Tuatara: Volume 8, Issue 2, May 1960
The compound word, mycorrhiza, was coined in 1885 by a German botanist to describe a compound organ which he had found prevalent in forest trees. As the name implies, the organ is a combination of fungus and root and in many plants takes the place of roots. Several types of mycorrhizas exist differing in the morphology of the organ and of the fungus.
Ectotrophic mycorrhizas are obvious as such to the naked eye. Fungal mycelium becomes woven round the root at an early age like a glove round a hand and extension of the root occurs inside the weft of mycelium, or mantle as it is called, which grows to accommodate it. Growth of the root is reduced in length but its branches become more numerous and are stouter than those of uninfected roots (Pl. 2). Part of the additional thickness is due to the fungus mantle and part is due to the greater radial length of the epidermal cells. Thus a non-mycorrhizal root of Nothofagus menziesii has a radius of approximately 160μ while a mycorrhiza on the same species can have a radius of 22μ including a mantle 30μ thick (Pl 1). The hyphae intrude between the epidermal cells and surround each cell with a network usually only one hypha in thickness - this is the Hartig-net, named after its discoverer. Typically the fungus does not penetrate further than the epidermis and only occasionally enters the cells themselves. In some, however, penetration is frequent and these are termed ectendotrophic mycorrhizas.
Ectotrophic mycorrhizas are formed by basidiomycetous fungi, Polypores (e.g. Boletus), Agarics (e.g. Amanita) and puff-balls all being involved. The frequent occurrence of fruiting bodies of these fungi around the base of beeches, oaks, poplars and pines is due to the concentration of the mycelium around the roots of these plants. Not many other plants form ectotrophic mycorrhizas - willows and eucalypts have them occasionally.
Some 90% of the remaining plants from the Thallophyta to the Spermatophyta carry endotrophic mycorrhizas, some with great consistency, others much more casually. Obviously some sort of grouping it required and indeed is easily arrived at.
(a)Orchid mycorrhizas. Although orchids vary from those that grow on soil to those that grow epiphytically on trees, from those with chlorophyll to those lacking chlorophyll at all stages of growth, from those with roots page 78 to those lacking roots - nevertheless all orchids are naturally infected with mycorrhizal fungi in that organ which is in contact with the substratum.
The fungus which infects orchids is placed in the form genus Rhizoctonia and much has been written on the relationship between the two organisms. In all cases the fungus invades the cortical cells of the organ, all hyphae being within the cells, but is kept in check by some form of defence within the orchid. Thus hyphae may become enclosed in cellulose, fungicide may be excreted by the cells, or cells immediately round the infection may die and thus restrict the spread of the endophyte. Typically the hyphae are digested by the host cells and obviously represent a gain to the host.
The morphology of the fungus is best described by taking a specific example. Earina autumnalis is a native epiphytic orchid, the roots of which are invariably infected by Rhizoctonia (Pl. 1). The fungus enters the velamen and ramifies through it before entering a transfusion cell in the exodermis on its way to the cortex. Once in the cortex the hyphae form coils, or pelotons, within the cell— a characteristic feature of orchid mycorrhizas. As the hyphae age moniliform filaments arise which are typical of the fungus Rhizoctonia (Pl. 1). In many of the cortical cells the hyphae are obviously disintegrating as a result of intra-cellular digestion.
(b)Ericaceous mycorrhizas: Described first in 1885 the mycorrhizas of the Ericaceae and Epacridaceae have never ceased to be a fruitful subject of investigation. Almost without exception the members of these two families possess very fine roots which are converted to mycorrhizas by invasion of the outermost layer of cells by a tangled mass of fine hyphae (Pl. 2). At first the hyphae are too fine for cross walls to be seen, if indeed they do exist at this time, but as the hyphae age they become thicker and septa are then apparent. As is common to all mycorrhizas the hyphae do not penetrate the root apical meristem but infection begins just behind this region. In the older parts hyphae undergo degeneration and the contents are released into the host cells.
