Why did parasitic plants evolve?



The evolution of fungi depends largely on the further development and spread of green plants. The majority of the fungus species live saprophytic, a few parasitic. These are, at least in certain phases of their life cycle, dependent on a range of active ingredients (e.g. a range of vitamins) that can only be provided in this form by living cells. It is striking that plant diseases are relatively rare in nature. The reasons for this are effective defense mechanisms that prevent the spread of parasitic fungi. First and foremost, the solid cell wall, including all deposits and deposits (e.g. cuticle), should be mentioned, which prevents the penetration of fungi, bacteria, viruses, etc. into the tissue and the cell lumina. On the other hand, reference should be made to the wide range of secondary plant substances, many of which are fungicidal and / or bactericidal. Such substances are often only produced after induction, i.e. only after infection (e.g. the phytoalexins).

It is also noteworthy that plant parasites (this applies equally to fungi, bacteria and viruses) are strictly host-specific. Some of them are dependent on a host change, as a result of which one stage of development of the parasite takes place in one, the next in another (not phylogenetically closely related to the first). Because of their high host specificity, parasites can cause far more damage in monocultures (agriculture) than in species-rich plant communities. The annual loss of food is equivalent to enough to supply 300 million people.

Parasitic fungi have at least three strategies to get hold of herbal ingredients:

  1. They produce cell wall and cuticle degrading enzymes and.

  2. They produce toxins that reduce the activity of host cells, possibly even completely inhibit them.

  3. They produce the plant's own substances, e.g. hormones, and thus intervene in the hormonal balance in the plant cell, which results in disruptions in the growth and differentiation process of the cells and tissues. An example: Gibberella fujikuroi secretes gibberellins which influence the growth of the host plants (here rice). The investigations carried out at the time led to the discovery of the phytohormone class of gibberellins.

There is a very extensive literature on parasitic fungi. Much of the work was created out of economic considerations. The majority deal with the classification of fungi, their life cycles, the symptoms of diseases in plants and their diagnosis, the host spectrum, and the search for resistance factors of the hosts. On the other hand, the molecular mechanism of action of the reaction of plants after fungal infection has so far only been clarified using a few examples and because of the variety of possibilities these cases can only be generalized to a limited extent.

The resistance of the hosts is based - apart from the general (unspecific) defense mechanisms outlined at the beginning - on the formation of specific (directed against certain fungi), genetically determined products. Genetic analyzes showed that the resistance is based on the one hand on the presence of dominant alleles of the corresponding genes and on the other hand there are resistance genes that can be inherited independently of one another.


A distinction is made between two forms of parasitism:

Necrotrophic parasitism: The infection leads to tissue destruction and thus to the death of the plant. The fungi are usually only facultatively parasitic; they can just as well reproduce saprophytically in dead or dying plant material.

Biotrophic parasitism: Here, the parasite and host live together, at least for longer periods of time. The parasite takes nutrients and growth substances from the host, but does not kill it. Most biotrophic fungi are obligate parasites. They can only survive a saprophytic phase to a limited extent; especially the formation of the fruiting body is linked to the presence of the host. Only in exceptional cases was it possible to cultivate individual (vegetative) stages of these fungi in cell-free nutrient media.

Parasitic fungi are found in all classes of fungi, hosts in all systematic groups of plants (and blue-green algae). The realm of the mushrooms is divided into the Myxomycota or slime molds and the Eumycota, the actual mushrooms.

One of the most economically important representatives of the parasitic is Plasmodiophora brassicae, the causative agent of carbonic hernia, a disease whose symptoms are manifested in the root area of ​​numerous species of crucifera. The changed differentiation pattern of the infected host tissue is based, among other things, on an increase in the auxin content of 50-100 times, an increase in cytokines by 10-100 times and (as a result?) An increase in the degree of ploidy in the nuclei of those tissue cells (IC TOMMERUP and DS INGRAM, 1971).

Spongospora subterraneae the causative agent of the powdered scab of potatoes, can serve as a vector for the potato mop-top virus.

