In conventional agricultural practices, agrochemicals, including synthetic fertilizers, pesticides, and other soil conditioners optimize crop production and combat insect-pest damage to satisfy the food demands of constantly growing global human populations. Long-term usage of expensive agrichemicals contaminates the soils and destroys biodiversity, deteriorating soil fertility, and microbiome-plant ecosystems. In this context, nanotechnology offers effective and powerful tool against factors that limit the legume production due to the compact size, ease of transport and handling, long shelf life, and high efficiency of nanomaterials. Thus, the application of nanoparticles in agricultural sectors are gaining momentum in developing nano-biosensors, nanoformulations (nanofertilizers/nanopesticides- NPs), and plant nutrient management. Indeed, nanotechnology is set to transform crop production systems, because NPs significantly reduce the environmental release of active ingredients. Unlike conventional fertilizers and pesticides, which often fail to reach their target sites and contribute to environmental contamination, NPs offers a more precise and eco-friendly solution. This review provides a broad view of the complex interactions between nanoparticles and phytomicrobiome-legumes, focusing both on the potential benefits and risks associated with the widespread use of nanoparticles. The emerging field of nanotechnology, especially nanoformulations, offers a green alternative to conventional fertilizers and pesticides, optimizing yields and managing legume diseases.

The effect of crop rotation on soil-borne diseases is a representative case of plant-soil feedback in the sense that plant disease resistance is influenced by soils with different cultivation histories. This study examined the microbial mechanisms inducing the differences in the clubroot (caused by Plasmodiophora brassicae pathogen) damage of Chinese cabbage (Brassica rapa subsp. pekinensis) after the cultivation of different preceding crops. It addresses two key questions in crop rotation: changes in the soil bacterial community induced by the cultivation of different plants and the microbial mechanisms responsible for the disease-suppressive capacity of Chinese cabbage. Twenty preceding crops from different plant families showed significant differences in the disease damage, pathogen density, and bacterial community composition of the host plant. Structural equation modelling revealed that the relative abundance of four key bacterial orders in Chinese cabbage roots can explain 85% and 70% of the total variation in pathogen density and disease damage, respectively. Notably, the relative dominance of Bacillales and Rhizobiales, which have a trade-off relationship, exhibited predominant effects on pathogen density and disease damage. The disease-suppressive soil legacy effects of preceding crops are reflected in compositional changes in key bacterial orders, which are intensified by the bacterial community network.

The application of rhizobia-legume symbioses is a sustainable approach to alleviate water stress and restore damaged areas. In this context, three strains Bradyrhizobium sp. BA2, RDI18 and RDT46 previously isolated from root nodules of Retama dasycarpa grown in the Moroccan High Atlas Mountains, were selected to investigate their prominent drought-tolerance capacity and significant plant growth-promoting (PGP) traits under drought stress. Subsequently, we analyzed the impact of individual or combined inoculations by the three strains on R. dasycarpa responses to three water regimes (40, 70, and 100 field capacity). The three strains tolerate different concentrations of PEG 6000 and possess different PGP activities, including phosphate solubilization, production of siderophore, exopolysaccharides, and auxin, under osmotic stress. The inoculation had a positive impact on plant response under all applied water regimes as it improved shoot and root length biomass, and chlorophyll content. The water stress reduced shoot length and dry weight of all plants. However, the inoculated plants maintained the highest values. The water stress reduced the infectivity of strains BA2 and RDI18, but not strain RDT46, which is not competitive at any water regime. Furthermore, water stress had no effect on the three strains' symbiotic efficiency, whereas it increased considerably the efficiency index of strains BA2 and RDI18. Proline and protein content increased in non-inoculated plants; whereas the inoculation significantly increased the catalase activity in plants under 40 % FC. These results show that the inoculation with appropriate strains such as BA2 and RDI18, enhance plant resilience to drought season.

Both sulfur (S) supply and legume-rhizobium symbiosis can significantly contribute to enhancing the efficiency of phytoremediation of heavy metals (HMs). However, the regulatory mechanism determining the performance of legumes at lead (Pb) exposure have not been elucidated. Here, we cultivated black locust ( Robinia pseudoacacia L.), a leguminous woody pioneer species at three S supply levels ( i.e., deficient, moderate, and high S) with rhizobia inoculation and investigated the interaction of these treatments upon Pb exposure. Our results revealed that the root system of Robinia has a strong Pb accumulation and anti-oxidative capacity that protect the leaves from Pb toxicity. Compared with moderate S supply, high S supply significantly increased Pb accumulation in roots by promoting the synthesis of reduced S compounds ( i.e., thiols, phytochelatin), and also strengthened the antioxidant system in leaves. Weakened defense at deficient S supply was indicated by enhanced oxidative damage. Rhizobia inoculation alleviated the oxidative damage of its Robinia host by immobilizing Pb to reduce its absorption by root cells. Together with enhanced Pb chelation in leaves, these mechanisms strengthen Pb detoxification in the Robinia-rhizobia symbiosis. Our results indicate that appropriate S supply can improve the defense of legume-rhizobia symbiosis against HM toxicity.

Microorganisms associated with plant roots significantly impact the quality and quantity of plant defences. However, the bottom-up effects of soil microbes on the aboveground multitrophic interactions remain largely under studied. To address this gap, we investigated the chemicallymediated effects of nitrogen-fixing rhizobia on legume-herbivore-parasitoid multitrophic interactions. To address this, we initially examined the cascading effects of the rhizobia bean association on herbivore caterpillars, their parasitoids, and subsequently investigated how rhizobia influence on plant volatiles and extrafloral nectar. Our goal was to understand how these plantmediated effects can affect parasitoids. Lima bean plants (Phaseoulus lunatus) inoculated with rhizobia exhibited better growth, and the number of root nodules positively correlated with defensive cyanogenic compounds. Despite increase of these chemical defences, Spodoptera latifascia caterpillars preferred to feed and grew faster on rhizobia-inoculated plants. Moreover, the emission of plant volatiles after leaf damage showed distinct patterns between inoculation treatments, with inoculated plants producing more sesquiterpenes and benzyl nitrile than noninoculated plants. Despite these differences, Euplectrus platyhypenae parasitoid wasps were similarly attracted to rhizobia- or no rhizobia-treated plants. Yet, the oviposition and offspring development of E. platyhypenae was better on caterpillars fed with rhizobia-inoculated plants. We additionally show that rhizobia-inoculated common bean plants (Phaseolus vulgaris) produced more extrafloral nectar, with higher hydrocarbon concentration, than non-inoculated plants. Consequently, parasitoids performed better when fed with extrafloral nectar from rhizobiainoculated plants. While the overall effects of bean-rhizobia symbiosis on caterpillars were positive, rhizobia also indirectly benefited parasitoids through the caterpillar host, and directly through the improved production of high quality extrafloral nectar. This study underscores the importance of exploring diverse facets and chemical mechanisms that influence the dynamics between herbivores and predators. This knowledge is crucial for gaining a comprehensive understanding of the ecological implications of rhizobia symbiosis on these interactions.