rhizobia

Rhizobia are a group of soil bacteria capable of fixing atmospheric nitrogen in a symbiotic relationship with leguminous plants. In compatible hosts, rhizobia colonize the root system and form specialized structures called nodules, where nitrogen becomes available to the plant as ammonia, enriching soil nitrogen content.

Symbiosis begins when the plant exudes flavonoids that induce rhizobial nodulation (nod) genes. In response, rhizobia produce Nod factors that trigger root curling and infection thread formation, leading to bacterial entry and the development of nodules. Inside nodules, rhizobia differentiate into bacteroids and express nitrogenase, an oxygen-sensitive enzyme that reduces N2 to ammonia. The plant supplies carbon and maintains a low-oxygen environment through leghemoglobin to protect nitrogenase.

Rhizobia encompass several genera, including Rhizobium, Bradyrhizobium, Sinorhizobium (Ensifer), Mesorhizobium, and Azorhizobium. Each genus has varying host ranges, with some strains nodulating a broad spectrum of legumes and others exhibiting tighter specificity. Nodulation and nitrogen fixation are regulated by plant factors and bacterial genes (nod, nif, and fix) and can be affected by soil chemistry and oxygen availability.

Agriculturally, rhizobia are exploited in inoculants to improve legume yields and reduce synthetic nitrogen fertilizer use. Successful inoculation depends on compatible strains, soil pH, moisture, nutrient status, and absence of competing native strains. High nitrate levels and certain soil conditions can suppress nodulation and N2 fixation.

Ecologically, rhizobia contribute to the global nitrogen cycle and soil fertility. They are found in the rhizosphere and root nodules, often forming complex interactions with plant roots. The study of rhizobia has advanced understanding of plant-microbe symbioses and sustainable farming practices.