Top— Left: Inflated hyphal segments in the cortical cells of Earina autumnalis (X 320). Right: T.S. velamen and outer cortex of Earina autumnalis. Note hyphal disintegration in the cortex (X 100). Centre— Left: T.S. mycorrhiza of Nothofagus menziesii. Right: T.S. non-mycorrhizal root of Nothofagus menziesii (both X 250), (m, mantle; ep, epidermis; c, cortex; en, endodermis). Bottom— Left: Arbuscule in cortex of Tmesipteris tannensis (X 230). Right: Intercalary vesicle in root of Clematis indivisa (X 280).
(c)Vesicular-arbuscular mycorrhizas. In a survey of the roots of plants growing in New Zealand it has been concluded that about 90% carry endotrophic mycorrhizas of this general type. Some have all their roots always infected, e.g. Griselinia (Pl 2), others have a sparser rather less constant infection, e.g. Coprosma, while still others are only occasionally infected and then very sparsely. Fungal infections of this general type have also been described in Ginkgo, ferns (mycorrhizomes), Psilotum and Tmesipteris, Lycopodium prothalli (mycothalli) and liverworts.
Hyphae of the fungal component of these mycorrhizas ramify through the cortex but as in all mycorrhizas they do not enter the stele. Within the cells they may form a coil or they may run straight through, but characteristically they anastomose to form a closed network. The characteristic structures of the fungus are the vesicles and arbuscules. Vesicles are spherical but may become irregularly shaped due to confinement in the relatively small space of a cell. They usually develop terminally (Pl. 2) on the hyphae, but in some cases a swelling occurs along a hyphae (e.g. Clematis, Pl. 1). They are not cut off by a septum. The contents of the vesicle become rounded off into spore-like bodies (Pl 2) which, however, soon disappear along with the wall. Occasionally when a hypha enters a cell it forms a profusely branched tree-like organ, an arbuscule, the small ultimate branches of which disintegrate, followed by disintegration of the walls of the whole structure (Pl. 1). At this stage the hyphal protoplasm is set free in the cell but it soon disappears and the cell protoplasm remains unaffected by the whole process.
The endophyte in these mycorrhizas has been placed in the Phycomycete genus Rhizophagus.
Mycorrhizas and Root Hairs
Top— Griselinia littoralis seedlings grown in an infertile soil. Mycorrhizal seedlings on left (3), non-mycorrhizal seedlings on right (6). Centre— Left: Pinus radiata seedlings grown in an infertile soil. Left, non-mycorrhizal; right, mycorrhizal. (X 1/6.) Right: T.S. root of Griselinia littoralis showing zone of Rhizophagus infection (X 45). Bottom— Left: Vesicle in root of Podocarpus hallii (X 1,000). Centre: Typical mycorrhiza of Nothofagus menziesii (X 3). Right: T.S. mycorrhiza of Pernettya macrostigma showing hyphae confined to outer layer of root (X 490).
Physiology of Mycorrhizas
The literature in this field is an object lesson in the danger of making generalisations. There are many mycorrhizas, all fundamentally the same admittedly, but differing in important physiological respects and even on the same plants differing according to the environment it happens to be growing in. It is small wonder then that every possible explanation of the function of mycorrhizas has been offered and that many of these are almost diametrically opposed. We will try to see some order in this chaos.
