Spring Upshaw
Biology Web Project
Due August 10, 2006
Mycorrhizae
Scientists estimate that plants evolved between 400 and 500 million years ago
(Krogh, 2005). The first plants were bryophytes. Overtime, plants began to develop
adaptations such as a water vascular system, which allowed them to grow taller and wider.
Fossil records show fungi with the earliest land plants, suggesting that they may have had a
mutual dependency.
During the time of Aristotle, classifying based on resemblance led there to be only
the plant and the animal kingdoms. If any organisms were green or grew forming roots
they were classified as plants, if not they were animals. After Aristotle’s time, Darwin
collected evidence to support his theory of natural selection and classification became
based on evolutionary relationships. Later in the 19th century, a three kingdom system was
developed. The third kingdom, protists, included organisms not resembling plants and
animals. Because yeast and bacteria are unicellular, they were placed in the protist
kingdom. Later in the history of classification, evidence from fossil records, electron and
light microscopes, and molecular structures led the protist kingdom to be divided into
monera and protocista, (Schwartz, 2001). This division was based on the organism being a
eukaryote or prokaryote. All fungi were moved from the plant or protist kingdom to the
protocista kingdom because they are immotile and are eukaryotes. Later fungi were
classified in a kingdom of their own because of their shape, reproduction, and ability to
absorb food.
The current system of classification still focuses on evolutionary relationships only
with more physical and molecular evidence. Physical and molecular structures of fungi
prove that it is more closely related to animals than plants. The following chart shows
basic characteristics of fungi and compares them to plants and animals.
Characteristic
Fungi
Plants
Animals
Number of cells
Multicellular except
Multicellular
Multicellular
unicellular yeast
Obtain energy
Heterotrophs
Autotrophs
Heterotrophs
Cell Structures
Cell wall made of
Cell Wall made of
Exoskeletons of
Chitin
Cellulose
arthropods is made
of chitin
Energy Storage
Glycogen
Starch
Glycogen
Reproduction
Sexually (with
Sexually and
sexually
spores) and
asexually
asexually
In addition to the characteristics listed above fungi is made up of many filaments called
hyphae. Within the hyphae are many nuclei. Depending on the species there may be cross
walls forming cells that each contain one nucleus. Because of the length and number of
hyphae, surface area is increased allowing for more nutrient absorption.
Fungi have a complex reproductive system that may consist of sexual and asexual
stages. In asexual reproduction, “spores may be produced or the hyphae may form
fragmentations and each fragment will become a new fungus” (Miller-Levine, 1995, p.
408). A basic explanation of sexual reproduction is as follows:
1. Two hyphae fuse forming a dikaryotic cell ( a cell with two haploid nuclei)
2. The two haploid nuclei fuse to form a diploid zygote.
3. Meiosis produces either four haploid nuclei or four haploid cells.
Sexual reproduction in fungi is unique because when the hyphae from different fungi fuse,
it will form hyphae with many nuclei in one cell if there are no cross walls. Because the
nuclei are coming from the hyphae of different fungi, the genetic material will be different.
The cell with many different nuclei will have more diversity than either of the “parent”
hyphae.
http://sps.k12.ar.us/massengale/fungi_notes_b1.htm
The fungi kingdom includes organisms that may be harmful or helpful. Examples
of helpful fungi include those that make the antibiotics penicillium and streptomycin.
Yeast can also be helpful because their fermentation produces sauerkraut, soy sauce, bread,
and cheese. Harmful fungi include those that causing ringworm, athlete’s foot, and plant
diseases.
Fossil records from the Rhynie Chert ecosystem in the Lower Devonian period,
show fossil fungi-plant interactions about 400 million years ago (Taylor, 2000). “All of the
land plants in the Rhynie Chert ecosystem where mycorrhizal” (Taylor, 2000).
Mycorrhizae is a symbiotic relationship forming between fungi and 80% of all land plants
today (Miller-Levine, 1995). Two major types of mycorrhizae are ectomycorrhizae and
arbuscular mycorrhizae. According to the fossil evidence, vesicular arbuscular
mycorrhizae, which is most common today, is the most primitive type of mycorrhizae.
Both groups surround the root of the host cell with their hyphae and extend into the soil.
Plants benefit from the association with fungi because of the increased surface area giving
more water and nutrients to the plants than the plant could receive using its own roots.
