Sesbania rostrata: a case study of natural variation in legume nodulation

Legumes acquired the ability to engage in a symbiotic interaction with soil-borne bacteria and establish a nitrogen-fixing symbiosis in a novel root organ, the nodule. Most legume crops and the model legumes Medicago truncatula and Lotus japonicus are infected intracellularly in root hairs via infection threads that lead the bacteria towards a nodule primordium in the root cortex. This infection process, however, does not reflect the great diversity of infection strategies that are used by leguminous plants. An alternative, intercellular invasion occurs in the semi-aquatic legume Sesbania rostrata. Bacteria colonize epidermal fissures at lateral root bases and trigger cortical cell death for infection pocket formation and subsequent intercellular and intracellular infection thread progression towards the primordium. This infection mode evolved as an adaptation to waterlogged conditions that inhibit intracellular invasion. In this review, we discuss the molecular basis for this adaptation and how insights into this process contribute to general knowledge of the rhizobial infection process.