Hormogones

Hormogones are short, motile filaments produced by certain cyanobacteria as dispersal propagules. They detach from a longer vegetative trichome and move across surfaces by gliding motility, using extracellular polysaccharides and surface structures rather than flagella. Typically composed of only a few cells, hormogones are specialized for spreading to new substrates and initiating new colonies.

The primary function of hormogones is dispersal. After detach­ment, they migrate to nearby or distant sites where conditions may support growth, photo­synthesis, and colony formation. Upon locating a suitable niche, hormogones can regenerate into a vegetative trichome that resumes normal growth and photosynthetic activity. In some lineages, especially Nostoc, hormogonia can give rise to longer filaments capable of forming heterocysts for nitrogen fixation when appropriate conditions apply. In many symbiotic contexts, hormogonia are the form that colonizes plant hosts, such as the roots of cycads, enabling the establishment of endosymbiotic relationships.

Formation and triggers for hormogone development vary with species but commonly include environmental cues such as nutrient limitation, changes in light, temperature, or moisture, and signals from neighboring colonies or potential hosts. Fragmentation of a filament can also generate hormogones, providing a rapid means to propagate and colonize new areas.

Ecologically, hormogones contribute to the spread of cyanobacteria in soils, freshwater, and marine ecosystems. They play a key role in biocrust formation and in establishing and maintaining symbiotic relationships with plants and lichens, thereby supporting nitrogen cycling and primary production in diverse habitats.