For over a century, the scientific community has had a comprehensive understanding of how rhizobia can promote the growth of legumes by forming nitrogen fixing nodules. Despite this knowledge, the interaction of rhizobia with non-legumes has remained largely ignored as a subject of study until more recent decades. In the last few years, research has shown that rhizobia can also associate with non-legume roots, which ultimately leads to the stimulation of growth through diverse direct and indirect mechanisms. For example, rhizobia can enhance growth through phytohormones production, the improvement of plant nutrient uptake, such as the solubilization of precipitated phosphorus, the production of siderophores to address iron needs, and also the reduction of ethylene levels through the ACC deaminase enzyme to cope with drought stress. Additionally, rhizobia can improve, indirectly, non-legume growth through biocontrol of pathogens and the induction of systemic resistance in the host plant. It can also increase root adherence to soil by releasing exopolysaccharides, which regulate water and soil nutrient movement. The objective of this review is to assess and analyze the existing knowledge and information regarding the mechanisms through which rhizobia promote the growth of non-legumes. By conducting a comprehensive analysis of these findings, we aim to gain new insights into the development of Rhizobium/non-legume interactions.

Plant growth-promoting rhizobia are known to improve crop performance by multiple mechanisms. However, the interaction between host plants and Rhizobium strains is highly influenced by growing conditions, e.g., heat, cold, drought, soil salinity, nutrient scarcity, etc. The present study was undertaken to assess the use of Rhizobium as plant growth promoters under abiotic stress conditions. Fifteen Rhizobium strains isolated from lentil root nodules were tested for phosphate solubilization activity (PSA) and phytohormones production under salt and drought conditions. The results showed that 15 Rhizobium strains were significant phosphate solubilizers, and indole acedic acid (IAA) and gibberellic acid (GA3) producers based on least significant difference (LSD) analysis (p ≤ 0.05). The highest rate of PSA was attributed to three strains namely, 1145N5, 1159N11, and 1159N32 with a range of 144.6 to 205.6 P₂O₅ (µg/mL). The highest IAA production was recorded in the strain 686N5 with 57.68 ± 4.25 µg/mL as compared to 50.8667 ± 1.41 µg/mL and 37.32 ± 12.59 µg/mL for Rhizobium tropici CIAT 899 and Azospirillum brasilense DSM-1690, respectively. Strain 318N2111 produced 329.24 ± 7.84 µg/mL of GA3 as against 259.84 ± 25.55 µg/mL for A. brasilense DSM-1690. R. tropici CIAT 899 showed tolerance to salt (5% NaCl) and drought (ψ = −2.6 MPa) stress, whereas strain 686N5 showed an extremely high level of salt-tolerance (5% NaCl) and moderate level of drought tolerance (ψ = −0.75 MPa). These results indicate different pathways for drought and salt tolerance mechanisms. The assessment of plant growth promoting (PGP) activities of Rhizobium showed differences between bacterial viability and bacterial PGP activity in terms of abiotic stress tolerance where bacterial PGP activity is interrupted before reaching the bacterial tolerance threshold. These results integrate a new concept of PGPR screening based on PGP activity under abiotic stress.

This study investigated the impact of soil properties under greenhouse conditions on the growth and productivity of two chickpea (Cicer arietinum) genotypes (V1 and V2) using two distinct soils collected from Marchouch and Beni Mellal sites. Soil analysis revealed significant differences in organic matter, phosphorus, potassium, and nitrogen levels between the two sites. Marchouch soil, characterized by higher nutrient content, especially phosphorus, demonstrated a more favorable environment for chickpea growth, resulting in enhanced plant height, leaf number, chlorophyll content, seed number, and seed weight. Variety V2 showed slightly better performance than V1 across both soil types, particularly in terms of seed yield and mineral content. This research highlights the importance of soil nutrient availability. Furthermore, this study emphasizes the important role of phosphorus in chickpea growth, with Marchouch soil having a higher phosphorus level (62.9 mg kg⁻¹), significantly boosting plant development and yield. Although soil mineral characteristics and genotypes had little effect on most minerals, zinc (19.77 mg uL⁻¹) and iron (69.43 mg uL⁻¹) levels stood out as significant exceptions. Therefore, further studies should focus on examining additional soil characteristics and expanding genotype selection. Based on the findings, Marchouch soil appears to be more favorable for chickpea cultivation. However, more research is needed on the effect of soil and genotypes on Rhizobium activity.