GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

17. MOLECULAR NITROGEN FIXATION

17.1. NITROGEN FIXATION BY SYMBIOTIC (NODULE) BACTERIA

Bacteria responsible for ROOT nodule formation in legumes (rhizobia) belong to the genus Rhizobium (Sinorhizobium). Based on host Specificity, several species of nodule bacteria are distinguished: Rhizobium leguminosarum, Rhizobium meliloti, Rhizobium trifolii, Rhizobium lupini, and others. Plant infection occurs exclusively through young root hairs (Figure). Bacteria colonize the tip of the root Hair and grow inward toward its base in the form of an infection thread. Subsequently, these Cellulose-sheathed threads penetrate the thin Cell walls of the young epidermal Cells into the root cortex and proliferate, resulting in nodule formation. Bacteria inside the nodules rapidly multiply and differentiate into irregularly shaped cells known as bacteroids. Molecular nitrogen can be fixed solely by nodules containing the pigment leghaemoglobin. This protein is structurally similar to haemoglobin and exhibits a high affinity for oxygen. It is believed that leghaemoglobin facilitates oxygen diffusion through The plant cell to the bacteroid.

The Specificity of the Rhizobium-plant Symbiosis is based on the interaction between legume Lectins and rhizobial Polysaccharides. In this symbiotic relationship, the plant supplies the bacteria with nutrients (sugars) and provides optimal conditions for their activity.

Certain non-leguminous plants also develop nitrogen-fixing root symbioses involving prokaryotes, specifically actinomycetes of the genus Frankia. Notably, cyanobacteria can also serve as nitrogen-fixing partners in symbiosis with higher plants.

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Symbiotic Nitrogen Fixation in legume root nodules:

A — pea root with nodules; Б — cross-section of nodules; В — cross-section of a plant cell filled with Rhizobium bacteria; Г — bacteroids (bacteria acquiring unusual shapes within cells); Д — bacterial invasion through root hair tips and the growth of infection threads