Transcript Symbioses and plant nutrition

Symbiosis and Plant Nutrition
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Symbioses and plant nutrition
Symbioses
• ‘living together of two dissimilar organisms’
• can be mutualistic or parasitic
In this lecture consider the mutualistic symbioses
• increase availability of nutrients to plants
• involved in N and P nutrition
– MYCORRHIZAL FUNGI - uptake of N (organic) and P (organic or
inorganic) from soil
– N2 FIXING PROCARYOTES - fixation of gasous N2 from
atmosphere
Mycorrhizas: basic characteristics
SYMBIOSIS BETWEEN FUNGI AND PLANT ROOTS
– four main types
THE MOST COMMON UNDERGROUND SYMBIOSIS
– very high proportion of plant species (90%)
NORMAL NUTRIENT ABSORBING ORGANS OF PLANTS
– increases uptake of P, N, Zn etc
HELP PLANTS ABSORB NUTRIENTS FROM SOIL
– ectomycorrhizas (organic and inorganic nutrients)
– arbuscular mycorrhizas (inorganic nutrients only)
Four main types of mycorrhizas
• Ectomycorrhizas (autotrophic hosts)
• Ericoid mycorrhizas (autotrophic hosts,
mainly)
• Orchid mycorrhizas (hosts often
heterotrophic)
• Arbuscular mycorrhizas (AM)
Altogether about 90% of terrestrial plants form
mycorrhizas
Some plants are not mycorrhizal
• Cruciferae – oilseed rape, cabbage and
relatives including Arabidopsis
• Proteaceae e.g. Banksia, Hakea etc
• Chenopodiaceae (some) e.g. Beta, Mariana
(blue bush), Atriplex (salt bush)
• Cyperaceae and Juncaceae - rushes
• Caryophyllaceae (some) -
Ectomycorrhiza
• FORMED MAINLY BY WOODY PERENNIALS (trees)
(around 10% of plant species, covering very large
areas of forest)
Myrtaceae, e.g. Eucalyptus
Pinaceae, e.g. Pinus
Fagaceae, e.g. beech, oak, Nothofagus
Betulaceae, e.g. birch, alder,
Dipterocarpaceae, e.g. Shorea
• FUNGI ARE MAINLY BASIDIOMYCETES
(toadstools/truffles/puffballs)
plus a few ASCOMYCETES
• CHARACTERISED BY FUNGAL SHEATH
SURROUNDING ROOTLETS
Fungi are mainly Basidiomycetes and
Ascomycetes, which form toadstools, puffballs
and truffles
Amanita
muscaria
Tuber spp.
Pisolithus
tinctorius
Ectomycorrhizal root
system
extensive external mycelium
• increases volume of soil
from which nutrients
available
• increases uptake of
inorganic N and P
• Increases uptake of organic
N and P
from Smith & Read, 1997
Structure of
Ectomycorrhizas
mantle or sheath
short absorbing rootlets
are completely covered
by a mantle of fungal
hyphae
various sources
Hartig net
Inorganic P uptake by Ectomycorrhizas
2
• Fungi depend on sugars
from the plant
• All nutrients from soil
pass through fungal
sheath to reach the
plant
• Fungal partner plays
major roles in
acquisition of inorganic
nutrients (Pi, NH4 and
NO3)
Dry weight (g)
a
a
ab
bc
1
cd
d
d
0
3
P content (mg)
a
a
a
2
b
1
bc
bc
c
0
ECM
AM
-P
+P
Eucalyptus ; Jones et al. 1988
Ectomycorrhizal fungi colonise soil organic
matter
(14CO2 feeding to seedling)
Bending & Read 1995
Fermented Horizon Organic Matter (FHOM)
Ectomycorrhizal fungi mobilise of N from
organic pool in soil
15
uncolonised
µg
N/mg
12
DW
FHOM
Thelephora
terrestris
Suillus
bovinus
9
6
start
42 days
Bending & Read 1995
Ericoid mycorrhizas
• Host plants: Ericales (heath plants like Erica, Calluna,
Epacris)
• Fungi: Ascomycetes (e.g. Hymenoscyphus ericae;
Oidiodendron maius)
• Saprotrophs/biotrophs, culturable in vitro and can
mineralise organic forms of P and N
• Environment: heathlands - many have high organic
matter, low nutrient concentrations, low pH
Ericoid mycorrhizas are formed by members of
the Ericaceae and Epacridaceae
Photo J. Wrigley. Australian National
Botanic Gardens
Epacris impressa
Transverse section of a mycorrhizal root of
Calluna vulgaris (Heather)
stele
single layer of
cortical cells filled
with fungus
1 mm
Smith and Read 1997
Ericoid mycorrhizas and plant nutrition
• fungi mobilise inorganic N and P from organic
sources in soil
• increase uptake of N and P by plants
• receive sugars from the plant
Arbuscular mycorrhizas (AM)
the most common type
PLANTS
•
•
•
•
•
very high proportion of land-plant species
– (around 80% quoted)
most families are mycorrhizal
includes Angiosperms, Gymnosperms, Pteridophytes
(ferns) and also some ‘lower plants’
includes trees, shrubs, herbs
high diversity in responsiveness (see later lecture)
some plants do not (apparently) benefit from symbiosis, for others
the association is obligate
Arbuscular mycorrhizas-structures inside roots
illustrations from M. Brundrett and S. Smith
intercellular hypha
arbuscule
Fungi that form AM
• Glomeromycota (separate phylum, distinct from
Zygomycetes, Ascomycetes and
Basidiomycetes)
• about 150 species described (may be
underestimate)
• Asexual
• Form large, multinucleate spores
• Unculturable (obligate symbionts)
Importance of arbuscular mycorrhizas in
plant nutrition
mycelium in soil
soil
particle
hyphae
root
from I. Jakobsen
• absorbs inorganic P, Zn, N
• accesses small pores
• increases volume of soil
exploited
• improves soil structural
stability
Increases plant nutrient
uptake and growth
(see later lecture)
Nitrogen fixing organisms
• convert N2 gas to NH4 which plants can use
• all are prokaryotes
– some bacteria and cyanobacteria
• some are free-living
• some are symbiotic in:
–
–
–
–
lichens
Azolla (water fern)
Cycads
root nodules of legumes, Casuarina, Myrica etc
Why is symbiotic N2 fixation important?