The physiology of mycorrhizas can be described by answering three questions:
|(a)||Under what conditions are mycorrhizas formed?|
|(b)||What does the fungus gain from the association?|
|(c)||The most important to us— what does the higher plant gain from the association?|
It has been found for ectotrophic mycorrhizas (which have been studied widely in this respect) that the density of infection bears an inverse relationship to the fertility of the medium in which the plant is growing. In the few endotrophic mycorrhizal plants which have been investigated in this regard the same thing holds. At one time this was regarded as sufficient reason for the formation of mycorrhizas, i.e. that mycorrhizas were most abundant where they did most good. But a more rational explanation was wanted. It had been noticed that ectotrophic mycorrhizal infection was reduced when plants were grown in shade. That this was an effect due to the reduced carbohydrate status of the roots was substantiated by ring-barking plants growing in good light, thereby cutting off the supply of carbohydrates from leaf to root and demonstrating that infection was again decreased. The effects of shading on infection could be somewhat counteracted by supplying sugar to the plant or by lowering its supply of nitrogen. The hypothesis was put forward that mycorrhizas are formed if the host roots contain a surplus of soluble carbohydrates. It was further assumed that a deficiency of nitrogen and/or phosphorus in the host plant would retard the formation of amino acids and proteins from carbohydrates and these latter would thus accumulate and stimulate mycorrhizal formation. Manurial experiments have shown quite conclusively that a shortage of either nitrogen or phosphorus enhances mycorrhiza formation whether the mycorrhiza is ectotrophic or endotrophic. This theory, which we owe to a Swedish worker, Bjorkman, in one stroke provides an explanation for the known frequency of occurrence of mycorrhizas and explains what the fungus gains from the association. Somewhat substantiating this theory is the fact that mycorrhizal Basidiomycetes characteristically cannot break down higher carbohydrates such as cellulose and therefore depend on a supply of simple carbohydrates.page 83
At this point I should make it clear that the foregoing does not apply to orchid mycorrhizas. There the boot is precisely on the other foot. It was shown that seeds of all orchids, with but two or three exceptions, would not germinate in the absence of a mycorrhizal fungus. Seeds of orchids are extremely small— up to 3,000,000 being produced in one capsule— and they therefore contain little food reserves. It was further shown that the vast majority of orchid seeds could be made to germinate in the absence of the fungus if soluble sugars were supplied in its place. It is known that Rhizoctonia unlike some other mycorrhizal fungi can excrete cellulytic enzymes and the conclusion seems justified that the fungus supplies soluble carbohydrates to the host in this particular case. It is tempting to ascribe a similar function to the endophyte of the colourless gametophytes of Lycopods, Psilotum and Tmesipteris; indeed it is difficult to perceive what other means these may have of obtaining carbohydrates. In a few orchids seed germination depends upon the provision of accessory substances as well as carbohydrates.
To the question— what does the fungus gain from the association— we have supplied one, if not the only answer. Literature on this point is sparse but again most of the work in the field has been done with ectotrophic mycorrhizas. Obviously until we know the growth requirements of endotrophic mycorrhizal fungi, and this has as a prerequisite the growing of the fungus in pure culture, we are only guessing. This is not the case with ectotrophic mycorrhizal fungi. Many such fungi are heterotrophic for specific vitamins, especially thiamine, and in addition there are growth stimulating compounds obtained from forest litter and forest tree roots which have, however, not yet been identified.
We are probably justified therefore in picturing mycorrhizal fungi as obtaining simple carbohydrates and some accessory compounds from the roots that they infect.
Are Mycorrhiza a Benefit to the Higher Plant?
The case for heterotrophic orchids has already been answered, but what about the remainder? There can be little doubt that under specific conditions mycorrhizas mean the difference between certain death of the plant and normal healthy vigour (Pl 2). These conditions are those of nutrient starvation. In forest soils and in dense turfs, where root competition is fierce, in very infertile soils and in root-bound condition in a pot, mycorrhizas will show their greatest effect. In this respect the availability of phosphorus appears to be all important with nitrogen suspected of being fairly important but the other essential elements of doubtful significance.
Pines, with their relatively high growth rate, are good experimental plants and most of the work concerned with mineral nutrient uptake by mycorrhizas has been done with these plants. Thus many experiments have compared by chemical analysis the mineral content of mycorrhizal and non-mycorrhizal plants after a period of growth in infertile soils. These page 84 have consistently shown that mycorrhizal plants under these conditions contain at least double the amount of phosphorus and slightly more nitrogen than non-mycorrhizal plants per unit of dry weight. Since all the absorbing roots in an ectotrophic mycorrhiza are clothed in fungal hyphae all nutrients entering the plant must pass through or between the hyphae. It is therefore not surprising that these hyphae have been shown to be capable of transferring quantities of phosphorus, nitrogen, calcium, sodium and sulphur (all as radio-active or stable isotopes) from a distant source to the host plant. This does not answer the question of how a mycorrhiza increases the plant's uptake of nutrients, phosphorus in particular.
