Genetics and functional genomics of legume nodulation

Abstract

Gram-negative soil bacteria (rhizobia) within the Rhizobiaceae phylogenetic family (a-proteobacteria) have the unique ability to infect and establish a nitrogen-fixing symbiosis on the roots of leguminous plants. This symbiosis is of agronomic importance, reducing the need for nitrogen fertilizer for agriculturally important plants (e.g. soybean and alfalfa). The establishment of the symbiosis involves a complex interplay between host and symbiont, resulting in the formation of a novel organ, the nodule, which the bacteria colonize as intracellular symbionts. This review focuses on the most recent discoveries relating to how this symbiosis is established. Two general developments have contributed to the recent explosion of research progress in this area: first, the adoption of two genetic model legumes, Medicago truncatula and Lotus japonicus, and second, the application of modern methods in functional genomics (e.g. transcriptomic, proteomic and metabolomic analyses). IntroductionNodulation is a highly host-specific interaction in which, with few exceptions, specific rhizobial strains infect a limited range of plant hosts. Plants secrete (iso)flavonoids that are recognized by the compatible bacteria, resulting in the induction of nodulation genes. These nodulation genes encode enzymes that synthesize a specific lipochitin nodulation signal (Nod signal), which activates many of the early events in the root hair infection process [1][2][3][4][5]. During the infection process, the bacteria enter the plant via the root epidermis and induce the reprogramming of root cortical cell development and the formation of a nodule. In the most well-studied cases, infection occurs through root hairs. The first observable event in the infection process is the curling of the root hair, which likely occurs through the gradual and constant reorientation of the direction of root hair growth. The bacteria become enclosed within the root hair curl where the plant cell wall is degraded, the cell membrane is invaginated and an intracellular tubular structure (i.e. the infection thread) is initiated. It is within this infection thread that the bacteria enter the root hair cell and eventually ramify into the root cortex. Before the infection thread reaches the base of the root hair cell, the root cortical cells are induced to de-differentiate, activating their cell cycle and causing them to divide to form the nodule primordium. In addition to the cortical cells, pericycle cells are also activated and undergo some cell divisions. When the infection thread reaches the cells of the developing primordium, the bacteria are released into cells via endocytosis. Inside a plant cell, the bacteria are enclosed in vacuole-like structures (symbiosomes) in which they differentiate into bacteroids. It is within these symbiosomes that the bacteria convert N 2 to NH 3 . The nodule is a true organ in which there is cellular specialization. For example, in addition to infected plant cells, uninfected plant cells also carry out the function of nitrogen as...

Community shifts of soybean stem-associated bacteria responding to different nodulation phenotypes and N levels

“…However, many questions regarding the driving forces and ecological rules underlying these relationships remain unanswered (Saito et al, 2007;Hardoim et al, 2008). During their evolution, legumes developed nodulation and mycorrhization systems to attain mutual symbiosis with rhizobia and mycorrhizae (Harrison, 2005;Stacey et al, 2006). Although plants are known to control the degrees of nodulation and mycorrhization of roots by rhizobia and mycorrhizae, respectively, through autoregulation systems (Carroll et al, 1985;Meixner et al, 2007), the effects of the autoregulation systems on the other plant-associated microorganisms in the phytosphere remain unclear (Parniske, 2000).…”

Investigation of the Demographic and Selective Forces Shaping the Nucleotide Diversity of Genes Involved in Nod Factor Signaling in Medicago truncatula

“…Functional interpretation of a potential event of selection in DMI1: The current working model of Nod factor signaling (Stacey et al 2006) assumes that DMI1 plays a role in initiating Ca 21 spiking and that the spiking mediates the gene activation (through DMI3) required for the formation of the infection thread (the first step mediating the entry of the symbiotic bacteria into the roots of Fabaceae). It is not known whether DMI1 is activated by NORK directly or indirectly, or if it is activated by independent means.…”

Calcium et signalisation Nod

“…Le signal se décompose en une phase d'élévation très rapide de la concentration en Ca 2+ , suivie d'un retour progressif à l'état initial. Ce schéma bi-phasique pourrait correspondre à l'ouverture d'un canal calcique sollicitant un pool interne, puis à sa fermeture et à un re-pompage actif du [8]). Le phénotype du mutant nfp (absence totale de réponse aux facteurs Nod) et la présence, dans le domaine extracellulaire de la protéine, de motifs compatibles avec la fonction biochimique de reconnaissance du squelette oligosaccharidique des facteurs Nod, font de NFP le récepteur présomptif des facteurs Nod.…”