Mesorhizobium loti

Mesorhizobium loti partners with Lotus japonicus, forming a model symbiosis that complements the Sinorhizobium-Medicago system. Where Medicago forms indeterminate nodules that grow continuously from a persistent meristem, Lotus forms determinate nodules that reach fixed size. These two nodule architectures represent different solutions to the same challenge, and comparing them reveals which features are essential to nitrogen-fixing symbiosis and which are variable.

The M. loti-Lotus system has become particularly important for genetic studies because both partners have small, sequenced genomes and are amenable to genetic manipulation. Forward genetic screens—mutating genes randomly and looking for symbiosis defects—identified dozens of plant genes essential for nodulation. These discoveries revealed that nodulation evolved by repurposing an ancient pathway for arbuscular mycorrhizal symbiosis, itself dating back 450 million years. Plants didn't invent nodulation machinery from scratch; they modified existing partnership infrastructure.

M. loti acquired its symbiosis genes on a mobile 'symbiosis island'—a large chromosomal region that can transfer between bacterial strains. This horizontal gene transfer means that non-symbiotic soil bacteria can acquire symbiotic capability in a single transfer event. The existence of symbiosis islands explains how nitrogen-fixing symbiosis spreads across bacterial lineages and suggests that engineering nitrogen fixation might target these transferable genetic modules.