Diazotrophs

Diazotrophs are a group of prokaryotic organisms with the ability to fix atmospheric nitrogen into ammonium or ammonia, forms usable to plants. There are two types of terrestrial diazotrophs: those free living in the soil, and those that form symbiotic relationships with plants.

Nitrogen makes up the majority of the Earth’s atmosphere, but is mostly found in a form unusable to organisms. Atmospheric nitrogen (N2) is found as two nitrogen atoms held together by a triple bond. In this form, nitrogen is inaccessible. Diazotrophs have the ability to split, or “fix” these bonds, freeing the nitrogen molecules.

Symbiotic Diazotrophs

The enzymes that are needed to fix nitrogen are easily damaged by oxygen, so some diazotophs form symbiotic relationships with plants. In this relationship, diazaotophs are protection from oxygen's damaging properties, while also being supplied carbon, and energy. In exchange for this, plants are able to benefit from the nitrogen.

Nod Factor

Bacteria from the genus Rhizobium form a relationship with leguminous plants. Parasponia is the only non-legume genus that can form a symbiotic relationship with these bacteria. They do this by the nodulation, or nod, factor - a signal of amino acids that alert the plant. When the bacteria sends out this signal, the plant responds with root hairs curling around the bacteria. As the infection threads from the bacteria begin to grow into the plants, the plant begins nodule formation. The bacteria begin to form intracellular infection threads, which contain reproducing rhizobia, and become surrounded by a membrane from plant material. Symbiosome is the name for this compartmentalization of bacteria within the plant, and it is the symbiosome that acts as the pathway between the plant and the bacteria, allowing for nutrient exchange.

References

[1] Streng, Arend, et al. “Evolutionary Origin of Rhizobium Nod Factor Signaling.” Plant Signaling & Behavior, vol. 6, no. 10, 2011, pp. 1510–1514., doi:10.4161/psb.6.10.17444.