Biological nitrogen fixation (BNF) refers to a microbial mediated process based upon an enzymatic “Nitrogenase” conversion of atmospheric nitrogen (N2) into ammonium readily absorbable by roots. N2-fixing microorganisms collectively termed as “diazotrophs” are able to fix biologically N2 in association with plant roots. Specifically, the symbiotic rhizobacteria induce structural and physiological modifications of bacterial cells and plant roots into specialized structures called nodules. Other N2-fixing bacteria are free-living fixers that are highly diverse and globally widespread in cropland. They represent key natural source of nitrogen (N) in natural and agricultural ecosystems lacking symbiotic N fixation (SNF). In this review, the importance of Azotobacter species was highlighted as both important free-living N2-fixing bacteria and potential bacterial biofertilizer with proven efficacy for plant nutrition and biological soil fertility. In addition, we described Azotobacter beneficial plant promoting traits (e.g., nutrient use efficiency, protection against phytopathogens, phytohormone biosynthesis, etc.). We shed light also on the agronomic features of Azotobacter that are likely an effective component of integrated plant nutrition strategy, which contributes positively to sustainable agricultural production. We pointed out Azotobacter based-biofertilizers, which possess unique characteristics such as cyst formation conferring resistance to environmental stresses. Such beneficial traits can be explored profoundly for the utmost aim to research and develop specific formulations based on inoculant Azotobacter cysts. Furthermore, Azotobacter species still need to be wisely exploited in order to address specific agricultural challenges (e.g., nutrient deficiencies, biotic and abiotic constraints) taking into consideration several variables including their biological functions, synergies and multi-trophic interactions, and biogeography and abundance distribution.

Background Higher absorption and translocation of sodium (Na) and chlorine (Cl) ions in plant tissue can lead to serious physiological and biochemical changes. However, salicylic acid (SA) is a natural signaling molecule responsible for the induction of environmental stress tolerance in plants. Spraying SA could provide protection against several types of stress such as salinity. This study aimed to show the influence of SA spraying (0.5 and 1 mM) on the damaging effects of NaCl toxicity (150 mM) in Salvia officinalis L. plants. Results The results showed that salinity strongly inhibited the growth of aerial and root parts and this inhibition was accompanied by a significant decrease in the production of chlorophyll pigments (by 63%). There was also a significant accumulation of Na, mainly in the roots. This accumulation of Na+ ions was accompanied by a decrease of calcium (Ca), potassium (K) and phosphorus (P) concentrations. However, SA mainly at 0.5 mM, greatly improved plant growth, essential oils and chlorophyll pigments synthesis. Besides, SA led to a decrease in Na content and an improvement in Ca, K and P content in the leaves and roots. Salt stress decreased the essential oil yield from 1.2% (control) to 0.4% (NaCl). Furthermore, gas chromatography–mass spectrometry analysis of essential oils exhibited that the 1,8-cineol, α-thujone, and camphor were identified as the main components of essential oils under all treatments. However, we noted in stressed plant treated or not with SA the appearance of the new majority compound thujanone. Salt stress decreased the major compounds content. SA spray under stress condition increased the content of major compounds compared to stressed plants untreated with SA. The histological study in scanning electron microscopy showed the peltate glands density decreased strongly under NaCl toxicity. However, SA application on stressed plants increased peltate glands density. On the other hand, the glands of stressed plants often show certain anomalies in the morphology: the first anomaly observed was the presence of glandular structures characterized by deformations in the form of small protuberances located on the head of the gland. The second, a less common abnormality is the morphological change in certain glands that change from a spherical to an ovoid shape. On another hand, all these anomalies were not detected in stressed plants sprayed with SA. Therefore, the absence of these anomalies under the effect of SA showed the repairing effect of this growth regulator. Conclusion The findings of the present work suggest that spraying of SA may be useful for improving the plant growth in NaCl-contaminated areas.

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