• significant transfer of N from inert atmospheric pool to
forms that plants can use
• 120 x 106 tonnes N2 fixed per year
• uses energy from plants
• converts NN  2 NH4+
other conversions involve
• N2 fixation by free-living organisms 50 x 106 tonnes
per year
• energy from lightning (25 x 106 tonnes per year)
• industrial conversion by Haber Bosch process to
produce fertilisers; needs very high energy inputs
N2 fixation needs a lot of energy and
reducing power
N2 + 8 e- + 16ATP + 10H+  2NH4+ + H2 + 16 ADP
+ 16 Pi
nitrogenase enzyme
• requires both Fe and FeMo cofactors
• is inhibited by free oxygen (O2)
N2 fixation requires special conditions
• highly reducing (anaerobic) conditions
• nitrogenase is protected by delivery of O2 by
leghaemoglobin in nodules (low partial
pressure in nodules)
• large amounts of energy supplied by aerobic
respiration  ATP and NADPH
• in legumes energy source comes from plant
as organic acids (e.g. succinate)
Nodulation in legumes
• Nodules are formed in the roots of legumes by soil
bacteria (Azorhizobium, Rhizobium, Bradyrhizobium etc)
• Bacterial species/strains are specific for different legume
groups (must have the correct strain)
• Plants need good supplies of nutrients other than N
– major nutrient P
– micronutrients Fe and Mo
• Nodulation and N2 fixation are inhibited by high NO3 or
NH4 in soil
Stages in nodule
development
from Sprent and Sprent, 1990
Stages in nodule development
•modified bacteria (bacteroids)
•are enclosed in apoplastic
compartments (symbiosomes)
1. rhizobium
attaches to
root hair
•surrounded by peribacteroid
membrane (PBM)
bacterioid
PBM
2. root hair curls
and rhizobium
invades cortex
3. invaded cortical
cells divide to make
nodule
Organisation of symbiotic compartments
(symbiosomes) in root nodules
bacteriods are enclosed in
apoplastic compartments called
symbiosomes
surrounded by peribacteroid
membrane (PBM)
N2 is fixed in the bacteroids and
NH4 transported to the plant
cell
Energy for N2 fixation is provided
by respiration of succinate
which is transported from the
plant cell into the symbiosome
bacteroid
NH4
plant
cell
succinate
symbiosome
Symbiotic mycorrhizal fungi and nitrogen-fixing
bacteria by-pass soil pools and reduce losses
crop
atmosphere N2
N2 fixation
labile
organic
N/P
stable
inorganic
mycorrhizas
labile
inorganic
microbes
stable
organic
soil
solution
leaching
N/P
erosion
Summary
• Symbioses between plants and other organisms can
increase nutrient acquisition from pools that are ‘difficult’ for
plants alone to use
• Symbiotic pathways by-pass soil microbial pathways,
improve ‘short-cycling’ and reduce potential losses
• Gasous N2 in atmosphere by nitrogen fixing procaryotes
• Organic and inorganic N and P pools by ericoid and
ectomycorrhizal fungi
• Inorganic P pools by arbuscular mycorrhizal fungi
Useful references
Bending, G. D. and Read, D. J. 1995 The structure and function of the
vegetative mycelium of ectomycorrhizal plants. V. Foraging behaviour and
translocation of nutrients from exploited litter. New Phytologist. 130: 401-409.
Jones, M. D.et al. 1998 A comparison of arbuscular and ectomycorrhizal
Eucalyptus coccifera: growth response, phosphorus uptake efficiency and
external hyphal production. New Phytologist. 140: 125-134.
Smith, S. E. and Read, D. J. 1997. Mycorrhizal Symbiosis, Ed 2nd. Academic
Press Ltd, Cambridge, UK
Sprent, J.I and Sprent, P. 1990. Nitrogen fixing organisms. Pure and applied
aspects. Chapman Hall, London.