Competition, Nodule Occupancy, and Persistence of Inoculant Strains: Key Factors in the Rhizobium-Legume Symbioses

Abstract

Biological nitrogen fixation by Rhizobium-legume symbioses represents an environmentally friendly and inexpensive alternative to the use of chemical nitrogen fertilizers in legume crops. Rhizobial inoculants, applied frequently as biofertilizers, play an important role in sustainable agriculture. However, inoculants often fail to compete for nodule occupancy against native rhizobia with inferior nitrogen-fixing abilities, resulting in low yields. Strains with excellent performance under controlled conditions are typically selected as inoculants, but the rates of nodule occupancy compared to native strains are rarely investigated. Lack of persistence in the field after agricultural cycles, usually due to the transfer of symbiotic genes from the inoculant strain to naturalized populations, also limits the suitability of commercial inoculants. When rhizobial inoculants are based on native strains with a high nitrogen fixation ability, they often have superior performance in the field due to their genetic adaptations to the local environment. Therefore, knowledge from laboratory studies assessing competition and understanding how diverse strains of rhizobia behave, together with assays done under field conditions, may allow us to exploit the effectiveness of native populations selected as elite strains and to breed specific host cultivar-rhizobial strain combinations. Here, we review current knowledge at the molecular level on competition for nodulation and the advances in molecular tools for assessing competitiveness. We then describe ongoing approaches for inoculant development based on native strains and emphasize future perspectives and applications using a multidisciplinary approach to ensure optimal performance of both symbiotic partners.

A selective bottleneck during host entry drives the evolution of new legume symbionts

“…At this stage, we cannot distinguish between two hypotheses, namely if these mutations do not exist (proliferation is always linked to nodulation and host entry) or if they were not yet selected in this experiment due to the dominance of nodulation competitiveness in the first steps of adaptation. Future functional analyses of the adaptive mutations identified in this study will expand our understanding of the molecular bases of nodulation competitiveness in rhizobia [65][66][67] , and its relationship to within-host proliferation. Interestingly, these two traits primarily selected by the plant, although necessary for the symbiotic interaction, are not, in themselves, beneficial for the host.…”

“…Moreover the identification of plant receptors (LYK3, NFR1, Sym37, SYMRK) and transcription factors (NIN, ERN1, ERN2) involved in both nodule organogenesis and rhizobial intracellular infection [67][68][69][70] supports the existence of common mechanisms controlling the two processes on the plant side. Future functional analyses of the adaptive mutations identified in this study will expand our understanding of the molecular bases of nodulation competitiveness in rhizobia 60,71,72 , and its relationship to within-host proliferation. Including mutualistic traits (nitrogen fixation and host growth promotion) in the analysis of the genetic couplings (or trade-offs) between the different symbiotic traits will be important to fully characterize the genetic constraints shaping the evolution of rhizobium-legume interactions [72][73][74] .…”

Assessment of Bacterial Inoculant Delivery Methods for Cereal Crops

“…Two inoculation methods suitable for field planting, alginate and 12 h coating were tested under field condition but only a negligible amount of inoculated bacteria were detected in the rhizosphere. This may have been because the bacterial isolate concentration used to coat the seeds (10 3 -10 4 CFU per seed) was too low to facilitate their establishment in sorghum rhizosphere under non-sterile conditions where there is competition from the natural microbial communities (Mendoza-Suárez et al, 2021). Although a standard of 10 4 rhizobial cells per seed is widely used to inoculate legumes with medium-size seed (e.g., mung bean and pigeon pea) like sorghum (Lupwayi et al, 2000), this may not be applicable for the inoculation of non-rhizobia species and for non-legumes.…”

Designing Synergistic Biostimulants Formulation Containing Autochthonous Phosphate-Solubilizing Bacteria for Sustainable Wheat Production

“…Although an elite strain with a high and consistent PS activity is an essential prerequisite for the development of successful inoculant, the non-biological components for bioformulations are still key bottlenecks in commercial development of the inoculants (Mendoza-Suárez et al, 2021 ). The choice of carrier material is fundamental, because carrier is the sole delivery vehicle of live microorganism from the production unit to the plants in the field (Vassilev et al, 2020 ).…”