Download Lecture 19 VA and Orchid Mycorrhizas

Lecture 19
VA and Orchid Mycorrhizas
Objectives: A large percentage of plants have VAM. VAM date back 400
million years to the first land plants. Understand the basic structure and
function of all mycorrhizas. What is unique about VAM? What characteristics
of the root system favor mycorrhizas? What types of roots are likely to be
non-mycorrizal? VAM appears to be a balanced mutualism - is it? When do
orchid mycorrhizas tip from mutualism to antagonism?
Required Reading:
http://mycorrhizas.info/index.html
Rodriguez R and Redman R. 2008. More than 400 years of evolution and
some plants still canʼt make it on their own. Journal of Experimental Botany
59: 1109-1114.
Optional Reading:
Parniske M. 2008. Arbuscular mycorrhiza: the mother of all plant root
endosymbioses. Nature Reviews Microbiology 6: 763-775.
What is Symbiosis?:
Different species of organisms living together – to good, bad or
neutral results for one or all interacting species.
http://en.wikipedia.org/wiki/Symbiosis
Mycorrhizas are symbiotic associations essential for one or both partners, between
a fungus and a living plant. The key role of mycorrhizas is nutrient transfer. The
intimate contact between fungus and plant, generally within the root, results from
synchronized plant-fungus development.
Since first land plants, 400 mya, roots have evolved as habitats for
mycorrhizal fungi .
A perennial woodland plant (Smilacina racemosa) with a
metacutinised root cap (M) and mycorrhizal fungus hyphae (arrows).
Note how dark-staining suberin in the root cap functions as an
extension of the exodermis to completely encase the root for
protection during periods of inactivity. Cleared root stained with
chlorazol black E. 85x http://mycorrhizas.info/index.html#what
Mycorrhizas are mutualists both synchronized with and
controlled by their plant hosts.
1. The structure and development of mycorrhizal fungus hyphae is substantially
altered in the presence of roots of host plants. These root-borne hyphae are
different from hyphae which are specialised for growth in soil.
2. All mycorrhizas have intimate contact between hyphae and plant cells
in an interface where nutrient exchange occurs.
3. The primary role of mycorrhizas is the transfer of mineral nutrients
from fungus to plant. In most cases there also is substantial transfer of
metabolites from the plant to fungus.
4. Mycorrhizas require synchronized plant-fungus development, since
hyphae only colonise young roots (except orchid mycorrhizas and
exploitative VAM).
5. Plants control the intensity of mycorrhizas by root growth, digestion of
old interface hyphae in plant cells (AM, orchid), or altered root system form
(ECM).
Mycorrhizal associations by fungi in the Glomeromycota* are termed
arbuscular mycorrhizas, or vesicular-arbuscular mycorrhizas, abbreviated as
AM or VAM.
*http://tolweb.org/Glomeromycota
Structures in Soil
Hyphae A network of hyphae forms in
the soil with thicker hyphae which
function as conduits.
Absorptive hyphaeThin highly branched
hyphae which are thought to absorb
nutrients.
Spores Large (for a fungus) asexual
spherical structures (20-1000+ um
diameter) formed on hyphae in soil, or in
roots.
Structures in Roots
Hyphae these are non-septate when
young and ramify within the cortex.
Arbusculesintricately branched haustoria
in cortex cells.
Vesiclesstorage structures formed by
many fungi.
http://mycorrhizas.info/vam.html#intro
Mycorrhizas - Part I
Part of a colony of a VAM
fungus (Glomus sp.) with
hyphae, arbuscules (A)
and vesicles (V) growing
from an entry point
(arrow). A colony is
defined as the network
originating from one
entry point.
http://mycorrhizas.info/vam.html#
Orchid mycorrhizas consist of coils of hyphae within roots or stems of
orchidaceous plants. The fungi also cause root disease in a wide variety of
plants - HOW do the fungi and the orchids balance between mutalism and
antagonism?
http://mycorrhizas.info/index.html#
http://en.wikipedia.org/wiki/Swordleaved_Helleborine
Bitardondo M & DJ Read. 2008. Fungal
specificity bottlenecks during orchid
germination and development. Mol.
Ecol. 17:3707-3716.
Benefits to plants (see url below for complete list and referencing):
Increased plant nutrient supply by extending the volume of soil accessible
to plants.
Increased plant nutrient supply by acquiring that would not normally be
available to plants.
Root colonisation by ECM and VAM fungi can provide protection from
parasitic fungi and nematodes.
Significant amounts of carbon transfer through fungus mycelia connecting
different plant species has been measured (Simard et al. 1997). This could
reduce competition between plants and contribute to the stability and
diversity of ecosystems.
Networks of hyphae supported by dominant trees may help seedlings
become established or contribute to the growth of shaded understorey
plants (Hogberg et al. 1999, Horton et al. 1999).
http://mycorrhizas.info/roles.html#important
Fine roots and long root hairsThe
phosphorus (P) depletion zone
around mycorrhizal and
nonmycorrhizal roots are similar,
so mycorrhizal benefits are likley
to be small, as is typical of
facultatively mycorrhizal species.
Nonmycorrhizal plants also tend to
have long roots hairs.
Coarse roots and no root hairsThe
P depletion zone is much larger
when compared to the
nonmycorrhizal condition, due to
fungal hyphae. This is typical of
plants with a high degree of
mycorrhizal dependency when
grown in soils where nutrients
are an important limiting factor
for plant growth.
See links by moving onlline to this URL:
http://mycorrhizas.info/roles.html
Do VAM or do NM benefit more in a range of soil phosphate levels?
