Nodule Organogenesis in Lotus japonicus

Abstract

Interactions between legumes and rhizobia are one of the best-characterized symbioses. Legumes develop nodules in response to rhizobia infection. Nod factors are essential for the recognition by legumes of rhizobia. Nodule primordia arise from root cortex, where vacuolated cells are redifferentiated. Rhizobia infect legumes via root hairs, penetrating via infection threads provided by the plant. After their release from infection threads into plant cells, rhizobia fix aerial nitrogen by differentiating to bacteroids. The mechanism of nodule organogenesis is poorly understood. This is partly because there has been l i l e molecular analysis on nodulation mutants. Recent advances in the use of model legumes, however, have enabled one to characterize nodulation mutants by molecular genetics. In this review, we will focus on the early steps of nodule organogenesis in relation to the analysis of a model legume, Lotus japonicus.

Enhanced Nodulation and Nitrogen Fixation in the Abscisic Acid Low-Sensitive Mutant enhanced nitrogen fixation1 of Lotus japonicus

“…Many legumes establish nitrogen-fixing root nodules following reciprocal signal exchange between the plant and rhizobia (Hayashi et al, 2000;Hirsch et al, 2003). The host plant produces chemical compounds, frequently flavonoids, which induce rhizobial nod genes, whose products are involved in the synthesis and secretion of Nod factor.…”

“…Eventually, the rhizobia are released into nodule cells, enclosed within a membrane, and differentiate into nitrogen-fixing bacteroids that reduce atmospheric nitrogen into ammonia. In return, the host plant supplies photosynthetic products, to be used as carbon sources, to the rhizobia (Zuanazzi et al, 1998;Hayashi et al, 2000).…”

crinkle, a Novel Symbiotic Mutant That Affects the Infection Thread Growth and Alters the Root Hair, Trichome, and Seed Development in Lotus japonicus

“…A valuable tool for understanding the nodulation process at the molecular level is the characterization of symbiotic mutants. In L. japonicus, detailed analyses of nodule organogenesis have been reported (Szczyglowski et al, 1998;Hayashi et al, 2000;van Spronsen et al, 2001) that provide the basic framework for the evaluation of nodulation mutants.To date, several L. japonicus mutants with altered nodule phenotypes have been isolated and characterized (Imaizumi-Anraku et al, 1997;Schauser et al, 1998;Szczyglowski et al, 1998; Bonfante et al, 2000;Wopereis et al, 2000;Kawaguchi et al, 2002), but few symbiotic genes have been cloned. The L. japonicus nodule inception (Nin) gene, identified using a transposon-tagged symbiotic mutant, was the first plant gene responsible for nodule formation to be isolated (Schauser et al, 1999).…”

“…A valuable tool for understanding the nodulation process at the molecular level is the characterization of symbiotic mutants. In L. japonicus, detailed analyses of nodule organogenesis have been reported (Szczyglowski et al, 1998;Hayashi et al, 2000;van Spronsen et al, 2001) that provide the basic framework for the evaluation of nodulation mutants.…”

Shedding light on an underground problem

“…Inside the nodule, the internalized bacteria differentiate into a specialized form that can fix atmospheric nitrogen. The decision of whether to nodulate is influenced by the availability of nitrogen in the soil, ethylene levels in the root, how many nodules it already has, and probably much more (12)(13)(14).…”