Symbiosis is the phenomenon of living together where both the partners are benefited. The microsymbionts derive freshly prepared food from the host plant which lack in soil. The macrosymbionts get certain nutrients from microsymbiont which are not readily available such as trace elements, nitrogen, phosphorous, etc. As a result of interaction of microorganisms with plant roots there may or may not develop symbiotic structure. Some of the symbiotic associations are:

Cyanobacterial symbiosis:

The term cyanobacteria includes the member 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 cicadae is associated with the coralloid roots of Cycas. It is present in cortex in a well defined region which is known as algal zone.

Bacteria symbiosis:

Among bacteria there are two categories of symbiosis, one that does not form symbiotic structure i.e. root nodules, and the second group which forms root nodules.

Associative symbiosis:

The first group includes the species of Azospirillum which are intimately associated with their host. As a result root hairs are deformated. 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 due to phytochrome production. The other non-nodule forming associative symbionts are Azatobacter paspali, Beijerinckia etc.

Plant Growth promoting Rhizobacteria:

The bacteria which colonize the rhizosphere of root are commonly known as rhizobacteria. The non-symbiotic beneficial rhizobacteria wich affect the plant growth favourably are called PGPR. PGPR belong to genera of Pseudomonas, Bacillus and Streptomyces, and most of them are fluorescent Pseudomonas. The other types are non-fluorescent Pseudomonas e.g. Serrantia and Arthrobacter. The most common species of bacillus are B. polymyxa, B. circulans, B. macerens.

These bacteria increase the growth of host plants. The increase in plant growth is due to a. changes of rhizosphere and other harmful microorganisms producing an indirect effect on the crop, b. control of pathogens and other harmful microorganisms in the rhizosphere, c. Production of growth hormones like gibberellin and indole acetic acid, d. release of nutrients from soil, e. possible production of vitamins or conversion of materials to a usable form by the host, f. possible nitrogen fixation by rhizobacteria.

Legume- Rhizobium symbiosis:

Rhizobium, a soil bacterium, enters in symbiosis with leguminous plants. It develops root nodules which are the site of N2 fixation.

Actinomycete-Non-legume Symbiosis:

From this class the species of Frankia are known to develop nodules which are known as Actinorrhiza. Nitrogen fixing nodulatde non-legumes are: the species of Alnus, Casuarina, Comptonia, Hippophae, Discaria, Dryas, Elaeagnus, Myrica, Purshia, Shepherdia etc. These plants grow in such a condition where the concenteration of nitrogen is low.

Fungal Symbiosis:

Mycorrhiza (fungus root) has been defined as a structure developed as a result of symbiotic association between fungi and plant roots. Mycorrhizal associations are diverse in both structure and physiological function. Mycorrhizal fungi can be grouped in to ecological category of root-inhabiting fungi.

Mycorrhizae can be divided in to ectotrophic and endotrophic, on the basis of trophic levels. On the basis of strictly morphological and anatomical features, mycorrhizae are divided in to three broad groups: ectomycorrhiza, endomycorrhiza and ectendomycorrhiza .

Endomycorrhizae can be divided into five distinct types: a. vesicular- arbuscular (VA) mycorrhiza, b. arbutoid mycorrhiza, c. monotropoid mycorrhiza, d. ericoid mycorrhiza, and e. orchid mycorrhiza.

Micorrhizosphere (the close vicinity of ectomycorrhizae) shows increased microbial community leading to mycorrhizosphere effect. Mycorrhizae increase the absorptive surface of root resulting in increased uptake of water and nutrients from the soil. The products of photosynthesis move from host to the fungal symbiont. Some of the ectomycorrhizal fungi produce indole acetic acid which is involved in morphogenesis and longevity of roots.

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