| Objectives for Today's Class | Plant Diseases caused by Protists and chromists | |
|---|---|---|
| Reference: Agrios Chapter 11 | Mastery Topics |
The eukaryotic kingdom Protista is not a monophyletic group; it contains organisms which are more closely related to members of other kingdoms than to those within this kingdom. More often organisms are placed in the protista because they lack characters (i.e. no complex development from embryos, little or no cell differentiation). This is not an ideal basis for placement and will undoubtedly lead to movement in, and out, of the kingdom as more information is obtained about any given organism. Therefore, by default, Protista is the "catch-all" kingdom. If an organism is a eukaryote but not a plant, animal, or fungi then, by default, it is a Protist. Consequently the lower taxon differentiations, while they may constitute bona fide groupings in and of themselves, may not be necessarily indicate any relationship among the groups of the kingdom.
The ribosomes are 80S characteristic of eukaryotic organisms. All other sub-cellular
organelles are typical of eukaryotes including; nuclear membranes, mitochondria, vacuoles, etc.
The Plant Pathogenically important Oomycetous Chromista are without pigmentation; achromic chromista.
The cell wall composition is a diagnostic and is a major character that places
the oomycetes in a different kingdom from the true fungi. Oomycetous
cell walls are composed of complex carbohydrates but do not contain chitin.
The thallus, vegetative part, of a filamentous fungus is called a Mycelium (plural
mycelia).
An individual strand of a mycelium is called a Hypha (plural hyphae).
Fungal growth
takes place at the apex of a hyphae. Spitzenkorpers are vacuolar bodies
formed within the the hyphae and migrate where they fuse with the plasma membrane;
in a process likened to reverse
pinocytosis. Fusion/addition to the membrane facilitates the growth process.
This process has been elegantly demonstrated in a number of fungi using pulse-chase
labeling experiments which
resulted in hyphae with bands of radioactivity perpendicular to the axis of each
hyphae.
The concept presented above is very simplified and not entirely accepted by
all researchers. The facts appear to be that the walls of the chromistan hypha
are uniform in thickness and not thinner towards the tip. It also appears
that the very tip of the hypha is impervious to enzymes (proteases and lignases)
that would aid in "softening" the tip allowing growth. Additionally, pulse/chase
experiments with radiolabeled precursors has shown that growth occurs in a
growth ring penultimate to the tip, conceptually similar to the region of
elongation of a growing root. This raises questions of the role of the
Spitzenkorpers as membrane bearers but does not eliminate them as
assisting in delivering materials to the growing area. Additionally, they
may play a role in endocytosis and exocytosis.
It is interesting that research on "fungal" growth has been conducted on Chromistan species; therefore, one wonders what may be the differences, or similarities, with the true fungi.
Typically, actively growing hyphae of oomycetes are coenocytic in
that they do not have cross walls that segment hyphae into discrete cellular units. In this condition it is easy to perceive the multinucleate condition of these
organisms. When observing living species by means of a microscope it is
easy to see the flow of cytoplasm, in one direction for a short time and
then the flow reverses. Interestingly enough, the contents of a hypha do not
flow out if the hypha is severed. If fact one can blend hyphae in a "Waring
Blender" and the fragments will grow when placed on a growth medium.
Reproduction
In nature, the swimming of zoospores is not random, though it may appear so
when viewing under a microscope. Zoospores are chemotactic and swim towards
amino acid sources such as those on roots in area of growth and wounds. Once
having arrived at a nutrient source, the zoospores withdraw their flagella and
encyst. The encystment provides protection for the organism because the
free-swimming zoospores is without a protective wall and is very vulnerable
to adverse environmental conditions. The cyst wall is pigment and provide both
immediate protection and a means of surviving an unfavorable environment. Once
the cyst germinates, its hypha is chemotactically attracted a the nutrient
source, whether a root or decaying matter.
Sexual reproduction takes place by the formation of an Oospore. From two adjacent
hyphae an antheridial initial forms on one hyphae and the oogonial intial from
the other hyphae grows through the antheridium, in Phytophthora sp..
In Phytophthora, some species (e.g. P. infestans) mating types occur while
others do not require mating types. In Pythium sp. the antheridium
and oogonium are borne on the same sporophoric hyphae and the antheridium
attached to the side of the oogonium, at or above the hemisphere.After fertilization, the thin walled oogonium develops into a thickened and pigmented oospore. Oospore germination maybe direct to hypha or to a sporangium.
Structures of Fungi courtesy: Dr. Donald C. Erwin
; University of California - Riverside
This page is authored and maintained by:
Dr. J.E. Partridge, Department of Plant Pathology, University of Nebraska-Lincoln
Mail
Home Page
References