Damiano Spagnuolo

In sustainable agriculture, plant nutrients are the most important elements. Biofertilisers introduce microorganisms that improve the nutrient status of plants and increase their accessibility to crops. To meet the demands of a growing population, it is necessary to produce healthy crops using the right type of fertilisers to provide them with all the key nutrients they need. However, the increasing dependence on chemical fertilisers is destroying the environment and negatively affecting human health. Therefore, it is believed that the use of microbes as bioinoculants, used together with chemical fertilisers, is the best strategy to increase plant growth and soil fertility. In sustainable agriculture, these microbes bring significant benefits to crops. In addition to colonising plant systems (epiphytes, endophytes and rhizospheres), beneficial microbes play a key role in the uptake of nutrients from surrounding ecosystems. Microorganisms, especially fungi, also play a protective function in plants, enhancing the responses of defence systems, and play a key role in situations related to soil iron deficiency or phosphorous solubilisation. Plant-associated microbes can thus promote plant growth regardless of natural and extreme conditions. The most frequently used strategies for growth-promoting microorganisms are nitrogen fixation, the production of growth hormones, siderophores, HCN, various hydrolytic enzymes and the solubilisation of potassium, zinc and phosphorous. Research on biofertilisers has been extensive and available, demonstrating how these microbes can provide crops with sufficient nutrients to increase yields. This review examines in detail the direct and indirect mechanisms of PGPR action and their interactions in plant growth and resistance.