This response curve was produced by growing mycorrhizal and nonmycorrhizal
plants of the Australian native Cassia pruinosa at a range of soil P levels. Note
that the benefit provided by mycorrhizal associations to this species is
dependant on soil P levels and can be defined by comparing these two curves
(data from Jasper et al. 1994).
Selected other roles in ecosystems
 Soil hyphae are likely to have an important role in nutrient cycling by helping to prevent
losses from the system, especially at times when roots are inactive (Lussenhop & Fogel
1999).
 Hyphae are conduits that may transport carbon from plant roots to other soil organisms
involved in nutrient cycling processes. Thus, cooperating with other members of the
decomposition soil food-web.
 Soil hyphae may have an important role in nutrient cycling by acquiring nutrients from
saprophytic fungi (Lindahl et al. 1999).
 Epigeous and hypogeous sporocarps of ECM and VAM fungi are important food
sources for placental and marsupial mammals as food sources and as food
sources and habitats for invertebrates.
 Hyphae of VAM fungi are considered to contribute to soil structure. Their role in
mechanical aggregation has been questioned but secretions such as glomalin may be
more important (Wright & Upadhyaya 1998). Hyphal mats produced by ECM fungi
considerably alter soil structure (Griffiths et al. 1994.
 Mycorrhizal fungi contribute to carbon storage in soil by altering the quality and quantity of
soil organic matter (Ryglewicz & Andersen 1994).
Some of the roles in human affairs:
 Fungal diversity is a bio-indicator of environmental quality.
 Fungi which have adapted to local soil conditions are required for agriculture, horticulture
and forestry.
http://mycorrhizas.info/roles.html#important
In Vesicular Arbuscular Mycorrhizal plant roots, the fungus forms a
haustorium, termed an arbuscule.
Remember that a resistance reaction in the host would likely lead to the
hypersensitive response (see Gene-for-Gene flash animation from rust
lecture), a kind of programmed cell death, that starves out the invading
fungal symbiont. That does not happen in VAMs.
Each fungal arbuscule is surrounded by a plantderived peri-arbuscular membrane (PAM).
The apoplast between the PAM and the fungal
plasma membrane is the peri-arbuscular space
(PAS).
The PAS includes both fungal and plant cellwall material.
The arbuscule communicates with the plant
cell.
Parniske M. 2008. Arbuscular mycorrhiza: the
mother of all plant root endosymbioses.
Nature Reviews Microbiology 6: 765-775
Parniske’s model:
- During the evolution of plants, the genetic program for Vesicular
Arbuscular Mycorrhizas (VAM or AM) has been subsequently recruited
for other types of plant root symbioses:
- Functional adaptation of a plant receptor kinase essential for VAM
symbioses paved the way for nitrogen-fixing bacteria to form
intracellular symbioses with plant cells.
PPA = plant pre-penetration apparatus
Plant factor induces VAM fungus spore
germination and growth & seed
germination of parasitic plants, eg. Striga.
MAMP (= Microbe-associated molecular patterns,
eg. chitin in VAM fungus wall
actively prepares the plant for invasion.
- Guides fungus through cells to apoplast
between cortical cells.
- VAM haustoria (arbuscules) will develop
within plant cortical cells.
Plant MAMP receptors recognize Nod
factors and chitin from fungi, among a
variety of MAMPs. Nod factors induce
calcium spiking in plants.
Perception of AM fungal or rhizobia N-fixing bacteria-derived
signals sets off signaling mediated by at least 7 shared factors.
SYMRK is a receptor kinase
acting before the Nod and Myc
factors induced calcium spikes as
Ca is released from storage
around the nucleus.
CASTOR & POLLUX, K
permeable channels, may
balance the charge with K+.
NUP85 & NUP133, nucleoporins,
are required for calcium spiking.
The calcium-calmodulindependent protein kinase
(CCaMK) complexes with
CYCLOPS, a phosphorylation
substrate, in the nucleus.
Nod factor receptor kinases
NFR1 & NFR5 are required for
Nod factor perception by the
plant. (Likely similar with Myc).
The intracellular structures in bacterial and fungal root endosymbioses are similar. In VAM fungi,
the prepenetration apparatus (PPA) is a tube of cytoplasm lined with ER, that bridges the
vacuole of the plant cell; the PPA precedes fungal invasion. The plant cell nucleus positions in
the front of the PPA and directs the orientation of the PPA.
In N-fixing nodulating endosymbiotic bacteria, there is a PIT which is like a PPA. The PIT is
induced by bacterial signals. The PIT is hypothesized to be evolutionarily derived from the PPA.
Given the benefits of mycorrhizas, how can we exploit them in forestry and
agriculture? This is a decision tree from Brundrett
http://mycorrhizas.info/roles.html
Ectomycorrhizas (ECMs)and myco-heterotrophy
Objectives: Understand the basic structure and function of all
mycorrhizas. What is unique about ECMs? Compare them with
VAMs and Orchid mycorrhizas. Understand what mycoheterotrophy is and why the non-photosynthetic plant in the
tripartite association is “cheating.” How does the invasive plant,
garlic mustard, interfere directly with seedling establishment and
indirectly with regrowth in the forest?
Required Reading: http://mycorrhizas.info/index.html
Great References:
Simard SW. et al. 1997. Net transfer of carbon between tree species in
the field. Nature 388: 579-582.
Whitfield J. 2007. Underground networking. Nature 449: 136-138.
Merckx V, Bitartondo MI and Hynson NA. 2009. Myco-heterotrophy:
when fungi host plants. Annals of Botany, online at
http://plantbio.berkeley.edu/~bruns/papers/merckx2009.pdf
Figure is from:
Martin F and Selosse M-A. 2008. The Laccaria genome: a symbiont
blueprint decoded. New Phytologist 180: 296-310.