Nutrients received by the plant include phosphorus, water, and nitrogen. In return, the fungi
get carbohydrates that are made by the plants. Differences between ectomycorrhizae and
arbuscular mycorrhizae include its host plants, its ability to survive independently of plants,
its attachment to the plants, and its ability to produce toxins to protect the host. The
mycorrhizal relationship is able to form with the dispersal of fungal spores or when the
hyphae divide. Fungal spores land on the ground and as the developing hyphae grow
underground, it will wrap itself around the root tip of a plant.
Ectomycorrhizae is less common than arbuscular mycorrhizae. Ectomycorrhizae
can form a symbiotic relationship with plants and can live independently of plants as a
decomposer. Most of the plants that have an association with ectomycorrhizae are
evergreens. It is classified into the taxonomic groups basidiomycota, with mushrooms, and
ascomycota, with truffles, depending on the species. Some species of ectomycorrhizae are
host dependent, only being able to grow with a specific species of a woody plant, whereas
others can grow on a variety of woody plant stems. Ectomycorrhizae wrap its hyphae
around the root tip of its host and may grow between the cells to provide more nutrients
and get more carbohydrates. Ectomycorrhizae also produce growth regulators, (auxins and
cytokinins) which change the shape of the hosts’ root tip (Trappe, 2006). The root tip
becomes branched and the root hairs are suppressed (Trappe, 2006). Without the
association with ectomycorrhizae, the plant has a single root tip with lots of hairs. The
hairs on the root tip are a disadvantage in comparison to the hyphae of ectomycorrhizae
because they are shorter in length which means that it will not be able to absorb as many
nutrients as ectomycorrhizae. The root hairs also have a short life span. A single root tip
serves as a disadvantage compared to the branched root tip that is induced by
ectomycorrhizae because it covers less surface area. Not only do the hyphae of
ectomycorrhizae serve to aid in a greater amount of nutrient absorption, it protects roots
from disease causing agents by producing antibodies (Trappe, 2006).
Vesicular arbuscular mycorrhizae are most common in terrestrial herbaceous plants,
such as sunflower, basil, rosemary, and sage. They are also common among evergreens
that are not host of ectomycorrhizae. It belongs to the taxonomic group of Zygomycota,
which includes molds. Because vesicular arbuscular mycorrhizae lack color, unlike
ectomycorrhizae, and because it grows within the roots of the host, it is not easily
observable. Fossil evidence from the Rhynie Chert ecosystem suggests that this fungus is
more primitive than others. Some believe that during the time land plants were evolving,
arbuscular mycorrhizae provided them with nutrients that they needed to fill unoccupied
niches. Vesicular arbuscular mycorrhizae are less host specific than ectomycorrhizae. It
also differs from ectomycorrhizae because they don’t produce growth regulators, they don’t
protect the root tips by producing antibiotics and creating barriers, and they don’t change
the root shape of the host (Trappe, 2006). “Instead of suppressing root hair formation, they
enter the rootlet through the root hairs” (Trappe, 2006, par.13)
HYPHAE SURROUNDING THE ROOT OF A PINE TREE
http://www.treehealers.com/roots/mychor.htm
Works Cited
Krogh, David (2005). Biology:A Guide to the Natural World.3rd edition, New Jersey.
Prentice Hall
Miller, R & Levine, Joseph (1995). Biology 3rd edition. New Jersey: Prentice Hall.
Karlene V. Schwartz, “Classification, biological”, in AcessScience@McGraw-Hill,
Http://proxy.libray.upenn.edu:9757, DOI 10.1036/1097-8542.139450, last modified: May
17, 2001.
Thomas N. Taylor, “Plant-fungi interactions (paleobotany)”, in AccessScience@McGrawHill, http://proxy.library.upenn.edu:9757, DOI 10.1036/1097-8542.YB001190, last
modified: December 26, 2000.
James M. Trappe, “Mycorrhizae”, in AccessScience@McGraw-Hill,
http://proxy.library.upenn.edu:9757.DoI10.1036/1097-8542.441900.last modiefied: May
30,2006
Google images. Hyphae surrounding the root of a pine tree
http://www.treehealers.com/roots/mychor.htm
Google images. Hyphae with and without crosswalls.
http://sps.k12.ar.us/massengale/fungi_notes_b1.htm