Scientists Estimate That Plants Evolved ______ Years Ago

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.

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.

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

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@McGraw-Hill, 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

Google images. Hyphae with and without crosswalls.