Myxostelida - Spores (resting stages), releasing myxamoeba - Trichiales - young sporangia of Trichia decipiens - Trichiales - Plasmodiocarp (polynuclear Plasmodium) of Hemitrichia serpula
© Bryce KENDRICK

Phytophthora-Species are dependent on Solanaceae as host plants. Phytophthora infestans is the causative agent of late blight in potatoes. An early destruction of the foliage, and the resulting reduction in the rate of photosynthesis, leads to serious harvest losses. The host spectrum is unlike that of some others Phytophthora-Species extremely narrow (potato, tomato and a few others). The fungus was the cause of a great famine in Ireland in the middle of the last century (1845/47); the consequence of this was a strong wave of emigration to the USA. Phytophthora cambivora is for a z. Currently responsible for the extinction of alder in Germany (root rot of the black alder).

There are four types of asci that look different, with two basic types: unitunicat and bitunicat. The figure shows the bitunicate type.
© Bryce KENDRICK

The ascomycetes include numerous species that cause leaf curl disease in various hosts, including Taphrina insititae (Host: plums and others), T. betulina (Birch), T. cerasi (Cherry), T. deformans (Peach). Most of these species form haustoria (exception T. deformans), i.e. outgrowths into the interior of the cell lumen. The most important prerequisite for the formation of haustoria is local perforation of the plant cell wall. After penetrating the cell lumen, the haustorium expands into a bubble-shaped structure; the plasmalemma of the host cell is not penetrated. Therefore, haustoria do not grow into the plasma of the host cells. However, as electron microscopic examinations on various house oriente types have shown, the plasma lemma changes structurally, it lays in folds, and a lot of electron-dense material is stored in the membrane. These observations indicate an active defense reaction of the cells. Leaf curl is based on increased but uneven growth of individual leaf zones. Various studies indicate that the content of IES and cytokinins in the leaves is increased.

To the Taphrina-Species also include those that cause the so-called witch's broom on numerous host trees. This phenomenon is due to an increase in the number of vegetation points, which reveals an irregular, tufted branching pattern on the affected branches.

The order Erysiphales includes the family Erysiphaceae (powdery mildew, or powdery mildew). Powdery mildew is a collective name for a large number of Erysiphacean species that parasitize on numerous angiosperms: Erysiphe graminis (Powdery mildew on grass), E.communis (on pumpkin), E. polygoni (on peas, clover and legumes). The fungal mycelium usually spreads on the upper side of the leaf (or the underside), only a few haustoria are sunk into the epidermal cells.

   
Erysiphales - Scanning electron microscope image of conidia of Erysiphe on the leaf of a host plant (picture left) - dichotomously branched acrosomal processes of microsphaera - asci from microsphaera, with dichotomously branched processes
© Bryce KENDRICK

Of Erysiphe graminis numerous subspecies (races, varieties?) specialize in cereals, some for example grow on wheat but not on barley; the reverse is true for others.

Ceratocystis ulmi and related species are responsible for Dutch elm disease. The disease was discovered in England in 1927, between 1930 and 1940 around 10 percent of all elms were infected there. In the dry summer of 1947, it spread epidemically over Germany.

Botrytis cinerea (Gray mold) is a slightly specialized parasite; In damp weather it attacks lettuce leaves as well as a large number of juicy fruits (tomatoes, strawberries, etc.).

Basidium of a Basidiomycete, Gastrocybe, with symmetrical arrangement of the basidiospores.
© Bryce KENDRICK

The Basidiomycetes contain two important orders of parasitic fungi: Ustilaginales (smut fungi) and Uredinales (rust fungi). The order Agaricales mainly includes the mycorrhizal fungi already discussed, only a few species are parasitic (e.g. the Hallimasch). The Ustilaginales are parasites with over 1000 species on hosts from over 75 families of angiosperms. They produce dark, powder-like traces on leaves, sprouts, flowers and fruits, which consist of a string of their fruiting bodies. Usually no haustoria are formed. An extensive mycelial system spreads in the intercellular spaces of the host plants. The Uredinales (approx. 4000 species in 100 genera) are characterized by a reddish-brown color (rust!) Of their spores. They form haustoria and affect a wide variety of angiosperms, gymnosperms, and pteridophytes. Puccinia graminis is the classic example of a mushroom with host change. Haploid (monokaryotic) mycelium grows up Berberis (Barberry), dikaryotic on various grasses. The species is characterized by a complex alternation of generations, in the course of which up to five different spore forms arise. Stands Berberis not available, may be Puccinia graminis can be kept on grass indefinitely in the dikaryotic phase.