Uptake of Phosphorus by Mycorrhizas
Although some of the theories of the mechanism of enhanced nutrient uptake due to mycorrhizas are not specifically concerned with phosphorus uptake they must be capable of explaining the outstanding effect of mycorrhizas on absorption of this element.
Early in mycorrhizal literature it was proposed that the effect of mycorrhizas was due to their greater absorption area as compared with non-mycorrhizal roots. It was pointed out earlier that mycorrhizas were of greater diameter and were more branched than non-mycorrhizal roots so that per unit length mycorrhizas would present a much larger surface to the soil than non-mycorrhizal roots. But it was also pointed out that mycorrhizas do not extend as much as non-mycorrhizal roots and it has been calculated that this at least compensates for the increase in other dimensions.
The theory that mycorrhizal plants have a greater transpiration rate than non-mycorrhizal plants has never been popular and lacks confirmatory evidence.
More recently it has been proposed that mycorrhizas excrete more H + ions than non-mycorrhizal roots due to the known greater metabolic rate of the former. It is argued that these ions will release greater quantities of bases from base exchange material in soils and thus increase the uptake of the bases. Similarly it has been shown that bacteria and fungi excrete organic acids into the substrate and these acids act as chelating (solubilising) agents for insoluble phosphorus.
It is claimed that this is the means of increased phosphorus uptake by mycorrhizas over that of non-mycorrhizal roots.
In the last few years radio-active phosphorus has become a major tool in solving this problem and advances have certainly been made with it. Thus we now know that the effect of temperature, metabolic poisons, and the course of phosphorus uptake by the host plant over short periods of time all demonstrate that phosphorus is actively (metabolically) absorbed by the hyphae in mycorrhizas before being passed on to the host.
In addition the fungus acts as a reservoir of stored phosphorus for the host since removal of mycorrhizal plants to a medium entirely lacking in phosphorus does not substantially alter the rate of phosphorus absorption by the host from the fungal hyphae. Non-mycorrhizal plants lack this page 85 reservoir. This evidence indicates that increased phosphorus uptake by mycorrhizas is due to the greater avidity of fungi for phosphorus compared with non-mycorrhizal roots.
Nitrogen Uptake and Mycorrhizas
The growth of some ectotrophic mycorrhizal fungi in pure culture is stimulated by the addition of amino acids which they are able to utilise better than inorganic nitrogen. It has also been shown that hyphae will transfer organic nitrogen compounds from an isolated source to pine seedlings. This has been interpreted as showing that mycorrhizal fungi in forest soils are able to absorb and break down organic nitrogen compounds and provide the host with otherwise unavailable nitrogen since in such soils there is intense competition for the small amount of inorganic nitrogen available there. But the evidence is less strong than that showing enhanced uptake of phosphorus by mycorrhizas.
In orchids we believe that fungal infection is necessary for seed germination by virtue of the carbohydrates supplied. In heterotrophic orchids this relationship probably exists during the whole life of the plant but we do not know what part the fungus plays in the nutrition of adult autotrophic orchids.
In ectotrophic, and in those intensely infected endotrophic mycorrhizas which have been experimentally studied, conditions combining maximum photosynthesis with a mild deficiency in some mineral nutrients induce the greatest infection. In these same plants, the presence of mycorrhizas can overcome a deficiency in phosphorus which would mean death to plants lacking mycorrhizas. Mycorrhizal fungi are also suspected of supplying the host with nitrogen derived from the soil's reservoir of organic nitrogen.
The conditions producing maximum infection or the degree of essentiality of mycorrhizas have not been assessed for plants in which mycorrhizal infection is not intense or constant.
I wish to thank Professor G. T. S. Baylis and Mr. R. F. R. McNabb for permission to use some of their research material and diagrams in this review.
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