Hormogonia are small, motile filaments formed by some cyanobacteria in the family Nostoceae.
Cyanobacteria differentiate into hormogonia when exposed to an environmental stress or when placed in new media.
Hormogonium differentiation is crucial for the development of nitrogen-fixing plant cyanobacteria symbioses, in particular that between cyanobacteria of the genus Nostoc and their hosts. In response to a hormogonium-inducing factor (HIF) secreted by plant hosts, cyanobacterial symbionts differentiate into hormogonia and then dedifferentiate back into vegetative cells after about 96 hours. Hopefully, they have managed to reach the plant host by this time. The bacteria then differentiate specialized nitrogen-fixing cells called heterocysts and enter into a working symbiosis with the plant.
Motile hormogonium filaments function as propagules for colonization of new habitats and are the infective units of cyanobacterial symbiotic associations.
Hormogonium and heterocyst differentiation are mutually exclusive developmental processes; hormogonia also form as a consequence of global differentiation, while heterocysts reflect patterned differentiation.
Hormogonium differentiation is mutually exclusive to heterocyst differentiation and plants appear to restrict hormogonium differentiation in the mature symbiotic association.
The vegetative cells are 5 to 6 m m in diameter, hormogonium cells 1.5 to 2 m m, heterocysts 6 to 10 m m and akinetes 10 to 20 m m.
The strain was isolated from symbiotic association with the gymnosperm cycad Macrozamia sp.
During the establishment of this endosymbiosis fungal hyphae incorporate a specific stage of the N. punctiforme life cycle, formed during the differentiation from the motile hormogonium to the immotile primordium stage.
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