Nodule Organogenesis and Symbiotic Mutants of the Model Legume Lotus japonicus

Abstract

A detailed microscopical analysis of the morphological features that distinguish different developmental stages of nodule organogenesis in wild-type Lotus japonicus ecotype Gifu B-129-S9 plants was performed, to provide the necessary framework for the evaluation of altered phenotypes of L. japonicus symbiotic mutants. Subsequently, chemical ethyl methanesulfonate (EMS) mutagenesis of L. japonicus was carried out. The analysis of approximately 3,000 M1 plants and their progeny yielded 20 stable L. japonicus symbiotic variants, consisting of at least 14 different symbiosis-associated loci or complementation groups. Moreover, a mutation affecting L. japonicus root development was identified that also conferred a hypernodulation response when a line carrying the corresponding allele (LjEMS102) was inoculated with rhizobia. The phenotype of the LjEMS102 line was characterized by the presence of nodule structures covering almost the entire root length (Nod++), and by a concomitant inhibition of both root and stem growth. A mutation in a single nuclear gene was shown to be responsible for both root and symbiotic phenotypes observed in the L. japonicus LjEMS102 line, suggesting that (a) common mechanism(s) regulating root development and nodule formation exists in legumes.

Development of N2‐fixing nodules on the wetland legume Lotus uliginosus exposed to conditions of flooding

“…One route for infection occurs in lateral roots of both flooded and non-flooded L. uliginosus, which were infected via root hairs. This is the most commonly observed infection process in typical indeterminate and determinate temperate and\or crop legumes , where newly-formed root hairs are the most infectible sites (Hirsch, 1992), and has been reported previously with non-flooded L. corniculatus and L. japonicus (Ranga Rao, 1977 ;Vance et al, 1982 ;Szczyglowski et al, 1998).…”

“…Since L. pedunculatus is a synonym for L. uliginosus (Grant & Small, 1995), the latter observation is particularly relevant to the present study. Thus, all available evidence (Ranga Rao, 1977 ;Pankhurst et al, 1979 ;Vance et al, 1982 ;Szczyglowski et al, 1998 ; this study) suggests that rhizobial infection through both root hair and nonroot hair processes is common among Lotus spp., and that the relative occurrence of these two routes may vary according to species and\or growth conditions (e.g. flooding).…”

“…In these nodules, infection threads were much smaller and relatively scarce ( Fig. 4c) compared with earlier stages and, as with L. corniculatus and L. japonicus (Vance et al, 1982 ;Szczyglowski et al, 1998), further development of the infected tissue was by division and expansion of already infected cells, rather than by new infections. In contrast to MAC236, MAC265 labelled infection threads and infection droplets relatively lightly (compare the silver-enhanced sections in Fig.…”

“…As a very low O # concentration is necessary for the expression and function of nitrogenase (Gallon, 1992), it is essential that an O # -diffusion barrier be formed in young nodules before N # fixation can occur within them (Sheehy & Thornley, 1988) potential role of the MAC236\MAC265 glycoprotein in the formation of such a barrier has previously been shown in terrestrially grown lupin nodules by James et al (1997), and the pattern of localization shown by the present study suggests that it may have a similar function in Lotus nodules, even under the low O # concentrations created by flooding. Although large infection threads have been recently reported by Szczyglowski et al (1998) in young nodules of L. japonicus, most previous studies of the development of nodules on Lotus (Pankhurst et al, 1979 ;Vance et al, 1982 ;Wood et al, 1985) have suggested that infection threads and infection droplets are relatively scarce and fairly small structures, at least in comparison to indeterminate nodules such as those found on pea (Newcomb et al, 1979 ;Rae et al, 1992). However, the present study has shown that, for a short period during nodule development, there is massive proliferation of quite large infection threads within L. uliginosus, with many of them releasing MAC236-labelled material into host cells more or less simultaneously, and hence producing the dense immunolabelling shown in Fig.…”

Invasion of Lotus japonicus root hairless 1 by Mesorhizobium loti Involves the Nodulation Factor-Dependent Induction of Root Hairs

“…japonicus interacts with its natural microsymbiont M. loti to form nitrogen-fixing nodules in a root hairdependent manner (Szczyglowski et al, 1998). On the cytological level, the major initial steps in this interaction comprise modification of root hair growth and initiation of the infection process that culminates in the development of an IT.…”