrographs
http://www.sunderland.ac.uk/~es0man/tem2.htm
Clover root with nodules
Part of a clover root system bearing naturally occurring
nodules of Rhizobium. Each nodule is about 2-3 mm long.
http://helios.bto.ed.ac.uk/bto/microbes/nitrogen.htm
In symbiotic nitrogen-fixing organisms such as Rhizobium, root
nodules can contain oxygen-scavenging molecules such as
leghaemoglobin, which shows as a pink colour when the active
nitrogen-fixing nodules of legume roots are cut open.
Leghaemoglobin may regulate the supply of oxygen to the nodule
tissues in the same way as haemoglobin regulates the supply of
oxygen to mammalian tissues
Leghaemoglobin
Clover root nodules.
Leghaemoglobin is
found only in the
nodules and is not
produced by either the
bacterium or the plant
when grown alone.
Anabaena with heterocysts
Anabaena sp. with symbiont bacteria
(possibly Zoogloea) around heterocysts
In some
cyanobacteria
nitogen fixation
occurs in
heterocycts. These
cells only have
Photosystem I
The other cells have both photosystem I and
photosystem II, which generates oxygen when light
energy is used to split water to supply H2 for
synthesis of organic compounds.
http://www-cyanosite.bio.purdue.edu/images/images.html
Some legumes are better at fixing nitrogen than others.
Common beans are poor fixers (less than 50 lbs per acre)
and fix less than their nitrogen needs. Maximum economic
yield for beans in New Mexico requires an additional 30-50
lbs of fertilizer nitrogen per acre. However, if beans are not
nodulated, yields often remain low, regardless of the amount
of nitrogen applied.
Other grain legumes such as peanuts, cowpeas, soybeans, and
faba beans are good nitrogen fixers, and will fix all of their
nitrogen needs. These legumes may fix up to 250 lbs of nitrogen
per acre and are not usually fertilized. If large amounts of
nitrogen are applied, the plant literally slows or shuts down the
nitrogen fixation process.
There are many research programs
attempting genetic improvement of
nitrogen fixation, e.g., alfalfa
General N fixation and Alfalfa
Myco rrhizae
Fungus
Root
Mycorrhizas are highly evolved, mutualistic associations between
soil fungi and plant roots.
The host plant receives mineral nutrients while the fungus obtains
photosynthetically derived carbon compounds.
Almost 80 percent of all terrestrial plants can form mycorrhizal
associations.
There are two major types ectotrophic mycorrhizae and
endotrophic mycorrhizae
Ectomycorrhizae
Mycorrhizal fungi produce a hyphal network in soils consisting of
individual strands of hyphae or relatively undifferentiated bundles
of hyphae called mycelial strands. Some fungi can produce
rhizomorphs, which contain specialised conducting hyphae, or
sclerotia, which are resistant storage structures. Soil hyphae acquire
nutrients.
Fungal structures in soil
Mycellial
strand
Absorptive hyphae
Scleridia
Mycorrhizal
root
Rhizomorphs
Soil mycellium
Example of ECM short roots (arrows) of birch
(Betula alleghaniensis), an angiosperm tree. The
mycorrhizal short roots are thicker than other
laterals of the same order due to the mantle and
Hartig net.
Early stage of
colonisation of pine short
root by Pisolithus
tinctorius. Hyphae
(arrows) have contacted
the root and are starting
to proliferate on its
surface near the apex (A).
SEM image showing the
next stage of pine root
colonisation by Pisolithus
tinctorius. Mantle hyphae
(arrows) have formed a
dense covering on the root
surface (arrows).
http://www.ffp.csiro.au/research/mycorrhiza/ecm.html
Pinus radiata and Amanita
muscaria ECM synthesised
under sterile conditions.
This association has highly
branched short roots with
many root tips (arrows).
Eucalyptus maculata and
Astraeus pteridis association
synthesised under sterile
conditions with relatively
unbranched ECM and
attached mycelial strands
(star).
Hand section
cleared and
stained with
Chlorazol
black E and
viewed with
interference
contrast
microscopy
Populus tremuloides ECM root cross section showing labyrinthine
Hartig net hyphae (arrows) around elongated epidermal cells. This
complex hyphal branching pattern is considered to increase the
fungal surface area in contact with the root.
http://www.ffp.csiro.au/research/mycorrhiza/ecm.html
Vesicular arbuscular mycorrhizae
The network of hyphae in the
soil is only connected to roots by
the entry points that initiate
mycorrhizal associations
Mycorrhizal root
system washed carefully
from coarse sand to
reveal the intact
network with external
hyphae (arrow) with
spores (S) produced by
Glomus mosseae.
http://www.ffp.csiro.au/research/mycorrhiza/ecm.html
Appressorium
At entry point
Epidermis
Hypodermis
Intracellular
hyphae
Intercellular
hyphae in air
channel
Arbuscules
Vesicle
Cortex
A colony refers to hyphal growth within a
root resulting from the same external hyphae
(1 or more connected entry points). These are
also called infection units.
