Nodule-associated bacteria alter the mutualism between arbuscular mycorrhizal fungi and N2 fixing bacteria

Abstract

Legumes can interact simultaneously with multiple microbial partners in symbiosis, including arbuscular mycorrhizal fungi (AMF), N2-fixing bacteria, and nodule-associated bacteria. However, our understanding of their functional diversity and interactions remains limited. The objective of this study was to evaluate individual and combined effects of these three symbioses on the abundance and function of the symbionts, including the growth, nitrogen content, nodulation and root traits of Mimosa scabrella (Bracatinga), a common leguminous tree in South America. We grew seedlings in a growth chamber for 90 days using sterile soil inoculated with all combinations of AMF, N2 fixing and nodule-associated bacteria. We found that the tripartite symbiosis formed by host, AMF and N2 fixing bacteria increased shoot biomass 3.7x and N content 5.2x relative to non-inoculated controls. This response was 71% greater than the sum of the respective individual treatments, confirming for the first time in M. scabrella a synergistic effect of such dual inoculation. In contrast, the presence of nodule-associated bacteria in a tetrapartite symbiosis resulted in a 57.7% reduction in the number of nodules in any treatment where N2 fixing bacteria were present. Concurrently, total AM root colonization decreased by approximately 50% in the tetrapartite compared to the tripartite symbiosis. We conclude that the tripartite symbiosis formed by M. scabrella, N2 fixing bacteria, and AMF can act synergistically in promoting plant biomass and N2 fixation. However, nodule-associated bacteria can act “opportunistically”, causing a detrimental effect on the tripartite mutualism. Our findings highlight the functional complexity of multipartite symbioses and how interactions among partners can determine biological processes and outcomes.