Arbuscular mycorrhizal fungi (AMF) establish symbiotic associations with most terrestrial plants. These soil microorganisms enhance the plant’s nutrient uptake by extending the root absorbing area. In return, the symbiont receives plant carbohydrates for the completion of its life cycle. AMF also helps plants to cope with biotic and abiotic stresses such as salinity, drought, extreme temperature, heavy metal, diseases, and pathogens. For abiotic stresses, the mechanisms of adaptation of AMF to these stresses are generally linked to increased hydromineral nutrition, ion selectivity, gene regulation, production of osmolytes, and the synthesis of phytohormones and antioxidants. Regarding the biotic stresses, AMF are involved in pathogen resistance including competition for colonization sites and improvement of the plant’s defense system. Furthermore, AMF have a positive impact on ecosystems. They improve the quality of soil aggregation, drive the structure of plant and bacteria communities, and enhance ecosystem stability. Thus, a plant colonized by AMF will use more of these adaptation mechanisms compared to a plant without mycorrhizae. In this review, we present the contribution of AMF on plant growth and performance in stressed environments.

Salinity is one of the main abiotic stresses limiting plant growth and development. However, the use of salt-tolerant plants combined with beneficial soil microorganisms could improve the effectiveness of biological methods for saline soil recovery. The aim of this study is to identify theCasuarina obesa/ Arbuscular Mycorrhizal fungi (AMF)/Plant Growth Promoting Rhizobacteria (PGPR) association that could be used in salt-land rehabilitation programs. Thus, the plants were grown under greenhouse on sandy soil, inoculated either with PGPR (Pantoea agglomeransandBacillussp.), or with AMF (Rhizophagus fasciculatusandRhizophagus aggregatum) or co inoculated with PGPR and AMF and watered with a saline solution (0, 150, and 300 mM). After 4 months of cultivation, the plants were harvested and the results obtained showed that inoculation improves the survival rate, height and biomass of the plants compared to the control plants. The results also showed that inoculation increases the total amount of chlorophyll and the accumulation of plant proline at all levels of salt concentration. However,P. agglomeransandBacillussp. strains alone or in combination withR. fasciculatusincreased plant growth. This study showed that these strains of PGPR, whether or not associated with AMF, could be biological tools to improveC. obesaperformance under saline stress conditions.

Land salinization is a major constraint for the practice of agriculture in the world. Considering the extent of this phenomenon, the rehabilitation of ecosystems degraded by salinization has become a priority to guarantee food security in semi-arid environments. The mechanical and chemical approaches for rehabilitating salt-affected soils being expensive, an alternative approach is to develop and utilize biological systems utilizing salt-tolerant plant species. Casuarina species are naturally halotolerant, but this tolerance has been shown to be improved when they are inoculated with arbuscular mycorrhizal fungi (AMF) and/or nitrogen-fixing bacteria (Frankia). Furthermore, Casuarina plantations have been proposed to promote the development of plant diversity. Thus, the aim of the current study was to evaluate the impact of a plantation comprising the species Casuarina inoculated with AMF and Frankia on the diversity of the sub-canopy and adjacent vegetation. Work was conducted on a plantation comprising Casurina equisetifolia and C. glauca variously inoculated with Frankia and Rhizophagus fasciculatus prior to field planting. The experimental area of 2500 m² was divided into randomized blocks and vegetation sampling was conducted below and outside of the Casuarina canopy in 32 m² plots. A total of 48 samples were taken annually over 3 years, with 24 taken from below the Casuarina canopy and 24 from outside the canopy. The results obtained show that co-inoculation with Frankia and Rhizophagus fasciculatus improves the height and survival rate of both species. After 4–5 years, there was greater species diversity and plant biomass in the sub-canopy environment compared with that of the adjacent environments. Our results suggest that inoculation of beneficial microbes can improve growth of Casuarina species and that planting of such species can improve the diversity of herbaceous vegetation in saline environments.

Plants of the Casuarinaceae family are widely known for their ability to tolerate salt stress. Casuarinaceae are able to develop symbiotic association with arbuscular mycorrhizal fungi and with the soil bacteria Frankia. The aim of this study was to evaluate adaptation mechanisms of Casuarina to tolerate salt stress when interacting Rhizophagus fasciculatus and Frankia. Two species showing different salt tolerance levels, i.e. C. equisetifolia and C. obesa, were grown in sandy sterile soil, inoculated with R. fasciculatus and/or Frankia and then watered gradually with increased concentrations of saline solutions. Total antioxidant activity, antioxidant enzyme concentration, and salt effects on cellular ultrastructure of shoots were evaluated. C. obesa has a better salt stress tolerance compared to C. equisetifolia. Co-inoculation (R. fasciculatus and Frankia) improved the performance of both plant species in saline. Higher antioxidant activity was observed in C. obesa. At 400 mM of NaCl C. obesa revealed a maintenance of cellular integrity whereas cell membrane rupture and disintegration of cellular contents were observed in C. equisetifolia tissues. Our results suggest that a selection of appropriate plant species is important to improve plant performance in saline soils.