How rhizobial symbionts invade plants: the Sinorhizobium–Medicago model

Abstract

Symbiotic nitrogen-fixing rhizobial bacteria and leguminous plants have evolved complex signal exchange mechanisms that allow a specific bacterial species to induce its host plant to form invasion structures through which it enters the plant root. Once these invasion structures reach the target cells in the interior of the plant root, the bacteria are endocytosed within a host cell membrane-derived compartment. In the microaerobic environment provided by the host cell, the bacteria differentiate into a specialized form called a bacteroid. The bacteroid form expresses the oxygen-sensitive enzyme nitrogenase that catalyzes the conversion of atmospheric nitrogen to ammonia. The dissection of the bacterial and plant signalling pathways that are involved in each stage of the invasion process has been facilitated by the complete genomic sequencing of Sinorhizobium meliloti and the near complete sequencing of the genome of the model host plant Medicago truncatula. Rhizobial bacteria interact very differently with the plant innate immune system than other groups of bacteria. Rhizobia lack some of the microbial molecular patterns that provoke plant defence responses. Additionally, legume plants differ from other plant families in that they lack the ability to perceive and respond defenceively to other microbial molecular patterns. Symbiotic rhizobial bacteria are similar to pathogenic bacteria such as Brucella spp, in that they both form chronic infections of eukaryotic cells within a host-derived membrane compartment, and require some of the same bacterial factors for survival within the host. These factors include the correct structure of the lipopolysaccharide core and lipid A, presence of cyclic β-glucans, and a common bacterial regulatory circuitry.