Lakhasly

Global fertilizer demand, driven by population and economic growth, has environmental consequences. High costs and negative impacts (nitrate contamination, soil acidification, greenhouse gas emissions) associated with nitrogen fertilizers fuel interest in plant growth-promoting bacteria (PGPB) inoculants. While plant-microbe interactions are well-studied, their agricultural application remains limited. Chemical phosphate is inefficiently used by plants, with 75-90% becoming fixed in soil. Biological nitrogen fixation (BNF) offers a sustainable alternative, being less prone to loss and used in situ. The management of plant-soil-bacteria interactions offers increased crop productivity, reduced fertilizer use, and better environmental resource conservation. Beneficial bacteria enhance plant growth, seed germination, and disease resistance. Rhizobia, particularly in soybean-Bradyrhizobium associations, show high BNF efficiency, reducing the need for nitrogen fertilizers and saving billions annually (e.g., ~$7 billion in Brazil for soybeans). Azospirillum species, effective in wheat and maize, also demonstrate plant growth-promoting properties, with field trials showing positive results and leading to commercial inoculant authorization in some countries. Partial fertilizer replacement with Azospirillum could save an estimated US$ 1.2 billion.

The demand for chemical fertilizers in agriculture has historically been influenced by
interrelated factors such as population growth worldwide, economic growth, agricul- tural production, among others (Morel et al., 2012). Interest in the use of inoculants containing PGPB that promote plant growth and yield has increased because nitrogen fer- tilizers are expensive and can damage the environment through water contamination with nitrates, acidification of soils and greenhouse-gas emissions (Adesemoye et al., 2009; Hungria et al., 2013). Plant-microorganism associa- tions have long been studied, but their exploitation in agri- culture for partially or fully replacing nitrogen fertilizers is still low (Hungria et al., 2013). Moreover, plants can only use a small amount of phosphate from chemical sources, because 75-90% of the added P is precipitated through metal-cation complexes and rapidly becomes fixed in soils (Sharma et al., 2013). Approximately 42 million tons of ni- trogenous fertilizers are applied annually on a global scale for the production of the three major crop cereals: wheat, rice, and maize. Annually, 8x 1010 kg of NH3 are produced by nitrogenous fertilizer industries, while 2.5 x 1011 kg of NH3 are fixed through BNF (Cheng, 2008). The nitrogen provided by BNF is less prone to leaching, volatilization and denitrification, as this chemical is used in situ and is therefore considered an important biological process that contributes to sustainable agriculture (Dixon and

Kahn. 2004).

The management of bacteria, soil and plant interac- tions has emerged as a powerful tool in view of the biotech- nological potential of these interactions, evidenced by increased crop productivity, reduction of production costs by reducing the volume of fertilizers applied and a better conservation of environmental resources. Moreover, ino- culants are composed of beneficial bacteria that can help the plant meet its demands for nutrients. As previously dis- cussed, these bacteria increase plant growth, accelerate seed germination, improve seedling emergence in response to external stress factors, protect plants from disease, and promote root growth using different strategies (Table 1). Whether gram- negative or gram-positive, these bacteria re- quire isolation in culture media and analysis
of various genotypic and phenotypic aspects, as well as analysis re- garding their beneficial interaction with the host plant in experimental and natural conditions.

Rhizobia species are well investigated because of their symbiotic relationship with leguminous plants and their agronomical application as inoculants in the cultiva- tion of economic crops (Alves et al., 2004; Torres et al., 2012). The soybean-Bradyrhizobium association is a good example of the efficiency of BNF, and B. elkanii and B. japonicum are species commonly used to inoculate this le- guminous plant. In this system, the BNF is so efficient that attempts to increase grain yields by adding nitrogenous fer- tilizers are not successful in plants effectively inoculated with the recommended Bradyrhizobium strains (Alves et al., 2004). In Brazil, where approximately 70% of the ni- trogenous fertilizers are imported, the costs of mineral N utilization in agriculture are high, and inoculants are a more cost-effective alternative, particularly for soybean crops. It is estimated that, in this
culture alone, Brazil saves approxi- mately US$ 7 billion per year thanks to the benefits of
BNF (Hungria et al., 2013).In the last few decades, a large array of bacteria asso- ciated with non-leguminous plants, including Azospirillum species, have demonstrated plant growth-promoting prop- erties (Okon and Labandrera-Gonzalez, 1994; Garcia de Salamone et al., 1996; Bashan et al., 2004; Cassán and Gar- cia de Salamone, 2008; Hungria et al., 2010). Azospirillum might promote the growth, yield and nutrient uptake of dif- ferent plant species of agronomic importance, particularly wheat and maize (Hungria et al., 2010). Inoculants contain- ing Azospirillum have been tested under field conditions in Argentina, with positive results regarding plant growth and/or grain yield (Cassán and Garcia de Salamone, 2008). In Brazil, field experiments designed to evaluate the perfor- mance of A. brasilense strains isolated from maize plants showed effectiveness in both maize and wheat. These re- sults were grounds for the authorization of the first strains of inoculants to be produced and commercially used in wheat and maize in this country. According to the authors (Hungria et al., 2010), the partial (50%) replacement of the nitrogenous fertilizer required for these crops in association with Azospirillum sp. inoculation would save an estimated US$ 1.2