Arbuscules (A) and
convoluted hyphae
(arrow) in the inner
cortex of an Asarum
canadense root.
Arbuscules only form in
the innermost cortex
cell layer next to the
endodermis in this
species.
Vesicles (V) produced
by a Glomus species in a
leek root. This root also
contains many
intercellular hyphae.
(Bar = 100 um)
http://www.ffp.csiro.au/research/mycorrhiza/ecm.html
Lichens
Lichens are symbiotic associations between
photosynthetic micro-organisms held in a mesh of fungi.
Cladonia fimbriata
Different species of lichens
growing on a boulder.
The fungus gives the lichen
its shape.
The alga provides the fungus with food. In some lichens the ‘alga’
is a Cyanobacteria and fixes nitrogen. Lichens are colonizers of
bare rock but some species are epiphytes found in tree canopies.
There are some 16,000 species of lichen
In contrast to the many thousands of lichen fungi, there are
only about 100 photosynthetic partners.
The algal components can live in isolation with the fungal
component – so the symbiotic relationship is not obligate for
them. However, no zoospores (the algal resting stage) or
gametes are produced while the alga is in the symbiotic
relationship though they can produce them when cultured
outside of it.
In contrast the symbiotic relationship is obligate for the fungal
component. The mycelium of a few species may grow under
laboratory conditions, on agar, but no lichen specific thallus is
produced and no fruiting body develops.
The oldest certain fossil lichen is EarlyDevonian (about 400
million years old) from the Rhynie Chert.
Fruiting body of the fungus
Cross-section through a lichen
Fungal hyphae:
a filament,
composed of
single cells
joined together
end to end
See also Fig. 17.18B (but note names used here)
Xanthoria parietina
The foliose lichen Xanthoria parietina, which grows on surfaces, including
concrete, and rocks subjected to sea spray. Much of the surface is covered with
bright orange fungal fruiting bodies about 3-5 mm diameter (arrowheads). The
orange colour is due to production of the pigment parietin at the lichen surface.
Cross section of one of a lobe viewed by phase-contrast microscopy. The
photosynthetic zone (p) is a distinct band of green algal cells. Above this band is
the cortex (c) of densely packed fungal cells . The lower part of the thallus consists
of a medulla (m) with conspicuous air pockets (a), and a thin lower cortex (lc).
Like many foliose lichens, Xanthoria produces rhizinae (r) that penetrate into
crevices and help to anchor the lichen to a surface.
http://helios.bto.ed.ac.uk/bto/microbes/lichen.htm
Lichens are unique as a symbiotic relationship
because they look and behave quite differently from
their component organisms. Lichens are regarded as
organisms in their own right and are given generic
and species names.
The fungus is termed the mycobiont
The green alga or a cyanobacterium is termed the
photobiont.
The "body" of a lichen is termed the thallus, and its
general shape is used to group lichens into four
broad categories.
Foliose Fruticose Squamulose Crustose
Classification by form
Foliose lichens
Foliose lichens have a flat,
leaf-like structure
Parmelia physodes, growing on
the twigs of a shrub.
Fructicose and Squamulose
Fruticose lichens have
an erect or pendulous,
bushy structure
Squamulose lichens have a
thallus consisting of minute,
scale-like squamules
Lecanora muralis
with ascocarps pink-brown
Crustose lichens
Crustose lichens produce a
flat crust on or beneath rock
or tree surfaces
Trebouxia
Trebouxia
The most common photobiont is a single-celled green algae of the
genus Trebouxia found in many lichens of temperate, arctic and
alpine regions, including all species of the common lichen genus
Cladonia. Trebouxia species seldom grow as free-living cells in nature
but seem to be specialised lichen symbionts.
Trebouxia secretes about 8% of the carbohydrates it produces when
living free in experiments, but up to 40% when it is a photobiont.
The increased secretion rate is caused by the fungus.
In lichens with Trebouxia the fungal hyphae can produce short
branches that penetrate through the algal wall to serve as
nutrient-absorbing haustoria.
Lichens are remarkable for their ability to withstand
prolonged drying and to resume activity rapidly after
rewetting. The rewetting of lichens is thought to start by
water absorption in the gelatinous matrix of the cortex.
The maximum rates of nutrient release from the
photobiont occur in optimal moisture conditions, whereas
it retains most of the carbohydrate in conditions of water
stress. It has been suggested that cycles of wetting and
drying may be advantageous in maintaining a lichen
symbiosis, because both partners could gain sufficient
carbohydrate at different stages of this cycle.
They are extremely intolerant to SO2
Sections you need to have read
16.20 17.18 32.11 36.5
Courses that deal with this topic
Botany 371/372 Plant physiology laboratory