**Background:** Nitrogen is one of the most limiting macronutrients for plant growth, being a vital component of nucleic acids, amino acids, and chlorophyll. While plants can directly take up nitrogen from soil via nitrate transporters (e.g., NRT1.3) or ammonia transporters (e.g., AMT2;1), a subset of plants form symbioses with atmospheric N2-fixing bacteria. The legume-rhizobia symbiosis, where rhizobia are housed in root-derived nodules, is the most extensively studied. However, other plants form similar relationships, including actinorhizal plants with Frankia, cycads with cyanobacteria, and five non-legume species within the Parasponia genus. This review explores these alternative nodulation systems and the additional benefits that symbiotic bacteria provide beyond nitrogen fixation.
**Methods:** This is a narrative review synthesising recent discoveries in plant-microbe nitrogen-fixing symbioses. The authors discuss the well-characterised legume-rhizobia system as a foundation, then examine non-legume nodulation (Parasponia andersonii, actinorhizal-Frankia, cycad-cyanobacteria), nod-factor-independent nodulation in legumes, and non-root nodulation (stem nodules, leaf nodules, aerial roots). They also review evidence for defence priming and abiotic stress resistance conferred by symbiotic bacteria, drawing on published experimental studies.
**Key Results:** The Parasponia genus contains the only five non-legume species that form rhizobia-interacting N2-fixing nodules. P. andersonii has 290 putative orthologs of Medicago truncatula genes with enhanced expression in nodules of both species, supporting a single gain-of-nodulation hypothesis. P. andersonii is relatively promiscuous compared to legumes but still requires compatible rhizobial Nod factors and has NF receptors orthologous to those in legumes (e.g., LysM domain receptor kinase LYK3). Some legumes, including Aeschynomene indica and Arachis hypogaea, can form nodules without Nod factors, using effectors secreted through the bacterial type III secretion system. Actinorhizal plants (8 families, 25 genera) form N2-fixing nodules with Frankia bacteria, accounting for an estimated 15–25% of global N2 fixation. Cycads, the oldest extant seed plants (evolved ~300 million years ago), house cyanobacteria in coralloid roots within a cortical cell layer called the cyanobacterial zone. A recent discovery identified Candidatus Celerinatantimonas neptuna in the roots of the seagrass Posidonia oceanica, with up to 98% of fixed nitrogen transferred to the plant. Maize brace roots can accommodate N2-fixing bacteria within mucilage that can provide as much as 82% of plant-required nitrogen. For defence priming, Rhizobium etli protected Phaseolus vulgaris from Pseudomonas syringae, halving bacterial populations in leaf tissue and reducing lesions by 75% in inoculated plants; this priming was passed to the next generation, with approximately three-fold decrease in pathogen abundance and ~80% reduction in lesion size. In Medicago truncatula and Pisum sativum, inoculation with Sinorhizobium meliloti and Rhizobium leguminosarum increased free salicylic acid by approximately 10% and 30%, respectively, when exposed to Erysiphe pisi. For herbivory, Bradyrhizobium japonicum inoculation of Glycine max led to higher jasmonic acid production; Helicoverpa zea larvae preferred plants with lower rhizobial colonisation despite similar total N levels, with higher larval growth rates in high-inoculum plants (0.881 ± 0.059 g/g/day) compared to high soil N (0.677 ± 0.051 g/g/day). For abiotic stress, Mesorhizobium huakuii allowed Astragalus sinicus to produce more arginine linked to drought tolerance. Salt-tolerant rhizobia in Phaseolus vulgaris resulted in shoot dry weight almost 30% higher after 7 days of drought compared to lower salt-tolerant strains. In Vicia faba, rhizobia with higher salt tolerance had higher N2 fixing capacity (32.28 ± 1.47 µmol h−1 Plant−1) compared to lower salt tolerance strains (13.83 ± 1.65 µmol h−1 Plant−1). Sesbania rostrata can form N2-fixing nodules on both roots and stems, with similar nitrogenase activity in both, enabling survival under both non-flooded and flooded conditions.
**Clinical Implications:** This is a plant biology review with no direct clinical implications. The findings have agricultural and environmental significance, particularly for developing sustainable crop production. Understanding diverse nodulation mechanisms could inform efforts to transfer nodulation capacity to non-legume crops, reducing fertiliser use. Rhizobial defence priming could help replace harmful pesticides. Enhanced abiotic stress tolerance through rhizobial symbiosis is relevant for mitigating climate change impacts including drought, flooding, and soil salinity, which reduce cereal production by 10% on average globally and cause major losses in crops such as rice.