Rhizobia–Legume Symbiosis Increases Aluminum Resistance in Alfalfa

Abstract

Alfalfa is the most important forage legume with symbiotic nitrogen-fixing nodule in roots, but it is sensitive to aluminum (Al), which limits its plantation in acidic soils. One rhizobia clone of Sinorhizobium meliloti with Al tolerance (AT1) was isolated from the nodule in AlCl3-treated alfalfa roots. AT1 showed a higher growth rate than the standard rhizobia strain Sm1021 under Al-stressed conditions. Alfalfa growth was improved by inoculation with AT1 under Al-stressed conditions, with increased length and fresh weight in shoots and roots. High nitrogenase activity and pink effective nodules were obtained in AT1-inoculated plant roots under Al stress, with increased total nitrogen compared with the non-inoculated control. The application of exogenous NH4+-nitrogen increased the Al resistance in alfalfa. It is suggested that rhizobia’s increase of the Al resistance in alfalfa is associated with its improved nitrogen status. Inoculation with Al-tolerant rhizobia is worth testing in an acidic field for improved alfalfa productivity.

Symbiosis under Abiotic Stress and Its Challenges

“…In this order, Al is the most abundant metal, becomes more soluble as acidity increases, and is often the major toxic element in acidic soils and water. Many hypotheses have been proposed for the mechanism of Al toxicity in animals and plants [52][53][54][55]. However, Al-microorganism interactions have been receiving attention in recent years.…”

“…However, Al-microorganism interactions have been receiving attention in recent years. Previous work on rhizobia has investigated direct and indirect mechanisms of Al-toxicity [53][54][55]. It has been shown that Rhizobium cells suggest that the repair mechanism used to overcome damage by Al to DNA in tolerant cells is quite different from that in sensitive cells, indicating that DNA is a possible site of action of Al in the common soil bacterium as Rhizobium [53]; this is an important factor for ecological applications.…”

“…It has been shown that Rhizobium cells suggest that the repair mechanism used to overcome damage by Al to DNA in tolerant cells is quite different from that in sensitive cells, indicating that DNA is a possible site of action of Al in the common soil bacterium as Rhizobium [53]; this is an important factor for ecological applications. Many experiments demonstrated that using acid and Al-tolerant nitrogen-fixing bacteria is one alternative way to increase productivity in the field [53][54][55]. For example, soybean inoculated with B. japonicum strain BJ11 increased the production under acid-aluminum soil (pH 5.0-5.5) [56].…”

“…Al-toxicity is a major problem affecting soil fertility, microbial diversity, and nutrient uptake of plants. Rhizobia response and legume interaction under Al conditions are still unknown [54], and it is important to understand how to develop and improve legume cultivation under Al stress [55]. The rhizobia response was recorded under different Al concentrations, with decreasing rhizobial cell numbers as Al concentration increased.…”

Competitiveness and symbiotic efficiency in alfalfa of Rhizobium favelukesii ORY1 strain in which homologous genes of peptidases HrrP and SapA that negatively affect symbiosis were identified

“…Failures in alfalfa (Medicago sativa) production have been attributed to soil acid pH, aluminium toxicity (Bouton, 2012;Jaiswal et al, 2018;Shi et al, 2022) and the presence of parasite rhizobia strains that inefficiently nodulate alfalfa (Torres Tejerizo et al, 2011;Eardly et al, 2022). These parasite strains, known as Oregon-like, have tolerance to acidic conditions, a host broad range (Eardly et al, 1992;Del Papa et al, 1999), and its presence has also been confirmed in USA and Canada (Bromfield et al, 2010).…”