UREDINALES: Teleutospore camp. The stalked two-cell teleutospores arise after a change of host (e.g. in Puccinia) in late summer on cereal plants and overwinter on the ground. The actual, septate basidia develop from these probasidia (photo: W. KASPRIK).


The prerequisite for this, however, are mild winters, because the dikaryotic asexual uredospores ("summer spores") cannot endure long, cold winters. On the other hand, fungal spores can be spread over thousands of kilometers by wind, so that local extermination of the barberry does not provide any permanent protection Puccinia offers. This species also consists of a number of subspecies, one specializing in wheat, another in oats, a third in rye, and so on. Puccinia graminis is one of the few parasitic fungi that can be kept cell-free on agar (with the addition of yeast extract).

      

Uredinales - Uredospores of Puccinia graminis tritici on the epidermis of a wheat leaf (Triticum). - Cross section through a sorus with uredospores of Puccinia graminis tritici in a wheat leaf. - Cross-section through an aecidium of Puccinia graminis tritici.

Dacrymycetales - Basidia, looking like tuning forks from Dacrymyces in different stages of development
© Bryce KENDRICK

This is a collective term for fungal species that do not develop fruiting bodies and that were therefore very difficult to classify. Based on ultrastructural studies, it is now assumed that they (most of them?) Can be assigned to the ascomycetes. This subheading includes some species that have been used in recent years as test objects to study the molecular processes involved in the infection of plants. We will therefore have to deal with them in detail in the next but one section.

Since parasitic water fungi are economically insignificant, they are relatively unknown. There are only a few scientists who have dealt with them intensively.

Over 200 filamentous marine fungi have been described, about a quarter to a third of which parasitize on algae. The majority of the mushrooms belong to the ascomycetes. Brown, red and green algae and diatoms can be considered as hosts. bes at Lagenisma coscinodisca, a diatom parasite, documented with an electron microscope,

The parasites of freshwater algae belong primarily to the chytridia, which were discovered by A. BRAUN (Berlin) in 1856 and which he describes as follows:

"The whole plant consists of a simple bubble-like cell, which often penetrates the cells of the nutrient organism with a root-like extension."

Chytridia do not form hyphae, instead a (non-cellular), sometimes quite extensive rhizoid system is formed, with which the host cells are captured, in some cases also enclosed. A specific parasite could be found for almost every known species of algae. The fungal attack should not be underestimated; In many cases, towards the end of a "water bloom" (the massive occurrence of a species of algae), an infection of almost all of the remaining cell colonies was found.

Chytridia attack blue-green algae (Cyanophyta) as well as Volvocales and Chlorococcales. Since the three groups of algae mentioned differ from one another in the chemical composition of their cell walls, the corresponding parasites must have a wide variety of wall-degrading enzyme systems at their disposal. A cell is only destroyed after cell-cell contact. This means that the fungus does not secrete any lytic agent into the surrounding medium in order to destroy all cells of a colony surrounded by jelly.

Years of population-dynamic studies (in lakes of the southern English Lake District) on diatom and cyanophycean parasites have shown that fungal infestation at the beginning of a population growth can drastically reduce the number of individuals and accelerate the subsequent decline of the population. The reduction in the number of individuals of one species of algae is, however, usually compensated for by an increase in the population size of another species (H. M. CANTER; Freshwater Biological Association, Ambleside). Such quantitative analyzes - and greater attention to the consequences of fungal infestation - are possible explanations for the fact that algalologists often find out that one type of algae is dominant in one year, but does not appear at all or only little in the following year.

The infestation of algae colonies by chytridia is by no means always the sole reason for the disintegration of a water bloom; other reasons, e.g. nutrient deficiency (phosphate deficiency) or weather-related changes, are often decisive, especially when the fungal infestation is limited and only a few cells in a colony (e.g. blue-green algae Micocystis aeruginosa) are destroyed.