**3. Symbiotic association between plant and cyanobacteria**

 In a symbiotic relationship, both organisms can benefit from each other in various ways. The filamentous cyanobacteria live in symbiosis with a wide range of eukaryotic hosts, including plants and fungi [ 13 ]. The cyanobacteria that will form symbiotic relationships with the plants are called cyanobionts, which can grow inside the host or more or less firmly attach themselves to the host [ 14 ]. The plants provide cyanobacteria with carbon sources, for example, sucrose. Cyanobacteria have the ability to fix nitrogen from the air in heterocysts, which benefits plants by supplying them with nitrogen ( **Figure 2** ). Therefore, the symbiotic cyanobacteria are mostly heterocyst-forming strains. They are virtually entirely associated with the genera Nostoc and Anabaena [ 15 ]. Cyanobacteria provide plants with about 88% of the fixed nitrogen in the form of NH 3 and keep only 12% for themselves [ 16 ]. In addition to a symbiotic relationship between cyanobacteria and whole plants, there is also a symbiotic relationship between cyanobacteria and plant tissues. Cyanobacteria were found to have colonized different areas of wheat, where they were abundantly present around the root, in the spaces between root epidermal cells and cortex, and as single cells within the stem or on the surface of leaves [ 17 ]. Due to the symbiotic relationship between Gunnera and Nostoc, the number of heterocysts formed increased by up to 80%. This is evidence that the symbiotic relationship has different effects on the growth and development of cyanobacteria. Leghemoglobin concentration in chickpea root nodules increases as a result of the simultaneous inoculation of the

 **Figure 2.**  *Nitrogen fixation in cyanobacterial heterocyst cell.* 

cyanobacterium *Anabaena laxa* and the rhizobia *Mesorhizobium cicero* [ 18 ]. The assimilation of ammonium in cyanobacterial heterocysts is carried out by the enzyme glutamine synthetase. Heterocysts in the *Nostoc-Anthoceros* symbiosis showed a 3- to 4-fold reduction in Glutamine synthetase activity [ 19 ]. The reduction of abiotic stress and plant protection against diseases are factors that encourage the development of symbioses from a plant's side. Plants benefit from the general improvement of soil conditions.
