Symbiotic associations with different groups of microorganisms are discussed below:
(i) Cyanobacterial Symbiosis:
The term cyanobacteria is of recent origin which includes the members of cyanophyceae. They may be both heterocystous and non-heterocystous forms. Heterocyst is the site of nitrogen fixation. The non-heterocystous forms also fix nitrogen. Anabaena cycadae is associated with the coralloid roots of Cycas. It is present in cortex in a well defined region which is known as algal zone.
(ii) Bacterial Symbiosis:
Among bacteria there are two categories of symbiosis, one that does not form apparent symbiotic structure i.e. root nodules, and the second group which forms root nodules.
However, there is a third group which enhances plant growth without entering in symbiosis:
(a) Associative symbionts:
The first group includes the species of Azospirillum which are intimately associated with their host. These have been isolated from the rhizoplane region. As a result of infection root nodules are not formed but pictures of root hair deformation are known.
Moreover, Azospirillum also invades cortical and vascular tissues of host, and enhances the number of lateral root hairs. This results in an increase in mineral uptake which are probably due to phytochrome production rather than N2 fixation.
Host specificity of Azospirillum differs from that of Rhizobium. Due to intimate association of Azospirillum with roots of several non-leguminous plants, Azospirillum and the other such bacteria are called associative symbiont.
The other non- nodule forming associative symbionts are Azotobacter paspali (found on roots of tropical grasses), Beijerinckia (shows host specificity with sugarcane root), Azospirillum (with roots of com, wheat, sorghum), etc.
Much work has been done on Azospirillum. It is associated with roots in such a way that a gentle washing does not dislodge the nitrogen metabolizing activity. It has been estimated that A. paspali contributes 15-93 kg N/ha/annum on sugarcane root. It saves nitrogen fertilizer equivalent to 20-40 kg/ha.
(b) PGPR (Plant Growth Promoting Rhizobacteria):
The bacteria which colonize the rhizosphere of root are commonly known as rhizobacteria. The non-symbiotic beneficial rhizobacteria which affect the plant growth favourably are called PGPR. The PGPR have been discovered by Kloepper (1980).
PGPR belong to genera of Pseudomonas, Bacillus and Streptomyces, and most of them are fluorescent pseudomonads. The other types are non-fluorescent pseudomonads, e.g. Serratia and Arthobacter. The most common species of Bacillus are B. polymyxa, B. circulans and B. macerens. These bacteria increase the growth of host plants.
The increase in plant growth is due to:
(i) Changes in balance of rhizosphere microflora producing an indirect effect on the crop,
(ii) Control of pathogens and other harmful microorganisms in the rhizosphere,
(iii) Production of growth hormones like gibberellin and indole acetic acid,
(iv) Release of nutrients from soil,
(v) Possible production of vitamins or conversion of materials to a usable form by the host, and
(vi) Possible nitrogen fixation by rhizobacteria.
(c) Legume-Rhizobium Symbiosis:
Rhizobium, a soil bacterium, enters in symbiosis with leguminous plants. It develops root nodules which are the site of Ns fixation.
(iii) Actinomycete-Non-Legume Symbiosis:
From this class the species of Frankia are known to develop nodules which are known as actinorhiza. Nitrogen fixing nodulated non-legumes are: the species of Alnus, Casuarina, Cercocarpus, Comptonia, Hippophae, Discaria, Dryas, Elaeagnus, Myrica, Purshia, Shepherdia, etc.
These plants grow in such a condition where the concentration of nitrogen is low. One of the most extensively studied plants is the alder trees, Alnus nepalensis which grows in nitrogen-deficient soil. The extent of nitrogen gain by such angiosperms vanes with soil types, climatic conditions and plant age. The nitrogen gain with Alnus is 12-200 kg/ha/annum, and with Hippophae 27-179 kg/ha/annum.
(iv) Fungal Symbiosis (Mycorrhiza):
In 1885, it was a German Forest pathologist, A.B. Frank who for the first time coined the term mycorrhiza to denote plant-fungus association. Mycorrhiza (fungus-root) has been defined as an apparent structure developed as a result of symbiotic association between fungi and plant roots.
Mycorrhizal associations are diverse in both structure and physiological function. Garrett (1950) grouped the mycorrhizal fungi into the ecological category of root-inhabiting fungi. This indeed be regarded as end terms in the specialization of root- inhabitants, that is, of an ecological group that includes many important soil-borne plant pathogens.
Frank classified the mycorrhizae into ectotrophic and endotrophic ones on the basis of trophic levels. However, on the basis of strictly morphological and anatomical features, mycorrhizae are divided into the three broad groups: ectomycorrhiza, endomycorrhiza and ectendomycorrhiza which correspond to the older and still commonly used terms ectotrophic, endotrophic and ectendotrophic mycorrhizae. That is literally by outside, inside and outside-inside feeding, respectively.
Harley and Smith (1983) have recognised the endomycorrhizae into five distinct types:
(a) Vesicular-arbascular (VA) mycorrhiza,
(b) Arbutoid mycorrhiza,
(c) Monotropoid mycorrhiza,
(d) Cricoid mycorrhiza, and
(e) Orchid mycorrhiza.
Marks (1991) has recognised the seven forms of mycorrhiza (Table 28.3), the special features of which are briefly discussed as below:
Table 28.3 : Mycorrhizal types, their characters and distribution within the plant kingdom.
