FIG 1.

The life cycle of N. punctiforme. Vegetative filaments contain actively growing and dividing cells that perform oxygenic photosynthesis and have the capacity to differentiate into alternative cell or filament types. Depletion of nutrients such as phosphate can lead to the formation of spore-like akinetes, while depletion of fixed nitrogen stimulates the formation of heterocysts. The differentiation of hormogonia is constitutive under standard laboratory conditions but is also influenced by various environmental signals. Hormogonia are capable of surface motility and are the infective unit for establishing nitrogen-fixing symbioses with numerous plants.