(a) Ectomycorrhiza:
Only 5% vascular plants develop ectomycorrhiza which predominates in family Pinaceae, Fagaceae, Betulaceae, Juglandaceae and Myrtaceae and in other tropical and temperate families. Fungi that participate in ectotrophic association include agaric Basidiomycetes, Gasteromycetes, Ascomycetes, fungi imperfecti and occasionally phycomycetes.
Strict host specificity is rare and, therefore, one plant may form mycorrhizae with several fungi simultaneously. Therefore, over 5,000 fungi of Basidiomycetes- Ascomycetes involved in forming ectomycorrhizae on 2000 woody plants. The fungi interact with feeder roots which in turn, undergo morphogenesis.
The mycorrhizae may be unforked, bifurcated, nodular, multiforked or coralloid. Outside the root surface fungal mycelia form a compact and multilayered covering known as mantle (Photoplate 28.1A, B). It prevents the direct contact of root tissues with rhizosphere.
A typical ectomycorrhizal symbiosis has been shown in Figs. 28.1 A-B:
Thickness of mantle varies from 20-40 mm depending on mycorrhizal fungi, temperature, nutritional factors, etc. The fungus forms a network of mycelia in cortex which is known as Hartig net. The mycelia never enter the endodermis.
The fungi forming ectomycorrhiza are Amanita muscaria, Boletus edulis, Cenococcus geophilus, Inocybe rimosa, Laccaria laccata, Leccinum, Lepiota, Russula spp., Pisolithus tinctorius, Suillus spp., Scleroderma citrinum, Rhizopogon spp.,
(b) Ectendomycorrhiza:
Ectendomycorrhiza shares the features of both ecto-and endo-mycorrhiza. They have less developed external mantle. The hyphae within the host penetrate its cells as well as grow within them. These are found in both gymnosperms and angiosperms. Very little is known about the fungi involved in this types of association due to little researches on them.
(c) Vesicular-Arbascular Mycorrhiza (VAM):
Over 90% of vascular plants of world flora form VA mycorrhiza. The mycosymbionts are widespread among both cultivated and wild plants, and found in bryophytes, pteridophytes, gymnosperms and angiosperms. The fungi forming VAM belong to family Endogonaceae of Zygomycotina. Hyphae are aseptate, inter-and intra-cellular in cortex.
The intracellular hyphae either become coiled or differentiated into densely branched arbuscules. Arbuscules function as haustoria and perhaps involved in interchange of materials between plant and fungus.
In addition, large, multinucleate, terminal or intercalary oil-rich vesicles may be produced on both inter-and intra-cellular hyphae. VAM are formed by about hundreds of fungal species. All of them belong to only six genera viz., Acaulospora, Gigaspora, Glomus, Entrophospora, Sclerocystis and Scutellospora. Diagrams of Glomus and Gigaspora are given in Fig. 28.2.
(d) Ericoid Mycorrhiza:
Ericoid mycorrhiza occurs throughout the fine root systems (hair roots) in the tribe Ericoidae of family Ericaceae (except tribe Arbutoidae). Many genera such as Epachris, Leucopogon, Monotoa, Rhododendron, Vaccinum, etc. develop ericoid mycorrhiza.
Plants are woody shrubs or small trees found in open or acid peaty soil. They have usually fine roots on which the fungus established to outermost layer of cortical cells forming dense intracellular cells. The fungi may all be ascomycetes, for example Pezizella, Clavaria spp., etc.
(e) Arbutoid Mycorrhiza:
Mycorrhiza of the tribe Arbutoidae of family Ericaceae was first described from Arbutus unedo. The host plants are mostly woody shrubs and trees. Roots are typically herorhizic (the short roots being converted into mycorrhiza with a well defined sheath and a Hartig net), the fungus penetrates cortical cells where it forms extensive coils of hyphae.
The mycosymbionts are of Basidiomycetes. Many of fungal symbionts which form symbiosis in these plant, also form mycorrhiza with conifers. It has been suggested that a transition between ecto- and endo- mycorrhizae exists in the arbutoid type of mycorrhiza, accounting for the term ectendomycorrhiza sometimes applied to this phenomenon.
(f) Monotropoid Mycorrhiza:
The family Monotropaceae which includes achlorophyllous plants (e.g. Monotropa hypopitys), develops monotropoid mycorrhiza. These plants completely depend on mycorrhizal fungi for carbon and energy. Roots form ball throughout which fungal mycelium ramifies enclosing the mycorrhizal roots of neighbouring green plants.
The root ball is the survival organ of Monotropa during winter and after return of favourable conditions it gives rise to flowering shoots. With the root growth, a sheath and Hartig net are formed.
From the hyphae a peg like haustoria push into epidermal and cortical cells. In the start, host cell wall invaginates to include fungal pegs, but finally pegs penetrate cell wall and emerge into cells. The structure and function of monotropoid mycorrhiza change with seasonal development of the host plants.
(g) Orchid Mycorrhiza:
In nature, orchids germinate only with infected endomycorrhizal fungi that subsequently colonize the host plants. The fungi are mostly the form genus Rhizoctonia with perfect state Ceratobasidium, Sebacina and Tulasnella occurring in Basidiomycetes (mainly Tulasnales) and Ascomycetes.